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Selective Sorption and Fixation of Cations by Clay Minerals: A Review
Abstract
Investigations concerning selective sorption and fixation of K and similar cations by clay minerals and soil clays and the mechanisms of these reactions are reviewed. In particular, recent observations on selective sorption of these ions in dilute solutions by weathered micas and vermiculite in relation to the interlayer structures are discussed in detail. Also, implications of the resistance to weathering of small mica particles to cation selectivity by soils are described. Despite the increased understanding of sorption and fixation reactions, the following aspects remain unclear.
First, the mechanism of the collapse of alternate layers in vermiculite on K or Cs sorption has not been unequivocally established. Second, factors that impart stability to the central core of mica particles so that K extraction becomes progressively difficult are not known. Third, inability of Ca or Mg ions to expand interlayers of Cs-saturated vermiculite in contrast to K-saturated vermiculite is not completely understood.
... Soils with elevated CDR values are more susceptible to dispersion, which means they can be easily transported by water or wind, especially when the soil is loose in texture (Bajracharya et al., 1992). Cations with low hydration, such as K + and Cs + in soils, are directly associated with soil clays (Sawhney, 1972). The findings of the present study reveal that CDR in GS1 and GS2 is comparatively higher than in FS, which provides evidence of an increased amount of 137 Cs allied with finer particles in GS1 and GS2 compared to FS. ...
... Monovalent cations (K + , NH 4 + , Rb + , and Cs + ) with low hydration tend to become immobilized within interlayer positions due to their ability to induce interlayer dehydration. When these monovalent cations (specifically, K + and Cs + ) saturate the interlayer, it leads to a collapse of the interlayer structure, and releasing K + and Cs + from interlayer sites becomes difficult (Sawhney, 1972). However, Bolt et al. (1963) Fig. 6. ...
... Consequently, minerals saturated with Cs + are more resistant to expansion by hydrated polyvalent ions, which often cause the layers held by K + ions to expand. As a result, the replacement of Cs + ions by polyvalent ions is not as readily achieved as it is with K + ions (Sawhney, 1972). The SHMP extracted a substantial amount of K + (40-60%) alongside a relatively small fraction of labile Cs + (1-2%) from soils. ...
... Weathered micaceous minerals and especially illite exhibit frayed edges which refer to the transition zones between collapsed and open interlayer space. Since Sawhney (1972), particular reactive properties have generally been attributed to this speci c space. Due to its con guration, it can accept only low hydration energy cations or small size cations including K + and Cs + compared to Ca 2+ and Mg 2+ . ...
... Furthermore, dehydrated K + originally in the interlayer space of micaceous minerals remains non exchangeable in contrast to those of expanding phyllosilicate. Only the weathering process partially releases interlayer cations from micas Sawhney, 1972). Then, superimposed to chemical ion-exchange, a physical mechanism leads to the capture of cations at low hydration potential into the mineral. ...
... The involvement of the speci c structural site, usually named frayed edge site (FES) (Sawhney 1972) was investigated by arti cially reducing the interlayer distance by incorporating K during the conditioning process. By reducing the vermiculite CEC with K xation, it mimics a sequence between true vermiculite and vermiculitized-mica. ...
Vermiculite and micaceous minerals are relevant Cs sorbent in soils and sediments. To understand Cs bioavailability in soils resulting from multi-cation exchanges, Cs sorption onto clay minerals have been carried out in batch experiments with solutions containing Ca ²⁺ , Mg ²⁺ and K ⁺ . A sequence between a vermiculite and various micaceous structures were achieved by conditioning a vermiculite at various amounts of K. Competing cation exchanges were investigated according to the concentration of Cs. The contribution of K on trace Cs desorption is probed by applying different concentrations of K on Cs-doped vermiculite and micaceous structures. Cs sorption isotherms at chemical equilibrium were combined with elemental mass balances in solution and structural analyses. Cs replace easily Mg ²⁺ > Ca ²⁺ and competes scarcely with K. Cs is strongly adsorbed on the various matrix and a K/Cs ratio about a thousand is required to remobilize Cs. Cs is exchangeable as long as the clay interlayer space remains open to Ca ²⁺ . However, excess of K, as well as Cs, in solution leads to the collapse of the interlayer spaces that locks the Cs into the structure. Once K and/or Cs collapse the interlayer space, the external sorption sites are then particularly involved in Cs sorption. Subsequently, Cs ⁺ exchanges preferentially with Ca ²⁺ rather than Mg ²⁺ . Mg ²⁺ is extruded from the interlayer space by Cs ⁺ and K ⁺ adsorption, excluded from short interlayer space and replaced by Ca ²⁺ as Cs ⁺ desorbs.
... With hot particles (fuel particles in Chernobyl [5,7,27,33,34,36] and CsMPs in Fukushima [52][53][54]) disintegrated, radiocesium incorporated in these particles transfers to the solution. In the solution, radiocesium is sorbed by the soil and sediment particles by ion exchange [5,7,27,53,[55][56][57][58][59][60][61][62][63]. Exchangeably sorbed radiocesium is fixed by micaceous clay minerals due to the replacement of the interlayer K-cations by Cs-cations [5,7,55,57,58,64,65]. Fixation of radionuclides is understood as the transformation of their exchangeable form to nonexchangeable. ...
... In the solution, radiocesium is sorbed by the soil and sediment particles by ion exchange [5,7,27,53,[55][56][57][58][59][60][61][62][63]. Exchangeably sorbed radiocesium is fixed by micaceous clay minerals due to the replacement of the interlayer K-cations by Cs-cations [5,7,55,57,58,64,65]. Fixation of radionuclides is understood as the transformation of their exchangeable form to nonexchangeable. ...
... The exchangeable sorption of radiocesium onto the soil and sediment particles can be selective or nonselective depending on the type of sorption sites [7,27,55,56,72]. The diversity of sorption sites can be divided into two main types: regular exchange sites (RESs) and selective sorption sites occurring on the frayed edges of the neighboring layers of micaceous clay crystal lattice (FES). ...
In the wake of Chernobyl and Fukushima accidents, radiocesium has become a radionuclide of most environmental concern. The ease with which this radionuclide moves through the environment and is taken up by plants and animals is governed by its chemical forms and site-specific environmental characteristics. Distinctions in climate and geomorphology, as well as 137 Cs speciation in the fallout, result in differences in the migration rates of 137 Cs in the environment and rates of its natural attenuation. In Fukushima areas, 137 Cs was strongly bound to soil and sediment particles, with its bioavailability being reduced as a result. Up to 80% of the deposited 137 Cs on the soil was reported to be incorporated in hot glassy particles (CsMPs) insoluble in water. Disintegration of these particles in the environment is much slower than that of Chernobyl-derived fuel particles. The higher annual precipitation and steep slopes in Fukushima-contaminated areas are conducive to higher erosion and higher total radiocesium wash-off. Among the common features in the 137 Cs behavior in Chernobyl and Fukushima are a slow decrease in the 137 Cs activity concentration in small, closed, and semi-closed lakes and its particular seasonal variations: increase in the summer and decrease in the winter.
... Another important factor that could influence the accumulation of 137 Cs in lake sediments is their mineralogical composition (Ashraf et al. 2014;Tanaka et al. 2015;Huon et al. 2018;Hagiwara et al. 2020). This influence is due to the sorption and fixation of Cs is particularly determined by the type and amount of phyllosilicate minerals (Sawhney 1972;Zachara et al. 2002;Tachi et al. 2020a;Park et al. 2021). Depending on their different physicochemical properties, such as chemical composition, expandability, cation exchange capacity (CEC), and layer charge, they are able to bind Cs to varying degrees (Cornell 1993;Mukai et al. 2016;Park et al. 2021). ...
... Depending on their different physicochemical properties, such as chemical composition, expandability, cation exchange capacity (CEC), and layer charge, they are able to bind Cs to varying degrees (Cornell 1993;Mukai et al. 2016;Park et al. 2021). Moreover, in the case of clay minerals, the sorption of Cs is highly controlled by the specific sorption at frayed edge sites (FES) and the exchangeable sorption at regular exchange sites (RES) (Sawhney 1972;Cornell 1993;Zachara et al. 2002;Tachi et al. 2020b). The mineralogical composition of lake sediments can be additionally used to trace the origin of particles contaminated with 137 Cs (Hagiwara et al. 2020). ...
