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Uranium geochemistry on the Amazon Shelf: Chemical phase partitioning and cycling across a salinity gradient
Abstract
The size distribution of U was examined in surface waters of the Amazon shelf. Water samples were collected during a low discharge river stage across a broad salinity gradient (0.3–35.4%) and fractionated by planar filtration and tangential-flow ultrafiltration into (1) solution (Us, <10,000 MW; ∼1–10 nm), (2) colloidal (Uc 10,000 MW-0.4 μm), (3) dissolved (Ud, <0.4 μm), and (4) particulate (Up, >0.4 μm) phases.Concentrations of colloidal U comprise up to 92% of the dissolved U fraction at the river mouth and attain highest values (∼0.45 μg/L) in the productive, biogenic region of the Amazon shelf (salinities above ∼20%). Ud and Uc distributions are highly nonconservative relative to ideal dilution of river water and seawater, indicating extensive removal at salinities below ∼10%. The distribution of Us also shows some nonconservative behavior, yet removal, if any, is minimal. Saltwater-induced precipitation and aggregation of riverine colloidal material is most likely the dominant mechanism of U removal in the low salinity, terrigenous region of the Amazon shelf. There is evidence of a substantial colloidal U input (∼245% of the riverine Uc flux into surface waters above 5%. Such Uc enrichment most likely is the result of colloidal U-rich porewater diffusion/advection from the seabed and fluid muds or shelf-wide particle-colloid disaggregation. Removal of solution and dissolved phase U via a colloidal intermediate and Uc aggregation was examined in terms of coagulation theory. The highly reactive nature of all U phases on the Amazon shelf suggests that remobilization and fractionation of U may also occur in other river-influenced coastal environments.
... In addition to the variable concentrations and isotopic signatures of dissolved and particulate Mo concentrations in different rivers (Revels et al., 2021), potentially also removal and desorption processes during estuarine mixing of river water and seawater could impact Mo isotopic signatures as well as the terrestrial fluxes of Mo into the oceans. Uranium isotopes have also been used as redox proxies (Andersen et al., 2017;Kendall, 2021), and some studies observed local depletion in the low salinity range for example in the Amazon estuary (Swarzenski et al., 1995), Delaware and Chesapeake estuaries (Sarin and Church, 1994), Tamar estuary (van den Berg et al., 1991) and Forth estuary (Toole et al., 1987). Scavenging of U at low salinity waters could be due to adsorption of U to metal oxides (Fe and Mn) or salt-induced aggregation of riverine colloidal material (Swarzenski et al., 1995). ...
... Uranium isotopes have also been used as redox proxies (Andersen et al., 2017;Kendall, 2021), and some studies observed local depletion in the low salinity range for example in the Amazon estuary (Swarzenski et al., 1995), Delaware and Chesapeake estuaries (Sarin and Church, 1994), Tamar estuary (van den Berg et al., 1991) and Forth estuary (Toole et al., 1987). Scavenging of U at low salinity waters could be due to adsorption of U to metal oxides (Fe and Mn) or salt-induced aggregation of riverine colloidal material (Swarzenski et al., 1995). The main removal mechanism of U from the oceans is precipitation of insoluble U upon reduction to U(IV) in anoxic sediments (Klinkhammer and Palmer, 1991). ...
... At higher salinity where U behaved either conservatively (S ~ 13-26, i.e., where dissolved concentrations deviated <20% from the DL) or largely conservatively (S > 26, where dissolved concentrations deviated between 20 and 40% from the DL) during mixing (Fig. 3A), however, U was predominantly in the soluble phase (<0.015 μm, Fig. 5B). Scavenging of U at low to intermediate salinity waters could be due to precipitation and coagulation of riverine colloidal material (Swarzenski et al., 1995;Swarzenski and McKee, 1998). As the U solidphase concentration is high in the estuary and on the shelf and considerably reduced in the NBC offshore region (Fig. 7), this corroborates the observations from the water column that U might be exported to the sediments in the low salinity region by either scavenging of U, after its reduction to U(IV), with Fe oxyhydroxides or microbially mediated conversion of both species (e.g. ...
The Amazon and the Pará are two major rivers that carry dissolved and suspended particulate trace metals to the Atlantic Ocean. In the dynamic mixing zone of the estuary, competing processes of trace metal sorption and release play a role, which might affect transport to the open ocean. Here we investigate the behavior of dissolved (<0.2 μm), soluble (<0.015 μm) and truly dissolved (<10 kDa and < 1 kDa) molybdenum (Mo), uranium (U), and vanadium (V) during estuarine mixing between river water (S < 1) and seawater (S > 35) end members during the high discharge period, as well as during aging of the plume in its northward flow along the coast. Molybdenum behaved conservatively during estuarine mixing and showed no colloidal fraction, suggesting Mo is solely present in the soluble or even truly dissolved fraction. Uranium behaved mostly conservatively but showed removal in the low salinity range (ca. S < 9). This is potentially due to colloidal flocculation at low salinities, as indicated by colloidal (0.015–0.2 μm) fractions of up to 30% for U but decreasing with increasing salinity until no significant difference could be discerned at S > 10. Vanadium shows a general conservative mixing, but with more scatter in the data than for Mo and U and potential removal at low to mid-salinities. Removal of V to the sediments is also indicated by surface sediment data from the mid-salinity region of the estuary but no size fractionation in the dissolved phase could be observed. Hence, V seems to be predominantly present in the soluble or even truly dissolved phase and export to the sediments might take place through particles >0.2 μm. No considerable removal or release of Mo, U and V was observed in their water column depth profiles, indicating a conservative behavior in the water column of the estuaries studied here. Additionally, we present a comparison of differential pulse adsorptive stripping voltammetry and inductively coupled plasma – mass spectrometry analyses for Mo and V, which showed excellent agreement within analytical uncertainty in this challenging sample material covering the full salinity range from freshwater to seawater.
... The mode and magnitude of U cycling are expected to be diverse among estuaries because every estuary provides a unique environmental setup with the characteristic variation of parameters and processes such as water discharge, suspended matter load, redox conditions within the estuary and across the sediment-water interface, biological productivity and respiration. Existing studies indicate both conservative and nonconservative behavior of U. The removal of U has been observed mainly in the low salinity zones of the estuaries (Borole et al., 1982;Maeda and Windom, 1982;Toole et al., 1987;Carroll and Moore, 1994;Sarin and Church, 1994;Swarzenski et al., 1995Swarzenski et al., , 2004Swarzenski and McKee, 1998;Andersson et al., 2001;Smith et al., 2004;Rahaman et al., 2010) and also in salt-marsh estuaries (Church et al., 1996;Windom et al., 2000;Moore and Shaw, 2008). The removal of U at low salinity has been generally ascribed to its scavenging onto the colloids of the organic phases (e.g., humic acid) and/or Fe-Mn oxyhydroxides (Swarzenski et al., 1995). ...
... Existing studies indicate both conservative and nonconservative behavior of U. The removal of U has been observed mainly in the low salinity zones of the estuaries (Borole et al., 1982;Maeda and Windom, 1982;Toole et al., 1987;Carroll and Moore, 1994;Sarin and Church, 1994;Swarzenski et al., 1995Swarzenski et al., , 2004Swarzenski and McKee, 1998;Andersson et al., 2001;Smith et al., 2004;Rahaman et al., 2010) and also in salt-marsh estuaries (Church et al., 1996;Windom et al., 2000;Moore and Shaw, 2008). The removal of U at low salinity has been generally ascribed to its scavenging onto the colloids of the organic phases (e.g., humic acid) and/or Fe-Mn oxyhydroxides (Swarzenski et al., 1995). A few studies have also reported the release of U from the Fe-Mn oxyhydroxide in the mid-salinity zones (McKee et al., 1987;Marques Jr. et al., 2003). ...
... However, the salinity range over which U is removed varies between the studies and is different even between periods of similar water discharges. The decreasing trends of U in the low-salinity zones can be ascribed to the cycling of U with colloids of Fe-Mn oxyhydroxide or organic colloidal phases, or both (Swarzenski et al., 1995). Interestingly, dissolved U and Mn concentrations exhibit positive correlations ( Fig. 3c-d) for the range of salinity over which both [U] diss and [Mn] diss display decreasing trends. ...
In this study, we discuss the cycling of uranium (U) in the Ganga (Hooghly) River estuary through investigation of the composition of the water samples of six periods of contrasting water discharge, the bulk and exchangeable phases of the coexisting suspended particulate matter (SPM) and the bed sediments. In addition, we evaluate data on urban and industrial effluent waters that drain into the estuary. Similar U concentrations of freshwaters and effluent waters do not indicate the anthropogenic source to be an important contributor of U to the estuary. The removal of dissolved U in the low-salinity zones, observed for five of six study periods, is primarily linked to the cycling of the Fe-Mn oxyhydroxide phases. The compositions of exchangeable phases of SPM provide supportive evidence for the removal of dissolved U. Mass balance calculations show that the degree of removal of dissolved U can be sustained by the observed enhancement of exchangeable U in the SPM. Loss of U from the bed sediments is inferred from the distributions of exchangeable U concentrations as a function of salinity. Multiple observations based on compositions of the exchangeable and bulk phases of bed sediments indicate mobilization and loss of both U and Mn through suboxic diagenesis in the sediment columns followed by tidally-induced sediment resuspension. We estimate that 40-60% of riverine U is removed in the low-salinity zone during the monsoon and post-monsoon periods. More importantly, the salinity range over which U removal occurs varies between the periods of similar water discharge. The estimated U flux from the estuary is ~70% higher in 2012 than in 2013. A significant difference between the annual U fluxes, driven mainly by considerable variation in the magnitude of U removal, suggests that estimates of meaningful annual U fluxes would require data at higher temporal resolution. This study highlights the importance of processes operating in the water column and below the sediment-water interface in the cycling of U in the Hooghly River estuary, where the solute-particle interaction driven by high SPM load is known to influence the cycling of trace metals.
... Those changes also lead to various biogeochemical processes (e.g., precipitation-dissolution, sorption-desorption, flocculation, sedimentation) affecting the abundance and composition of suspended particulate matter (SPM) and dissolved components during estuarine mixing (Wang and You 2013;Jeandel and Oelkers 2015). These processes result in the removal or addition of chemical species, such as nutrients, organic carbon, trace metals, and radionuclides, in estuaries (Swarzenski et al. 1995;Wang et al. 2010;Lin et al. 2012). As a result, these Communicated by Mead Allison estuarine processes will affect chemical budgets in estuaries and the resulting fluxes into the ocean (Boyle et al. 1974;McKee et al. 1987;Brunskill et al. 2003). ...
... Dissolved U enrichment, on the other hand, has been documented for the Yellow River (Huanghe) Estuary, China (Xu and Zhou 1987;Jiang et al. 2007;Sui 2015). The different behaviors of U in various estuaries will result in different flux calculations of dissolved U transport from rivers to the ocean (Swarzenski et al. 1995;Sui 2015). Thus, it is important to quantify the fluxes of U delivered into the ocean after estuarine mixing in order to validate mass-balance calculations for U. Jeandel and Oelkers (2015) reported that materials released from SPM transported by rivers to the ocean have a large effect on the global cycles of many elements. ...
... The dissolved 238 U activity concentrations were somewhat below the theoretical mixing line in the low salinity regions (≤ 5, water sample with salinity~5 was used in experiment C), showing slight removal during the mixing experiment. This mixing behavior of dissolved U was similar to that observed in the Alafia River Estuary (Swarzenski and Baskaran, 2007) and in the Amazon River plume (Swarzenski et al. 1995(Swarzenski et al. , 2004. The removal of dissolved U in both experiments A and B was similar in spite of the fact that there was no SPM in experiment B. ...
In order to examine the mixing behavior of dissolved uranium (U) in estuaries under different suspended particulate matter (SPM) regimes, three laboratory-based experiments were conducted by mixing seawater with river water containing different concentrations of SPM. Comparing this study with other field and laboratory-based experiments, dissolved U behaved differently depending upon the concentration of SPM. When SPM concentrations are > 0.8 g/L in the Yellow River, desorption/dissolution of U from SPM becomes predominant and dissolved U is enriched relative to the theoretical mixing line. However, when SPM concentrations are < 0.8 g/L, dissolved U behaves conservatively with some extent of removal during estuarine mixing. ²³⁴U/²³⁸U activity ratios were somewhat constant showing no measurable isotopic fractionation during physical mixing and U sorption/desorption to/from particles. Addition of dissolved ²³⁸U desorbed/dissolved from SPM during the annual Yellow River water-sediment regulation scheme (Jun 30th–Jul 14th, 2014) was estimated at 6.4 × 10¹¹ dpm, about 9% of the total riverine flux of dissolved ²³⁸U during that same period. This study represents a contribution to studies of dissolved U in muddy rivers and estuaries throughout the world. Results reported here provide not only a perspective to better estimate U flux from rivers to the ocean but also new insights into better understanding its estuarine mixing behavior and controlling factors.
... To model REE release across the salinity gradient at the Honokohau Harbor and Kiholo Bay sites, we employ the method developed by for trace elements that behave nonconservatively in estuarine systems. The approach follows from the work of , , and , whereby the release rate of a particular nonconservative trace element is estimated by constructing a chord or a tangent line from the ocean endmember through the highest salinity point(s) on the nonconservative trace element concentration versus salinity distribution curve for the estuary, or in our case, the salinity gradient of the mixing experiment Maeda and Windom, 1982;Swarzenski et al., 1995). Commonly, the tangent line is approximated by linear regression from the ocean endmember through the more saline water samples and back to 0 salinity Swarzenski et al., 1995;Nozaki et al., 2001). ...
... The approach follows from the work of , , and , whereby the release rate of a particular nonconservative trace element is estimated by constructing a chord or a tangent line from the ocean endmember through the highest salinity point(s) on the nonconservative trace element concentration versus salinity distribution curve for the estuary, or in our case, the salinity gradient of the mixing experiment Maeda and Windom, 1982;Swarzenski et al., 1995). Commonly, the tangent line is approximated by linear regression from the ocean endmember through the more saline water samples and back to 0 salinity Swarzenski et al., 1995;Nozaki et al., 2001). The resulting y-intercept of the tangent line, or computed linear regression line, is an estimate of the effective terrestrial water endmember concentration of the nonconservative trace element Head, 1985, and references therein). ...
... where [Ln] TotSGD , [Ln] TSGD , and [Ln] MarineSGD are the individual REE concentrations in the total, terrestrial sourced, and marine sourced SGD, and J ½Ln desorb is the desorption (i.e., release rate) flux of each individual REE from suspended particles or colloids to solution (e.g., Swarzenski et al., 1995). Combining Eqs. ...
... To model REE release across the salinity gradient at the Honokohau Harbor and Kiholo Bay sites, we employ the method developed by for trace elements that behave nonconservatively in estuarine systems. The approach follows from the work of , , and , whereby the release rate of a particular nonconservative trace element is estimated by constructing a chord or a tangent line from the ocean endmember through the highest salinity point(s) on the nonconservative trace element concentration versus salinity distribution curve for the estuary, or in our case, the salinity gradient of the mixing experiment Maeda and Windom, 1982;Swarzenski et al., 1995). Commonly, the tangent line is approximated by linear regression from the ocean endmember through the more saline water samples and back to 0 salinity Swarzenski et al., 1995;Nozaki et al., 2001). ...
... The approach follows from the work of , , and , whereby the release rate of a particular nonconservative trace element is estimated by constructing a chord or a tangent line from the ocean endmember through the highest salinity point(s) on the nonconservative trace element concentration versus salinity distribution curve for the estuary, or in our case, the salinity gradient of the mixing experiment Maeda and Windom, 1982;Swarzenski et al., 1995). Commonly, the tangent line is approximated by linear regression from the ocean endmember through the more saline water samples and back to 0 salinity Swarzenski et al., 1995;Nozaki et al., 2001). The resulting y-intercept of the tangent line, or computed linear regression line, is an estimate of the effective terrestrial water endmember concentration of the nonconservative trace element Head, 1985, and references therein). ...
... where [Ln] TotSGD , [Ln] TSGD , and [Ln] MarineSGD are the individual REE concentrations in the total, terrestrial sourced, and marine sourced SGD, and J ½Ln desorb is the desorption (i.e., release rate) flux of each individual REE from suspended particles or colloids to solution (e.g., Swarzenski et al., 1995). Combining Eqs. ...
