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Radiochemical assessment of the readsorption and redistribution of lead in the SM&T sequential extraction procedure
Abstract
The objective of this study was to employ radiochemical methods to assess the importance and extent of the readsorption and redistribution of lead in the standards, measurement and testing programme’s (SM&T) sequential extraction procedure. Step-wise tests indicated that 60% to over 90% of the lead extracted from the exchangeable fraction was readsorbed. Approximately 35% to over 85% of the lead extracted from the reducible fraction was readsorbed, while negligible readsorption was detected for the oxidisable fraction. Statistically significant correlations were observed between readsorption from the first two fractions and both the total iron content and the organic carbon matter even at low organic contents (i.e. less than 6 wt.%). By following the radiotracer activity through the first two steps of the method, the reducible fraction was shown to play an important role in scavenging lead released from the exchangeable soil fraction. The radiotracer partitioning results were employed to “correct” for the redistribution effects yielding the “true” fractionation of the Pb in the samples.
... For each element, the sum of concentrations from fractions 1 to 4 divided by the bulk concentrations (i.e., recovery of the leaching experiments) was better than 82%. Re-adsorption of dissolved ions on residual solid phases may occur during sequential leaching experiments (Rendell et al., 1980; Sholkovitz, 1989; Gilmore et al., 2001). The fractions of metals re-adsorbed on Fe oxides and insoluble phases (silicate and aluminosilicates) in the hydroxylamine hydrochloride extraction step were determined using 54 Mn, 58 Co, 139 Ce, and 146 Gd radioisotope tracers (Ambe et al., 1995; Takahashi et al., 1997 Takahashi et al., , 1999). ...
... The Co–Mn association, confirmed below by EPMA and XANES spectroscopy, indicates that the positive Co–Fe/Mn correlation inFig. 1 Koschinsky and Halbach, 1995; Koschinsky and Hein, 2003 ). This interpretation should be viewed cautiously , however, because Pb is prone to re-adsorption (Gilmore et al., 2001), and this side-effect is expected to be larger than for REE, such as Ce and Gd (Sholkovitz, 1989). Cerium extraction data are conclusive for D535 and D1-X1 because Ce and Mn were eluviated together: 75% Ce and 89% Mn in D535, 62% Ce and 87% Mn in D1-X1. ...
The oxidation state and mineral phase association of Co, Ce, and Pb in hydrogenetic, diagenetic, and hydrothermal marine ferromanganese oxides were characterized by X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy and chemical extraction. Cobalt is trivalent and associated exclusively with the Mn oxide component (vernadite). Cerium is tetravalent in all genetic-type oxides (detection limit for Ce(III) ∼ 5 at. %), including Fe-rich areas (ferrihydrite) of hydrogenetic oxides, and is associated primarily with vernadite. Thus, the extent of a Ce anomaly does not result from variations in redox conditions, but appears to be kinetically controlled, decreasing when the growth rate increases from hydrogenetic to diagenetic to hydrothermal oxides. Lead is divalent and associated with Mn and Fe oxides in variable proportions. According to EXAFS data, Pb is mostly sorbed on edge sites at chain terminations in Fe oxide and at layer edges in Mn oxide (ES complex), and also on interlayer vacancy sites in Mn oxide (TCS complex). Sequential leaching experiments, spectroscopic data, and electrochemical considerations suggest that the geochemical partitioning in favor of the Mn oxide component decreases from Co to Ce to Pb, and depends on their oxidative scavenging by Mn and Fe oxides.
... Another well documented artefact that occurs in sequential extraction is the possibility of readsorption or redistribution between phases (Kheboian and Bauer, 1987;Nirel and Morel, 1990). It has, however, been shown that the readsorption for most metals is much less than suspected and will not disqualify the use of sequential extraction provided that the limitations of the technique are considered (Belzile et al., 1989;Ho and Evans, 2000;Gilmore et al., 2001). Readsorption of copper and lead on the oxidisable fraction has, however, been shown to be a significant drawback leading to a possible overestimation for the association of these metals to organic matter (Belzile et al., 1989;Ho and Evans, 2000;Gilmore et al., 2001). ...
... It has, however, been shown that the readsorption for most metals is much less than suspected and will not disqualify the use of sequential extraction provided that the limitations of the technique are considered (Belzile et al., 1989;Ho and Evans, 2000;Gilmore et al., 2001). Readsorption of copper and lead on the oxidisable fraction has, however, been shown to be a significant drawback leading to a possible overestimation for the association of these metals to organic matter (Belzile et al., 1989;Ho and Evans, 2000;Gilmore et al., 2001). Despite these findings, sequential extraction has been extensively used with considerable success on a multitude of solid systems with different hydrogeochemical conditions; soil (Li et al., 1995;Bellanca et al., 1996), mining waste (Clevenger, 1990) and wetland sediments (Dollar et al., 2001). ...
Several roadside soil samples were collected at two field sites in Sweden. They were analysed for total elemental content (using both ICP-MS and XRF) and stable lead isotopes. Extraction with deicing salt solution and sequential extraction were performed in order to elucidate the potential mobility due to the use of deicing agents. The total concentrations of elements, especially lead, have decreased and lead is presently almost at background concentrations (15-51 ppm for surface samples). However, the isotopic signature indicates that old gasoline lead still is left at the site constructed prior to 1975. The field site constructed in 1992 showed, however, no 206Pb/207Pb ratio below 1.14. Only minor amounts were leached using deicing salt solutions; for lead only 0.29%, on average, was extracted indicating that the mobile fraction already was released. Sequential extraction indicated that lead mainly was associated with reducible (34.4%) and oxidisable (35.4%) fractions. Exchangable and acid soluble fractions contained 20.3% while 10.0% was found in the residual fraction. The salt extraction released, however, very low concentrations indicating that most in fraction 1 is acid soluble (e.g. carbonates). Tungsten was also found at high concentrations indicating a possible impact from studded tires. For tungsten the following composition was obtained: residual (48.0%) > oxidisable (47.6%) > reducible (3.3%) > exchangeable/acid soluble (1.1%). From the isotopic studies it was also suggested that the order for incorporating anthropogenic lead into soils is exchangeable/carbonates > (hydr)oxides > organic matter > residual. The multivariate technique principal component analysis (PCA) seems promising for evaluating large sequential extraction datasets.
... Step 1 of the BCR extraction of a CRM, it was found that only 20-30% of copper and zinc, 125 but 60-90% of lead, 126 were scavenged from the acetic acid solution by the (solid) reducible fraction. When 212 Pb was instead added to the reagent at the start of BCR Step 2, 35-85% of the activity partitioned into the solid phase but, when added at the start of BCR Step 3, the tracer remained in solution. ...
... When 212 Pb was instead added to the reagent at the start of BCR Step 2, 35-85% of the activity partitioned into the solid phase but, when added at the start of BCR Step 3, the tracer remained in solution. 126 Incomplete dissolution of carbonates during Step 1 of the BCR protocol has been demonstrated by X-ray analysis of the residue at the end of the extraction step, for both urban canal sediment 86 and soil. 84 The latter study focused on a calcareous soil, where the amount of acetic acid added was insufficient to dissolve the ca. ...