... Apart from the particle size distribution, the mineralogical composition has a very significant influence on the distribution of 137 Cs in bottom sediments in aquatic ecosystems (Kim et al. 2006;Fan et al. 2014;Mukai et al. 2016;Hagiwara et al. 2020;and others). Many studies have shown that Cs binding is especially strong with micaceous minerals and their weathered minerals, such as illites and vermiculites (Sawhney 1972;Cremers et al. 1988;Cornell 1993;Poinssot et al. 1999;Park et al. 2021). In our case, clay minerals have a dominant effect on the sorption and thus the distribution of 137 Cs in the bottom sediments of Koronowo Lake, which may be evidenced by a significant positive correlation between the content of 137 Cs and clay minerals (R s = 0.6, p value < 0.05) ( Table 1; Fig. 8). ...
Purpose
The main aim of this study was to investigate factors influencing the long-term distribution of ¹³⁷ Cs activity concentrations in the bottom sediments of the dam lake, Koronowo Reservoir, 32 years after the Chernobyl nuclear power plant accident. For this purpose, selected properties of the collected sediment samples, such as grain size, mineralogical composition, and organic matter (OM) content, were investigated.
Materials and methods
The samples of lake sediments were collected with a Kayak-type gravity corer. The spatial and vertical distributions of ¹³⁷ Cs and ⁴⁰ K activity concentrations in the bottom sediments were investigated based on gamma spectrometry measurements. The particle size distribution of surface lake sediments was determined using a laser particle size analyzer. SEM and XRD were used for the mineralogical analysis of the collected sediment samples. Additionally, the content of organic matter was examined in all samples using an elemental analyzer.
Results
The ¹³⁷ Cs content was significantly elevated in the case of fine-grained (< 63 µm) surface lake sediments (classified as silts, which are deposited in the profundal zone of Koronowo Lake) and ranged from 12.5 ± 4.1 to 29.2 ± 4.0 Bq kg ⁻¹ . It was found that the increased concentration of ¹³⁷ Cs activity is more closely related to the content of the silt fraction (2–63 µm) than to the clay fraction (< 2 µm) in the collected surface lake sediments. The content of clay minerals also showed a significant positive correlation with ¹³⁷ Cs activity concentration in the surface lake sediments of Koronowo Lake. A similar relationship was noticed for the OM content, but it may be suspected that it is the result of radiocesium-bearing particle accumulation in OM-rich sediments.
Conclusion
The most important factor influencing the spatial distribution of ¹³⁷ Cs activity concentrations in the surface lake sediments of Koronowo Lake, apart from the bottom morphology and grain size of sediments, is the content of clay minerals. Moreover, the increased detrital inflow to the lake after the construction of the dam could have probably affected the vertical distribution of ¹³⁷ Cs activity concentrations in the bottom sediments, as evidenced by, e.g., the measurements of ⁴⁰ K activity concentration.
... On the other hand, biotite, vermiculite, and aluminous smectite are known to strongly adsorb Cs + compared to other cations (Sakalidis et al. 1988;Cornell 1993;Maes et al. 1999;Mukai et al. 2016;Durrant et al. 2018;Wissocq et al. 2018;Ogasawara et al. 2019;Kitayama et al. 2020). Furthermore, illite and partially vermiculitized biotite sorb Cs + more strongly than vermiculite (Sawhney 1972;Brouwer et al. 1983;Maes et al. 1999;Kitayama et al. 2020;Latrille and Bildstein 2022). Indeed, these clay minerals provide selective sorption sites that enhanced Cs + sorption (Brouwer et al. 1983;Park et al. 2019), which in some cases appear as Cs fixation sites (Kikuchi et al. 2015). ...
... Weathered micaceous minerals and in particular illite exhibit frayed edges which refer to the transition zones between collapsed and open interlayer space. Since Sawhney (1972), particular reactive properties have generally been attributed to this specific space. Due to its configuration, it can only accept cations with low hydration energy or small size cations, notably K + and Cs + , compared to Ca 2+ and Mg 2+ . ...
... At trace concentrations, Cs + is easily replaced by Mg 2+ Tamura et al. 2015) on vermiculite, while Cs + is more strongly adsorbed on illite (Wissocq et al. 2018) but remains exchangeable (Latrille and Bildstein 2022). At high Cs + concentrations, it has been assumed that weakly hydrated Cs + sorbed to the frayed edges of micaceous minerals is dehydrated leading to a partial collapse of the interlayer (Mukai et al. 2016;Sawhney 1972;Kikuchi et al. 2015;Park et al. 2019). Likewise, Cs + incorporated in the vermiculite interlayer by ion exchange with Mg 2+ , Ca 2+ , and Na + induces subsequent interlayer collapse in vermiculite (Sawhney 1972;Lee 1973Lee , 1974Kogure et al. 2012;Dzene et al. 2015;Yin et al. 2017;Kitayama et al. 2020). ...
Vermiculite and micaceous minerals are relevant Cs⁺ sorbents in soils and sediments. To understand the bioavailability of Cs⁺ in soils resulting from multi-cation exchanges, sorption of Cs⁺ onto clay minerals was performed in batch experiments with solutions containing Ca²⁺, Mg²⁺, and K⁺. A sequence between a vermiculite and various micaceous structures has been carried out by conditioning a vermiculite at various amounts of K. Competing cation exchanges were investigated as function of Cs⁺ concentration. The contribution of K⁺ on trace Cs⁺ desorption is probed by applying different concentrations of K⁺ on Cs-doped vermiculite and micaceous structures. Cs sorption isotherms at chemical equilibrium were combined with elemental mass balances in solution and structural analyses. Cs⁺ replaces easily Mg²⁺ > Ca²⁺ and competes scarcely with K⁺. Cs⁺ is strongly adsorbed on the various matrix, and a K/Cs ratio of about a thousand is required to remobilize Cs⁺. Cs⁺ is exchangeable as long as the clay interlayer space remains open to Ca²⁺. However, an excess of K⁺, as well as Cs⁺, in solution leads to the collapse of the interlayer spaces that locks the Cs into the structure. Once K⁺ and/or Cs⁺ collapse the interlayer space, the external sorption sites are then particularly involved in Cs sorption. Subsequently, Cs⁺ preferentially exchanges with Ca²⁺ rather than Mg²⁺. Mg²⁺ is extruded from the interlayer space by Cs⁺ and K⁺ adsorption, excluded from short interlayer space and replaced by Ca²⁺ as Cs⁺ desorbs.
... The Hayama Lake catchment area is located within the main inland radioactive contamination plume resulting from the Fukushima Daiichi Nuclear Power Plant accident in March 2011 (Kato et al., 2019). Once deposited, 137 Cs strongly and quasi-permanently binds to fine soil particles such as silts and clays (Sawhney, 1972;He and Walling, 1996), which has been confirmed in the soils of Fukushima Prefecture (Saito et al., 2014;Nakao et al., 2014). ...
... Soils were sampled with a plastic trowel and consisted of 10 composited sub-samples of topsoil (1-2 cm uppermost layer). All soil samples were dried at 40 • C for about 48 h and then sieved to 63 µm to isolate the fraction concentrating 137 Cs (i.e. to silt and clay minerals (Sawhney, 1972;He and Walling, 1996)). ...