Groundwater and seawater samples were collected from nearshore wells and offshore along the Kona Coast of the Big Island of Hawaii to investigate rare earth element (REE) behavior in local subterranean estuaries. Previous investigations showed that submarine groundwater discharge (SGD) is the predominant flux of terrestrial waters to the coastal ocean along the arid Kona Coast of Hawaii. Groundwater and seawater samples were filtered through 0.45 μm and 0.02 μm pore-size filters to evaluate the importance of colloidal and soluble (i.e., truly dissolved ionic species and/or low molecular weight [LMW] colloids) fractions of the REEs in the local subterranean estuaries. Mixing experiments using groundwater collected immediately down gradient from a wastewater treatment facility (WWTF) proximal to the Kaloko-Hanokohau National Historic Park, and more “pristine” groundwater from a well constructed in a lava tube at Kiholo Bay, were mixed with local seawater to study the effect of solution composition (i.e., pH, salinity) on the concentrations and fractionation behavior of the REEs as groundwater mixes with seawater in Kona Coast subterranean estuaries. The mixed waters were also filtered through 0.45 or 0.02 μm filters to ascertain the behavior of colloidal and soluble fractions of the REEs across the salinity gradient in each mixing experiment. Concentrations of the REEs were statistically identical (two-tailed Student t-test, 95% confidence) between the sequentially filtered sample aliquots, indicating that the REEs occur as dissolved ionic species and/or LMW colloids in Kona Coast groundwaters. The mixing experiments revealed that the REEs are released to solution from suspended particles or colloids when Kona Coast groundwater waters mix with local seawater. The order of release that accompanies increasing pH and salinity follows light REE (LREE) > middle REE (MREE) > heavy REE (HREE). Release of REEs in the mixing experiments is driven by decreases in the free metal ion activity in solution and the concomitant increase in the amount of each REE that occurs in solution as dicarbonato complexes [i.e., Ln(CO3)2⁻] as pH increases across the salinity gradient. Input-normalized REE patterns of Kona Coast groundwater and coastal seawater are nearly identical and relatively flat compared to North Pacific seawater, indicating that SGD is the chief source of these trace elements to the ocean along the Kona Coast. Additionally, REE concentrations of the coastal seawater are between 10 and 50 times higher than previously reported open-ocean seawater values from the North Pacific, further demonstrating the importance of SGD fluxes of REEs to these coastal waters. Taken together, these observations indicate that large-scale removal of REEs, which characterizes the behavior of REEs in the low salinity reaches of many surface estuaries, is not a feature of the subterranean estuary along the Kona Coast. A large positive gadolinium (Gd) anomaly characterizes groundwater from the vicinity of the WWTF. The positive Gd anomaly can be traced to the coastal ocean, providing further evidence of the impact of SGD on the coastal waters. Estimates of the SGD fluxes of the REEs to the coastal ocean along the Kona Coast (i.e., 1.3 – 2.6 mmol Nd day⁻¹) are similar to recent estimates of SGD fluxes of REEs along Florida’s east coast and to Rhode Island Sound, all of which points to the importance of SGD as significant flux of REEs to the coastal ocean.
... Previous studies isolated colloidal material using methods that were dependent upon the nature of the colloidal material, such as by adsorption on XAD resins. Isolation of colloidal material based on the molecular sizes from large volume water samples was developed using cross-flow ultrafiltration cartridges to investigate the size distribution of dissolved organic carbon and of trace metal carriers in coastal water (e.g., Moran and Moore 1989;Whitehouse et al. 1990;Baskaran et al. 1992;Swarzenski et al. 1995;Guo and Santschi 1996;Guo et al. 2000Guo et al. , 2001Wen et al. 1997). Application of different ultrafiltration techniques that separate particles and colloids from dissolved species have been applied in a variety of hydrological and geochemical settings such as the Amazon and the sub-arctic boreal Kalix River system (Swarzenski et al. 1995;Porcelli et al. 1997;Andersson et al. 2001). ...
... Isolation of colloidal material based on the molecular sizes from large volume water samples was developed using cross-flow ultrafiltration cartridges to investigate the size distribution of dissolved organic carbon and of trace metal carriers in coastal water (e.g., Moran and Moore 1989;Whitehouse et al. 1990;Baskaran et al. 1992;Swarzenski et al. 1995;Guo and Santschi 1996;Guo et al. 2000Guo et al. , 2001Wen et al. 1997). Application of different ultrafiltration techniques that separate particles and colloids from dissolved species have been applied in a variety of hydrological and geochemical settings such as the Amazon and the sub-arctic boreal Kalix River system (Swarzenski et al. 1995;Porcelli et al. 1997;Andersson et al. 2001). ...
... Release of uranium from bottom sediments on the shelf was suggested to be a source of dissolved (<0.4 µm) uranium. However, subsequent studies in the Amazon also demonstrated that U removal (Fig. 3) occurred at salinities <12 (Swarzenski et al. 1995. Overall, it was established that the behaviour of U is highly variable; examples have been found of conservative behavior as well as both additions and removal of U by interaction with sediments. ...
Rivers carry the products of continental weathering, and continuously supply the oceans with a broad range of chemical constituents. This erosional signature is, however, uniquely moderated by biogeochemical processing within estuaries. Estuaries are commonly described as complex filters at land-sea margins, where significant transformations can occur due to strong physico-chemical gradients. These changes differ for different classes of elements, and can vary widely depending on the geographic location. U- and Th-series nuclides include a range of elements with vastly different characteristics and behaviors within such environments, and the isotopic systematics provide methods for investigating the transport of these nuclides and other analog species across estuaries and into the coastal ocean.
There are numerous types and definitions of estuaries (see Dyer 1973). In this paper, it is simply considered to be a region where freshwater and seawater mix, to emphasize the chemical focus of the issues involved here. This region can either be contained within a river channel or extend onto the shelf, with effects often extending well into an adjoining ocean basin (Fairbridge 1980). These are complex ecological, biogeochemical, and hydrodynamic systems, with strong gradients in the concentrations and composition of micro- and macro particulate matter as well as dissolved organic and inorganic species. Particle-reactive radionuclides and trace elements being transported across estuaries are subject to partitioning between the dissolved, colloidal and particulate phases. The variable chemical nature of the U- and Th-series radionuclides is evident in their distributions in an estuarine environment. Such radionuclides can effectively serve as tracers to identify the sources, fate and transport of particles and colloids as well as other pollutants that behave similarly to these nuclides. These nuclides also have a wide range of half-lives that can be used to examine processes over various time scales. For example, Th and Ra daughter isotopes have …
... (Swarzenski et al., 1995), Narbada ( Borole et al., 1982) and the Gironde (present work). 68 (Libes, 1992) 11 Table 3.1. ...
... (Moore, 1992) Investigations of uranium concentrations in several estuaries have found this element to be conservative across a salinity gradient with neither a source nor a sink for uranium (Martin et al., 1978;Borole et al., 1982;Porcelli et al., 1997;Feng et al., 1999;Windom et al. 2000;Balakrishna et al., 2001). However, non-conservative removal or input in estuaries has also been observed by several workers (Maeda and Windom, 1982;McKee et al., 1987;Carroll and Moore, 1994;Sarin and Church, 1994;Swarzenski et al., 1995;). ...
... Non-conservative behaviour can vary between different estuaries, as well as with space and time within the same estuary. Indeed, uranium removal has been found to show seasonal variation, with removal taking place during periods of low river discharge (Maeda and Windom, 1982;Sarin and Church, 1994;Church et al., 1996), while others simultaneously exhibit uranium removal at low salinities and input at higher salinities (Swarzenski et al., 1995). ...
Recent studies have confirmed that marine systems are much more dynamic than previously considered with regard to the cycling of radionuclides. Marine particles provide sites for sorption reactions and efficient binding of dissolved radionuclides. These particles grow through aggregation of successive layers of oxyhydroxide precipitates with their surface-bound organic and inorganic species attached. Once formed, they may remain in suspension as small colloids, or they may aggregate or associate with other particles until their density is such that they fall vertically through the water column. These chemical processes partially account for the ultimate removal of radionuclides from surface waters to seabed sediment.
A significant understanding of particle scavenging processes has developed through studies of naturally occurring radionuclide tracers. Particularly suited to studies of chemical scavenging in marine systems is the observed disequilibrium in the 234Th/238U pair as 234Th, with a half life of 24.1 days, provides a mechanism to study processes with temporal ranges of days to months. Uranium and thorium have markedly different solubilities in sea water. Uranium is soluble and relatively unreactive with respect to particle sorption processes; as a result, uranium is conservative in sea water, its concentration varying as a function of salinity. Its decay gives rise to a constant rate of production of its thorium daughter. The preferential scavenging of particle reactive 234Th, while its soluble parent 238U remains nearly constant, provides an appropriate tool for assessing temporal variations in the rate of removal of particles from surface waters.
In this dissertation, a practical and reliable radioanalytical procedure for the determination of trace quantities of 234Th in environmental samples was tested and optimised in the laboratory. This technique was then applied to a number of diverse marine environments with a view to gaining a better understanding of the mechanisms controlling scavenging and particulate transport processes. In particular, the concentration of 234Th was measured in north-east Atlantic and in Arctic fjord (Kongsfjord, Svalbard) surface and deep waters. At each of the sampling locations 234Th concentrations were found to be significantly less than their source 238U, indicating preferential removal. This removal was observed to decrease with increasing water depth.
The observed 234Th/238U activity ratios, along with the corresponding 234Th particulate concentrations, for both the Atlantic and Svalbard study areas, were used to calculate the mean residence times of 234Th with respect to scavenging onto particles and removal from the surface. The results indicate that Svalbard fjord waters are much more active with regard to particle scavenging than are Atlantic waters. This has been attributed to the action of large terrigenous particles, deriving from 'dirty ice' melting within the Kongsfjord system.
In addition, data on the role of colloids in scavenging processes and the role of colloids in the partitioning between solid and dissolved fractions, have been gathered in the course of research expeditions carried out in the North East Atlantic and in Kongsfjord. The results of these experiments revealed that in north-east Atlantic surface waters, a significant fraction of the thorium in the operationally-defined dissolved phase (< 0.45 m) is in colloidal form. In contrast, little colloidal thorium was observed in the inner waters of Kongsfjord. The results of these experiments are consistent with the colloidal pumping model originally developed by Honeyman and Santschi (1989). This model assumes that 234Th is rapidly absorbed onto colloidal particles, which then aggregate (at a slower rate) into larger filterable particles. In essence, colloids act as intermediates in the transition from the fully dissolved to the filter retained phases. Thus, the time for fully dissolved 234Th to appear in the filter retained fraction is dependent on the rate of colloidal aggregation. The observed scavenging times in the Atlantic were longer than those observed in Kongsfjord due to the higher colloidal concentration in the latter.
New data on the behaviour of uranium at the freshwater - sea water interface, with particular emphasis on the conservative/non-conservative behaviour of this element in an estuarine environment, are reported. Examination of the 238U concentrations in the dissolved phase (< 0.45 m) along a salinity transect in the Gironde estuary revealed that there was some removal of uranium in the 0–16 ‰ salinity range. This non-conservative behaviour appears to be a seasonal effect related to the level of river discharge during the sampling period. The application of tangential flow ultrafiltration to examine the role of colloids in this process showed that there was no 238U present in colloidal form at the time of sampling. Thus, the observed uranium distribution in the estuary is most likely a result of the adsorption of uranium onto large particles (> 0.45 m).
... Previous studies isolated colloidal material using methods that were dependent upon the nature of the colloidal material, such as by adsorption on XAD resins. Isolation of colloidal material based on the molecular sizes from large volume water samples was developed using cross-flow ultrafiltration cartridges to investigate the size distribution of dissolved organic carbon and of trace metal carriers in coastal water (e.g., Moran and Moore 1989; Whitehouse et al. 1990; Baskaran et al. 1992; Swarzenski et al. 1995; Guo and Santschi 1996; Guo et al. 2000 Guo et al. , 2001 Wen et al. 1997). Application of different ultrafiltration techniques that separate particles and colloids from dissolved species have been applied in a variety of hydrological and geochemical settings such as the Amazon and the sub-arctic boreal Kalix River system (Swarzenski et al. 1995; Porcelli et al. 1997; Andersson et al. 2001). ...
... Isolation of colloidal material based on the molecular sizes from large volume water samples was developed using cross-flow ultrafiltration cartridges to investigate the size distribution of dissolved organic carbon and of trace metal carriers in coastal water (e.g., Moran and Moore 1989; Whitehouse et al. 1990; Baskaran et al. 1992; Swarzenski et al. 1995; Guo and Santschi 1996; Guo et al. 2000 Guo et al. , 2001 Wen et al. 1997). Application of different ultrafiltration techniques that separate particles and colloids from dissolved species have been applied in a variety of hydrological and geochemical settings such as the Amazon and the sub-arctic boreal Kalix River system (Swarzenski et al. 1995; Porcelli et al. 1997; Andersson et al. 2001). ...
... Release of uranium from bottom sediments on the shelf was suggested to be a source of dissolved (<0.4 µm) uranium. However, subsequent studies in the Amazon also demonstrated that U removal (Fig. 3) occurred at salinities <12 (Swarzenski et al. 1995). Overall, it was established that the behavior of U is highly variable; examples have been found of conservative behavior as well as both additions and removal of U by interaction with sediments. ...
... While conservative behaviour of U has been reported for some localities, such as western India (Borole et al., 1982), non-conservative behaviour has been reported at other localities, such as the Amazon River and the Kalix River entering the Baltic Sea (Swarzenski et al., 1995, 2003, 2004; McKee et al., 1987; Barnes and Cochran, 1993; Porcelli et al., 2001). Both decreases and increases in U concentration have been reported to occur in this transfer zone, as a result of different absorption and release processes (Swarzenski et al., 2003; McKee et al., 1987; Barnes and Cochran, 1993; Swarzenski and Baskaran, 2006; Breckel et al., 2005) although in general there appears to be a net loss (Barnes and Cochran, 1993). ...
... ) and the Amazon Delta (Swarzenski et al., 1995, 2003, 2004) where U removal occurs. A detailed study found that low salinity U removal in the Kalix River estuary zone is likely to be a consequence of absorption onto settling flocculated colloids of organic material and Fe, with U losses occurring of up to 50% (Andersson et al., 2001). ...
... It has been shown that colloidal U in coastal waters is strongly bound to DOC and can be removed from the water column (Chabaux et al., 2003; Swarzenski et al., 1995; Mann and Wong, 1993). The Mackenzie River contains a significant amount of DOC, with a mean concentration of 375 μM measured during 2003–2004, although the DOC of Arctic Rivers is highly variable on Fig. 5 ...
... Additional 234 U sources into seawater may include: (a) pore water diffusion from pelagic sediments (Ku, 1965;Ku et al., 1977), (b) river inputs including in situ leaching of riverine sediments by seawater (Bhat and Krishnaswami, 1969;Joshi and Ganguly, 1976;Joshi and Mishra, 1981), (c) low-temperature alteration of oceanic basalts (Aumento, 1971;MacDougall, 1977;MacDougall et al., 1979), and (d) hydrothermal vent emissions (Edmond et al., 1979). As a source term for 234 U, marine pore water U data are inconclusive in that U profiles can show either diffusion out of sediments into the water column (Swarzenski et al., 1995;McKee et al., 1996) or downward diffusion into sediments (Cochran, 1982;Anderson et al., 1989;Klinkhammer and Palmer, 1991;Nagao et al., 1992). Although additional work may help clarify such disparities, current knowledge suggests that diffusion from deep-sea sediments alone cannot offset the 234 U excess (Turekian and Chan, 1971;Borole et al., 1982), and that the global contribution of U from river-influenced coastal sediments via diffusion/ advection (McKee et al., 1987;Barnes and Cochran, 1990;1993) still needs further refinement. ...
... There is increasing evidence that U can be actively removed during its transport across many estuarine gradients which can vary considerably in geographical location, size and physico-chemical character (Carroll and Moore, 1994;Sarin and Church, 1994;Somayajulu, 1994;Swarzenski et al., 1995;Porcelli et al., 1997;Swarzenski and McKee, 1999;Andersson et al, 2001). The degree of U removal appears to be dependent on several key criteria, including, (a) a long freshwater-seawater mixing time relative to the kinetics of U sorption/desorption, (b) the presence of reactive U-carrier phases (e.g., Fe/Mn oxides, organic coatings), (c) water column oxygen depletion, and (d) high-energy regime of a particular system (i.e., resuspension). ...