Since their introduction in the late 1970s, sequential extraction procedures have experienced a rapid increase in use. They are now applied for a large number of potentially toxic elements in a wide range of sample types. This review uses evidence from the literature to consider the usefulness and limitations of sequential extraction and thereby to assess its future role in environmental chemical analysis. It is not the intention to provide a comprehensive survey of all applications of sequential extractions or to consider the merits and disadvantages of individual schemes. These aspects have been covered adequately in other, recent reviews. This review focuses in particular on various key issues surrounding sequential extractions such as nomenclature, methodologies, presentation of data and interpretation of data, and discusses typical applications from the recent literature for which sequential extraction can provide useful and meaningful information. Also covered are emerging developments such as accelerated procedures using ultrasound- or microwave energy-assisted extractions, dynamic extractions, the use of chemometrics, the combination of sequential extraction with isotope analysis, and the extension of the approach to non-traditional analytes such as arsenic, mercury, selenium and radionuclides.
... Thus, a speciation analysis of contaminant metals should provide useful information regarding the chemical nature or potential mobility and bioavailability of a particular metal, which can improve the accuracy of estimates of the environmental impact of these contaminants. Despite a lack of selectivity and re-adsorption problems (Gilmore et al., 2001;Chomchoei et al., 2002) associated with the presently available methods of solid phase fractionation (Tessier et al., 1979;Kersten and Förstner, 1986), sequential extraction procedures are a useful tool for predicting long-term adverse effects in contaminated soils and sediments and for assessing the differences in mobility and bioavailability in an environmental context (Hursthouse, 2001). ...
... By comparison, only about 30% of the total Pb concentration was associated with the reducible phase in the non-mining road dusts. Redistribution of trace metals among the BCR phases has been shown to be linked to a predominance of Mn oxides in a sample matrix (Gilmore et al., 2001;Filgueiras et al., 2002). However, chemical and optical microscopy analysis showed that Fe is nearly 50 times more concentrated than Mn in our samples suggesting that Fe oxides are the major controlling phase for Pb adsorption and not Mn oxides. ...
Airborne particulate material collected from seventeen rural unsurfaced roads in Missouri's agricultural and resource mining areas were characterized using the BCR sequential extraction procedure and simulated in vitro body fluids to determine the phase partitioning and bioaccessibility of Pb associated with roadway dusts. Results show that dusts produced from driving over unsurfaced roads in the mining area has a substantial portion of the Pb concentration associated with the more mobile exchangeable-plus-carbonate and reducible geochemical phases. By comparison, unsurfaced road dusts outside the resource mining area have lower metal contents, as expected, and a larger portion of the total Pb concentration associated with the immobile oxidizable and non-silicate bound residual phases. SEM/EDS analysis suggests the minerals associated with the more mobile Pb components include cerussite, Pb oxides and sulfates. Compared with the coarser >1 μm size fraction of dust, the <1 μm fraction contains a substantially higher concentration of Pb in association with clay minerals. Extraction tests using simulated body fluids show that gastric fluid can mobilize as much as 69% of the total Pb concentration in mining area road dust samples after five hours. Simulated alveolar lung fluid also was an efficient extractor of Pb from the <1 μm sample dust fraction, dissolving up to 100% of the available Pb after 100 h. Regression analysis suggests that aqua regia total Pb concentration is a good predictor of mobility and bioaccessibility and can be used to minimize costs associated with monitoring suspended dust contamination.
... Thus, a speciation analysis of contaminant metals should provide useful information regarding the chemical nature or potential mobility and bioavailability of a particular metal, which can improve the accuracy of estimates of the environmental impact of these contaminants. Despite a lack of selectivity and re-adsorption problems (Gilmore et al., 2001;Chomchoei et al., 2002) associated with the presently available methods of solid phase fractionation (Tessier et al., 1979;Kersten and Förstner, 1986), sequential extraction procedures are a useful tool for predicting long-term adverse effects in contaminated soils and sediments and for assessing the differences in mobility and bioavailability in an environmental context (Hursthouse, 2001). ...
This study was conducted to assess the anthropogenic impact on metal concentrations in the bottom sediments of the Juam reservoir, Korea, and in stream sediments in its catchment, and to estimate the potential mobility of selected metals (Fe, Mn, Cu, Ni, Pb and Zn) using sequential extraction. A comparison of the metal concentrations in the stream sediments with mean background values in sediments collected from first- or second-order creeks shows that Pb, Cu and Ni are the most affected by anthropogenic inputs. The 206Pb/207Pb ratios of the bottom and core sediments (means: 1.2320 ± 0.0502 and 1.2212 ± 0.0040, respectively) suggest that Pb contamination is mainly due to the waste discharge of abandoned coal and metal mines rather than industrial and airborne sources. Considering the proportion of metals bound to the exchangeable, carbonate and reducible fractions, the comparative mobility of metals is suggested to decrease in the order Mn > Pb > Zn > Ni > Fe ≫ Cu.
... The preferential co-extraction of Pb together with Fe and Mn was proved by its high contents in the reducible fraction (F3). However, Pb is more prone to re-adsorb on the soil residue during extraction and this side-effect is expected to be more important compared to the other elements analyzed (Gilmore, et al., 2001). For all 16 samples with the Pb content ranging from 8000 mg kg −1 to 100000 mg kg −1 , the differences between the results obtained by the two methods were within the limits of agreement regardless the Pb concentration and the negative bias towards XRF was very low compared to the value computed for the Pb content. ...
The Tessier extraction method was used for speciation of Cu, Pb, Zn, As, Fe and Mn in a large concentration range in contaminated
soil with various mineralogical compositions. The results were compared by X-ray fluorescence spectrometry (XRF) as a reference
method using the Bland and Altman test. A sum of five fractions (exchangeable, bound to carbonates, Fe-Mn oxides, organic
matter and residual forms) was compared with the total content determined on solid matrix by the reference method. A good
agreement between the methods in the whole concentration range was found for Cu, Zn, As, and Fe. For Mn and Pb, XRF was found
suitable to verify the sequential extraction only for concentrations above 250 mg kg−1. This was a consequence of a poorer reproducibility of Pb extraction using the Tessier scheme due to a great difference in
the mineralogical composition and the diversity of the Pb species identified in soil. The poorer result of Mn was attributed
to the spectral interference of Fe in XRF.
... dithionite, hydroxylamine hydrochloride, and ascorbic acid) are used to determine which TMs are associated with which phases (Schwertmann and Pfab, 1994;Trolard et al., 1995;Koschinsky and Hein, 2003;Neaman et al., 2004). This approach has many shortcomings, including incomplete dissolution of target phases, dissolution of nontarget species, and potential redistribution of elemental species among remaining minerals in the sample (Taylor and McKenzie, 1966;McCarty et al., 1998;Ostergren et al., 1999;Gilmore et al., 2001). Alternatively, instrumental microchemical techniques, including scanning transmission electron microscopy (STEM), particle induced X-ray emission (PIXE), and synchrotron-based X-ray radiation fluorescence (SXRF), have been used to determine directly chemical associations at the nanometer to micrometer scale in undisturbed or minimally disturbed samples (Przybylowicz et al., 2001;Manceau et al., 2002b;Sutton et al., 2002;Jeong and Lee, 2003;Utsunomiya et al., 2003). ...