In a context of accelerated soil erosion and sediment supply to water bodies, sediment fingerprinting techniques have received an increasing interest in the last 2 decades. The selection of tracers is a particularly critical step for the subsequent accurate prediction of sediment source contributions. To select tracers, the most conventional approach is the three-step method, although, more recently, the consensus method has also been proposed as an alternative. The outputs of these two approaches were compared in terms of identification of conservative properties, tracer selection, modelled contributions and performance on a single dataset. As for the three-step method, several range test criteria were compared, along with the impact of the discriminant function analysis (DFA). The dataset was composed of tracer properties analysed in soil (three potential sources; n = 56) and sediment core samples (n = 32). Soil and sediment samples were sieved to 63 µm and analysed for organic matter, elemental geochemistry and diffuse visible spectrometry. Virtual mixtures (n = 138) with known source proportions were generated to assess model accuracy of each tracer selection method. The Bayesian un-mixing model MixSIAR was then used to predict source contributions on both virtual mixtures and actual sediments. The different methods tested in the current research can be distributed into three groups according to their sensitivity to the conservative behaviour of properties, which was found to be associated with different predicted source contribution tendencies along the sediment core. The methods selecting the largest number of tracers were associated with a dominant and constant contribution of forests to sediment. In contrast, the methods selecting the lowest number of tracers were associated with a dominant and constant contribution of cropland to sediment. Furthermore, the intermediate selection of tracers led to more balanced contributions of both cropland and forest to sediments. The prediction of the virtual mixtures allowed us to compute several evaluation metrics, which are generally used to support the evaluation of model accuracy for each tracer selection method. However, strong differences or the absence of correspondence were observed between the range of predicted contributions obtained for virtual mixtures and those values obtained for actual sediments. These divergences highlight the fact that evaluation metrics obtained for virtual mixtures may not be directly transferable to models run for actual samples and must be interpreted with caution to avoid over-interpretation or misinterpretation. These divergences may likely be attributed to the occurrence of a not (fully) conservative behaviour of potential tracer properties during erosion, Published by Copernicus Publications on behalf of the European Geosciences Union. 110 T. Chalaux-Clergue et al.: Comparison of tracer selection methods transport and deposition processes, which could not be fully reproduced when generating the virtual mixtures with currently available methods. Future research should develop novel metrics to quantify the conservative behaviour of tracer properties during erosion and transport processes. Furthermore, new methods should be designed to generate virtual mixtures closer to reality and to better evaluate model accuracy. These improvements would contribute to the development of more reliable sediment fingerprinting techniques, which are needed to better support the implementation of effective soil and water conservation measures at the catchment scale.
... Ca 2+ was found to be most effective for the release of Li from rock (Green River Shale) into pore fluid. This can be addressed by that fast hydration shell exchange rate of Ca 2+ made it easier to exchange for bound Li [41,42]. This is also because that heavier element with higher ionic charge is preferable to be fixed in the interlayer of clay under humid condition by exchanging with lighter element with lower ionic charge [22,41,42]. ...
... This can be addressed by that fast hydration shell exchange rate of Ca 2+ made it easier to exchange for bound Li [41,42]. This is also because that heavier element with higher ionic charge is preferable to be fixed in the interlayer of clay under humid condition by exchanging with lighter element with lower ionic charge [22,41,42]. Abundant presence of Ca 2+ in Marcellus Shale brines in Table 1 is in accordance with the results of this hydrothermal reaction experiment, by providing the background of high Li concentration in brines. ...
In order to mitigate climate change, diversifying the sources of lithium supply is crucial for the decarbonization
of energy sector through enhanced renewable electricity generation and electrified transportation. Shale brines
have been recently found to be containing significant amount of lithium, but relevant subsurface phenomena
regarding its origin, fate, and transport are unknown. Here we present a suite of geochemical experiments to
elucidate the initial presence of lithium in shale rocks and its release mechanism from solid phase into fluid, and
numerical modeling to estimate the resources of lithium in shale brines by addressing its fate and transport. We
find that the majority of lithium is inorganically bound as an interlayer cation of clay in shale rock, while a
sparingly small portion is organically bound. Hydrothermal reaction experiments for leaching lithium reveal that
calcium ion in fluid has strongest impact on lithium to be released into fluid, while sodium ion has minimal
impact. From the numerical modeling combined with the experimental findings, average concentration of
lithium in shale brines mimicking Marcellus Shale system is estimated to be about 135 ppm under calcium ion
dominancy in pore fluid, which shows excellent match with actually measured values from produced Marcellus
Shale brines. This study provides the understanding of fundamental phenomena addressing release, transport,
and accumulation of lithium in geologic system, and hence contributes to the enhancement of sources of lithium
supply for energy decarbonization.
... Compared with litter layers (organic layers including OL, OF, and OH horizons), mineral soil consists of not only organic materials but also mineral particles. Clay minerals can strongly, almost irreversibly, adsorb 137 Cs, which is responsible for a large portion of the reduction in the mobility and bioavailability of 137 Cs in mineral soil (Sawhney, 1972;Cremers et al., 1988;Kruyts and Delvaux, 2002). However, some studies suggest that the interaction between 137 Cs and clay minerals cannot solely explain the retention and migration behavior of 137 Cs in mineral soil; the presence of soil organic matter (SOM) in mineral soil can inhibit the fixation of 137 Cs by clay minerals (e.g., Kruyts and Delvaux, 2002;Rigol et al., 2002;Matsunaga et al., 2013;, which makes 137 Cs mobile and bioavailable in mineral soil. ...
... Therefore, this process is not responsible for the long-term retention of 137 Cs in soils. On the contrary, clay minerals play a vital role in the long-term retention of 137 Cs in soils through almost irreversible adsorption (fixation) of 137 Cs (Sawhney, 1972;Cremers et al., 1988). This fixation not only limits the migration of 137 Cs in soils but also reduces availability for the uptake of 137 Cs by plant roots (Delvaux et al., 2000;Fesenko et al., 2001). ...
Understanding the fate of the vertical distribution of radiocesium (137Cs) in Japanese forest soils is key to assessing the radioecological consequences of the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. The 137Cs behavior in mineral soil is known to be primarily governed by interaction with clay minerals; however, some observations suggest the role of soil organic matter (SOM) in enhancing the mobility of 137Cs. Here we hypothesized that soil organic carbon (SOC) concentration profile determines the ultimate vertical pattern of 137Cs distribution in Japanese forest soils. In testing this hypothesis, we obtained soil samples that were collected before the FDNPP accident at four forest sites with varying SOC concentration profiles and quantified the detailed vertical profile of 137Cs inventory in the soils roughly half a century after global fallout in the early 1960 s. Results showed that the higher the SOC concentration in the soil profile, the deeper the 137Cs downward penetration. On the basis of the data for surface soils (0-10 cm), the 137Cs retention ratio for each of the 2-cm thick layers was evaluated as the ratio of 137Cs inventory in the target soil layer to the total 137Cs inventory in and below the soil layer. A negative correlation was found between the ratio and SOC concentration of the layer across all soils and depths. This indicates that the ultimate fate of 137Cs vertical distribution can be predicted as a function of SOC concentration for Japanese forest soils, and provides further evidence for SOM effects on the mobility and bioavailability of 137Cs in soils.
... This interaction acts on two scales. On the scale of the individual clay mineral, (Sawhney 1972) describes how the introduction of cations with low hydration energy (e.g. K + , Rb + and Cs + ) will result in clay mineral interlayer collapse and even fixation of the cations in the mineral, while cations with higher hydration energy (e.g. ...
... It would therefore seem that the extreme expansion from Na + observed by (Sridharan et al. 1986) and(di Maio 1996) is related to the smectite content. As described by (Sawhney 1972) and (di Maio 1996) K + can fixate in the smectite structure causing "illitization", which compresses the individual clay minerals, thereby causing the significant difference observed here by (Sridharan et al. 1986) and(di Maio 1996). However, when the mineral composition is already illitic this effect diminishes, which complies with the observations. ...
... This effect is further considered in Section 3.2, but we largely emphasize comparison between model and measured elemental and isotopic ratios to avoid such complications (e.g., Gaillardet, Dupré, Louvat, & Allègre, 1999;Meybeck, 1987;Stallard & Edmond, 1983). For this purpose, the solute concentrations of the stream are normalized to Na, which is generally not retained in the byproducts of silicate weathering (Sawhney, 1972). This Na-normalization eliminates the effects of dilution and/or evaporation which could otherwise impact absolute solute concentrations (e.g., Dessert et al., 2003;Gaillardet, Dupré, Louvat, & Allègre, 1999). ...