... This paper incorporates data from two Amazon River/shelf cruises (1991,1996) as well as one Fly River/estuary expedition (1997). A description of estuarine processes relevant to U cycling on the Amazon shelf has been previously published (McKee et al., 1987;Swarzenski et al., 1995). ...
Natural concentrations of U-238 and delta(234)U values were determined in estuarine surface waters and pore waters of the Amazon and Fly (Papua New Guinea) Rivers to investigate U transport phenomena across river-dominated land-sea margins. Discharge from large, tropical rivers is a major source of dissolved and solid materials transported to the oceans, and are important in defining not only oceanic mass budgets, but also terrestrial weathering rates. On the Amazon shelf, salinity-property plots of dissolved organic carbon, pH and total suspended matter revealed two vastly contrasting water masses that were energetically mixed. In this mixing zone, the distribution of uranium was highly non-conservative and exhibited extensive removal from the water column. Uranium removal was most pronounced within a salinity range of 0-16.6, and likely the result of scavenging and flocculation reactions with inorganic (i.e., Fe/Mn oxides) and organic colloids/particles. Removal of uranium may also be closely coupled
... In Flüssen schwanken die Urangehalte von beispielsweise 0,03 µg/L im Amazonas bis 3,9 µg/L im Ganges. Im Meerwasser ist Uran mit einer durchschnittlichen Konzentration von 3,2 µg/L nachweisbar (Swarzenski et al. 1995), (Merkel et al. 2002), (Lide 2008). ...
... Hierbei spielt der Rückhalt von kolloidal gebundenem Uran die wesentliche Rolle. In Flusswässern kann dieser Anteil des partikulären gebundenen Urans mehr als die Hälfte ausmachen (Swarzenski et al. 1995), (Guo et al. 2007). Dieser Effekt kann auch technisch gezielt genutzt werden: So wurde in Laboruntersuchungen gezeigt, dass eine Uranentfernung durch Bindung des Urans an spezielle Makromoleküle und anschließende Abtrennung der Kolloide mittels Ultrafiltration möglich ist (Colloid-Enhanced Ultrafiltration, CEUF) (Roach et al. 2009). ...
Mit Inkrafttreten des Grenzwertes für Uran im Trinkwasser seit November 2011 werden für die betroffenen Wasserwerke Aufbereitungstechnologien benötigt, die eine wirksame und möglichst einfache Entfernung von Uran gewährleisten. In der vorliegenden Arbeit wird deshalb die Eignung von Granuliertem Eisenhydroxid (GEH) als Adsorptionsmittel zur Uranelimination untersucht. Anhand von Adsorptionsisothermen konnte gezeigt werden, dass GEH prinzipiell in der Lage ist, Uran adsorptiv zu binden und im Vergleich mit anderen oxidischen Sorbentien die höchsten Beladungen erzielt. Die experimentellen Daten zur pH-Abhängigkeit der Uranadsorption für die Modellsysteme U(VI) / H2O und U(VI) / H2O / CO2 lassen sich mit Hilfe des Oberflächenkomplexierungsmodells und unter Verwendung des angepassten Parametersatzes für Ferrihydrit erfolgreich darstellen. Es wurde festgestellt, dass die Adsorptionskapazität stark von der verwendeten Wassermatrix abhängig ist und sich für reale Grund- oder Trinkwässer deutlich verringert. Als wesentliche Einflussparameter wurden neben dem pH-Wert und der damit verbundenen Carbonatkonzentration die Konzentrationen von Calcium, Phosphat und in geringerem Ausmaß von Sulfat und Huminstoffen ermittelt. Die Verringerung der Kapazität wird durch Konkurrenzadsorption und durch eine Veränderung der chemischen Speziierung des Urans in Abhängigkeit der Wassermatrix erklärt. Sowohl die Adsorptionskinetik als auch die Adsorptionsdynamik lassen sich erfolgreich mit Hilfe des Oberflächendiffusionsmodells (HSDM) beschreiben. Die charakteristischen Transportkoeffizienten für die Filmdiffusion und die Korndiffusion wurden über empirische Korrelationen und auf experimentellem Wege ermittelt. Die vergleichsweise langsame Kinetik im Adsorptionsfilter wird durch den geschwindkeitsbestimmenden Schritt der Korndiffusion verursacht und führt zu einem relativ flachen Verlauf der Durchbruchskurve. Mit Hilfe von Kleinfilterversuchen nach der RSSCT-Methode wurde der Urandurchbruch im GEH-Festbettadsorber im Labormaßstab simuliert und für eine Pilotanlage in einem betroffenen Wasserwerk erfolgreich prognostiziert. Die ermittelte Durchbruchskurve für dieses Grundwasser zeigt eine Überschreitung des Urangrenzwertes nach einem spezifischen Durchsatz von 5.000 Bettvolumina. Eine Regeneration von uranbeladenem GEH konnte im Labormaßstab mittels Natronlauge erfolgreich durchgeführt werden. Bei der Wiederbeladung des Adsorbens zeigte sich aber eine Verringerung der Adsorptionskapazität, sodass die Anzahl der Nutzungszyklen vermutlich begrenzt ist. Im direkten Vergleich mit Anionenaustauschern kann festgestellt werden, dass GEH deutlich geringere Uranbeladungen erzielt und somit ein ökonomischer Einsatz zur Uranentfernung in der Trinkwasseraufbereitung generell fraglich ist. Für spezielle Anwendungsgebiete, bei denen ein Einsatz von Austauscherharzen nicht möglich ist, stellt GEH aber eine wirtschaftliche Alternative dar. Gedruckte Version im Papierflieger Verlag GmbH, Clausthal-Zellerfeld erschienen, ISBN 978-3-89720-564-2, ISSN 1864-5984
... Although the pH value of the W5 sample increased to 8.6, the adsorption of U was also affected by the Eh value, and the Eh value at W5 reflected a reducing environment. Therefore, U (VI) was still reduced to U (IV) and precipitated [20,[36][37][38][39]. Therefore, the U content in the river water was very low and did not change much (Figure 3b). ...
This work aims to understand the distribution and migration mechanisms of U and Mo between surface sediment and water in acid mine drainage (AMD) regions near a molybdenum mining region. A series of river sediment and water samples near the Jinduicheng mining area, one of the largest deposits of molybdenite ore in Asia, were collected and analyzed. Our results indicate that: (1) The pH value of river water samples increases with distance from upstream to downstream, while the pH has poor correlation with Mo and U; (2) The content of Mo and U in the sediment are significantly higher than the background value, which suggests potential pollution; (3) The content of Mo and U in the water and sediments in the lower reaches of the Wenyu river is significantly linearly related with the distance from the mining area, suggesting that AMD is a potential source; (4) BCR analysis shows that Mo in river sediments mainly existed as the residual, while U mainly existed as the non-residual; (5) The distribution coefficient Kd(Mo) exceeds Kd(U), indicating that Mo is enriched in sediments while U is more prone to porewater.
... Although a significant U input of 5.5 Mmolyr'' has been reported, in subsequent studies a net flux of this magnitude was not observed and the impact of the Amazon Shelf on the U budget remains unclear (McKee, 1987;Swarzenski et al, 1995;Swarzenski and McKee, 1998^ ...
p>Of the major processes that act to regulate the chemistry of the Ocean-Earth system, hydrothermal circulation and the associated ridge flank sedimentation processes are possibly the least well constrained. This research focuses on sedimentation processes and low temperature hydrothermal fluid flow across crust aged 0.36Ma at ~14<sup>o</sup>S on the east flank of the East Pacific Rise. The sediments are dominantly oxic and comprise biogenic carbonates, hydrothermal plume fall-out, and locally sourced basaltic debris, with minor barite and scavenged Al. The Fe/Mn ratio of the plume material decreases with distance from the ridge axis (3.6±0.4wt/wt at 0.36Ma to 2.5±0.3 wt/wt by 4.6Ma), consistent with the slower oxidation kinetics of Mn<sup>2+</sup> with respect to Fe<sup>2+</sup>. It is demonstrated that excess barium does not give an accurate estimate of biogenic barite in regions with a significant input of Fe (hydro)oxides. Comparison of the Al/Ti ratio to other biogenic parameters suggests that scavenged Al reflects a dominant association with the biogenic particle flux. Early diagenetic alteration of the plume derived Fe (hydr)oxides by dominated by transformation of ferrihydrite to geothite. Differential behavioural of divalent transition metals and oxyanions during alterations was observed. Cu and Zn are incorporated into the goethite structure without discrimination. Ni and Co are strongly associated with the Mn phases. V is discriminated against during transformation, where rejected V is retained in the sediment. Significant loss of P appears to occur during transformation of colloidal ferrihydrite to a more structured but still amorphous Fe (hydr)oxide phase. The distribution of U is a result of post depositional enrichment during early diagenesis followed by remobilisation and loss of U from the sediment column. The loss of U appears to reflect the influence of basement fluids on the sediment column, where burn-up of the sediment U enrichment is observed, leading to high concentrations of porewater U and the upwards mobilisation of sedimentary U over time. An assessment of the oceanic U budget suggests that U may provide a good tracer of low temperature hydrothermal circulation.</p
... In addition, numerous studies explored the application of natural and artificial radionuclides to evaluate processes, such as: geochronology of sediment (e.g., Swarzenski et al., 2006;Appleby, 2008); contaminant uptake, retention and trophic transfer through organisms, including model predictions relative to results from independent field observations (e.g., Fisher et al., 1988Fisher et al., , 2000Roditi et al., 2000;Baines et al., 2002;Luoma and Rainbow, 2005;Baumann and Fisher, 2011); sediment plume dynamics (Swarzenski et al., 1995); carbon flux (Buesseler et al., 2006); water mass ventilation (Matsumoto and Key, 2004), GEOTRACES/GEOSECS/JGOFS programs (Broecker and Peng, 1982;Bowles and Livingston, 1997;Moffett and German, 2018); runoff; and groundwater discharges (Charette, 2001;Moore, 2006;Swarzenski, 2007;IAEA, 2010). Furthermore, by coupling radiotracers (e.g., 222 Rn and 226 Ra) with stable isotopes of water (e.g., δ 18 O), one can discriminate source terms of submarine groundwater discharge and distinguish fresh groundwater discharge from saline, recirculated groundwater. ...
Radioisotopes have been used in earth and environmental sciences for over 150 years and provide unique tools to study environmental processes in great detail from a cellular level through to an oceanic basin scale. These nuclear techniques have been employed to understand coastal and marine ecosystems via laboratory and field studies in terms of how aquatic organisms respond to environmental stressors, including temperature, pH, nutrients, metals, organic anthropogenic contaminants, and biological toxins. Global marine issues, such as ocean warming, deoxygenation, plastic pollution, ocean acidification, increased duration, and intensity of toxic harmful algal blooms (HABs), and coastal contamination are all impacting marine environments, thereby imposing various environmental and economic risks. Being able to reliably assess the condition of coastal and marine ecosystems, and how they may respond to future disturbances, can provide vital information for society in the sustainable management of their marine environments. This paper summarizes the historical use of radiotracers in these systems, describes how existing techniques of radioecological tracing can be developed for specific current environmental issues and provides information on emerging issues that would benefit from current and new radiotracer methods. Current challenges with using radioecological tracers and opportunities are highlighted, as well as opportunities to maximize the application of these methods to greatly increase the ability of environmental managers to conduct evidence-based management of coastal and marine ecosystems.
... , 210 Po =3. 210 Pb c [13] 88 [16∼18] , yN, 777T [14] . Carvalho [1,9] influence of industrial releases in the Seine river and estuary. ...
... Uranium average concentration in seawater is 3.3 ppb (e.g. Ku et al., 1977), while in common surface waters it presents temporal and spatial variability strongly dependent on the local redox conditions (Plater et al., 1992), but still predominantly in the order of ng/L, as documented in Swarzenski et al. (1995), Andersson et al. (2001) or Guo et al. (2007). U concentration in Río Tinto was already shown by Casas-Ruiz and Barbero (2015), that reported values up to 200 μg/L at pH < 2.5 after 0.45 μm filtering. ...
... However, data on the mechanisms of uptake and siting of U on river and estuarine particles is limited. Release of particulate U during estuarine mixing (McKee et al., 1987; Swarzenski et al., 1995 ) and removal of U by settling particles may have substantial impact upon the delivery of U to the oceans. Recent work in the Baltic has documented evidence for removal of U from both anoxic and oxic waters (Andersson et al., 1995) and suggested that Fe-Mn-oxyhydroxide particles may be important for redistributing U in the water column. ...
... Ultrafiltration usually requires an initial pre-filtration step at a pore size of 0.45 µm (Tanizaki et al., 1992;Sigleo and Helz, 1981;Sigleo et al., 1982). Recently ultrafiltration has been performed in cross-flow mode (Martin et al., 1995 for fresh-waters; Moran and Moore, 1989;Sempere et al., 1994 for seawater; Swarzenski et al., 1995 for estuarine waters). Crossor tangential flow filtration, where the suspension is passed across the filter surface, may have certain advantages over traditional ultrafiltration, such as reducing concentration polarisation and increasing the flow rate (Moran and Buessler, 1993;Yeats and Strain, 1990). ...
The objective of this thesis was to study element speciation and migration in the boreal environments (Karelia and Central Russia). Specific goals were to i) characterize the molecular weight distribution of trace elements and organic matter (OM) in various types of natural waters, ii) reveal the TE colloidal forms dependence in the continuum soil solution - bog - stream - feeding and terminal lakes in the surface waters of boreal watersheds, and iii) quantify the dependence of stable and radiogenic element isotopic composition on the size fraction, element speciation and physico-chemical conditions of surface waters. The main originality of the work is to apply the method of cascade filtration to study the molecular weight distribution of trace elements in surface waters and to combine the geochemical and isotope techniques, for the first time on the same natural objects, and particularly within the watershed continuum soil solution - bog - stream - feeding and terminal lakes, in order to better understand the factors that control biogeochemical cycling of elements in the subarctic region. The first part is devoted to general introduction and the insight into the problems. The second chapter is aimed at studying natural organic matter size distribution in the natural waters with different redox and hydrodynamic conditions in the continuum soil solution - bog - stream - feeding and terminal lakes for a small watershed with homogeneous bedrock composition. In particular, we investigate organic matter concentrations, weight-average molecular weights, optical properties and carbon/nitrogen ratios to get the information about the origin of DOM (autochthonous / allochthonous), its aromaticity, and its hydrophility / hydrophobicity. The third chapter describes the study the molecular weight distribution of trace elements and it provides quantitative characterization of TE colloidal speciation in pristine, organic-rich rivers and surface waters. We demonstrate that, using TE correlations with iron and OM in the filtrates, one can characterize colloids distribution and compare partition coefficients in the continuum soil solution - bog - stream - feeding and terminal lakes, and also compare natural and modeling data. The fourth chapter describes the results of variations of stable (Cu, Mg) and radiogenic (Sr, Nd) isotopic composition in filtrates and ultrafiltrates of various size fraction in the different types of natural waters of south and north of European Russia boreal zone allowing to identify the factors influencing the isotopic composition variation along the landscape continuum in different size fractions. Finally, the fifth part is devoted to study the variations of iron isotopic composition of different water types and colloidal size fractions. In particular, we investigate the dependence of isotopic composition of filtrates and ultrafiltrate son iron and OM concentrations, pH and other physico-chemical factors. The sixth chapter presents the main conclusions and perspectives. Investigations carried out in this study should contribute to the prediction of TE and OM behavior in the Arctic and boreal regions due to climate warming mostly pronounced in these latitudes.
... Wen et al. (1996) showed that the mass balance is much better for Fe and Hg with a 10-kD cartridge (93710% for Fe and 96716% for Hg) rather than 1-kD (7672% for Fe and 60710% for Hg). Swarzenski et al. (1995) reported 87 to >95% mass balance with 10-kD for U in the Amazon shelf. Since a 10-kD cartridge was used for this work, we will only compare our results with those obtained using 10-kD cartridge, as mass balance issue is a major concern with 1-kD cartridges. ...