The mineralogy of natural ferromanganese coatings on quartz grains and the crystal chemistry of associated trace elements Ni, Zn, Ba, and As were characterized by X-ray microfluorescence, X-ray diffraction, and EXAFS spectroscopy. Fe is speciated as ferrihydrite and Mn as vernadite. The two oxides form alternating Fe- and Mn-rich layers that are irregularly distributed and not always continuous. Unlike naturally abundant Fe–vernadite, in which Fe and Mn are mixed at the nanoscale, the ferrihydrite and vernadite are physically segregated and the trace elements clearly partitioned at the microscopic scale. Vernadite consists of two populations of interstratified one-water layer (7 Å phyllomanganate) and two-water layer (10 Å phyllomanganate) crystallites. In one population, 7 Å layers dominate, and in the other 10 Å layers dominate. The three trace metals Ni, Zn, and Ba are associated with vernadite and the metalloid As with ferrihydrite. In vernadite, nickel is both substituted isomorphically for Mn in the manganese layer and sorbed at vacant Mn layer sites in the interlayer. The partitioning of Ni is pH-dependent, with a strong preference for the first site at circumneutral pH and for the second at acidic pH. Thus, the site occupancy of Ni in vernadite may be an indicator of marine vs. continental origin, and in the latter, of the acidity of streams, lakes, or soil pore waters in which the vernadite formed. Zinc is sorbed only in the interlayer at vacant Mn layer sites. It is fully tetrahedral at a Zn/Mn molar ratio of 0.0138, and partly octahedral at a Zn/Mn ratio of 0.1036 consistent with experimental studies showing that the VIZn/IVZn ratio increases with Zn loading. Barium is sorbed in a slightly offset position above empty tetrahedral cavities in the interlayer. Arsenic tetrahedra are retained at the ferrihydrite surface by a bidentate-binuclear attachment to two adjacent iron octahedra, as commonly observed. Trace elements in ferromanganese precipitates are partitioned at a few, well-defined, crystallographic sites that have some elemental specificity, and thus selectivity. The relative diversity of sorption sites contrasts with the simplicity of the layer structure of vernadite, in which charge deficit arises only from Mn4+ vacancies (i.e., no Mn3+ for Mn4+ substitution). Therefore, sorption mechanisms primarily depend on physical and chemical properties of the sorbate and competition with other ions in solution, such as protons at low pH for Ni sorption.
... All these studies have increased our knowledge of these systems. However, extraction studies are plagued with artifacts such as removal of both Mn and Fe oxides and redistribution of Pb species among soil mineral phases (Chao and Theobald, 1976; Gilmore, 2001; McCarty et al., 1998; Ostergren et al., 1999; Taylor and R.M., 1966). Microprobe techniques are not surface sensitive and can be restricted by their detection limits for trace elements such as Pb (see Goldstein et al., 1992; Ostergren et al., 1999; Rauch et al., 2000). ...
Lead sorption efficiencies (sorption per specific surface area) were measured for a number of natural and synthetic Mn and Fe-oxides using a flow-through reactor. The Mn-oxide phases examined included synthetic birnessite, natural and synthetic cryptomelane, and natural and synthetic pyrolusite; the Fe-oxides studied were synthetic akaganéite, synthetic ferrihydrite, natural and synthetic goethite, and natural and synthetic hematite. The sorption flow study experiments were conducted with 10 ppm Pb with an ionic strength of either 0.01 M NaNO3 or 0.01 M KNO3, both at pH 5.5. The experimental effluent solution was analyzed using aqueous spectroscopic methods and the reacted solids were analyzed using microscopy (field emission scanning electron microscopy, FE-SEM), structure analysis (powder X-ray diffraction, XRD), bulk chemical spectroscopy (energy dispersive spectroscopy, EDS), and surface sensitive spectroscopy (X-ray photoelectron spectroscopy, XPS). Overall and under these conditions, the synthetic Mn-oxides have higher sorption efficiencies than the natural Mn-oxides, which in turn are higher than the natural and synthetic Fe-oxides. Only natural pyrolusite had a sorption efficiency as low as the Fe-oxides. Most of the natural and synthetic Fe-oxides examined in this study removed about the same amount of Pb from solution once normalized to BET N2 surface area, although synthetic akaganéite and hematite were significantly less reactive than the rest.
... This correlation was statistically significant for all three heavy metals at p b 0.05 except for Zn. The fractions I and II of SM&T are assumed to be representatives of exchangeable and weakly acid soluble and reducible fraction of heavy metals in the soils, re- spectively ( Gilmore et al., 2001). Thus, the DGT uptake was a good predictor of the sum of acid soluble and reducible fractions of heavy metals in the contaminated soils. ...
The applicability of diffusive gradients in thin-films (DGT) as a biomimic surrogate was investigated to determine the bioavailable heavy metal concentrations to earthworm (Eisenia foetida). The relationships between the amount of DGT and earthworm uptake; DGT uptake and the bioavailable concentrations of heavy metals in soils were evaluated. The one-compartment model for the dynamic uptake of heavy metals in the soil fitted well to both the earthworm (R(2)=0.641-0.990) and DGT (R(2)=0.473-0.998) uptake data. DGT uptake was linearly correlated with the total heavy metal concentrations in the soil (aqua regia), the bioavailable heavy metal concentrations estimated by fractions I+II of the standard measurements and testing (SM&T) and physiologically based extraction test (PBET, stomach+intestine). The coefficients of determination (R(2)) of DGT uptake vs. aqua regia were 0.433, 0.929 and 0.723; vs. SM&T fractions (I+II) were 0.901, 0.882 and 0.713 and vs. PBET (stomach+intestine) were 0.913, 0.850 and 0.649 for Pb, Zn and Cu, respectively. These results imply that DGT can be used as a biomimic surrogate for the earthworm uptake of heavy metals in contaminated soils as well as predict bioavailable concentrations of heavy metals estimated by SM&T (I+II) and PBET as a human oral bioavailable concentrations of heavy metals.
... Readsorption of Pb upon application of the BCR SES to soil and sediments has been demonstrated with the use of the 212 Pb radiotracer. 87 Using a modified Tessier scheme on oxic spiked natural sediments it was found that Cu, Hg and As suffered from strong readsorption in the most labile fractions (i.e. with MgCl 2 and NaOAc/HOAc as extractants). 85 Addition of complexing agents such as criptand and multidentate chelating agents to the conventional first extractant of the BCR SES has been proved useful to counteract readsorption/redistribution of Pb in soil models 88 and sediments. ...
A comprehensive review of the sequential extraction schemes for metal fractionation in environmental samples (ie., sediment, soil, sewage sludge, fly ash, etc.) is presented. The review contains more than 400 references and covers principally the literature published over the last decade. The use of each reagent involved in these schemes is looked at critically, and guidelines for their selectivity and extraction capacity are given. The operational character of these schemes is emphasised. Topics such as comparability between sequential extraction schemes of widespread use, harmonisation, acceleration, validation, etc. are addressed and future developments outlined.
... Since the extraction sequence of Tessier et al. (1979) is utilized in more studies than any other method, it has been the focus of many reviews. Some problems include readsorption of metals following extraction (Gilmore et al. 2001), reprecipitation (Calmano et al. 2001), non-selectivity of chemical reagents, and soil/solution ratio effects. Other problems may involve the sequence of the individual steps, specific "matrix effects" such as cross-contamination of the metal, and heterogeneity as well as physical associations (e.g. ...