Lithium isotope ratios (δ⁷Li) of rivers are increasingly serving as a diagnostic of the balance between chemical and physical weathering contributions to overall landscape denudation rates. Here, we show that intermediate weathering intensities and highly enriched stream δ⁷Li values typically associated with lowland floodplains can also describe small upland watersheds subject to cool, wet climates. This behavior is revealed by stream δ⁷Li between +22.4 and +23.5‰ within a Critical Zone observatory located in the Cévennes region of southern France, where dilute stream solute concentrations and significant atmospheric deposition otherwise mask evidence of incongruence. The water‐rock reaction pathways underlying this behavior are quantified through a multicomponent, isotope‐enabled reactive transport model. Using geochemical characterization of soil profiles, bedrock, and long‐term stream samples as constraints, we evolve the simulation from an initially unweathered granite to a steady state weathering profile which reflects the balance between (a) fluid infiltration and drainage and (b) bedrock uplift and soil erosion. Enriched stream δ⁷Li occurs because Li is strongly incorporated into actively precipitating secondary clay phases beyond what prior laboratory experiments have suggested. Chemical weathering incongruence is maintained despite relatively slow reaction rates and moderate clay accumulation due to a combination of two factors. First, reactive primary mineral phases persist across the weathering profile and effectively “shield” the secondary clays from resolubilization due to their greater solubility. Second, the clays accumulating in the near‐surface profile are relatively mature weathering byproducts. These factors promote characteristically low total dissolved solute export from the catchment despite significant input of exogenous dust.
... The determined reaction rate constants for different hydrothermal solutions and temperatures are listed in Table 3. CaCl 2 solutions showed a relatively larger reaction rate constant than other solutions. This was because Ca 2+ had the fastest hydration shell exchange rate, which made it easier to exchange for bound Li (Eberl 1980a;Sawhney 1972a). In addition, it is also known that heavier element with higher ionic charge is easier to be fixated in the interlayer of clay under humid condition by exchanging with lighter element with lower ionic charge (Sawhney 1972b;Vine 1975;Eberl 1980b). ...
To meet the extensive demand for lithium (Li) for rechargeable batteries, it is crucial to enhance Li production by diversifying its resources. Recent studies have found that produced water from shale reservoirs contains various organic and inorganic components, including a significant amount of Li. In this study, findings from hydrothermal reaction experiments were analyzed to fully understand the release of Li from organic-rich shale rock. Subsequently, numerical algorithms were developed for both pore-scale and continuum-scale models to simulate the long-term behavior of Li in shale brines. The experimental conditions considered four different hydrothermal solutions, including the solutions of KCl, MgCl2, CaCl2, and NaCl with various concentrations under the temperature of 130 °C, 165 °C, and 200 °C. The release of Li from shale rock into fluid was regarded as a chemical interaction of cation exchange between rock and fluid. The reactive transport pore-scale and upscaled continuum-scale models were developed by coupling the chemical reaction model of Li interaction between rock and fluid. The model was first implemented to investigate the release and transport of Li in the pore scale. Continuum-scale properties, such as effective diffusivity coefficients and Li release rate, were obtained as the field-averaged pore-scale modeling results. These properties were used as the input data for the upscaled continuum-scale simulation. The findings of this study are expected to provide new insight into the production of Li from shale brines by elucidating the release, fate, and transport of Li in subsurface formations.
... According to Sawhney (1972), Douglas (1989), Malla (2003), Skiba (2013), and Skiba et al. (2018) dioctahedral vermiculite adsorbs selectively (from multi-cationic solutions) and fixes cations having low hydration energy (i.e. K + , NH 4 + , Rb + , and Cs + ). ...
The aim of the present work was to study the clay mineralogy of the Tomanov´a Formation - a ~ 65 m thick
complex of inland freshwater mudstones and shales of the upper Triassic age occurring in the Tatra Mountains.
The formation was chosen for the study because it was assumed that, even though it underwent high-grade
diagenesis, it could contain dioctahedral vermiculite-bearing clays. The clays have unique properties concerning
the selective adsorption and the fixation of Cs+. Samples used in the present study were collected from
outcrops located within Czerwone Z˙ lebki in the Tomanova´ Valley in the Polish part of the Tatra Mountains.
Seven horizons rich in clays were selected for the detailed mineralogical study. Both bulk samples and the clay
fractions were analyzed using X-ray diffractometry, Fourier-transform infrared spectroscopy, and optical microscopy.
Layer charges of swelling clays separated from the samples were measured using the O–D method.
All studied samples contained dioctahedral R0 and R1 illite-vermiculite-smectite mixed layered minerals
mixed layered minerals (hereafter I-V-S) rich in dioctahedral vermiculite interlayers, kaolinite, quartz, anatase,
and rutile. Three of the studied samples contained also berthierine. The swelling clays’ measured mean layer
charge values were in the vermiculitic range (i.e. 0.64–0.68/O10(OH)2). The common presence of kaolinite
having a “poor degree of ordering” in the studied rocks indicated that quite an intense chemical weathering likely
occurred within the alimentation area. The vermiculite-bearing clays (i.e. I-V-S) identified in the studied rocks
were most likely also the products of the paleo weathering. The I-V-S minerals survived the high-grade diagenesis
most likely due to the potassium deficiency in the system preventing the clays from the illitization. Those
findings may suggest that deposits of dioctahedral vermiculite-bearing clays can be found even within geological
formations that underwent high-grade diagenesis, which is of exploration importance because dioctahedral
vermiculite could be possibly used/utilized as a molecular sieve for 137-Cs immobilization in environmental
applications.
... Several mechanisms have been proposed as possible causes, including ion competition between potassium (K + ) and ammonium (NH 4 + ) ions, leaching and adsorption inhibition associated with organic matter decomposition, and thermodynamic adsorption and desorption processes. Regarding ionic competition, most 137 Cs in SS are specifically adsorbed on the frayed edge site (FES) of clay mineral particles (mainly 2:1-type layered silicates) (Sawhney, 1972) and the number of 137 Cs adsorption sites including FES provided by clay minerals in the environment is more than sufficient to adsorb 137 Cs (Delvaux et al., 2000). The FES is known to be extremely selective for Cs + , K + , NH 4 + and Rb + , and among these weakly hydrating ions, the selectivity for Cs + is the highest. ...
... One of these radioactive contaminants, cesium-137 ( 137 Cs) is considered to be the most serious risk to the local population because it was emitted in high abundance and it has a relatively long half-life of 30 y (4,5). Once 137 Cs reaches the soil surface, it is quickly and almost irreversibly bound to fine soil particles (6)(7)(8) and therefore remains in the upper few centimeters of the soil profile, with very limited vertical migration (9). Research conducted on soil profiles collected in Fukushima Prefecture after the 2011 accident demonstrated that 95% of the 137 Cs in the soil is strongly bound to clay materials (10) and concentrated in the upper 2 cm of undisturbed soils (11,12), making it susceptible to downstream transfer by water erosion during rainfall events. ...
In the context of elevated concerns related to nuclear accidents and warfare, the lessons learnt from the Fukushima Daiichi Nuclear Power Plant accident in 2011 are important. In particular, Japanese authorities implemented an ambitious decontamination program to reduce the air dose rate in order to facilitate the return of the local inhabitants to previously evacuated areas. This approach contrasts the strategy adopted in Chernobyl, where the most contaminated areas remain off limits. Nonetheless, the effectiveness of the Japanese decontamination strategy on the dispersion of radioactive contaminant fluxes across mountainous landscapes exposed to typhoons has not been quantified. Based on the unique combination of river monitoring and modeling in a catchment representative of the most impacted area in Japan, we demonstrate that decontamination of 16% of the catchment area resulted in a decrease of 17% of sediment-bound radioactive fluxes in rivers. Decontamination operations were therefore relatively effective, although they could only be conducted in a small part of the area due to the dominance of steep forested slopes. In fact, 67% of the initial radiocesium contamination was calculated to remain stored in forested landscapes, which may contribute to future downstream radiocesium dispersion during erosive events. Given that only a limited proportion of the initial population had returned in 2019 (~30%), it raises the question as to whether decontaminating a small percentage of the contaminated area was worth the effort, the price, and the amount of waste generated?
... Therefore, adsorption process is not attributable to the K isotopic variation ∼0.2 in suspended sediments. Dur- ing continental weathering, K is more readily absorbed by clay minerals than Na (Sawhney, 1971). There is no systematic relationship between K/Na and δ 41 K in suspended sediments (Fig. 7b), ...