The phase partitioning of 234 Th between dissolved (o10-kiloDalton, kD), colloidal (10 kD—0.4 mm), and particulate (X0.5 mm) matter across a horizontal transect, from a coastal station to the deep Canada Basin, and a vertical profile in the deep Canada Basin of the western Arctic Ocean was investigated. Concentrations of suspended particulate matter (SPM), dissolved, colloidal and particulate organic carbon, particulate organic nitrogen and nutrients (silicate, phosphate and nitrate) were also measured to assess transport and scavenging processes. Total 234 Th (colloidal+particulate+dissolved) indicated deficiencies relative to secular equilibrium with its parent, 238 U in the upper 100 m, which suggests active scavenging of 234 Th onto particle surfaces. In contrast, at depths >200 m, general equilibrium existed between total 234 Th and 238 U. The inventory of SPM and the specific activity of particulate 234 Th in the Canada Basin was about an order of magnitude higher than the profile reported for the Alpha Ridge ice camp station. This higher concentration of SPM in the southwestern Canada Basin is likely derived from ice-rafted sedimentary particles. Inventories of nutrients, and dissolved organic carbon and nitrogen in the upper 100 m of the Canada Basin are comparable to the other estimates for the central Arctic Ocean. Comparison of the mass concentrations of colloidal and filter-retained particulate matter as well as the activity of 234 Th in these phases indicates that only a very small component of the colloidal material is actively involved in Th scavenging. Lower values of the conditional partition coefficient between the colloidal and dissolved phase indicate that the Arctic colloids are less reactive than colloidal material from other regions. The conditional partition coefficient between the filter-retained and dissolved phases (K f) is generally higher than that for other regions, which is attributed to the higher complexation capacity of glacio-marine sedimentary particles in these waters. The 234 Th-derived export of POC for the shelf and deep Canada Basin ranges between 5.6 and 6.5 mmol m À2 d À1 , and is in agreement with other estimates reported for the central Arctic Ocean and Beaufort Sea.
... Colloids have been separated using ultrafiltration techniques in various studies to investigate the size distribution of dissolved organic carbon (Buffle et al., 1978; Carlson et al., 1985) and of trace metal carriers in seawater (e.g., Moran and Moore, 1989; Whitehouse et al., 1990; Baskaran et al., 1992; Swarzenski et al., 1995) and freshwaters (e.g., Hoffmann et al., 1981; Salbu et al., 1985). A discussion of ultrafiltration methods is given by Buffle et al. (1992). ...
The importance of colloids and organic deposits for the transport of uranium isotopes from continental source regions and through the estuarine environment was investigated in the mire-rich Kalix River drainage basin in northern Sweden and the Baltic Sea. Ultrafiltration techniques were used to separate uranium and other elements associated with colloids > 10 kD and >3 kD from "solute"
uranium and provided consistent results and high recovery rates for uranium as well as for other elements
from large volume samples.
Uranium concentrations in 0.45 µm-filtered Kalix River water samples increased by a factor of 3 from near the headwaters in the Caledonides to the river mouth while major cation concentrations were relatively constant. ^(234)U/^(238)U ratios were high (δ^(234)U = 770-1500) throughout the basin, without showing any simple pattern, and required a supply of ^(234)U-rich water. Throughout the Kalix River, a large fraction (30-90%) of the uranium is carried by >10 kD colloids, which is compatible with
uranium complexation with humic acids. No isotopic differences were found between colloid-associated
and solute uranium.
Within the Baltic Sea, about half of the uranium is removed at low salinities. The proportion that is lost is equivalent to that of river-derived colloid-bound uranium, suggesting that while solute uranium behaves conservatively during estuarine mixing, colloid-bound uranium is lost due to rapid flocculation of colloidal material. The association of uranium with colloids therefore may be an important parameter in determining uranium estuarine behavior.
Mire peats in the Kalix River highly concentrate uranium and are potentially a significant source of recoil ^(234)U to the mirewaters and river waters. However, mirewater data clearly demonstrate that only small ^(234)U/^(238)U shifts are generated relative to inflowing groundwater. A simple box model of uranium accumulation in peat and transport through the mire that is compatible with the mire data demonstrates that with efficient removal of uranium from solution, only small shifts in ^(234)U/^(238)U ratios can be generated in mirewater uranium. The measurements and model calculations show that mirewaters are not the
primary source of the uranium in the river. Bedrock groundwaters with high ^(234)U/^(238)U ratios and uranium
concentrations must be the dominant source of riverine uranium.
... There exist many case studies of uranium displaying both conservative and non-conservative mixing behavior in surface estuaries. Several studies have shown the Amazon River estuary to be a net sink of U. Removal processes at low to intermediate salinities include colloidal aggregation, flocculation, and adsorption onto resuspended sediments (e.g., Swarzenski et al., 1995;Swarzenski and McKee, 1998). Swarzenski et al. (2004) observed nearly conservative mixing of U in the Fly River estuary (Paupa New Guinea), though U-isotopes indicated that the element is being actively cycled between the water column and sediments. ...
A comprehensive geochemical study of the subterranean estuary underlying Waquoit Bay (Cape Cod, MA, USA) has been undertaken. Sediment and pore water samples across the groundwater-seawater mixing zone show that the porous sediments support an active biogeochemistry. The redox cycles of iron and manganese are well developed with the oxidation of dissolved ferrous iron forming extensive deposits of iron-oxide coated sands. The strong relationship between the sediment concentrations of Fe and P reflects the sorption of phosphate from seawater and/or groundwater. Though two orders of magnitude less abundant than iron oxides, manganese oxides also precipitate in the subterranean estuary. Recirculation of seawater past the iron-rich sediments under reducing conditions makes this subterranean estuary a net sink for U from coastal seawater. In contrast, large scale release of Ba occurs in the mid to high salinity zone of the subterranean estuary, leading to a net source of Ba to the coastal ocean. While salt-induced desorption of Ba from the sediments may be an important process, a strong correlation with dissolved Mn suggests that sorbed-Ba is also released during the reductive dissolution of Mn oxides. Finally, the Ba results may provide insight into the processes controlling Ra cycling in the subterranean estuary and, therefore, its use as a tracer of submarine groundwater discharge to the oceans.
... The extent of loss of U and Mo in the Hooghly and the Mandovi estuaries are estimated following earlier approaches (Li and 1979; Maeda and Windom, 1982;Swarzenski et al., 1995) and using the U, Mo vs. salinity relationship (Figs. 4 and 5). The relevant relation is ...
Dissolved molybdenum (Mo) and uranium (U) concentrations were measured in five Indian estuaries; the Narmada, Tapi, Mandovi and the Mahi fall into the Arabian Sea and the Hooghly falling into the Bay of Bengal. Riverine Mo and U vary significantly, in the range of 1 to 90 and 0.02 to 19 nmol/kg respectively. The lowest Mo and U were observed in the Mandovi river, consistent with lateritic lithology exposed in its drainage, higher runoff and lower water-rock interaction. The Sabarmati has the highest U, 19 nmol/kg and sources from the groundwater having very high U concentration. The highest Mo (90 nmol/kg) is observed in the Mahi river and probably results from anthropogenic sources. Hooghly river seems to have high concentrations of Mo (14 nmol/kg) and U (8 nmol/kg) and are possibly derived from weathering of black shales of the Himalaya. Behaviour of Mo and U in all the estuaries analysed in this study is highly variable. Both Mo and U in the Narmada estuary during pre-monsoon and monsoon seasons and U in the Tapi estuary behave conservatively. Mo in the Tapi shows its addition in the mid salinity ranges (~ 4 to 12‰) and is sourced from anthropogenic activity probably related to effluent from industries situated along the estuary in this salinity range. Both dissolved Mo and U show their removal at lower salinity ranges in the Hooghly and the Mandovi estuaries. These estuaries are net sink of Mo in which significant amount of oceanic Mo along with riverine Mo is being lost. About 1.6 × 10⁶ and 2 × 10⁵ mol of Mo are being removed annually in the Hooghly and the Mandovi estuaries. Uranium removal is order of magnitude lower compared to Mo in both the Hooghly and the Mandovi estuaries. The loss of Mo and U in the estuaries are associated with mangrove swamps present in the estuaries which seems to be an important sink of the oceanic Mo and could represent its significant sink in oceanic budget of Mo.
... Assuming a uranium substitution as Uranyl (UO 2 2+ ) into the Ca 2+ site, uranium incorporation into coralline aragonite can be written as [Min et al., 1995] U/Ca in corals could be sensitive to pH and total CO 2 changes, as proposed by Min et al. [1995], but also to salinity changes [Robinson et al., 2004]. Indeed, a linear variation is expected between U concentration in seawater and salinity [Swarzenski et al., 1995;Robinson et al., 2004]. On the contrary, Sr incorporation in aragonite has been shown to be independent on SSS over a wide range (5-44 psu [Zhong and Mucci, 1989]). ...
We present a high-resolution study of five geochemical components of a New Caledonia coral core. Minor and trace elements (Sr/Ca, U/Ca, Mg/Ca, Ba/Ca), together with oxygen isotopes (δ18O), were investigated at near-fortnightly resolution. Geochemical measurements were compared to sea surface temperature (SSTTSG) and salinity (SSSTSG) recorded by a thermosalinograph (TSG) located less than 10 m from the coral. Results show that Sr/Ca, Mg/Ca, and δ18O are temperature-dependent (correlation coefficient to local SSTTSG between 0.74 and 0.84). Their robustness as SST tracers is tested against a satellite-based data set for a 4 year validation period. U/Ca has a more complex behavior and appears to be linked to both SSTTSG and SSSTSG, despite relatively small local SSS variations. For the first time this salinity-related imprint is quantified in a U/Ca coralline ratio. In addition to SST and SSS tracers, Ba/Ca measurements provide a possible way to investigate upwelling activity occurrences.
... TheSwarzenski and Baskaran, 2007). Although there is generally a strong relationship between U and salinity in the world oceans (Ku et al., 1977; Chen et al., 1986), non-conservative behavior of U has been observed in select estuarine and coastal waters (Maeda and Windom, 1982; McKee et al., 1987; Toole et al., 1987; Carroll and Moore, 1994; Swarzenski et al., 1995 Swarzenski et al., , 2004; summarized in Swarzenski and Baskaran, 2007). Non-conservative behavior of U has been observed in Tampa Bay waters and has been attributed, in part, to enhanced fluid exchange across the sediment/water interface (Swarzenski and Baskaran, 2007). ...
Historically, Tampa Bay has been impacted heavily by a wide range of anthropogenic perturbations that may include, agricultural-, shipping-, phosphate mining/distribution-related activities, as well as a burgeoning coastal population. Due to the presence of U-rich underlying sediments, elevated activities of U- and Th-series daughter products may be naturally released into this system. This region is also known for summer thunderstorms and corresponding increases in precipitation and surface water runoff. Only limited work has been conducted on the partitioning of particle-reactive radionuclides (such as 7Be, 210Pb, and 234Th) in such a dynamic coastal system. We investigated both the removal residence time and partitioning of these radionuclides between filter-retained particulate matter (≥0.5 μm) and the filtrate (
... Periodic or episodic reworking also results in pulse release of reduced reaction products (metabolites) and suboxically solubilized trace metals into overlying water (Robert et al., 2004). Such release may promote what is effectively distillation of colloidal Fe and associated trace constituents such as U and As from massive suboxic mud piles (McKee et 824 [62, 6 Journal of Marine Research al., 1987;Swarzenski et al., 1995;Sullivan and Aller, 1996). Reprecipitation of remobilized elements in adjacent regions of lower sedimentation, for example in shelf edge facies distal to mobilized muddy deltas, is clearly one potential mechanism for metal segregation and enrichment (Krauskopf, 1957;Trefrey and Presley, 1982). ...
Conceptual models of early diagenetic processes in sedimentary deposits guide interpretation and investigation of compositional patterns, elemental fluxes, and biogeochemical interactions. The ideas that sediments are open to exchange, are laterally homogeneous, and often accrete steadily upward underlie most diagenetic theory. Net accumulation rate of a deposit is thus a master variable controlling reactions, net fluxes, and sediment properties. These basic ideas and corresponding models have proved extraordinarily robust and useful. Large regions of the seafloor, however, can deviate significantly from some of the common assumptions of traditional diagenetic models, particularly along continent-ocean boundaries, where most sedimentary debris is processed. A spectrum of diagenetic facies representing a wide range of boundary conditions and internal transport-reaction regimes is typically present. Mobile muds are one of the major endmember diagenetic facies found in energetic, high sedimentation environments such as estuaries and deltas. These deposits often behave as episodically-mixed, fluidized batch reactors dominated by microbial biomass rather than, for example, classic advective plug flow reactors or geometrically complex, bioturbated bodies. Redox reaction patterns in mobile muds are unsteady. Suboxic conditions often dominate temporally, reflecting a balance between frequency of seafloor disturbance and the relative abundance and reactivity of recently entrained oxidants and reductants. Sedimentary dynamics, rather than net sedimentation, control the magnitude and nature of elemental fluxes and biogeochemical properties of mobile muds and the lateral exchange of material between diagenetic regimes. The understanding of elemental cycling in continental margins and their evolution as biogeochemical systems require consideration of the different dominant modes and the relative importance of diagenetic processing within and between individual facies.
... For example, in the Guem estuary, uranium may be removed at low salinity and released at higher salinity similar to the uranium behavior ob-Ž . served by Swarzenski et al. 1995 for the Amazon river-seawater mixing zone. ...
Data for uranium concentrations in 29 rivers and eight estuaries are presented. The river data expands the existing database on riverine uranium transport to include more smaller watersheds which collectively account for a large portion of material transport from the continent to the oceans. Riverine concentrations for these smaller watershed range from less than 50 to 660 pM. The results for these systems, when combined with previously published data on mostly larger rivers, do not change significantly the calculated global riverine flux and thus earlier estimates by Palmer and Edmond [Palmer, M.R., Edmond, J.M., 1993. Uranium in river water. Geochim. Cosmochim. Acta, 57, pp. 4947–4955] are substantiated. Uranium transport through eight diverse estuaries was studied to assess the importance of estuarine removal in the global marine uranium budget. Results indicate that uranium is conservatively transported in most systems studied. Results reported here for the Savannah estuary, however, indicate significant uranium removal. Our results suggest that uranium is removed in salt marsh estuaries at a rate of ca. 70 μmol/m2. This compares to a rate of 15 μmol/m2 for Delaware salt marshes [Church, T.M., Sarin, M.M., Fleisher, M.Q., Ferchlman, T.G., 1996. Salt marshes: an important sink for dissolved uranium. Geochim. Cosmochim. Acta, 60, pp. 3879–3887]. We suggest that uranium removal to salt marsh sediments is due to anaerobic microbially mediated processes. We use these results to estimate the global significance of the salt marsh sink in the oceanic budget of uranium. We estimate that 2.7×107 mol of uranium are removed to salt marshes annually as compared to an annual global riverine input of 3–6×107 mol estimated by Palmer and Edmond [Palmer, M.R., Edmond, J.M., 1993. Uranium in river water. Geochim. Cosmochim. Acta, 57, pp. 4947–4955].
In this study, we discuss the cycling of uranium (U) in the Ganga (Hooghly) River estuary through investigation of the composition of the water samples of six periods of contrasting water discharge, the bulk and exchangeable phases of the coexisting suspended particulate matter (SPM) and the bed sediments. In addition, we evaluate data on urban and industrial effluent waters that drain into the estuary. Similar U concentrations of freshwaters and effluent waters do not indicate the anthropogenic source to be an important contributor of U to the estuary. The removal of dissolved U in the low-salinity zones, observed for five of six study periods, is primarily linked to the cycling of the Fe-Mn oxyhydroxide phases. The compositions of exchangeable phases of SPM provide supportive evidence for the removal of dissolved U. Mass balance calculations show that the degree of removal of dissolved U can be sustained by the observed enhancement of exchangeable U in the SPM. Loss of U from the bed sediments is inferred from the distributions of exchangeable U concentrations as a function of salinity. Multiple observations based on compositions of the exchangeable and bulk phases of bed sediments indicate mobilization and loss of both U and Mn through suboxic diagenesis in the sediment columns followed by tidally-induced sediment resuspension.
We estimate that 40–60% of riverine U is removed in the low-salinity zone during the monsoon and post-monsoon periods. More importantly, the salinity range over which U removal occurs varies between the periods of similar water discharge. The estimated U flux from the estuary is ~70% higher in 2012 than in 2013. A significant difference between the annual U fluxes, driven mainly by considerable variation in the magnitude of U removal, suggests that estimates of meaningful annual U fluxes would require data at higher temporal resolution. This study highlights the importance of processes operating in the water column and below the sediment-water interface in the cycling of U in the Hooghly River estuary, where the solute-particle interaction driven by high SPM load is known to influence the cycling of trace metals.