Soil pH may influence speciation and extractability of Pb, depending on type of vegetation in urban soil environments. We investigated the relationship between soil pH and Pb extractability at forest and turf grass sites in Baltimore, Maryland. Our two hypotheses were: (1) due to lower pH values in forest soils, more Pb will be in exchangeable forms in forested than in turfgrass soils and (2) due to the greater lability of exchangeable Pb in equilibrium with soil solution in forest soils, concentrations of this form will increase with depth more so than in the turfgrass soils, as related to organic matter content and pH. Soil samples were collected from three forested and three turfgrass sites to depths of 20 cm. Lead forms were determined using a sequential extraction technique. Soils under turfgrass and forest vegetation differed in the extractability of soil Pb (P < 0.01) for the Mn(III, IV)- and Fe(III)(hydr) oxide fraction. A greater Pb concentration was bound to this fraction under turfgrass (211 mg kg(-1), 69% of total Pb) than forested soils (67 mg kg(-1), 61% of total Pb), perhaps due to soil pH differences of 5.9 and 5.0, respectively. In the forested soils, as depth increased, the ratio of exchangeable-to-total Pb increased and the ratio of organically bound Pb-to-total Pb decreased. The results suggest changes in pH and organic matter content with depth affect the extractability of Pb, and these soil properties are affected differentially by grass versus tree vegetation in the urban soils investigated.
The older layers of thick ferromanganese (FeMn) crusts from the central Pacific Ocean have undergone diagenetic phosphatization, during which carbonate fluorapatite (CFA) filled fractures and pore space and replaced carbonates. The effects of phosphatization on individual trace metal concentrations, speciation, and phase associations in FeMn crusts remain poorly understood yet may be important to metal enrichment mechanisms, paleoceanography, and extractive metallurgy. This study examines the concentrations, speciation, and mineral phase associations of Pb in phosphatized and nonphosphatized layers within three Pacific Ocean crusts using standard chemical and mineralogical techniques, in addition to bulk X-ray absorption spectroscopy (XAS) and microfocused X-ray fluorescence (μ-XRF) mapping. Our findings challenge the conclusions of previous works, which reported that most Pb in phosphatized crusts was associated with the CFA-dominated residual phase after sequential leaching experiments. These results were interpreted as an indication of Pb transfer from the oxide phases to diagenetic CFA during phosphatization. However, our results reveal an inverse correlation of Pb with Ca and P in the μ-XRF mapping of in situ FeMn crust sections and bulk chemistry. Furthermore, XAS measurements reveal that Pb speciation in bulk phosphatized FeMn crust layers is very similar to nonphosphatized layers, indicating only minor, if any, change in speciation during diagenetic phosphatization. Small differences in the EXAFS spectra for one highly phosphatized layer indicate that minor amounts of Pb may have been altered during phosphatization, but the new Pb-bearing phase was not unequivocally identified. Taken together, our results demonstrate that Pb speciation is not significantly affected by phosphatization.
Marine ferromanganese oxides contain a large amount of trace elements such as arsenic (As) and molybdenum (Mo). However, the host phases of tetrahedral AsO43- and MoO42- oxyanions therein have not been clearly identified so far. In this work, we explored the mineralogical components of hydrogenetic (HG) ferromanganese nodules and compared the distribution behaviors of As and Mo. The X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) analyses showed that the predominant manganese and iron phases were vernadite (δ-MnO2) and ferrihydrite, respectively. According to the sequential extraction examination, both As and Mo were associated with the iron (oxyhydr)oxide phases. However, the XAS analyses indicated that As was retained by the ferrihydrite phase via double corner-shared complexation, while Mo was preferentially adsorbed on δ-MnO2 via edge-shared complexation. The immobilization of As and Mo by HG ferromanganese samples was attributed to specific chemical binding (ΔGchem) rather than coulombic interaction (ΔGcoul) as proposed in previous studies. The comparison of Mo XAS spectra before and after extraction revealed the unreliability of sequential extraction approach to determine the host phase of trace elements due to the potential re-adsorption risk. The different distribution trends of As and Mo were due to their disparate intrinsic properties (e.g., electronic configurations, cation/anion radius ratios and averaged dissociation constants of conjugate acids) and the diverse properties (i.e., surface site densify, number of strong sites, adsorption equilibrium constant and crystalline structure) of ferrihydrite and vernadite (δ-MnO2). These research findings would be significant for evaluating the geochemical behaviors and environmental fate of trace elements in the marine systems.
It is generally recognized that the knowledge of the total concentrations is not sufficient to understand the behaviour of metals in soils and to predict their availability and mobility, which influence their release and their interaction with other environmental compartments. These characteristics are usually investigated with single or sequential extraction procedures. Single extractions are commonly performed with complexing agents, diluted acids or electrolytes, and in many applications are aimed at the estimation of the most potentially mobile metal fraction and/or of the proportion amenable for plant uptake. In sequential extraction procedures, the soil under investigation is treated with reagents of different chemical properties, which are usually applied in order of increasing strength and give rise to the leaching of metals and to the partitioning of their total content into different fractions; sequential extraction results provide information on metal association to soil components and help to predict the risks associated to metal contamination. This chapter deals with the state-of-the-art of single and sequential extraction techniques, with reference to the following topics: the types and properties of extracting reagents; batch and dynamic flow-through procedures; the possible drawbacks and limitations; other approaches to metal extraction, such as kinetic procedures and leaching with non-specific reagents. Examples of applications of single and sequential extractions to the investigation and management of metal-polluted soils are reported throughout the chapter and a case study on metal availability in polluted soils in Piedmont (Italy) is described in detail.
The role of calcium (Ca) in the cycle of phosphorus in water environment depends on the speciation of Ca in sediments, and reliable Ca sequential extraction method is useful for obtaining information about Ca transport processes from sediment to water. Experiments have been carried out to develop a simple but robust Ca sequential extraction method for identification of Ca portioning in sediments. The sequential extraction protocol was modified for extractants, extracting time, concentration and pH of the reagents in the first three steps of Tessier sequential procedure. With optimum conditions, there were 1:25 sediment to solution ratio and 3 h of extracting time shaken with NH4AC (1 mol L−1, pH 7.0) (exchangeable Ca fraction); 1:25 sediment to solution ratio and extracted with NH4AC (1 mol L−1, pH 5.0) for 5 h (carbonate-bound Ca fraction); and 1:30 sediment to solution ratio and extracted with NH2OH·HCl (0.5 mol L−1, pH 1.5) for 6 h (Fe and Mn oxides bound Ca fraction). The recovery for Tessier procedure ranged form 35.6 to 71.1 %, while the recovery for the modified Tessier procedure ranged form 46.1 to 86.9 %. These results revealed that Tessier procedure underestimated the proportion of Ca in exchangeable fraction. In sediments, Ca was mainly exchangeable and carbonate bound, which was considered as the most liable fractions.
A sequential extraction methodology, designed to measure the solid phase partitioning of metals in soils and sediments, is described. The method uses centrifugation to pass increasing concentrations of HNO 3 through the sample, followed by ICP-AES analysis of major and trace elements of the extracts. A data-processing algorithm is used to identify the number of physico-chemical components extracted, their composition and the proportion of each in each extract. The algorithm has been successfully tested on a synthetic data set and the combination of the extraction methodology and data-processing algorithm have been tested on a contaminated soil sample (NIST SRM 2710).