Mountain uplift produces steep slopes and rapid erosion rates, and hence exposes more silicate rocks to weathering and accelerates the drawdown of atmospheric CO2, driving global cooling over the Cenozoic. Riverine potassium (K) and lithium (Li) isotopes are two promising proxies for silicate weathering because the isotopes fractionate during secondary mineral formation associated with weathering and these two elements are highly enriched in silicate rocks. This study explores similarity and difference between K and Li isotope behaviors during weathering across the Lancang River in the southeastern Tibetan Plateau, in order to evaluate the role of these two isotopes in tracing silicate weathering in tectonically active mountains. Dissolved K and Li in the Lancang River mainstream are mainly derived from the dissolution of silicates. River waters are enriched in the heavy isotope of K (−0.35‰ to +0.05‰) and Li (+7.1‰ to +20.4‰) relative to suspended sediments (−0.56‰ to −0.42‰ and −2.3‰ to 0‰, respectively), and the element partitioning between dissolved and suspended loads controls dissolved δ7Li and δ41K values. Interesting, dissolved δ7Li and δ41K show the opposite evolution trends toward downstream. As the weathering intensity increases downstream (higher Si/(Na + K)* ratios), the dissolved δ7Li values increase from 7.4‰ to 13.6‰, whereas dissolved δ41K values decrease from −0.04‰ to −0.26‰. Compared to its headwater with limited weathering, more secondary minerals are formed in the lower reaches (e.g., floodplains) with low physical erosion rates and high annual precipitation, driving more 6Li incorporated into clays, thus high dissolved δ7Li values downstream. While dissolved δ41K values and the fraction of 39K incorporated into secondary minerals are low in floodplains, this is consistent with observations that illite is a main K-bearing phase among clay minerals, and its content in the clay fraction in river sediment decreases as weathering intensity increases. Our study further supported that the increase in Cenozoic seawater δ7Li is related to weathering in floodplains rather than weathering of mountains, whereas K isotope fractionation mainly occurs in mountain regions. If Cenozoic seawater δ41K rise, as with δ7Li change, weathering at higher elevation may exerts more effect on high riverine and seawater δ41K values. This study increases our knowledge of K-Li isotope behaviors during present-day silicate weathering, and the response of K-Li isotopes in sedimentary archives to silicate weathering over geological time.
... As the major elemental components of plagioclase, CaO* and Na 2 O are rapidly lost with the relatively rapid hydrolysis of plagioclase. The slight depletion of K 2 O may be attributed to its retention in clay minerals in the weathering products (Sawhney, 1972). Both TiO 2 and Fe 2 O 3 are enriched in all three stages. ...
... Monovalent NH4 + with a small ionic radius (1.48 Å) and low hydration energy (322 kJ/mol) may readily arrive at frayed edge sites (FES) and exchange RE 3+ . By contrast, divalent Mg 2+ with a large hydrated radius (4.28 Å) and high hydration energy (1828 kJ/mol) may have difficulty entering the wedge zone of FES (Coleman et al., 1963;Sawhney, 1972), so they are generally thought to desorb RE mostly from accessible external sites. The ion exchange capacity of NH4 + with RE 3+ is stronger than that of Mg 2+ , especially with RE 3+ in collapsed interlayers. ...
The exchangeability of rare earth (RE) in weathered crust elution-deposited rare earth ores largely depends on its interaction with clay minerals, which may be significantly influenced by various cations. Therefore, the effects of K<sup>+</sup>, Ca<sup>2+</sup> and Al<sup>3+</sup> on RE<sup>3+</sup> adsorption and desorption in binding sites of montmorillonite (MMT) were investigated. Through the pre-saturation, the interlayer ions of MMT had been replaced by K<sup>+</sup>, Ca<sup>2+</sup> or Al<sup>3+</sup>. RE<sup>3+</sup> can adsorb on the interlayer sites of Ca-MMT and K-MMT, but nearly not Al-MMT. The basal spacing of Ca-MMT is larger than K-MMT, which provides a smaller hinder effect of interlayer collapse for the interlayer diffusion of RE<sup>3+</sup>. The adsorption capacity followed the order: Ca-MMT＞K-MMT＞Al-MMT and La<sup>3+</sup>＞Y<sup>3+</sup>＞Eu<sup>3+</sup>. It can predict that the grade of the exchangeable RE in ores abundant in Ca<sup>2+</sup> is the most, followed by the ore rich in K<sup>+</sup> and Al<sup>3+</sup> the least. Clay minerals tend to adsorb light RE and hard to adsorb middle and heavy RE. The reversibility of RE adsorbed in interlayers, especially in collapsed interlayers, is far worse than that on externals. The desorption rates of RE were in the order of RE-Al-MMT＞RE-K-MMT＞RE-Ca-MMT and Eu<sup>3+</sup>＞Y<sup>3+</sup>＞La<sup>3+</sup>. For the desorption of interlayer RE<sup>3+</sup>, NH<sup>4+</sup> is better than Mg<sup>2+</sup> because the larger change of the basal spacings (Δd) provides more minor activation energy barriers (ΔE) for NH<sup>4+</sup> diffusion within interlayers. It can enrich the metallogeny theory of weathered crust elution-deposited rare earth ores and provide a certain theoretical basis for its efficient exploitation.
... Co: The average content of suspended Co in the water of the tributaries of Lake Onego for four hydrological seasons in terms of dry matter was 25 µg/g (the concentrations ranged from 4 to 56 µg/g). The obtained values are 1.3 times higher than the global average and exceed the values for the upper part of the continents (Figure 4), the average values correspond to the obtained values of Co in the suspended matter of the rivers of southern France [49], and the limits of fluctuations are similar to the data obtained for different sections of the Amazon River [50]. With the exception of the Kumsa, Vodla and Sheltozerka rivers, the maximum concentration of suspended Co was observed in the winter season for the rivers draining the territory of the FCS. ...
This paper presents the results of seasonal observations of the geochemical composition of the waters of the large tributaries of Lake Onego. The mineralogy and geochemistry of the suspended matter and the isotopic composition (oxygen-18 and deuterium) of the river waters were studied for the first time. The dependence of the chemical and isotopic compositions of the tributary water on the season and characteristics of the catchment area (swampiness and lacustrine) was revealed. It is shown that the river waters belong to the bicarbonate class of the calcium group and have low mineralization, high color and a similar composition to the main minerals of the suspended matter. It is determined that the difference between the multielement spectra of the water and suspended matter of the different rivers is closely related to the geological and geomorphological structures of river basins. It is established that the quantitative characteristics of the mineral and organic parts of the suspended matter, the ratios of the different minerals andthe size and patterning of the particles of detrital material in the tributaries differ. The change in the mineralogical and geochemical compositions of the suspended matter of each individual river over the year is insignificant. The influence of the river runoff on the formation of lake waters is manifested in the chemical composition of the lake waters. The quantitative ratios of the main ions, biogenic elements and microcomponents in lake water mainly correspond to their ratios in river waters. The mineral part of the dispersed sedimentary matter of the lake in its geochemical characteristics is close to the suspended matter of the river waters.
... During contamination (II), input solution had a constant cesium concentration C i (mol L − 1 ). sites") have strong affinity to cesium but low density (Brouwer et al., 1983;Eberl, 1980;Francis and Brinkley, 1976;Jackson, 1962;Maes and Cremers, 1986;Poinssot et al., 1999;Rich and Black, 1964;Sawhney, 1972;Zachara et al., 2002). Planar sites have much lower affinity to cesium but represent most of the cation exchange capacity "CEC" and correspond to the basal surface and edge sites (Cornell, 1993;Rigol et al., 2002;Staunton and Roubaud, 1997;Zachara et al., 2002). ...
This paper addresses the modelling of cesium sorption in non-equilibrium and nonlinear conditions with a two-site model. Compared to the classical Kd approach, the proposed model better reproduced the breakthrough curves observed during continuous-flow stirred tank reactor experiments conducted on two contrasted soils. Fitted parameters suggested contrasted conditions of cesium sorption between 1) equilibrium sites, with low affinity and high sorption capacity comparable to CEC and 2) non-equilibrium sites, with a fast sorption rate (half-time of 0.2-0.3 h), a slow desorption rate (half-time of 3-9 days) and a very low sorption capacity (0.02-0.04% of CEC). Comparison of EK sites densities with sorption capacities derived from the literature suggests that the EK equilibrium and kinetic sites might correspond to ion exchange and surface complexation of soil clay minerals respectively. This work stresses the limits of the Kd model to predict 137Cs sorption in reactive transport conditions and supports an alternative non-equilibrium nonlinear approach.