The Amazon River has the largest drainage basin in the world, making it a major source of trace elements and dissolved organic matter (DOM) to the Atlantic Ocean. However, despite the increasing anthropogenic impacts to the Amazon basin, few recent studies exist quantifying trace element data in this region. The aim of the study was to analyze the input and removal processes that influence the transport of Ni and Co species in the Amazon and Pará River estuaries and mixing zone. Toward this goal, this work provides a comprehensive mixing and speciation study for the trace elements Ni and Co. Samples were collected during a period of high river discharge on the RV Meteor cruise M147 (Amazon – GEOTRACES process study GApr11) in the Amazon and Pará River outflow regions, as well as the aging mixing plume to the north, a mangrove belt to the southeast and the North Brazil Current (NBC) seawater endmember. Here we present the results for labile particulate (>0.2 μm), labile and total dissolved (<0.2 μm), large colloidal (0.015–0.2 μm), soluble (<0.015 μm) and ultrafiltered (<1 and < 10 kDa) fractions of Ni and Co in surface waters (towed-fish) and along the water column at different depths (CTD) samples using comparative approaches by adsorptive cathodic stripping voltammetry (AdCSV) and inductively coupled plasma-mass spectrometry (ICP-MS). We observed good agreement between AdCSV and ICP-MS measurements for Ni, and to a lesser extent Co. In general, dissolved and soluble Ni and Co decreased with increasing salinity, however additional non-conservative removal was also observed and attributed to possible biological uptake and colloidal flocculation. Shipboard AdCSV measurements showed that dissolved Ni was present mostly in the “reactive” form as weak complexes, suggesting high bioavailability, while reactive dissolved Co was absent, indicating the presence of strong organic Co complexes. In both Ni and Co, an elevated colloidal fraction was observed at low salinity, suggesting removal of dissolved Ni and Co via colloidal flocculation upon seawater mixing, while the soluble species were transported to the Atlantic Ocean. At depth, the soluble phase dominated, and we observed concentration maxima at 500–1000 m, indicating the presence of Antarctic Intermediate Water (AIW) and possible biological regeneration. We also observed unique source signatures in dissolved and labile particulate Ni and Co species from the Amazon and Pará River outflow regions, in addition to a contribution from mangrove belt-associated groundwater.
The optimum multiparameter (OMP) method was often used to determine the percentages of water masses based on temperature, salinity and other parameters, like nutrient or dissolved oxygen (DO). There are a number of water masses in the East China Sea (ECS), a marginal sea of the western Pacific Ocean. However, it is difficult to clarify the proportion of water masses using traditional parameters, such as temperature, salinity, nutrient or DO because of the occurring of intensive biogeochemical processes in the near shore and shelf areas. Here, we reported the use of 234U/238U activity ratio embedded in the OMP method. The results indicate that seawater in the northern ECS mainly consisted of the estuarine water of Changjiang River (CEW), Kuroshio water (KW), and Yellow Sea Coastal Current (YSCC). In March 2017, the CEW only influenced the offshore waters shallower than 30 m; the KW affected the east edge and the YSCC contributed more than 75% in the northern ECS.
The aim of the study was to assess the ecological risk of five trace metals, i.e. Cd, Cu, Ni, Pb, Zn, accumulated in bottom sediments of young water reservoir – Bardowskiego Lagoon located in Warsaw, Poland. In this case, several indices were used, i.e. Geoaccumulation index (Igeo), Contamination factor (CF), Potential ecological risk factor (ER), Degree of contamination (DC), Pollution load index (PLI) and Risk index (RI). The average trace metals concentrations in sediments, were as follow: Zn > Pb > Cu > Cd > Ni. The obtained results revealed that bottom sediments were very highly to extremely high contaminated by Cd and the ecological risk related with this pollutant was the largest. They also indicated that water from the study area was characterized by a low ecological quality, especially due to the content of Cd and Pb. The research proves that problem of cadmium contamination in bottom sediments of young water reservoirs is real and poses a serious ecological risk.
The elements existing as "water soluble form" and "ion exchangeable and surface complexed forms" were extracted from weathered slates with different weathering level by water and diluted HNO3 (ca 3%), respectively. The sample (Spl) is almost non-weathered black slate sample. On the contrary, the sample (Sp3) is intensely weathered and oxidized fragile sample. The weathering level of the sample (Sp2) is intermediate between Spl and 3. The purpose of this study is to investigate the effects of weathering level on the states in rocks and dissolution behavior of each element.
The amounts of elements extracted by water and diluted HNO3 from slates with different weathered level are different, indicating that elemental states in slate change due to the chemical weathering.
Further, the solutions extracted by water were successively filtered through 0.45 mm, 0.20 mm and 100 kDa. The almost of all elements extracted from fresh black slate (Spl) exist in solution as truly dissolved species. On the other hand, truly dissolved species do not almost exist in solution extracted from weathered slate (Sp2 and 3), and almost of all elements are completely removed by the ultrafiltration except some of alkali and alkali earth elements (Li, Sr and Cs) and oxyanions (As and Sb). They are adsorbates on Al and Fe-bearing colloidal particles or their components.
In hydrothermal alteration area, arsenic and metals included in rocks can be expected to leach out from the outcrops. In order to perform risk assessments of leaching arsenic and metals, it is important to systematically understand the leaching behavior of arsenic and metals from rocks and soils. In this study, the geochemical behavior of arsenic and metals, which were leached out from rocks in a hydrothermal alteration area, was investigated. Monolith of outcrop was classified into layer A (weathered rocks), layer B (intermediate), and layer C (host rocks) based on soil classification, and then extraction experiments of arsenic and metals for individual layer were conducted. The results showed that in the hydrothermal alteration zone, arsenic and metals readily leached into the water from the rocks of layer C as ionic forms, while arsenic and metals from the rocks of layer A were relatively stable. That is, the leaching concentrations were lower, and leached arsenic and metals were colloidal. In addition, the arsenic leaching from the layer C into the water increased at a lower temperature. By considering the chemical forms of arsenic and metals and their geochemical behaviors under several environmental conditions, we can evaluate the risks of natural arsenic and metals and apply such knowledge to the treatment procedures.
Tropical radiochemical oceanography is the study of transport, behaviour and fate of radionuclides in tropical marine coastal and oceanic systems. Such studies provide a fundamental understanding of the incidence and biogeochemical behaviour of radionuclides in the marine environment to assist the design, conduct, and interpretation of radioecology experiments. This chapter is mostly directed at tropical marine systems; however, information from other climatic regions is also presented. For the most part, reference is only made to water column, that is, radiochemical oceanographic distributions and incident processes. Focus is given to several naturally-occurring and artificial radionuclides of specific interest, as well as information on global and regional databases. Case studies on applications of radionuclides as tracers of marine processes and recent incidents resulting in releases of radionuclides to the marine environment are also presented.
The colloidal speciation of some trace metal elements (Cu, Pb and Cd) was studied in the Penzé estuary (Western Channel, Brittany) over a seasonal cycle. This study was performed in order to understand the metal transfer mechanisms between the different fractions (particulate, colloidal and truly-dissolved). Two methods were used: (i) frontal ultrafiltration (FU) allowing the partition in 7 size fractions and (ii) stripping chronopotentiometry (SCP) allowing the determination of metal concentrations in the various fractions. FU was chosen for its easy implementation and because it is a non-destructive technique. The detection limits of SCP were improved in order to measure the low-metal concentrations. Ours results have shown an important occurrence of Cu, Pb and Cd in the colloidal phase. If Cd is rather found in the lower part of the colloidal size-spectrum (<50 kDa), Pb is mainly associated with high molecular weight compounds (>300 kDa); Cu having an intermediate distribution. Over the seasonal cycle, two major phenomena were identified: (i) a strong mobilisation of humic substances from soils during autumn and (ii) the degradation of particulate organic matter in the benthic compartment during spring. These processes affect not only the colloidal distribution of the studied elements but also play an important role in the quantities and the forms exported toward the coastal zone.
Research of the effect of salinity on the fate of radionuclides has been focused on seas or estuarine systems while there is almost no information on marine environments with a salinity higher than that of sea water. The hypersaline Bardawil lagoon is a concentration basin, with evaporation exceeding precipitation. This study presents the characteristics of some environmental factors including salinity and their influence on the distribution of natural and artificial radionuclides in different compartments of the lagoon. The concentrations of (238)U, (234)Th, (228)Ra and (137)Cs in sediments show some degree of dependency on the water's salinity. Migration of these radionuclides in the lagoon's sediments must take place from high salinity to low-salinity regions. Cluster analysis revealed the data structure for sediment by separating (137)Cs and (40)K from (232)Th, (226)Ra, and (234)Th and for sand by separating (40)K from the other radioisotopes.
The Tinto River estuary is one of the most contaminated coastal systems in the world due to the high amounts of pollutants this acidic river transports. The tidal influence and the fluvial discharges control the water mixing, which include a salt-induced process (that is typical of all estuaries) and an acid neutralization process (pH-induced mixing). These processes affect the geochemical behavior of uranium, which has a non-conservative pattern. Despite the high concentrations of dissolved uranium in the Tinto River, 234U/238U shows that uranium stems mostly from seawater (between 80 and 100%). Riverine uranium is adsorbed onto both Fe and Al precipitates when the acidic river water is mixed with seawater. Distribution coefficients (Kd) show that dissolved and particulate uranium are controlled by adsorption–desorption processes and the formation of carbonate complexes, both depending on pH. At low pH, uranium tends to be dissolved, and the adsorption by suspended particles is low. As the pH increases, the adsorption processes onto Fe and Al particles are more intense, increasing particulate uranium which reaches a maximum at pH 5.5, where the uranium solubility minimum occurs. At higher pH values, dissolved uranium increases by carbonate complexation.
This paper describes a comprehensive study of the behaviour of U in the Ría of Huelva estuary, formed by the Tinto and Odiel rivers. This ecosystem is conditioned by two hydrochemical facts: one connected with the acid mining drainage (AMD) generated in the first section of the river basins, and another one related to the fertilizer industry located at the estuary. AMD gives a singular character to these rivers; low pH and high redox potential that keep high amounts of toxic elements and radionuclides in dissolution. Most of the data for dissolved U in estuaries indicate conservative mixing, but there are examples of non-conservative behaviour attributed to oxidation/reduction processes or solubility variations. In the Ría of Huelva estuary the U shows a non-conservative behaviour due to solubility changes produced by variations in the pH. A complete removal of riverine dissolved U is observed in a pH range of 4-6. At higher pH values, U release from suspended matter, and probably also from sediments into the dissolved phase is found.
In the present study the activities of uranium were measured in water column, suspended particulate matter (SPM) and surface sediment along the Sungai Selangor, which is from river, estuary and near-shore. During high tide, the activities of uranium were found highly in the particulate phase (234U = 350.93±11.69 mBq g-1 and 238U = 243.55±9.33 mBq g-1), where the activity of uranium in particulate phases is slightly correlated with the amount of suspended particulate matters. A strong adsorption of uranium occurred onto the suspended particles matter was showed by the distribution coefficient (Kd) values as 2366 x 10-3 L g-1 and 1363 x 10-3 L g-1 for 234U and 238U, respectively.
Estuaries are zones of mixing between freshwater from rivers and saline ocean water. The estuarine zone contains strong gradients in salinity, turbidity, energy, and chemical composition. This chapter examines U- and Th-series series nuclides that enter estuaries, their fate within the estuary and their input to the ocean. The source of U- and Th-series nuclides to estuaries varies depending on elemental characteristics and half-life. Potential sources of U- and Th-series nuclides are rivers, the ocean, the atmosphere, sediments, and groundwater. Physically, estuarine mixing and transport are complex processes. Many chemically relevant changes occur along the estuary; including changes in pH, sediment size, temperature and salinity/conductivity. For U- and Th-series nuclides, half-lives and reaction rates differ widely relative to the residence time of water within an estuary. Sedimentary processes that influence the fate of U- and Th-series nuclides in estuaries include settling, deposition, resuspension and burial. These are intimately linked to geochemical processes such as sediment diagenesis and pore water diffusion. In addition to the dissolved phase, particulate phases also play important roles in the fate of U- and Th-series nuclides in estuaries. It is apparent that much progress has been made in terms of analytical precision and the ability to measure low concentrations in dissolved and colloidal phases. Much work has also been done to further develop extremely useful tools such as the radium quartet. These studies would improve the current state of knowledge about the estuarine control of global uranium budgets.
The role of particles for U isotope transport was investigated in the Kalix River watershed, a particle-poor, Fe/Mn-rich river in northern Sweden, and in the Baltic Sea estuary. Particles >0.45 mu m are strongly enriched in U and contain 20-50% of the total riverine uranium budget and <1% of the total U in brackish waters (3-7 PSU). The particles have high delta(234)U which is close to that of dissolved U in the associated water, indicating that U on particles is dominantly nondetrital and isotopically exchanges rapidly with the ambient dissolved U. Particles at the river mouth are dominated by nondetrital Fe-Mn oxyhydroxides. Uranium and Fe are strongly correlated, clearly demonstrating that secondary Fe-oxyhydroxide is the major carrier of U in river water. There is no evidence for significant association of U with Mn-oxyhydroxide. Apparent U distribution coefficients (<(K)over cap (Fe)(d)>) were calculated for U between the authigenic Fe on particles and the solution. These values appear to be relatively constant throughout the year. This suggests an equilibrium between Fe in solution and authigenic Fe-oxyhydroxides on detrital particles. High values of <(K)over cap (Fe)(d)> calculated for one summer as well as high U concentrations in brackish waters can be explained by U scavenging by biogenic phases with low authigenic Fe content. Copyright (C) 1998 Elsevier Science Ltd.
A 1-kDa (1 kDa = 1000 daltons) Osmonics spiral-wound cross-flow filtration (CFF) membrane has been evaluated for sampling Cd, Cu and Ni in 1-kDa fractions of prefiltered seawater. Size-fractionated samples were collected as part of a CFF intercomparison experiment (Buesseler et al., 1996-this issue) and in the Gulf of Maine. Mass-balance results averaged 93 ± 6% (Cd), 81 ± 19% (Cu), and 99 ± 18% (Ni) for the intercomparison experiment and 56 ± 17% (Cd), 119 ± 38% (Cu), and 91 ± 6% (Ni) for the Gulf of Maine samples. Differences in mass balance between these studies may be related, in part, to removal of sub-μm particles and associated trace metals during prefiltration. An Amicon CFF system showed higher (2–12 ×) colloidal concentrations for Cu and Ni compared to Filtron and Osmonics systems; all CFF systems showed similar Cd, Cu and Ni mass-balance results. A model based on particle impaction with a filter shows the potential for adsorption of sub-μm particles by a 1-kDa Osmonics CFF membrane and conventional (Nuclepore) filters.
The phase speciation of thorium and consequences for the residence times of colloids have been examined in seawater of the Middle Atlantic Bight (MAB) and the Gulf of Mexico. Two fractions of colloidal organic matter (COM), 0.2 μm > COM1 > 1 kD and 0.2 μm > COM10 > 10 kD, were sampled using cross-flow ultrafiltration techniques and measured for their 234Th activity and organic carbon concentration. The ratios of mass concentrations of COM1 to those of suspended particulate matter were as high as 10 in the MAB and 6–34 in the Gulf of Mexico. Higher concentrations of colloids may be of great importance in the biogeochemical cycling of many particle-reactive nuclides or trace elements owing to their high specific surface area and complexation capacity. A significant fraction of 234Th in the traditionally defined “dissolved” pool was found to be associated with colloids. On average, about 10% of “dissolved” 234Th was in the colloidal fraction of sizes between 10 kDa and 0.2 μm, and ∼50% was in the 1 kDa-0.2 μm fraction. Values of the partition coefficients [Kc: (0.5−4) × 106 ml g−1 for Kc1 and (0.5−7) × 106 ml g−1 for Kc10] of 234Th between truly dissolved (
This chapter reviews the inputs of key actinide elements to the natural environment to examine global cycling of U in the context of the global iron cycle and to review the possible roles of microbial processes in the cycling and remobilization behavior of the actinides. The mobility of the actinide elements in natural systems is controlled by their partitioning between aqueous and particulate phases, which is in turn affected by their aqueous speciation. Actinide elements that occur predominantly in the aqueous phase may be rapidly and widely dispersed via riverine discharge and ocean currents and may remain bioavailable indefinitely. By contrast, actinide elements associated with particulates may undergo more limited transportation, where sedimentation can occur near the point of discharge, with estuarine, tidal fiat, and salt marsh sediments acting as sinks. The mobility of aqueous actinide species in the natural environment is dependent on a range of competing mechanisms.