The 14 extracts from each duplicate experiment were analysed for 19 elements and data analysis identified nine chemically distinct soil components: pore-water residual solutes; organic, easily exchangeable; a Cu–Zn dominated phase; a Pb-dominated phase; amorphous Fe oxide/oxyhydroxide; crystalline Fe oxide; Fe–Ti oxide; and Mn oxide.
Element mobility and availability in natural solid matrices can be studied with single and sequential extraction procedures; such procedures provide reliable and useful information only if the experiments are correctly planned and executed and the results are properly interpreted. Chemometrics can be a valuable tool for these aims, especially taking into account the large amounts of data generated with extraction essays and the complexity of the processes under investigation. This review deals with the application of chemometrics in research studies involving single and sequential extractions on soils or sediments, for several purposes: the development and optimization of the extraction conditions, the calculation of element fractionation, the visual illustration of the experimental results, the acquisition of different areas of information, including relationships among variables, similarities and differences among samples, causes of the observed behaviour (e.g. source identification), risk assessment, models and predictions of future events. In Part I of the review, following an overview on extraction procedures, the applications of univariate and bivariate chemometric methods are reported; then the principles of multivariate techniques for pattern recognition based on variable reduction, their applications and the main findings obtained are addressed.
An exploratory study of the area surrounding a historical Pb-Zn mining and smelting area in Zawar, India, detected significant contamination of the terrestrial environment by heavy metals. Soils (n=87) were analyzed for pH, EC, total organic matter (TOM), Pb, Zn, Mn, and Cd levels. The statistical analysis indicated that the frequency distribution of the analyzed parameters for these soils was not normal. The median concentrations of metals in surface soils were: Pb 420.21 μ g/g, Zn 870.25 μ g/g, Mn 696.70 μ g/g, and Cd 2.09 μ g/g. Zn concentrations were significantly correlated with Cd (r=0.867), indicating that levels of Cd are dependent on Zn. However, pH, electrical conductivity and total organic matter were not correlated significantly with Cd, Pb, Zn, and Mn. To assess the potential mobility of Cd, Pb, and Zn in soils, single (EDTA) as well as sequential extraction scheme (modified BCR) were applied to representative (n=23) soil samples. The amount of Cd, Pb, and Zn extracted by EDTA and their total concentrations showed linear positive correlation, which are statistically significant (r values for Cd, Pb, and Zn being 0.901, 0.971, and 0.795, respectively, and P values being <0.001). The correlation coefficients indicate a strong relation between EDTA-extractable metal and total metal. These results appear to justify the use of 'total' metal contents as a useful preliminary indicator of areas where the risks of metal excess or deficiency are high. The EDTA extractability was maximum for Cd followed by Pb and Zn in soils from all the locations. As indicated by single extraction, the apparent mobility and potential bioavailability of metals in soils followed the order: Cd ≥ Pb > > Zn. Soil samples were sequentially extracted (modified BCR) so that solid pools of Cd, Zn, and Pb could be partitioned into four operationally defined fractions viz. acid-soluble, reducible, oxidizable, and residual. Cadmium was present appreciably (39.41%) in the acid-soluble fraction and zinc was predominantly associated (32.42%) with residual fraction. Pb (66.86%) and Zn (30.44%) were present mainly in the reducible fraction. Assuming that the mobility and bioavailability are related to solubility of geochemical forms of metals and decrease in the order of extraction, the apparent mobility and potential metal bioavailability for these contaminated soil samples is Cd > Zn > Pb.
The modified three-step sequential extraction procedure proposed by the Community Bureau of Reference (or Bureau Communautaire de Reference, BCR) was used to predict trace element mobility in soils affected by an accidental spill comprising arsenopyrite- and heavy metal-enriched sludge particles and acid waste waters. The procedure was used to obtain the distribution of both the major (Al, Ca, Fe, Mg, and Mn) and trace elements (As, Bi, Cd, Cu, Pb, Tl, and Zn) in 13 soils of contrasting properties with various levels of contamination and in the sludge itself. The distributions of the major elements enabled us to confirm the main soil fractions solubilized in each of the three steps, and, in turn, to detect the presence of pyritic sludge particles by the high Fe extractability obtained in the third step. Cadmium was identified as being the most mobile of the elements, having the highest extractability in the first step, followed by Zn and Cu, Lead, Tl, Bi, and As were shown to be poorly mobile or nonmobile. In the case of some of the trace elements, the residual fractions decreased at higher levels of contamination, which was attributed to the anthropogenic contributions to the polluted samples. Comparison with soil-plant transfer factors, calculated in plants growing in the affected area, indicated that a relative sequence of trace element mobility was well predicted from data of the first step.
A microwave-assisted continuous-flow sequential extraction system was developed for rapid fractionation analysis of heavy metals in soil. Insertion of pressure-adjusted air between the extractants provided stable flows of the extractants without mutual mixing and back-pressure influence of a column packed with soil, thereby facilitating reliable continuous-flow extractions. In addition, use of pure water as a pumping solvent removed metal contamination because of direct contact between corrosive extractants and the pump containing metallic materials. Focused microwave irradiation to the soil accelerated the selective extractions of the acid-soluble and reducible fractions of heavy metals in soil in the first and second steps of the sequential extraction conditions, as defined by the Commission of the European Bureau of Reference (BCR). The microwave-assisted continuous-flow extraction provided high correlations in amounts of six heavy metals except Zn in the first step and Cu in the second step extracted from a reference sludge soil, BCR CRM 483, with a conventional batchwise extraction proposed by BCR; continuous-flow extraction assisted by conductive heating provided lower correlations for all the six metals. The proposed method drastically reduced the time required for the sequence extraction to ca. 65 min without losing accuracy and precision of the fractionation analysis of heavy metals in soil, whereas the BCR batchwise method requires ca. 33 h.
Sequential extractions of contaminants from a soil or sediment have been shown to be cost-effective contaminated site assessment tools that provide information on contaminant partitioning within an environmental matrix. Such information is necessary for defining remediation alternatives and mitigation strategies. The typical sequential extraction approach involves a batch method, and known limitations include the possibility of contaminant readsorption to the remaining soil or sediment In this work, a flow-through reactor was constructed and tested for application in a sequential extraction scheme. The sequential extraction scheme used was one developed for actinide-contaminated materials. The flow-through approach gave partitioning results that were similar to the batch method for uranium. We also monitored the extraction of stable Ca, K, Fe, Al, Zr, and Sc and obtained partitioning results generally similar to those observed with the batch extraction, except for Ca. Our results indicate readsorption of Ca when using a batch approach is small but significant and is eliminated with our new flow-through method. A limitation of the flow-through method is the possibility of underextraction of certain phases and higher analytical uncertainties. These uncertainties are more difficult to minimize, as compared to the uncertainty obtained with a batch approach.
Direct determination of the chemical form of trace metals in soils still remains a challenge for instrumental analytical techniques. This paper examines the potential of EXAFS spectroscopy to speciate and quantify the form of trace metals in the solid fraction of soil materials using lead as a case study. Three soils contaminated by different sorts of industrial activities, including the synthesis of lead organometallics for gasoline antiknocks, Pb-Zn smelting, and recycling of lead acid battery, were investigated. In soil contaminated by alkyl-tetravalent lead compounds, lead was found to be divalent and complexed to salicylate
and catechol-type functional groups of humic substances. Lead sulfate and silica-bound lead are the predominant forms in the vicinity of the battery reclamation area. Near the smelter, lead was found to be divalent and coordinated to O,OH ligands. It is present in several chemical forms, which prevented them from being identified individually. The multiplicity of lead species in soils contaminated by smelting activities is thought to be due to long-term atmospheric emissions and to the variety of lead-containing phases simultaneously, and successively, emitted in the atmosphere. EXAFS can be applied to a wide variety of matrices including sediments, solid and liquid wastes, and fly ash particles.