... Sediment consists of clay, and its structure has a hexagonal cavity of an oxygen layer whose size fits the ionic diameter of K (Kevan, 2003;Robinson, 1962). This property of K might have increased the difficulty of ionic exchange with Ca (Sawhney, 1972). Additionally, the high solubility of Ca(OH) 2 likely led to the precipitation of Mg as magnesium hydroxide (Mg(OH) 2 ) (Eq. (5)) (Arai, 1996), making Mg non-detectable in CSM5 and CSM10. ...
The coastal in-situ capping method can sequester contaminated sediment and suppress sediment resuspension. Few studies have investigated the suppression of sediment resuspension induced by calcium eluted from in-situ capping materials. We investigated the physicochemical suppression of calcium on sediment resuspension. A resuspension experiment was conducted in an annular flume using coastal sediment mixed with 0 g (CSM0), 1 g (CSM1), 5 g (CSM5), and 10 g (CSM10) of Ca(OH) 2 under a stepwise increase in bottom shear stress. Calcium enhanced sediment erosion resistance, decreasing suspended sediment concentrations. Exponentially increased SSC in CSM0 and CSM1 was three times higher than that in linearly increased CSM10. Viscosity in CSM10 was approximately three times higher than that in CSM0 and CSM1. Calcium-induced cation exchange increased sediment viscosity via sediment structural rearrangement, calcium-silicate-hydrate production, and the development of larger aggregates. Consequently, calcium suppressed sediment resuspension by physiochemically changing the sediment properties.
... It is known that clay minerals form frayed edge sites (FESs) at increased frayed edges of neighboring layers of mica crystal lattices (Sawhney, 1972;Tamura and Jacob, 1960). FES selectively captured the nonhydrated size of Cs ions (Francis and Brinkley, 1976). ...
To mitigate radioactive cesium from soil to plant, increasing and maintaining the exchangeable potassium (ExK) level during growth is widely accepted after Tokyo Electric Company's Fukushima Dai-ichi Nuclear Plant accident in Japan. This is because the antagonistic relationship between soil solution K and ¹³⁴Cs + ¹³⁷Cs (RCs) concentrations changes the transfer factor (TF: designated as the ratio of radioactivity of plant organ to soil) of RCs. As the relationship between ExK and TF depends on the soil types, crop species, and other environmental factors, the required amount of ExK should be set to a safe side. Eleven years after the accident, as the activity of ¹³⁴Cs was almost negligible, ¹³⁷Cs became the main RCs in most of the agricultural fields in Fukushima Prefecture. We propose a new indicator, the concentration ratio of plant ¹³⁷Cs to soil exchangeable ¹³⁷Cs (Ex¹³⁷Cs), instead of TF, which showed a better correlation with ExK even among soils with different properties (or mineralogy).
... The similarity in the spatial distribution of the radionuclide concentration (correlation coefficient 0.51, p = 0.027) can be due to the fact that 137 Cs sedimentates mainly with the particulate matter of lithogenic origin [30,31]. The obtained values of the radionuclide concentration do not contradict the literature data. ...
Purpose. The aim of the work is to study spatial variability of the 137Cs and 40K concentrations in the bottom sediments of the Balaklava Bay, and to estimate the sedimentation rate and relative content of the biogenic fraction. Methods and Results. The results of the 137Cs and 40K concentration measurements in 5 columns of the bottom sediments sampled in various parts of the Balaklava Bay are represented. Activity of 137Cs and 40K in the samples was determined by the gamma spectrometric analysis. Based on the measurement data, spatial variability of the 137Cs and 40K concentration fields in the bottom sediments was studied, and the sedimentation rate was quantitatively estimated. Application of the balance equation provided quantitative estimates of the relative biogenic fraction content; its spatial variability was described. Quantitative estimates of the relationship between the biogenic fraction estimates resulted from use of the direct and indirect methods are represented. Conclusions. According to the obtained results, the 137Cs and 40K concentrations in the upper 5 cm layer of bottom sediments varied in space from 11 to 62 and from 155 to 562 Bq/kg, respectively. The maximum radionuclide concentrations were characteristic of the northern part of the bay, the minimum ones – of its southern part. The sedimentation average rate was 0.51 ± 0.06 cm/year. Relative biogenic fraction content in the sediments varied in space from 30 to 89% and averaged 46 ± 25%. The maximum values were observed in the southern part of the bay, the minimum ones – in the northern part.
... First, adhered clay minerals could be possible hotspots of 137 Cs on the plastics. It has been well documented that clay minerals have a strong affinity to interact with 137 Cs, and the mobility of 137 Cs is strongly limited by clay matrices (Sawhney, 1972;Tachi et al., 2020a). Although negatively charged clay minerals commonly adsorb cations such as K + and Mg + , weathering of the minerals releases the adsorbed cations into the environment and layered structure of aluminosilicates in minerals slightly open at the edge which named frayed edge site (Rai and Kawabata, 2020). ...
Plastics are one of the ubiquitous and artificial types of substrates for microbial colonization and biofilm development in the aquatic environment. Characterizing plastic-associated biofilms is key to the better understanding of organic material and mineral cycling in the “Plastisphere”—the thin layer of microbial life on plastics. In this study, we propose a new method to extract biofilms from environmental plastics, in order to evaluate the properties of biofilm-derived organic matter through stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope signatures and their interactions with radionuclides especially radiocesium (¹³⁷Cs). The extraction method is simple and cost-effective, requiring only an ultrasonic bath, disposable plastic syringes, and a freeze drier. After ultrasound-assisted separation from the plastics, biofilm samples were successfully collected via a sequence of syringe treatments, with less contamination from plastics and other mineral particles. Effective removal of small microplastics from the experimental suspension was satisfactorily achieved using the method with syringe treatments. Biofilm-derived organic matter samples (14.5–65.4 mg) from four river mouths in Japan showed ¹³⁷Cs activity concentrations of <75 to 820 Bq·kg⁻¹ biofilm (dw), providing evidence that environmental plastics, mediated by developed biofilms, serve as a carrier for ¹³⁷Cs in the coastal riverine environment. Significant differences in the δ¹³C and δ¹⁵N signatures were also obtained for the biofilms, indicating the different sources, pathways, and development processes of biofilms on plastics. We demonstrate here a straightforward method for extracting biofilms from environmental plastics; the results obtained with this method could provide useful insights into the plastic-associated nutrient cycling in the environment.
... The difference among the soil types is as large as 64 10 times between sandy soil and clay soil (Lembrechts, 1993). 65 It is known that clay minerals form frayed edge sites (FESs) at 66 increased frayed edges of neighboring layers of mica crystal lattices 67 (Sawhney, 1972;Tamura and Jacobs, 1960). FES selectively captured the 68 nonhydrated size of Cs ions (Francis and Brinkley, 1976). ...
... The metamorphic products of the mudstones, shales and phyllites are largely composed of different clay minerals, such as illite, vermiculite and chlorite, which may contain high levels of potassium. High initial concentrations of potassium in clay minerals, as well as their ability to exchange with soil potassium, can strongly influence K concentrations in rocks (Inoue 1983;Sawhney 1972). Among the samples of the SF group, the sample of copper shale from the Richtelsdorf Mountains has a very high Ra activity. ...
Natural building stones must be analyzed according to a legally defined procedure
to prove that the natural radioactivity they emit is below the so-called activity index given by European Commission. This applies in particular to rooms inside buildings or apartments that are used by people for longer periods of time. Gamma spectrometric measurements were determined for all nuclides involved, 40K, 226Ra and 232Th, which were finally summed up to a total dose for each sample. Measurements are prescribed to check whether the reference value of 1 millisievert per year can be exceeded by radionuclides in the respective building materials. Natural stones, 82 in total from all over the world, such as various igneous, metamorphic and sedimentary rocks, which are currently used as building stones for interior and exterior finishing, as kitchen worktops, floor tiles, wall cladding, table tops, furniture cover plates, paving stones, terrace slabs, paving slabs, stairs, street furniture, etc., were selected. Additionally, Rn-222 and Rn-220 exhaled per unit mass were determined simultaneously on 54 selcted from the total 82 rocks by using the accumulation method.