Colloidal (COM) or macromolecular organic matter makes up a significant portion of the bulk dissolved organic matter (DOM) pool in aquatic environments. Because of their high specific surface areas and complexation capacities, marine colloids are of great importance not only in the global carbon cycle but also in the biogeochemical cycling of many particle-reactive nuclides and trace elements in the ocean. However, the colloidal pool as a whole is still poorly understood and largely uncharacterized. Recently, cross-flow ultrafiltration and other separation techniques, which have been successfully used to isolate marine colloids, combined with a multitracer approach, have greatly advanced our understanding of the cycling of COM and its associated trace elements in marine environments. In this paper we focus on recent developments on isotopic and elemental composition of colloids which allow organic matter cycling in marine environments to be constrained. Major sections review sampling techniques for aquatic colloids, concentrations and distribution of COM, biochemical and elemental (organic and inorganic) characterization, and stable isotopic (C-13 and N-15) and radioisotopic (C-14 and Th-234) characterization of marine colloids. We discuss sources and turnover rates of organic matter in the ocean, importance of benthic boundary layer processes in the cycling of DOM, changes in the paradigms of marine organic matter cycling, and research needs for a better understanding of the biogeochemistry of marine colloids.
Surface water was collected from the J iulong Estuary for determ ination of activity concentrations of uranium isotopes in different size fractions, namely, greater than 53, 10¡« 53, 2¡« 10, 0. 4¡« 2ƒƒ m , 10 000 u¡« 0. 4 ƒƒ m and less than 10 000 u fractions by m icrofiltration and cross2flow ultrafiltration technologies. Results indicated that most of the dissolved uranium (< 0. 4ƒƒ m) exis2 ted in the low molecular mass fraction (< 10 000 u) , and the colloidal uranium 2238 (10 000 u¡« 0. 4ƒƒ m) only contributed less than 1% of the dissolved uranium2238. The fractions of colloidal uranium in the dissolved phases decreased with the increasing sa2 linity. A positive linear relationship bet ween uranium2238 activities and salinitieswas observed for the dissolved, colloidal and low molecular mass fractions, indicating a conservative behavior of uranium in the J iulong Estuary. In the particulate phases (> 0. 4 ƒƒ m) , the partitioning of uranium isotopes among different size fractions was controlled by the partitioning of particle concentra2 tions. In the regions w ith salinities below 20, the partitioning of uranium 2238 among different size fractions was as follow s: 10¡« 53ƒƒ m > 2¡« 10ƒƒ m > 0. 4¡« 2ƒƒ m greater than above 53ƒƒ m. However, the order at the offshore station w ith salinities above 30 changed as follow s: 0. 4¡« 2ƒƒ m > 10¡« 53ƒƒ m > 2¡« 10ƒƒ m greater than above 53 ƒƒ m. The fraction of the 0. 4¡« 2 ƒƒ m particles increased at the offshore station, suggesting the increased contribution of the authigenic uranium. The activity ratio of uranium2234 to uranium2238 in the dissolved phases, including the low molecular mass fraction and the colloidal fraction, was larger than uni2 ty, showing the occurrence of excess uranium2234. In contrast, the activity ratio of uranium 2234 to uranium2238 in all size frac2 tions of the particulate phase was close to the equilibrium value (1. 0). The observed different values of the activity ratio of urani2 um 2234 to uranium 2238 in the dissolved phase and the particulate phase were ascribed to the p referential leaching of uranium2234 and the small amount of the leaching particulate uranium. The mass ratio of thorium 2232 to uranium 2238 also showed different val2 ues bet ween the dissolved phase and the particulate phase. Mass ratio of thorium2232 to uranium2238 in the dissolved, colloidal and low molecular mass fractions was less than unity, while those in the different size fractions of particulate phases were larger than unity, reflecting a different behavior bet ween uranium and thorium during their transport into the ocean.
Data on the behaviour of dissolved (<0.45 μm) uranium isotopes in the Gironde estuary mixing zone, gathered in the course of a research campaign in summer 1999, are presented in this paper. The data clearly show uranium in the estuary to behave non-conservatively during periods of low river discharge, with removal taking place at salinities in the range 0-16‰. Results from measurements of the evolution of 238U concentrations at a sampling location within the non-conservative region during a tidal cycle suggest that this removal is most pronounced at low salinities. Ultrafiltration experiments revealed that the retention of 238U by a 10 kDa ultrafilter was negligible, indicating that there was little, if any, uranium associated with colloidal species in these waters. The mechanism responsible for the transport of uranium through the estuary is seasonally variable being dependent on the prevailing river discharge.
A general study on uranium present at trace levels in anoxic sediments derived from Authie Bay (in northern France) has been undertaken. For that purpose, concentrations of various uranium species in pore waters and recovered solutions (after mineralization of sediments) were determined by ICP-AES and ICP-MS. To access the extent of early diagenesis occurring in these sediments, reduced solid sulfur species were determined after their conversion into H2S gas following sequential extraction procedures. Our preliminary findings reveal that dissolved U(VI) precipitates rapidly with depth in pore waters certainly in the form of insoluble U(IV). Under stronger reducing conditions, new aqueous species [mostly and ] in which uranium is in oxidation state IV are formed in the pore water. Valuable interpretation of these particular properties of sedimentary U has necessitated a global examination of sediment biochemistry because of the influence of bacterial activities on the chemistry of Fe, Mn, S and more particularly U by metal-reducing bacteria.
The continuous and persistent weathering of the continents and the ensuing transport of both eroded and anthropogenic products by rivers account for almost all the dissolved solutes, particulates, and sediment-associated contaminants delivered to the ocean. Transported within this riverine load are essential nutrients (e.g. P, N, C), (oxy)hydroxides of Fe/Mn that act as highly efficient riverine scavengers, an ever-changing suite of man-made organic and inorganic constituents, as well as a wide variety of natural solutes (e.g. Si, Ca, Mg, Sr). The environmental behavior of these river-borne constituents is controlled by dynamic phase partitioning between operationally defined dissolved, colloidal, and particulate pools control. As most trace elements are bound onto river particulates, their fate is defined foremost by the nonconservative nature of such solids as they move along the particle-size continuum or are exchanged and remobilized in river bed and floodplain sediments. This paper describes the global riverine transport of sediment-associated contaminants to the ocean.
Keywords:
sediment-associated contaminants;
river discharge;
suspended sediment concentration;
phase partitioning;
storage;
remobilization;
scavenging;
redox cycling
The 234U/238U activity ratio in river water in southern Africa is generally higher than that reported for rivers in other regions of the world. This is interpreted as due to the prevailing environmental conditions: in this warm dry region mechanical weathering predominates over chemical weathering, causing the isotope activity ratio of leached uranium to be, on average, 2.03 ± 0.42 as compared to a ratio of 1.20 for river water in the more humid tropical and temperate regions. The isotopic composition of leachable uranium from river sediment is similar to that in the water. Rivers draining the Witwatersrand gold and uranium mining area clearly show pollution inputs characterised by high uranium content and low activity ratios.
The equilibrium partitioning of Th among dissolved, colloidal, and particulate size fractions was studied under conditions similar to those of coastal marine waters in laboratory experiments using 234Th tracer and ultrafiltration techniques. In colloid-free alumina suspensions (ie prepared in ultrafiltered seawater), distribution coefficients (K d's) were constant over the range of particle concentrations studied (0.1-5.0 mg 1 -1). Adsorption of Th by the alumina involved hydrolysis species that are retained by a 1000 nominal molecular weight ultrafilter. Th adsorption was decreased by the presence of natural dissolved organic matter (Phaeodactylum tricornutum exudates) and by pre-existing colloidal matter. -Authors
The dissolved concentration of uranium and the relative abundance of two uranium isotopes, **2**3**4U and **2**3**8U, vary over a wide range of values in natural waters. The concentration is controlled mainly by the redox potential of the environment and by CO//2. The mechanism of isotope fractionation is thought to be entrainment of **2**3**4U in the aqueous phase either by selective leaching of the solid phase or by direct recoil of the daughter nuclide. Ion exchange techniques and alpha-spectrometery permit the measurement of uranium at concentrations as low as pp 10**1**1 and the isotopic ratio to a few per cent. In oxidizing conditions the uranium isotopes behave in a chemically stable conservative manner such that separate groundwater sources may have identifiably different characteristics and mixing volume calculations may be made. Other potential uses of these isotopes include radiometric dating, tracing of hydrologic systems, ore prospecting and earthquake prediction.
REDUCTION of the soluble, oxidized form of uranium, U(VI), to insoluble U(IV) is an important mechanism for the immobilization of uranium in aquatic sediments and for the formation of some uranium ores1-10. U(VI) reduction has generally been regarded as an abiological reaction in which sulphide, molecular hydrogen or organic compounds function as the reductant1,2,5,11. Microbial involvement in U(VI) reduction has been considered to be limited to indirect effects, such as microbial metabolism providing the reduced compounds for abiological U(VI) reduction and microbial cell walls providing a surface to stimulate abiological U(VI) reduction1,12,13. We report here, however, that dissimilatory Fe(III)-reducing microorganisms can obtain energy for growth by electron transport to U(VI). This novel form of microbial metabolism can be much faster than commonly cited abiological mechanisms for U(VI) reduction. Not only do these findings expand the known potential terminal electron acceptors for microbial energy transduction, they offer a likely explanation for the deposition of uranium in aquatic sediments and aquifers, and suggest a method for biological remediation of environments contaminated with uranium.
THE MAJOR goals of the AmasSeds sediment transport group are to determine the dominant physical processes causing sediment transport on the Amazon Shelf and to understand the particle response to these processes. The study was mo-tivated by previous findings concerning such issues as: a) spatially variable sediment accumulation (from 0 to 10 cm y-l; Kuehl et al., 1986); b) ex-treme sediment reworking depths/surface mixed layer thicknesses of up to 150 cm at water depths of 15 m (Kuehl et al., 1986); c) sediment dispersal (only 60-70% of the Amazon sediment discharge is accounted for; Nittrouer et al., 1986); and d) the origin of fine-scale stratigraphy. Although dy-namic physical processes control the response of the sediments, the link between them can be complicated. Therefore a comprehensive study to document physical processes and simultaneous sediment response was initiated in this complex environment. Some preliminary findings are pre-sented in this paper. The field program was designed to measure the fluid, flow, and suspended-sediment characteris-tics throughout the water column and on varying time scales (e.g. semidiurnal, fortnightly, sea-sonal). Measurements fall into three major cate-gories: suspended-sediment distribution through-out the water column; boundary-layer processes; and particle characteristics. Oceanographic cruises were scheduled during each season and period of river discharge (see Fig. 3 of Nittrouer et al. this issue) and three of the four cruises have been completed. Cruise IV is scheduled for October-November, 1991, during minimum river dis-charge. The cruises consist of legs designated for large-scale (spatial) surveys and anchor-station (temporal) surveys. The large-scale surveys con-tain up to 50 water-column profiles spaced along seven shelf-normal transects positioned between the river mouth and the French Guiana border (Fig. 1). The anchor stations consist of hourly profiles repeated over a 25-h period at a specific location. Anchor stations have been occupied at inner-shelf locations across the topset beds of the River-Mouth and Open-Shelf Transects (see Fig. 1 of Nittrouer et al., this issue).
The pronounced desorption of Ba and 226Ra from river-borne
sediments in the Hudson estuary can be explained quantitatively by the
drastic decrease in the distribution coefficients of both elements from
a fresh to a salty water medium. The desorption in estuaries can
augment, at least, the total global river fluxes of dissolved Ba and
226Ra by one and nine times, respectively. The desorptive
flux of 226Ra from estuaries accounts for 17-43% of the total
226Ra flux from coastal sediments. Two mass balance models
depicting mixing and adsorption-desorption processes in estuaries are
discussed.
In the determination of dissolved 238U in aqueous solutions, a pre-concentration by chelating ion exchange chromatography with a Chelex-100 column at pH 4 (or Chelex-238U) and co-precipitation with iron hydroxide (or Fe238U) yielded indistinguishable results for pure solutions of inorganic uranium and samples of open-ocean waters. However, in coastal marine waters, the concentration of Fe238U could be significantly higher than that of Chelex-238U. The concentration ratio of Fe238U. The concentration ratio of Fe238U to Chelex-238U seemed to be higher in waters with higher concentration of dissolved organic carbon (DOC). A concentration ratio of Fe238U to Chelex-238U as high as 1.27 was observed. The concentration of 238U (UV-238U) in these coastal waters might be further elevated if dissolved organic matter in the sample was first mineralized, by subjecting it to UV irradiation in the presence of hydrogen peroxide under acidic conditions, before analysis. The ratio of the concentration of UV-238U to Chelex-238U increased even more dramatically with increasing concentration of DOC. The highest value observed was 1.83. The concentration of dissolved uranium thus increased in the order: Chelex-238U ⩽ Fe238U ⩽ UV238U. These results are consistent with the notion that: (a) only inorganic and weakly bound species are removed by the Chelex 100 resin so that Chelex-U represents inorganic and weakly bound uranium: (b) iron hydroxide co-precipitation can remove a fraction of dissolved organic matter so that FeU includes inorganic as well as a variable fraction of ‘strongly bound’ uranium; (c) the UV irradiation of a sample of water converts ‘strongly bound’ uranium to the readily detectable inorganic uranium: (d) ‘strongly bound’ uranium is more likely to be found in coastal marine waters where the concentration of dissolved organic matter is higher. Previously published data are almost exclusively Chelex-U and FeU. In the open oceans, if the amount of ‘trongly bound’ uranium is small, they should be essentially identical. In coastal waters, however, Chelex-U and FeU may represent different fractions of dissolved uranium. Thus, some of the conclusions on the behavior of uranium in estuarine and coastal waters drawn from these data may have to be re-examined to take into consideration the speciation of uranium in these waters and the analytical methods used. By determining Chelex-238U and UV-238U simultaneously in a sample, ‘strongly bound’ uranium may be estimated as the difference in concentration in these two measurements.
Soluble uranium concentrations in the Ogeechee and Savannah Rivers are 0.046 ± 0.005 and 0.026 ± 0.01 μg/l, respectively. Particulate concentrations are approximately similar in the Ogeechee River but are about four times the soluble concentration in the Savannah River. River end-member 234U/238U ratios range from 1.04 ± 0.04 to 1.08 ± 0.12. Observations suggest that uranium is removed from estuarine waters at low salinities during low discharge.
Experiments using radioactive tracers in microcosms of 150 l and 13 m3 volumes, which are designed to mimic Narragansett Bay, indicate that Th isotopes are good analogues for studying the removal behavior of “particle-reactive” pollutants such as Am, Pb, Po, Hg and Cr(III) in coastal environments. The removal of Th isotopes and Fe has been found to be closely linked in microcosms and Narragansett Bay. In addition, the settling velocities of tracer microspheres in the experimental tanks were found to be faster in spring and summer than expected from Stokes' law and similar to those of234Th-containing particles in Narragansett Bay [5]. It is concluded that aggregation of particles is important in accelerating the removal of surface-reactive elements during the warm season.
The estuarine geochemistry of rare earth elements (REEs) was studied using samples collected in the Amazon River estuary from the AmasSeds (Amazon Shelf SEDiment Study) cruise of August 1989. Extensive removal of dissolved (0.22 m filtered) trivalent REEs from river water occurs in the low (0-6) salinity region. Removal by the salt-induced coagulation of river colloids leads to fractionation among the REE(III) series; the order of removal is light REEs > middle REEs > heavy REEs. There also is the enhanced removal of Ce (relative to trivalent La and Nd) in the low salinity (0-6) zone and in the zone of high biological activity. This is the first field observation of strong Ce removal associated with coagulation of river colloids and biological productivity. The argument is made that the decrease in the Ce anomaly across a biological front is caused by biologically mediated oxidation of Ce(III) to Ce(IV). Coagulation of river colloids and biologically mediated oxidation of Ce (III) lead to fractionation of REE(III) and redox modification of Ce. These processes result in the REE composition becoming fractionated relative to the Amazon River water and crust and more evolved toward the REE composition of the oceans. This study implies that reactions in estuaries play significant, yet poorly understood roles in controlling the REE composition and Ce anomaly of the oceans.