Chemical extractdion experiments for estimating charactereistic particulate binding forms of heavy metals in anoxic marine and freshwater sediments were carried out under both the presence and absenceof atmospheric oxygen during the analytical procedure. Storage methods including aeration of anoxic sediments do not preserve the initial fractionation patterns of heavy metals and may both increase (Ni, Pb, Cu, Zn, Cd) and decrease (Fe, Mn) their mobility. Elutriate tests of methanogenic freshwater sedimenten from Hamburg harbour basins show that metals with increased carbonatic fractions (Fe, Mn) and those with predominantly sulphidic/organic fractions (Zn, Cu, Cd) are particularly sensitive to sample aeration. Transformations of oxidizable Ni (11%), zinc (62%) and Cd (67%) predominantly lead to easily reducible associations. Drying of the sediments decreases the proportion of the sulphidic/organic fraction of the metals to a stronger degree. Oxidized Zn and Cd is then found even in the least stable exchangeable fraction. If pretreatment of sediment samples is done adequately the chemical leaching approach may provide information on diagenetic effects and reaction kinetics, which influence the mobility of contaminants in highly complex matrices.
Two sequential extraction procedures (a 3-step protocol proposed by the Standards, Measurements and Testing programme (SM&T – formerly BCR) of the European Union and a modified Förtsner 5-step procedure) have been compared. The comparison has been carried out by analyzing a Certified Reference Material (CRM 601 – Lake sediment) and five different sediment samples collected in the Orbetello Lagoon (Central Italy). Prior to the comparison, the analytical performances of the laboratory have been evaluated on the CRM 601; this CRM has been recently certified for the content of Cd, Cr, Cu, Ni, Pb and Zn in the three fractions resulting by applying the SM&T 3-step procedure. Results of this evaluation showed the high reliability of the analytical performances of the laboratory for all considered metals. Results of the comparison between 3- and 5-step procedures showed no differences for Zn and Cd whereas significant differences have been found for Pb, above all, and for Cr, Ni and Cu relatively to the oxidisable fraction.
A number of model soils synthesized with various known amounts of natural minerals and humic acid were employed to study copper and lead redistribution during extraction by 1 M HOAc/NaOAc. The adsorption intensities of the individual constituent phases were measured from adsorption isotherms, and the redistribution behavior was studied by the standard additions method. Humic acid and pyrolusite are found to exhibit the highest binding affinity for Cu and Pb, respectively, and their relative importance relies upon both their binding ability and their abundance. The extractable concentrations of the metals and the extent of recovery depend largely upon the composition of the model soils, owing to different binding strength presented in the various systems investigated. The presence of dissolved humic acid during extraction has a significant influence on metal redistribution, as evidenced by the increase in the adsorption of Cu and decrease in the adsorption of Pb. In view of the diverse compositions and various competing reactions involved, it is recognized that redistribution behavior will vary widely among both natural soil systems and given metals.
By mixing carbon disulfide and diethylamine in chloroform prior to use, diethyldithiocarbamic acid forms. Such metal ions as CU2+, CO2+, Zn2+, Ni2+, Cd2+, Fe3+, and Mn2+ were quantitatively extracted from an aqueous sulfate medium as a diethyldithiocarbamate complex. The optimum extraction conditions, such as the pH, mixing ratio of reagents, and shaking time for equilibrium, were established.
Synthetic model soils were used to evaluate redistribution of lead (Pb) in both the three-stage sequential extraction procedure proposed by the Commission of the European Communities Bureau of Reference (BCR, now Standards, Measurements and Testing Programme) and the commonly applied procedure of Tessier et al. (1979). Redistribution of Pb in both schemes was found to be substantial, sufficient to render meaningless any attempts to quantify the original phase associations of Pb in cases where these are not known. In the model systems, the manganese oxide and humic acid phases were found to be responsible for most of this redistribution in both schemes. Despite the fact that both schemes allow severe Pb redistribution to occur, differences are apparent in the manner in which this occurs. Whereas Tessier et al.'s procedure in all cases yielded the highest recovery of Pb at the reducing step, the three-stage BCR procedure released the most Pb at the oxidizing step, both irrespective of where it originated. These differences are attributable to a combination of high temperature (96 °C) and the presence of acetic acid (25%, vv) in Tessier et al.'s reducing step. These conditions were found to reduce the sorptive capacity of humic acid by more than 50%, in addition to retaining more of the Pb in solution by acetate ions. Extensive redistribution in the three-stage BCR procedure was also observed in systems where natural soils were mixed with Pb-spiked synthetic phases. In keeping with the results related to synthetic soil models, the organic matter and Fe/Mn oxide fractions of the natural soils appear to dominate the redistribution of Pb during the extraction.
The interpretability of a selective extraction procedure for metals in aquatic sediments was tested by means of synthetic models. The major sedimentary geochemical phases were prepared and characterized by X-ray diffraction and scanning electron microscopy. A specific trace metal was doped into each phase by adsorption or coprecipitation, content being verified by atomic absorption (AA). The phases and metals were calcite (Pb), iron sulfides (zn), humic acid (Cu), and iron oxyhydroxides (Cu or Ni). Three different model sediments were prepared by combining phases and diluting with silica or illite. Measurements of extracts by AA and inductively coupled plasma (ICP) indicated that trace metals were not recovered in the appropriate fractions. This implies that selective extraction procedures as currently practiced may not be suitable for distinguishing the phase association of metals in real sediments.
The trace metal speciation in two standard reference soils from NIST was determined using the BCR sequential extraction procedure, and found comparable in the cases of Cd, Cu and Zn to previous results using the Tessier
Soils contaminated with lead and copper from three industrial-waste sites were characterized for particle-size distribution, heavy metal (Pb, Cu) content by particle size, sequential extraction for metal speciation, scanning electron microscopy for morphology, and energy dispersive X-ray microanalysis (SEM/EDX) for chemical composition. High concentrations of Pb and Cu were found in all size fractions of the test soils. Pb was observed to be associated primarily with the carbonate and the Fe-Mn oxide phases of the soil samples. Cu was associated with carbonate, Fe-Mn oxide, and organic fractions of the soils. In one of the test soils (Soil A), SEM/EDX analyses showed the presence of roughly spherical isolated Pb-enriched particles. In addition, a few irregularly shaped Cu-enriched particles were detected. Such particles were observed to increase with decreasing soil-particle size. The presence of small Pb particles or precipitates adhering to larger soil particles also was observed.