In addition, our results were statistically compared with literature data, according to which fesic igneous rocks, gneisses and clay mineral-rich rocks have the highest potential to be radiologically threatening. Limestones, marbles, quartzites, mafic igneous rocks, and sandstones are generally less radiologically harmful, but even here there are exceptions depending on the geologic formation history and mineralogical composition. The highest activity index of 1.87 was determined for the gneiss Giallo California. Model calculations to determine the expected radiation exposure for a human were used by applying two less complex approaches. Even with the extreme model "coffin" , which consists of the Gialo California, a human experiences only a maximum dose rate of 1.29 mSva-1, which is far below the maximum dose rate of 20 mSv a-1 allowed for occupational radiation exposure.
The 220Rn and 222Rn are often neglected, however their contribution to the dose rate received by the population may be significant. Model simulations for a room with 4m x 3m x2.5m where the walls and the floor coverd with tiles of 1cm thickness show a maximum indoor radon concentration of less then 100 Bg.m-3. With increasing thickness of the tiles up to 3cm, 10 cm and 20cm and for poorly ventilated rooms especially some granitoids, gneisses and the copper-rich shale are above the allowed limit of the European Commission.
We had previously developed a measurement method using an imaging plate (IP) to evaluate and address surface contamination caused by the release of radioactive materials during the Fukushima nuclear accident. The measurement units for the surface contamination density were in relative values [described as relative luminescence levels measured in luminescence arbitrary units (LAU)], but the evaluation was required in absolute values, such as Bq cm ⁻² , to enable appropriate control of exposure doses. This study establishes a method for converting the IP measurements of surface contamination density due to environmental radioactivity into absolute values. Soil contaminated with radioactive materials from the Fukushima nuclear accident was collected to create a working reference material (WRM). The conversion coefficient for surface contamination density was calculated using the WRM values measured with an IP and high-purity germanium detectors. The IP measurement values were converted into the surface contamination density using the conversion coefficient. The WRM values measured with the IP and high-purity germanium detectors were 324.1 LAU and 32.22 ± 2.27 Bq cm ⁻² , respectively. The surface contamination density conversion factor was calculated as 0.0994. The surface contamination density on the roof of the Tsukuba City facility was re-evaluated using the conversion factor. The average value of 29,972 Bq m ⁻² matched the amount of radioactive material fallen in Tsukuba City. By standardizing the measurement conditions for surface contamination when using IPs, we successfully quantified the surface contamination density with an accuracy comparable to that of conventional methods. This method is expected to make a significant contribution to efficient radiation safety management.
Sedimentation transport of 137Cs can lead to the significant accumulation of this radionuclide at depths that could not be reached through the vertical water exchange alone; therefore, a comparative assessment of its contents for different types of suspended matter (SM) and regions of the Black Sea is of particular interest. For this purpose, we have collected samples of SM and seawater at deep-water and coastal stations for the subsequent determination of the 137Cs activity in the surface water layer. To calculate the fraction of lithogenic matter, the potassium content in the SM was additionally determined. The range of 137Cs content on SM at different stations differed by more than an order of magnitude: from 7 to 111 Bq/kg for specific activity, and from 0.03 to 0.69% for its content on SM, in % of the total content in the surface water layer. Stations located farther away from the coast were characterized by the lowest percentage of 137Cs in SM, while its content at the coastal stations was more variable. The comparison of the lithogenic and biogenic contribution to SM and data on 137Cs for different stations suggests that the content of this radionuclide on SM is primarily determined by the dynamic variations of the lithogenic matter. Based on the 137Cs migration on SM, the Black Sea is divided into at least two types of regions. One regions are water areas far removed from sources of lithogenic matter where SM is formed mainly due to the hydrobiont activity. The 137Cs content in SM due to the predominance of biogenic matter and an insignificant concentration of lithogenic matter in this case accounts for hundredths of a percent of its total content in the surface water layer. Other regions are coastal and shelf water basins, which, on the one hand, are subjected to the significant coastal and river lithogenic runoff, and on the other, they are characterized by the elevated trophicity and biological productivity. In these water areas, 137Cs content on SM owing to the variability of biotic and abiotic factors is more variable and can fluctuate from values typical for the open sea to an order of magnitude higher.
To study the effect of soil erosion on the distribution and migration pattern of radionuclides, the levels of 239+240 Pu and 137 Cs in alpine meadow soil were measured in the Hongsongwa Nature Reserve, Hebei Province. The measured activities of 239+240 Pu and 137 Cs in surface soil ranged from 0.028 to 2.781 Bq/kg and from 1.3 to 59.8 Bq/kg, respectively. The distribution of 137 Cs and 239+240 Pu is uneven and significantly correlated with the organic matter content and altitude variations within the study area. Core samples were collected from both ridge and valley locations to assess erosion rates, revealing that ridge areas experienced approximately 2.5 times higher erosion rates (18.0 t ha −l a −1) compared to valleys (7.3 t ha −l a −1). The vertical migration behavior of 239+240 Pu and 137 Cs was quantitatively described by a convection-diffusion equation model. Results indicated that core samples taken from the ridge displayed significantly higher apparent diffusion coefficients (D Cs = 4.57 cm 2 /y; D Pu = 2.42 cm 2 /y) as well as apparent convection coefficient (ν Cs = 0.31 cm/y; ν Pu = 0.43 cm/y), which were approximately 10 and 2 times those observed in reference sample (D Cs = 0.33 cm 2 /y; D Pu = 0.32 cm 2 /y; ν Cs = 0.16 cm/y; ν Pu = 0.17 cm/y), respectively. The migration rate of 239+240 Pu is accelerated by 39% compared to that of 137 Cs due to soil erosion. The diffusion and convection rates of both isotopes in the valley sample are similar to those in the reference sample. In general, soil erosion significantly affects the horizontal and vertical migration of 239+240 Pu and 137 Cs.
We investigated cesium-137 (137Cs) transfer to Japanese mustard spinaches (spinach) due to resuspended particles across the multiple sites in Fukushima Prefecture, Japan. At a site located in difficult-to-return zone, 137Cs deposition in pots placed near the ground increased. However, there was no significant difference in the amount of 137Cs in spinach placed near the ground and those placed far from the ground. 137Cs adsorption to the spinach was less than 10% of the 137Cs deposition at each study site. For future study, it is necessary to evaluate the effects of weather and growth conditions on 137Cs absorption to the spinach.
The irreversible adsorption capacity of radioactive cesium in soil of granite origin was assessed via a desorption experiment. The results demonstrated that the cesium desorption only occurred when it reached 0.035% of the cation exchange capacity, despite the presence of the competing ion (K+). The fixation of cesium on frayed edge sites, primarily ascribed to weathered mica and interpretable via the dual-site Langmuir model, may contribute to this irreversible binding. Consequently, the extraction of these minerals from granite-origin soil possesses the potential to diminish the concentration and volume of radioactive soil waste contamination.
Das Element Caesium gehört zu den Alkalimetallen. Caesium wurde erstmals 1860/61 von Gustav Robert Kirchhoff und Robert Wilhelm Bunsen beschrieben. Caesium ist ein seltenes Element auf der Erde und ein extrem reaktives Metall. Es reagiert bei Luftkontakt spontan und macht eine Aufbewahrung in Glasampullen unter reinem Argon oder im Vakuum erforderlich. Einsatz findet Caesium nur in der Forschung (Glühkathode, Atomuhren, Photomultipliern). Eine biologische Bedeutung des Elements ist nicht bekannt, es kommt normalerweise nicht im Körper vor und ist nicht toxisch. Die Gefahr für Menschen geht von den instabilen Isotopen des Caesiums aus, welche z. B. durch Kernumwandlung bzw. Kernzerfall entstehen: Ein prominentes Beispiel dafür ist das Isotop Cs‐137.