Colloidal material in the size range 1.2 nm to 0.4 m was isolated by ultrafiltration from Chesapeake Bay and Patuxent River waters (U.S.A.). Temperature controlled, stepwise pyrolysis of the freeze-dried material, followed by gas chromatographic-mass spectrometric analyses of the volatile products indicates that the primary organic components of this polymer are carbohydrates and peptides. The major pyrolysis products at the 450°C step are acetic acid, furaldehydes, furoic acid, furanmethanol, diones and lactones characteristic of carbohydrate thermal decomposition. Pyrroles, pyridines, amides and indole (protein derivatives) become more prevalent and dominate the product yield at the 600°C pyrolysis step. Olefins and saturated hydrocarbons, originating from fatty acids, are present only in minor amounts. These results are consistent with the composition of Chesapeake phytoplankton (approximately 50% protein, 30% carbohydrate, 10% lipid and 10% nucleotides by dry weight). The pyrolysis of a cultured phytoplankton and natural particulate samples produced similar oxygen and nitrogencontaining compounds, although the proportions of some components differ relative to the colloidal fraction. There were no lignin derivatives indicative of terrestrial plant detritus in any of these samples. The data suggest that aquatic microorganisms, rather than terrestrial plants, are the dominant source of colloidal organic material in these river and estuarine surface waters.
In recognition of problems associated with traditional ultrafiltration techniques, cross‐flow ultrafiltration has been adapted to isolate colloidal material from natural aquatic environments, both freshwater and marine. The procedure is capable of delivering filtration rates on the order of tens of liters per hour without introducing significant quantities of organic or trace‐metal contaminants, and thus is suitable for large‐volume and trace‐component applications. Mass balance experiments in which concentrations were measured in ultrafiltrate and retained fractions show that both organic material and several trace metals are recovered without significant losses on the filters.
Sorption of uranium on amorphous ferric oxyhydroxide was investigated at 25° and 60°C from 0.01 M NaCl and 0.01 M NaHCO3 solutions over an initial U concentration range of ∼10−4M to 5 · 10−7M (23,800-93.2 ppb U). Uranium distribution coefficients ranged from more than 2 · 106 ml g−1 from 0.01 M NaCl at 25°C to ∼ 3 · 104 ml g−1 from 0.01 M NaHCO3 at 25°C and fell rapidly with increasing initial U solution concentration. The uranium sorption data fit a Dubinin-Radushkevich sorption isotherm.Sorption of radium on amorphous ferric oxyhydroxide also was investigated at 25° and 60°C from 0.01 M NaCl over an initial Ra concentration range of ∼ 5 · 10−7–5 · 10−10M (113-0.113 ppb Ra). The radium sorption data fit a Freundlich sorption isotherm with radium distribution coefficients ranging from a low of ∼1100 ml g−1 at 60°C to a high of >20,000 ml g−1 at 25°C, much lower than comparable uranium distribution coefficients.
Samples representing yearly averages of material transported by the Amazon and Yukon Rivers were analyzed to separate the transition metals (Cr, Mn, Fe, Co, Ni, and Cu) into the following transport phases: (1) crystalline particles, (2) metal hydroxide coatings, (3) solid organic material, (4) sorbed material, and (5) those in solution. The major transport phases are crystalline particles and metal hydroxide coatings, which, combined, carry 65% to 92% of the transition metals transported. Solid organic material, the next most important phase, transports between 5% and 19% of the total transported. Material carried in solution transports 0.6% to 17% of the total transported. Sorbed transition metals account for between 0.02% and 8% of the total transported. Metal hydroxide coatings represent the major transporting mechanism potentially available to organisms, since, for the Amazon and Yukon Rivers, respectively, 87% and 78% of the Fe, 69% and 73% of the Mn, and 71% and 69% of the Ni are transported in this form. Comparing the concentrations of transition metals carried to the oceans with the concentrations on the continents, a high Cu ratio (5 to 7) indicates continental depletion or river output enrichment; a moderate ratio (1.1 to 1.7) for Ni, Co, and Cr indicates intermediate depletion or enrichment, and a near-unity ratio for Fe and Mn indicates little depletion or enrichment. The sediments transport >97% of the total mass of transition metals to the world's oceans.
The Ganges-Brahmaputra river system supplies more dissolved uranium to the ocean than any other system in the world (SARIN et al., 1990; SACKETT et al. 1973). However, there have been no investigations to determine whether riverine supplies of uranium are altered by geochemical reactions in the river-ocean mixing zone. In this study, uranium and salinity data were collected in the Ganges-Brahmaputra mixing zone during a period of low river discharge. The uranium distribution with salinity shows that in waters < 12 ppt salinity, uranium activities are significantly lower than predicted from conservative mixing of river and seawater. This suggests that uranium is being removed within the mixing zone. The behavior of uranium in the Ganges-Brahmaputra is in sharp contrast to its in teh Amazon mixing zone where MCKee et at. (1978) found uranium activities significantly higher than predicted from conservative micxing. The contrasting behaviours for uranium in theses systems are due to the different locatons where mixing between river and seawater occurs. For the Amzon, mixing takes place on the continental shelf whereas for the Ganges-Brahamputra, mixing occurs within shoreline sedimentary environments. The physiochimical processes controlling uranium removal to sediment deposits in the Amazon are partly known. We discuss mechanisms which may be removing uranium to suspended and mangrove sediments in the Ganges-Brahmaputra.
The two major river systems on the west coast of India, Narbada and Tapti, their estuaries and the coastal Arabian sea sediments have been extensively studied for their uranium concentrations and 238 U / 238 U activity ratios. The 238 U concentrations in the aqueous phase of these river systems exhibit a strong positive correlation with the sum of the major cations, Na + K + Mg + Ca, and with the HCO 3 - ion contents. The abundance ratio of dissolved U to the sum of the major cations in these waters is similar to their ratio in typical crustal rocks. These findings lead us to conclude that 238 U is brought into the aqueous phase along with major cations and bicarbonate. The strong positive correlation between 238 U and total dissolved salts for selected rivers of the world yield an annual dissolved 238 U flux of 0.88 × 10 10 g / yr to the oceans, a value very similar to its removal rate from the oceans, 1.05 × 10 10 g / yr , estimated based on its correlation with HCO 3 - contents of rivers. In the estuaries, both 238 U and its great-grand daughter 234 U behave conservatively beyond chlorosities 0.14 g/l. These data confirm our earlier findings in other Indian estuaries. The behavior of uranium isotopes in the chlorosity zone 0.02-0.14 g/l, was studied in the Narbada estuary in some detail. The results, though not conclusive, seem to indicate a minor removal of these isotopes in this region. Reexamination of the results for the Gironde and Zaire estuaries (Martin et al ., 1978a and b) also appear to confirm the conservative behavior of U isotopes in unpolluted estuaries. It is borne out from all the available data that estuaries beyond 0.14 g/l chlorosities act neither as a sink nor as a source for uranium isotopes, the behavior in the low chlorosity zones warrants further detailed investigation. A review of the uranium isotope measurements in river waters yield a discharge weighted-average 238 U concentration of 0.22 g/l with a 234 U / 238 U activity ratio of 1.20 ± 0.06 is missing . The residence time of uranium isotopes in the oceans estimated from the 238 U concentration and the 234 U / 238 U A. R. of the rivers yield conflicting results; the material balance of uranium isotopes in the marine environment still remains a paradox. If the disparity between the results is real, then an additional 234 U flux of about 0.25 dpm/cm 2 ·10 3 yr into the oceans (about 20% of its river supply) is necessitated.
Work of the last 10 years has demonstrated that oceanic particle size distribution by volume tends to be flat at mid-water depths (equivalent to a cumulative particle number distribution with a slope of −3) and is peaked in nepheloid layers with active resuspension and in surface waters with active biological production. The observed loss of fine peaks from the suspensions to yield flat distributions requires aggregation of the material, as the fines settle slowly. Mechanisms leading to particle collision are examined; for interactions between particles of similar size, Brownian motion dominates below 1.5 to 8 μm. However, if large particles (such as ‘marine snow’) are present at realistic concentrations, they become important in the removal of fine particles by shear-controlled coagulation. The coagulation times calculated for shear are too long for steady state to be presumed while the size distributions evolve under the influence of coagulation mechanisms. Therefore suggestions that the flat size distributions are quasi-stationary results of shear-controlled coagulation are rejected, and the notion that there is sub-equal production of particles at different points in the spectrum is favoured. Such production and the subsequent scavenging of small particles by large settling ones confers great importance on components of biological origin in both providing elements of the total size spectrum and determining the distribution and sedimentation of others of lithogenic origin. In surface waters, filtration rates by zooplankton indicate that aggregation rates of particles above submicron sizes are biologically determined.
This 10-chapter book discusses components of aquatic systems, natural organic matter, aquatic organic compounds, complexation equilibria, homologous complexants, in situ distribution of chemical species, potentiometric methods, voltammetric methods, and nonelectrochemical methods. The references are from the 1970s and early 1980s. An index and an extensive reference section also are provided.
Uranium contamination of groundwaters resulting from U mining activities and the leakage of nuclear waste from storage facilities is a growing concern. In Framvern Fjord (southern Norway), dissolved [sup 238]U concentrations at the bacterial maximum layer (24 m), which is situated a few meters below the oxic-anoxic (i.e. oxygen-hydrogen sulfide) interface (18 m), are observed to be [approximately]60% lower than concentrations above and below this depth. Removal of U occurs well below the depth at which Fe and Mn oxyhydroxides are precipitated and under conditions inconsistent with abiological reduction of soluble U(VI) to particle-reactive U(IV). Our observations suggest that the microbial population in the anoxic waters near the O[sub 2]-H[sub 2]S interface in the fjord exerts an effective control on the aquatic biogeochemistry of U in this environment. 27 refs., 3 figs., 1 tab.
The behaviour of natural uranium isotopes during estuarine mixing has been studied in three British estuaries. Uranium exhibits conservative behaviour in the Clyde and Tamar estuaries but there is evidence of removal in the Forth estuary at salinities <10‰, where high particulate loads of up to 180 mg l−1 occur. Phosphate removal is also observed in the Forth in this salinity range. The uranium removal rate in the upper part of this estuary is estimated at 44 kg y−1.The uranium concentrations and corresponding activity ratios measured for the freshwater end-members of the three rivers are: 0·15 μg l−1 and 1·65 for the Clyde 0·04 g−1 and 1·44 for the Tamar and 0·09 μg l−1 and 1·50 for the Forth. Regression analysis gives an overall ‰ ratio of (9·53 ± 0·84) × 10−8 g g−1 over the salinity range 5·0–33·2‰ for all three estuaries, which is in excellent agreement with values obtained by other workers.
Chemical mass balence models of river solution chemistry, constrained by geologic data and thermodynamic models, provide a consistent description of the weathering processes that occur in the Amazon Basin. In areas with high weathering rates, such as the Andes, calcium, magnesium, sulfate, and alkalinity come from the weathering of evaporate minerals, sulfides, and carbonates. The inputs of calcium, magnesium, and alkalinity from exclusively carbonate terrains are limited by calcite, and perhaps by dolomite, saturation. When evaporites are present, only the alkalinity inputs are so limited because of additional weathering sources of calcium and magnesium. Dissolved silica, potassium, and sodium can be used to define mass balance relationships that in turn establish the nature of the clay mineral suites which are consistent with solute compositions. Use of thermodynamic constraints, specificially kaolinite and quartz stability, assists in data interpretation. At lowest weathering rates, all common primary minerals exposed to weathering are broken down to release silica and cations; quartz, kaolinite and (Fe,Al)-sesquioxides all appear to be unstable. At higher weathering rates, enough silica is available to stabilize kaolinite, then quartz. The stabilization of Si-bearing phases is reflected by a drop in the ratio of silica to other dissolved components. Finally, at the highest weathering rates, such as in the Andean catchments, high cation levels result in the formation of the 2:1 clays, which sequester even more silica, and any clear relationship between silica and other solutes degenerates.
Uranyl adsorption was measured from aqueous electrolyte solutions onto
well-characterized goethite, amorphous ferric oxyhydroxide, and hematite
sols at 25°C. Adsorption was studied at a total uranyl concentration
of 10 -5 M, (dissolved uranyl 10 -5 to 10 -8 M) as a function of
solution pH, ionic strength and electrolyte concentrations, and of
competing cations and carbonate complexing. Solution pHs ranged from 3
to 10 in 0.1 M NaNO 3 solutions containing up to 0.01 M NaHCO 3 . All
the iron oxide materials strongly adsorbed dissolved uranyl species at
pHs above 5 to 6 with adsorption greatest onto amorphous ferric
oxyhydroxide and least onto well crystallized specular hematite. The
presence of Ca or Mg at the 10 -3 M level did not significantly affect
uranyl adsorption. However, uranyl carbonate and hydroxy-carbonate
complexing severely inhibited adsorption. The uranyl adsorption data
measured in carbonate-free solutions was accurately modeled with the
surface complexation-site binding model of Davis et al . (1978),
assuming adsorption was chiefly of the UO 2 OH + and (UO 2 ) 3 (OH) + 5
, aqueous complexes. In modeling it was assumed that these complexes
formed a monodentate UO 2 OH + surface complex, and a monodentate,
bidentate or tridentate (UO 2 ) 3 (OH) + 5 surface complex. Of the
latter, the bidentate surface complex is the most likely, based on
crystallographic arguments. Modeling was less successful predicting
uranyl adsorption in the presence of significant uranyl carbonate and
hydroxy-carbonate complexing. It was necessary to slightly vary the
intrinsic constants for adsorption of the di- and tricarbonate complexes
in order to fit the uranyl adsorption data at total carbonate
concentrations of 10 -2 and 10 -3 M.
Adsorptive cathodic stripping square wave voltammetry was used to determine dissolved Ti at nanomolar to sub-nanomolar levels in estuarine samples over a range of salinities and dissolved organic matter concentrations. Dissolved Ti concentrations range from 0.2 to 13.7 nM and from 0.2 to 67 nM in the Delaware Bay and Winyah Bay (USA) estuarine systems, respectively.Early and late spring surveys in Delaware Bay show that dissolved Ti is largely removed during estuarine mixing. Dissolved Ti concentrations decrease with decreasing filter size fraction, suggesting that Ti is associated with particles or colloids. Mechanisms of Ti removal in estuaries may be similar to those of other particle-reactive elements, such as Fe. Some coherence between the dissolved Si and Ti distributions during a spring diatom bloom suggests that biological scavenging may also be important.In the organic-rich Winyah Bay system, Ti concentrations are patchy, indicating that both addition and removal occur during the mixing of multiple source waters. As biological productivity is low, a variety of physical and chemical processes probably control Ti behavior. Such processes include interactions between particles, dissolved organic matter, and fluxes from adjacent reservoirs.
Two observed characteristics of Th isotope and stable metal sorption in natural aquatic systems are seemingly at odds with physico-chemical adsorption theory: (1) characteristic sorption times of days to weeks and (2) Kds which are inversely related in magnitude to particle concentrations. In addition, sorption rate constants are positiveiy correlated with particle concentrations and Kd.
This paper presents a conceptual and mathematical model with which it is proposed that these metal sorption characteristics have the same underlying physical process in common: the coagulation of colloidal (nonfilterable) particles onto larger (filterable) particles. “Brownian pumping” (the transfer of truly dissolved metal species to filterable particles through a colloidal intermediate) consists of two rate steps: (1) rapid formation of metal/colloid surface site complexes (adsorption) and (2) slow coagulation of colloids with filterable particles. The Brownian-pumping model is tested against field and laboratory data. The field data, obtained from the literature, covers different regions of the oceans: deep ocean environments, euphotic zone, coastal and estuarine systems. The laboratory data involved ²²⁸Th sorption in suspensions of goethite and polystyrene latexes. Although the model has general applicability, results and discussions herein emphasize thorium isotope behavior. The Brownian-pumping model suggests that Th or other strongly sorbing elements may be useful as in situ “coagulometers” either at relatively high (e.g., greater than 5–10 mg/l) particle concentrations or when the mass ratio of colloids (C*p) to filterable particles (Cp) is known. The model also indicates that the ratio of colloids to filterable particles in marine systems, may be, by a first approximation, described by the relationship log C*p = 0.7 log Cp – 2.6 (in units of kg/l).