Methods are described for a sequential extraction scheme to dissolve selectively elements bound in soils and sediments in the following nominal forms: (1) adsorbed, exchangeable, carbonate (AEC); (2) amorphous iron oxyhydroxide (am Fe ox), including manganese oxides; (3) crystalline iron oxides (cry Fe ox); (4) organics and sulfides; and (5) residual (mainly silicates). This scheme has been applied in triplicate to a suite of ten international CRMs, i.e., soils SRM 2709–2711 and the SO-1–4 series, marine mud MAG-1, lake sediment LKSD-4 and the till sample TILL-2. Elements determined comprise: As (by HG–quartz tube AAS, HG–QTAAS); Be, Ca, Co, Cr, Cu, Ni, P, Ti and V (by ICP-AES); Mn, Fe and Zn (by FAAS); and Cd, Ce, La, Li, Tl, Pb and U (by ICP-MS). The precision obtained is excellent, generally in the range 2–10% RSD, at concentrations 10 × higher than detection limits. Most results for the element concentrations summed over the five phases agree, within statistical uncertainties, with the recommended total values for the CRMs. Those where recoveries are significantly below 90% are for elements such as Cr and V, which are known to be present in refractory minerals and would require fusion for complete dissolution. The results presented herein for samples SO-1–4 and MAG-1 do not agree well with those recently published using a scheme purported to dissolve similar phases. This highlights the need to be more definitive in describing the nature and extent of the phases actually extracted so that comparisons can be made between different laboratories and studies.
Single or sequential extraction procedures are often used for soil and sediment studies to determine forms or phases of elements (e.g. `bioavailable' forms of elements). The significance of the analytical results is highly dependent on the extraction procedures used, owing to the `operationally defined' character of these schemes which requires the adoption of standardized protocols. The first part of the present review discusses the aspects of standardization of leaching and extraction schemes as applied to environmental analysis. This second part focuses on the validation aspects of such procedures and describes the preparation and certification of soil and sediment reference materials certified for their extractable trace element contents, following standardized single and sequential extraction procedures.
Various potential sources of irreproducibility in the BCR three-stage sequential extraction procedure have been investigated using the lake sediment CRM 601. Of the variables considered, the pH of the hydroxylamine hydrochloride in Step 2 proved most important. Factors such as the type of acid used in pH adjustment, the temperature and duration of extraction, and working under nitrogen did not affect precision, although they did alter the amounts of metals extracted. Improved precision was obtained when the hydroxylamine hydrochloride concentration was increased from 0.1 to 0.5moll−1 and when the speed of the centrifugation was increased from 1500 to 3000×g. The addition of a MgCl2 wash between the steps of the extraction procedure gave rise to increased uncertainty. Although it did not adversely affect reproducibility, the use of filtration to separate solid and liquid phases was not recommended since it promoted dissolution of non-target phases. Neither ammonium hydrogen oxalate nor oxalic acid proved suitable alternatives in Step 2 owing to precipitation of insoluble lead salts, particularly in the presence of calcium.
Despite the progressive decrease in emissions coming from a variety of sources, lead continues to be one of the toxic metals more often found as environmental pollutants. Of particular concern is lead migration through the soil which may result in contamination of water supplies through the leaching caused by water infiltration.Carbonate minerals are frequently found in soils and those of heavy metals are usually insoluble so carbonates in the soil could act as a sink for heavy metals for as long as the environmental conditions guarantee their stability. The influence of soil composition on the fixation of lead from aqueous solutions of this metal has been studied. Two clayey soils with different carbonate content have been used in the tests. The kinetics and equilibrium of retention of lead in the soils under different conditions of pH and salinity have been determined using batch and fixed‐bed column experiments. Carbonate precipitation and ion exchange were the main processes responsible for the Pb uptake by the soils and concentrations of up to 50% were achieved. The results obtained show that carbonates confer the soil a high immobilization capacity for lead, impeding the advance of the contamination under natural conditions.
The sorption behavior of Zn(II) on the mineral fractions of a marine sediment collected from Sagami Bay (Japan) was investigated using a sequential extraction-radiotracer technique. The extractions used were: (i) 1 M CH3COONH4; (ii) 4 × 10−2 M NH2OHt·HCl/25% CH3COOH; (iii) 30% H2O2(pH 2); and (iv) 7 M HNO3. The residues remaining after the chemical treatment were collected, and used for the sorption experiments with 65Zn(II) as a tracer. The sorption isotherms of 65Zn(II) on all the fractions except for one with a strong acid treatment fitted a Langmuir type equation. The capacities of the individual mineral fractions for 65Zn(II) sorption were estimated from the isotherms. Treatment with 7 M HNO3 removed almost all the sorptive sites for Zn(II) ions.
The operational speciation of selected trace metals in several contaminated Toronto area soils was determined by a modified BCR (Community Bureau of Reference) sequential extraction procedure. This type of environmental analysis is now increasingly used, but its reliability is questioned due to concerns regarding redistribution of extracted metals occurring during the procedure. Using radiotracer techniques, the redistribution of Cd and Zn during the sequential extraction was directly quantified. For a fill material taken from downtown Toronto, metal partitioning behavior conformed with the theory and anecdotal evidence in the soil chemistry literature: Cd was highly mobile, while Cu and Pb were primarily associated with oxidizable organic matter, and Zn was distributed across all soil fractions. Soil from an abandoned metals recycling operation had unusual physical characteristics, and the trace metals within were largely nonextractable. In the soils examined, some 20−30% of the Cd and Zn released from the acid-soluble fraction of the soil was readsorbed to the reducible mineral oxide fraction. The extent of the phenomenon is less than previously suspected and does not invalidate sequential extraction results for these metals.
The behavior of ionic lead in soils of diverse characteristics is investigated. Soils have rather large capacities for immobilization of lead, and these may be reasonably predicted by a correlation function involving cation-exchange capacity and pH. Results demonstrate that the fixation of lead is principally caused by reactions involving essentially insoluble organic materials. Precipitation by carbonate and sorption by hydrous metal oxides appear to be of secondary importance. The conclusions are indirectly confirmed by previous reports involving plant uptake studies.
The importance of postextraction readsorption of As, Cd, Cu, Ni, Pb, and Zn has been evaluated for each step of a sequential extraction procedure by measuring the recovery of small amounts of trace elements added during the extraction of oxic lake sediments. For all the cases examined but one, the trace element spikes (<100% of the amount present in control samples) were recovered within the limits given by the experimental errors. These results contrast with the large percentages of postextraction readsorption observed for these trace elements in previous studies, a consequence of the use by previous investigators of either large spikes (greatly exceeding the amounts present in the control samples) during extractions of natural sediments or simple model sediments.
An analytical procedure involving sequential chemical extractions has been developed for the partitioning of particulate trace metals (Cd, Co, Cu, Nl, Pb, Zn, Fe, and Mn) into five fractions: exchangeable, bound to carbonates, bound to Fe-Mn oxides, bound to organic matter, and residual. Experimental results obtained on replicate samples of fluvial bottom sediments demonstrate that the relative standard deviation of the sequential extraction procedure is generally better than ± 10%. The accuracy, evaluated by comparing total trace metal concentrations with the sum of the five individual fractions, proved to be satisfactory. Complementary measurements were performed on the individual leachates, and on the residual sediments following each extraction, to evaluate the selectivity of the various reagents toward specific geochemical phases. An application of the proposed method to river sediments is described, and the resulting trace metal speciation is discussed.
A number of model soils synthesized with various known amount of natural minerals and humic acid were employed to study copper and lead redistribution during extraction by 1 M HOAc/NaOAc. The adsorption intensities of the individual constituent phases were measured from adsorption isotherms, and the redistribution behavior was studied by the standard additions method. Humic acid and pyrolusite are found to exhibit the highest binding affinity for Cu and Pb, respectively, and their relative importance relies upon both their binding ability and their abundance. The extractable concentrations of the metals and the extent of recovery depend largely upon the composition of the model soils, owing to different binding strength presented in the various systems investigated. The presence of dissolved humic acid during extraction has a significant influence on metal redistribution, as evidenced by the increase in the adsorption of Cu and decrease in the adsorption of Pb. In view of the diverse compositions and various competing reactions involved, it is recognized that redistribution behavior will vary widely among both natural soil systems and given metals.
Environmental studies on soil and sediment analysis are often based on the use of leaching or extraction procedures (e.g. single or sequential extraction procedures) which enable broader forms or phases to be measured (e.g. `bioavailable' forms of elements) and which are, in most cases, sufficient for the purpose of environmental policy. However, the lack of uniformity in the procedures used often hampers comparison of the results, owing to their `operationally defined' character. This implies that the `forms' of metals are defined by the determination of extractable elements, using a given procedure and, therefore, the significance of the analytical results is highly dependent on the extraction procedures used. The comparability of results can hence only be achieved if similar procedures are applied, which justifies efforts towards standardization. This article describes case studies of standardization approaches for single and sequential extraction procedures applied to soil and sediment analysis. In particular, the strategy followed by the Standards, Measurements and Testing program (formerly BCR) is fully described (feasibility study, interlaboratory studies and establishment of standardized protocols based on the results of tests and participants' experience).
A sequential extraction procedure, used to remove the heavy metals in specifically adsorbed and easily reducible manganese (Mn) oxide fractions, was used to study the transformation of heavy metals added to an alkaline soil. Most of the endogenous Cu (86%) and Pb (79%) were found in the residual fraction (RES) which was considered to be mainly highly crystalline Fe oxides and silicate minerals. The recently added Cu, Pb and Cd existed mainly on the surfaces of the soil particles as reactive fractions (water-soluble plus exchangeable and NaCaHEDTA-extractable fractions) and as highly stable forms (RES fraction). There was a particularly high affinity of Pb for Mn oxides. The concentrations of metals in the reactive fractions were in the order: Cd>Cu>Pb. When water-soluble heavy metals are added to the soil, they are rapidly retained by the soil. The reactive forms then slowly transform into highly stable forms. The processes associated with the transformation of added Cu and Pb can be described by a diffusion equation. The processes may be attributed mainly to the diffusion of the surface species into micropores and the entrapment in microporous solids. Unlike Cu and Pb, most of the exchangeable Cd transformed to the forms extracted with NaCaHEDTA and to residual forms. The slow processes of the transformation of Cd added to soil may be attributed to inner-sphere surface complexation via partial or complete dehydration of surface species. The relative diffusion rate coefficients (D/r2) were found to be of the order of 10−10 to 10−11 s−1. Addition of CaCO3 decreased the reactivity or extractability of added heavy metals through the increase in pH.
The present study considers a trace metal sequential extraction scheme based on the classical Tessier procedure in order to determine whether readsorption and redistribution processes occur during the metal extraction from oxic sediments of the Odiel Marshes Natural Park (Southwest Spain). The metals studied included As, Cd, Cr, Cu, Hg, Ni, Pb and Zn. The trace metal standard addition approach was used to assess these phenomena, it being concluded that both readsorption and redistribution occur and their extent depends on the characteristics of the sediment. This may lead to serious misinterpretation of extraction data because the metal concentration determined in the extract may not be representative of the metal levels in the sediment fractions. Readsorption was mainly observed for Cu, Hg and As, when both 1 M MgCl2 and 1 M NaOAc (HOAc, pH 5) were used as extractants. This artefact was not observed when either 0.4 M NH2OH·HCl (25% (v/v) HOAc) or H2O2 were used to extract Cd, Cr, Cu, Ni, Pb and Zn from sediments with a organic matter content lower than 7%.
A sequential chemical extraction method for the determination of the geochemical fractionation of Am, Pu, and U was evaluated rigorously on a single marine sediment standard (IAEA-135). Partitioning of actinides was operationally defined for five reagent fractions and stable element analyses (Al, Ca, Fe, Mn, Ti, etc.) were performed to assist in establishing the phase specificity of the extractions. The method produces, in general, results that agree to within a one standard deviation interval for replicate measurements for each fraction. Actinide readsorption, examined by a double-spiking technique, was found to be significant in some cases. Ethylenediamine-tetraacetic acid (EDTA) was examined as a hold-back reagent to lessen the effect of readsorption but seriously compromised the phase selectivity of the extractants. In addition, the use of NaOCl for the destruction of organic matter was found to dissolve carbonates out of sequence and is not recommended for carbonate-bearing materials.
Radiotracers provide a convenient tool to study the behaviour of metal ions and other contaminants in natural waters. Furthermore, the use of radiotracers in controlled laboratory experiments provides an ideal approach for investigating, in a systematic manner, the role of individual parameters on processes, at mass concentrations found in the environment. A simplistic procedure has been developed to produce a thorium radiotracer (234Th), in a similar chemical form (non-acidic) to that of the aquatic medium under investigation, to determine the kinetics of sorption to marine particulate material. The benefit of this procedure is that the system is likely to be perturbed to a lesser extent by changes in pH and dissolved Th species following tracer addition. Results from filtering different tracer batches through membranes, to determine the sorption characteristics prior to use, indicate that the physico-chemical behaviour of individual batches was extremely variable. Not surprisingly, the amount of uptake immediately following tracer additions to solutions containing marine particulate material was also variable. It is suggested that the role of tracer may be underestimated if added in acid form. Nevertheless, having accounted for the initial sorption, the subsequent uptake between different batches was reasonably reproducible. The usefulness of radiotracers is discussed in the context of these observations.
The adsorption of Cu, Pb, and Cd from selected extractant solutions, onto uncontaminated river sediments, has been investigated under typical extraction conditions. Significant adsorption of added metal was found to occur during overnight extraction with dilute HCl (pH>1.5), 0.1 M hydroxylamine hydrochloride (pH 2), 0.1 M sodium citrate (pH 4.6), 1 M ammonium acetate, 10% sodium citrate-1% sodium dithionite, and 25% acetic acid. Adsorption also occurred during a hydrogen peroxide digestion procedure. The inability of reagents to prevent losses of soluble metal in these experiments strongly suggests that a proportion of the metal actually released from a sediment sample during an extraction will be readsorbed. This may lead to serious misinterpretation of extraction data because the metal concentrations determined in the extract do not represent metal levels in the sediment fractions attacked.
The possibility of applying diethylammonium diethyldithiocarbamate and butyl acetate for the extraction of copper, cadmium, lead and bismuth from molar solutions of orthophosphoric acid, and from solutions containing up to 55% pyrophosphoric acid and 20% tripolyphosphoric acid has been investigated. Some characteristics of the various diethylammonium diethyldithiocarbamate-butyl acetate-phosphoric acid systems are described. Suitable conditions have been found for flame atomic-absorption spectrometric determination of the ions extracted. This extraction/AAS approach has been used to determine copper, cadmium, lead and bismuth in some iron-, aluminium- and tin-containing samples dissolved in concentrated orthophosphoric acid. The same approach is used for determining 10(-5)-10(-6)% copper, cadmium and lead in orthophosphoric acid and in alkali-metal mono and dihydrogen phosphates.
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