α-SPECTROMETRIC uranium isotopes analyses on groundwater samples from the highly fertilised Handschuhsheimer Feld area in Heidelberg suggest that the uranium content of the groundwater is primarily of natural origin; uranium derived from phosphate fertilisers is adsorbed on the uppermost soil layers and the uranium content of the water is correlated to the HCO−
3 content. A similar relationship between uranium and HCO−
3 also applies for world rivers and lakes. We show here that this indicates that a low uranium content is due to low HCO−
3 contents and not to lower contamination levels. On the basis of this correlation the average natural uranium content of world rivers, for 35% of the water input to the ocean, was calculated to be µg l−1.
ESTIMATES published in the past 20 years of the sediment load delivered to the sea by the Amazon River range from 4 to 10 × 108 tonnes yr−1 Estimates published around 1960, when few data were available on either the sediment concentration or the river discharge, were in the range 9–10 × 108 tonnes yr−1 (refs 1, 2). Estimates published in 1967 and 1968, after significant new data had been collected, were in the lower range of 4–5 × 108 tonnes yr−1 (refs 3, 4). Our more comprehensive data collected mostly since 1970 (and especially in 1977) suggest that the earlier higher estimates may have been more nearly correct. We show here that the mean annual load of suspended sediment at Óbidos, Brazil is between 8 and 9 × 108 tonnes yr−1. Most of this sediment is discharged onto the continental shelf.
âµâ¹Fe-labeled, spherical hematite particles not retained by filters because of their size coagulated to form aggregates, which were filter-retained. Batch systems containing hematite particles and a suite of radionuclides showed that, as the hematite coagulated, colloid-adsorbed radionuclide species were transferred from the colloid pool to the filter-retained particle pool. Sorptive equilibrium was rapidly achieved; thus, the rate-limiting step for the appearance of the radionuclides in the filter-retained particle pool was coagulation. The results indicate that, in many instances, slow sorption' kinetics may be due to a combination of particle aggregation and artifacts of incomplete phase separation. It is evident that what is an artifact in experimental systems can be an important process in natural aquatic systems. The results of this study therefore provide a framework for a better understanding of the role of coagulation coupled to sorption in regulating the fate of trace metals in natural systems.
In the Amazon Basin, substrate lithology and erosional regime (seen in terms of transport-limited and weathering-limited denudation) exert the most fundamental control on the chemistry of surface waters within a catchment. Secondary effects,, such as the precipitation of salts within soils and in stream beds, biological uptake and release, and cyclic salt inputs, are more difficult to discern. Samples can be separated into four principal grouping based on relationships between total cation charge (TZ+) and geology. (1) Rivers with O
Deposition in organic-rich sediments is a major sink for uranium in the ocean. Uranium concentrations and 234 U/ 238 U ratios of paniculate matter collected by sediment traps and U concentrations in sediment pore waters collected in situ in Saanich Inlet were measured to study the process of U deposition in anoxic marine basins. The results provided direct evidence for two previously hypothesized mechanisms of U deposition in organic-rich sediments. Dissolved U is incorporated into particles in surface waters, possibly in association with autochthonous organic matter. Fluxes of paniculate authigenic U ranged between 0.025 and 0.4 dpm 238 U cm -2 yr -1 , with lowest fluxes occurring during winter months. Particulate U fluxes collected in the upper oxic water column and the lower anoxic waters were not significantly different, indicating that chemical scavenging of U by particles settling through the anoxic H 2 S-bearing bottom waters of Saanich Inlet contributes negligibly to the total paniculate U flux. Dissolved U also diffuses into the sediments where it is precipitated. The calculated diffusive flux of U into the sediments (5.3 · 10 -3 dpm 238 U cm -2 yr -1 ) is much smaller than the particulate U flux from the water column, and the sum of these two fluxes is less than the rate of authigenic U burial in the sediments (0.48 dpm 238 U cm -2 yr -1 ), suggesting that an additional mechanism of U precipitation may be taking place at, or near, the sediment-water interface. The pore water data are consistent with thermodynamic predictions that U should be reduced and precipitated under the Eh and pH conditions of the sediments and suggest that previous studies which have indicated a flux of dissolved U out of anoxic marine sediments suffered from serious sampling artifacts.
The effects of imposed variations of pH and Eh on aqueous uranium mobility at 25°C have been studied in three simulations of natural water systems. Constituents tested for their effect on uranium mobility were: 1.(a) hydrous ferric oxide, to represent adsorptive solids which precipitate or dissolve in response to variations in pH and Eh.2.(b) kaolinite, representing minerals which, although modified by pH and Eh changes, are present as solids over the pH-Eh range of natural waters.3.(c) carbonate, to represent a strong uranium-complexing species.Uranium mobility measurements from each simulation were regressed against pH and Eh within a range appropriate to natural waters.Hydrous ferric oxide and kaolinite each affected uranium mobility, but in separate pH-Eh domains. Aqueous carbonate increased mobility of uranium, and adsorption of UO2(CO3)34− caused colloidal dispersion of hydrous ferric oxide, possibly explaining the presence of ‘hydrothermal hematite’ in some uranium deposits. Enhanced uranium mobility observed in the pH-Eh domains of thermodynamically insoluble uranium oxides could be explained if the oxides were present as colloids. Uranium persisting as a mobile species, even after reduction, has implications for the near surface genesis of uranium ores.
The kinetics of aggregation of riverine dissolved Fe colloids upon addition of seawater have been studied using samples from two New Zealand rivers.Results show that the kinetic data are not well-described by a second-order rate law as found in previous studies. A significant fraction of the dissolved Fe was aggregated immediately upon the addition of seawater, with the fraction aggregated in this way increasing as the salinity was increased. Storage of river water before salt addition for 8–48 hr was found to markedly decrease the fraction of Fe aggregated upon salt addition. The rate and extent of aggregation decreased as the pore size of the filters used to separate dissolved and particulate iron was increased. The rate and extent of aggregation were markedly decreased when pre-existing particles already larger than the nominal pore size of the filters were removed before addition of salt, showing that large particles are essential to the aggregation mechanism.A statistical model of particle collisions has been developed that explains this dependence of aggregation rate on the presence of large particles and on the pore size of the filter.
The concentrations of dissolved uranium and radium isotopes have been determined in the waters of several Indian rivers. The
concentration of uranium (U-238) is found to vary between 0·01–7·0 micrograms/litre, depending primarily on the terrain through
which the river flows. From the available data on the activity ratios, U-234/U-238, concentration of U-238 in rivers and the
amount of uranium depositing on the ocean floor, a material balance calculation has been attempted. This shows that the supply
of uranium to sea by rivers based on above measurements, is inadequate to explain the observed concentrations of U-238 and
U-234 in the oceans.
The Ra-226 concentration of river waters is found to be of the order of 0·1 dpm/litre (varying between 0·05–0·2 dpm/litre).
The activity ratios, Ra-228/Ra-226, range between 1 and 4. This large variation is clearly related to the fact that the concentrations
of the parent nuclides of Ra-226 and Ra-228 are different. The observed ratios are consistent with those expected from leaching
of clays/soils; however, analogous to the situation in the case of uranium, the observed concentrations of radium isotopes
in the oceans are too large to be explained by their influx by rivers.
The estuarine chemistry of several metals and anion-forming elements is compared. Differences in the estuarine behaviour of
nickel and copper appear to be caused by differences in the stability and reaction rates of organic complexes of these metals.
Nickel occurs partially in very stable organic complexes that have an estuarine pathway different from the electrochemically
labile (weakly complexed) fraction as the two fractions are not in equilibrium on the estuarine time scale; whereas, the complexes
of copper in the estuary are in equilibrium with each other. Titanium occurs largely in a nonlabile form and is subject to
removal at low salinities; it is released again at intermediate salinities (25 practical salinity units) and behaves conservatively
at higher salinities. Comparison of several anion-forming elements reveals differences; uranium, antimony, and molybdenum
show nonconservative behaviour and selenium shows conservative behavior. Voltammetric measurements of the chemical speciation
of molybdenum in estuarine waters indicated the unexpected presence of a nonlabile species for this element.
The characterization and quantitation of aquatic organic colloids isolated from sedimentary porefluids was performed by size exclusion chromatography (SEC) with UV detection and ultrafiltration (UF) with total organic carbon analysis and UV detection. Separation of the samples and several well-characterized humic substances by SEC was found to be highly dependent upon the ionic strength of both the mobile phase and sample matrix. The elution of samples containing indifferent electrolyte concentrations by low ionic strength mobile phases yielded multipeak chromatograms and may be subject to misinterpretation as true molecular weight distributions. A dynamic coiling/uncoiling mechanism is offered as an explanation for the observed phenomenon. For well-characterized humic substances eluted with high ionic strength mobile phases, number- and weight-average molecular weights as well as the polydispersivities were calculated and were found to be in good agreement with other recently published values. Molecular weight distributions for Suwanee fulvate determined by both sizing techniques appeared to be in reasonable agreement. Ultrafiltration was used to separate organic colloids from low molecular weight components in sedimentary porewaters. Vertical profiles in the cores showed depletion of colloids near the sediment/water interface and accumulation of macromolecules with depth. Porewater colloids collected using glass fritted samplers appeared to confirm this trend, although lower values were recovered than when extracted by centrifugation methods.
Gibbs free energies, enthalpies and entropies of 42 dissolved uranium species and 30 uranium-bearing solid phases have been critically evaluated from the literature and estimated when necessary for 25°C. Application of the data shows that the uranium in natural waters is usually complexed. At typical concentrations of chloride, fluoride, phosphate and sulfate, uranous (U4+) fluoride complexes are important in anoxic waters below pH 3–4. An intermediate Ehs (between about +0.2 and −0.1 V) and pH values 1–7, UO2+ ion may predominate. In oxidized waters, uranyl (U22+) fluoride complexes and uranyl ion predominate below pH 5; from about pH 4 to 7.5, UO2(HPO4)22− is the principal species; while at higher pHs, UO2CO30 and the di- and tri-carbonate complexes predominate. Uraninite [UO2-UO2.25], α-U3O8 and schoepite are the stable uranium oxides and hydroxides in water at 25°C. Coffinite, USiO4 (c), is probably stable relative to UO2(c) when dissolved silica exceeds about 60 ppm (as SiO2). At low Ehs and pH 4–6, the solubilities of stoichiometric crystalline uraninite and coffinite are below roughly 10−4 ppb. But at intermediate Ehs and neutral to alkaline pHs in the presence of phosphate or carbonate, the formation of uranyl phosphate or carbonate complexes can increase the solubilities of these minerals by several orders of magnitude. The uranyl minerals carnotite, tyuyamunite, autunite, potassium autunite and uranophane are least soluble at pHs in the range 5–8.5 and, in the case of carnotite and tyuyamunite, have solubilities as low as 0.2 and 1 ppb uranium, respectively. The autunites and uranophane are usually several orders of magnitude more soluble than this, consistent with their natural occurrences. Sorption of uranyl on to natural materials is maximal in the same pH range of 5–8.5.
Pore water uranium data from the NW Atlantic Ocean, Cariaco Trench, Black Sea and San Clemente Basin are used to calculate fluxes across the sediment-water interface. All samples were collected in situ and show depletions in pore water U relative to the overlying water concentration. The uptake rate of U, derived from pore water gradients, reflects the redox environment, with lowest fluxes (10 μg/cm2ky) in oxic/su☐ic sediments of the NW Atlantic dominated by NO3− reduction and higher fluxes (25–70 μg/cm2ky) in the anoxic, sulfate-reducing sediments of the Cariaco Trench, Black Sea and San Clemente Basin. The anoxic fluxes compare well with previous estimates based on solid phase data. Calculations indicate that the areally integrated removal of U into su☐ic, hemipelagic sediments is of greater magnitude than that into anoxic, sulfate-reducing sediments. These fluxes are discussed in the context of the oceanic U balance. The removal of U in all oceanic sediments is estimated to account for about 65% of the riverine input, and this sink plus removal of U in altered oceanic crust balances the most probable value for riverine input.
The most comprehensive data set on uranium, thorium, and radium isotopes in the Ganga-Brahmaputra, one of the major river systems of the world, is reported here. The dissolved 238U concentration in these river waters ranges between 0.44 and 8.32 μ/1, and it exhibits a positive correlation with major cations (Na + K + Mg + Ca). The ratio in waters is very similar to that measured in the suspended sediments, indicating congruent weathering of uranium and major cations. The regional variations observed in the [ ] activity ratio are consistent with the lithology of the drainage basins. The lowland tributaries (Chambal, Betwa, Ken, and Son), draining through the igneous and metamorphic rocks of the Deccan Traps and the Vindhyan-Bundelkhand Plateau, have [ ] ratio in the range 1.16 to 1.84. This range is significantly higher than the near equilibrium ratio (~1.05) observed in the highland rivers which drain through sedimentary terrains.
The colloid chemistry of iron and humic aggregation was studied in a series of laboratory experiments, which were related to seasonal aggregation extent data collected in a temperate estuary. Kinetically, the aggregation of iron occurs with a rapid initial reaction, finished within a few minutes, followed by an extensive, slow second reaction, which lasts for several hours. Organic carbon aggregation, however, occurs primarily in the first reaction. The first reaction is perikinetic while the second reaction is observed to be either perikinetic or orthokinetic, depending on the pore size of the filter used to clarify the suspension. The second reaction involves aggregation of small filtrable colloids (<0.5 μm) with larger aggregates formed in the first reaction, and follows pseudo-second order kinetics with respect to the filtrable iron concentration when finer filters (<0.5 μm) are used for clarification. Little or no temperature dependence is evident for the first reaction extent, while increasing temperature strongly enhances the second reaction. An activation energy of 9–11 kcal·mole−1 for the second reaction is due to a combination of transport and charge repulsion terms. Field data show a correspondingly greater aggregation extent of iron with salinity in warmer months relative to colder months.
Formation of fine-scale () strata on the Amazon continental shelf was investigated using radiographic and radiochemical (e.g. 210Pb) techniques. Three distinct sedimentary environments are found within the inner shelf ( water depth) mud deposit: (1) interbedded mud and sand in a corridor extending from the river mouth across the inner shelf; (2) faintly laminated muds on both flanks of the corridor; and (3) bioturbated muds comprising the remainder of the deposit. High rates of sediment accumulation () are found in the region of interbedded mud and sand, and in the regions of faintly laminated muds. Accumulation rates in the region of bioturbated muds are relatively low (0.1–2 cm/yr), and an inverse relationship is observed between accumulation rate and extent of sedimentary structure alteration by benthic fauna. A first-order sediment budget indicates that sediment accumulation on the Amazon shelf (between the Brazil-French Guiana border and the Para River) is nearly equal to estimates of sediment discharged by the Amazon River ().The model of Amazon shelf sedimentation most consistent with the radiographic and radiochemical data suggests that the inner shelf mud deposit is a subaqueous delta prograding seaward over relict basal sands and that a turbulent jet emanates from the river mouth with competence (during some flood periods) to carry sand as bedload across the inner shelf. The presence of cross-bedded sands in the corridor of interbedded mud and sand supports the hypothesis of a turbulent jet. The faintly laminated muds probably represent suspended sediment rapidly deposited on both sides of the jet.
Pore water profiles of uranium and thorium isotopes in the muddy sediments of Buzzards Bay, MA permit an assessment of the effect of diagenetic redox reactions on the geochemical behavior of these elements. Uranium shows a pronounced minimum pore water concentration (~ 1.2 dpm/kg) near the sediment water interface (0–3 cm) which coincides with the pore water Fe maximum. U concentrations increase with depth to a broad maximum which is greater than the overlying seawater value, then decrease. Laboratory sediment tank experiments maintained without macrofauna also show the near-interface minimum and increase, and sediment incubation experiments show removal of U from the pore water to a constant value by about 40 days. The activity ratio is equal to the seawater value in all samples. Oxidation state separations of pore water U in HCl solution (using anion exchange) demonstrate that U is present in the pore water as U(VI). The coincidence of the pore water uranium and Fe maximum is consistent with the hypothesis of reduction of U(VI) to U(IV) and removal from solution at about the same depth (or pe) as Fe is reduced. Increases in pore water U with depth appear to be related to release of authigenic (seawater) U(VI) to solution, possibly as alkalinity and pH increase and organic matter with which the U is associated is oxidized. Lower concentrations at depth in the sediment column may be linked to increases of effective U reducing agents like hydrogen sulfide in the pore water or to the activities of macrofauna.