The radioactive caesium concentration of brown rice can be accurately predicted from the amount of dissolved radioactive caesium in the soil when exchangeable potassium level is high enough not to affect absorption of radioactive caesium by rice plant. However, measuring the amount of dissolved radioactive caesium requires a lot of labor, such as collecting a large amount of soil solution, filtering, and concentrating. Therefore, we investigated whether it is possible to adsorb dissolved radioactive caesium in soil on an adsorption sheet supporting zinc-substituted Prussian blue and use it for simple prediction of radioactive caesium concentration in brown rice. A caesium adsorption disk coated with a membrane filter to prevent adhesion of soil particles was buried in the surface soil of a local paddy field for two weeks, and dissolved radioactive caesium was recovered. The amount of radioactive caesium recovered on the adsorption sheet was highly positively correlated with the concentration of radioactive caesium in brown rice. By embedding an adsorption sheet at the tillering stage of cultivation and analyzing the collected radioactive caesium concentration, it is possible to predict the radioactive caesium concentration in brown rice without the need to collect a large amount of soil solution or analyze the soil physicochemical properties.
For the disposal of high-level radioactive waste (HLW), the deep geological disposal is recognized as an effective method. The distribution coefficient (Kd) of radionuclides on buffer/backfill materials or host rock is one of the key parameters used in the safety assessment of geological repository. 137Cs is one of the high-yield (t1/2 = 30.1 y, 6%) fission products in spent fuels, its high solubility makes it likely to migrate through groundwater to the biosphere. Multibarrier system prevents leakage of radionuclides to the environment. The present review discusses the general mechanisms of cesium adsorption by minerals, elaborates the parameters which influence adsorption of cesium contain concentration of cesium, pH, humic acid, competitive cations and properties of minerals. Furthermore, we have collected the Kd values from cesium adsorption studies concerned with the minerals conducted during the past two decades, and analyzed by the probabilistic modelling to obtain the best-estimated Kd values of Cs adsorption on bentonite, granite and clay under different solution conditions.KeywordsSafety assessmentCesiumSorptionProbabilistic modelling
There is a concern of recontamination to agricultural field and crops by resuspended particles derived from the radioactive contaminated areas in Fukushima Prefecture. We investigated the cesium-137 (137Cs) transfer from the soil contaminated by resuspended particles to Japanese mustard spinaches. When the contribution of airborne particulate matters to 137Cs activity was large, the proportion of bioavailable 137Cs fractions in the particles increased. Consequently, 137Cs transfer factor of the Japanese mustard spinaches increased. It was considered that the difference in the type of particles might affected the proportion of the bioavailable 137Cs in the particles and its transfer to the crops.
Potassium bioavailability in soil is a major problem for efficient crop production, and it is directly linked to the behavior of potassium at the mineral–water interface. Potassium adsorption is known to occur in soil via outer-sphere as well as inner-sphere surface complexation, depending on the soil conditions (e.g., mineralogy). However, the main focus of this study is to identify novel mechanisms of potassium sorption to newly formed surface precipitates. Soil clay minerals and metal oxides can undergo dissolution on a short time scale (e.g., minutes to weeks) at circumneutral pH values and be subjected to surface precipitation. We hypothesize that the formation of surface precipitates can create new adsorption sites for potassium, thus affecting potassium mobility. The objective of this study is to determine how potassium sorption to aluminum and silicon oxides is affected by co-ions, dissolved silicate, and surface precipitation. Potassium sorption experiments were conducted at a pH of 8.5 using AlO(OH) and SiO2 sorbents in the presence of co-ions (magnesium, nickel, and zinc). The soils of the Texas High Plains, where potassium bioavailability is a significant problem, is alkaline (pH 7.8–8.2), and therefore, a relatively alkaline pH was selected for this study. Adsorption results indicate that SiO2 played an important role in increasing potassium sorption compared to AlO(OH) due to its role in the formation of silicated metal hydroxides. Surface area normalized adsorption values indicate that potassium adsorption increases up to 29% on average when dissolved silicate is present with surface precipitates. X-ray diffraction analysis of the reacted solids indicated that the presence of zinc and nickel compared to magnesium enhanced the formation of new surface precipitates, which were indexed to layered double hydroxides (LDHs). X-ray absorption near edge structure analysis indicated that potassium may be bound to these surface precipitates (e.g., LDHs) via inner-sphere surface complexation. Overall, these results provide novel insights into potassium adsorption and fixation in soil and clay mineral systems, and they have important implications for effective potassium fertilizer application and bioavailability in soil because of the influence that surface precipitates have on cation sorption.
Current radiocesium (137Cs) models to evaluate the risk of 137Cs transfer from soil to plants are based on the clay and exchangeable potassium (K) contents in soil. These models disregard the mineralogy of the clay fraction and are likely not capable of accurately predicting the 137Cs transfer factor (TF) in soils of contrasting parent rocks and weathering stages. The objectives of this study were to test that hypothesis and to identify whether quantitative information on mineralogy can improve the predictions. A pot cultivation experiment was set up with clay-sand mixtures in single and double clay doses that were fertilized, spiked with 137Cs and grown with ryegrass for 30 days. Four clays (illite, biotite, smectite and vermiculite) along with six deposits from clay-rich geological units were compared. The TF generally decreased with increasing clay dose for each of these ten different clay groups, however, the TF varied two orders of magnitude across clay groups and doses. The TF was highest for clays with little 137Cs specific sites such as bentonite and/or where the exchangeable K content was low compared to the other clays. The TF was well predicted from the soil solution 137Cs and K concentrations (R2 = 0.72 for log transformed TF), corroborating earlier findings in natural soils. The TF (log transformed) was statistically unrelated to total phyllosilicate content or 1:1 and 2:1:1 type phyllosilicate content while it significantly decreased with increasing 2:1 phyllosilicate content (R2 = 0.32). A multiple regression model with four different X-ray diffraction (XRD) based phyllosilicate groups yielded the strongest predictive power (R2 = 0.74). We conclude that XRD quantification is valuable for describing 137Cs bioavailability in plant substrates. These findings now await confirmation for natural soils.
The understanding of the swelling phenomenon of montmorillonite is essential to predict the physical and chemical behavior of clay-based barriers in radioactive waste disposal systems. This study investigated the key factors controlling crystalline swelling behavior of montmorillonite with different interlayer counter-ions by molecular dynamics (MD) simulations. On the basis of the comparisons between MD simulated and experimental results, the water content in the interlayer in five homoionic (Na-, K-, Cs-, Ca- and Sr-) montmorillonite was strongly correlated to the hydration number of each counter-ion. The analyses for these results offer insights that the hydration number is controlled by the hydration free energy, the volume and the distribution of each interlayer counter-ion. The systematic MD simulations with virtually variable parameters clarified that the hydration free energy and the charge of interlayer counter-ions compete as influencing factors, and the control the formation rate of an outer-sphere complex of each counter-ion. The empirical relationships between these key factors will allow essential insights into predicting the swelling behavior of montmorillonite with different interlayer counter-ions.
We subjected [Formula: see text]Cs-contaminated soil to stirring and particle classification to reduce the contaminated soil volume. [Formula: see text]Cs activity as a function of particle diameter changed drastically as the weight ratio of soil/water varied; 85% of all [Formula: see text]Cs was adsorbed to particles of minimum diameter after stirring. Soil argillites (smectites, vermiculites, and illites) adsorbed [Formula: see text]Cs. We used X-ray diffraction (XRD) analysis to measure the soil contents; the [Formula: see text]Cs distribution as a function of soil particle diameter mirrored the smectite levels revealed by the XRD peak intensities. Thus, smectic was the main absorbent of [Formula: see text]Cs.
Hydraulic fracturing to generate complex fracture networks is essential for shale reservoir development. However, the recovery of shale oil and gas is still low due to various engineering and geological factors. Acid treatment has been approved as a potential approach to enhance stimulated reservoir volume (SRV) by changing petrophysical and mechanical properties. Understanding the multiscale pore structure evolution behind the macro-performance change is critical in the application of acid treatment in shale reservoirs. In this study, cylindrical and powder shale samples from the Longmaxi formation are treated with 15 wt% hydrochloric acid (HCl) for 10 days. Before and after acid treatment, X-ray computed tomography (CT) and N2 adsorption techniques are used to characterize shale pore structure at microscale and nanoscale, respectively. Combined with the determination of variations in chemical compositions of shale samples and acid solutions, the mechanism of multiscale pore structure evolution induced by acid treatment is discussed. The N2 adsorption results uncover a considerable increase in volume and size of nanopores. All the nanopores increase in carbonate-rich shale, whereas the micropores and mesopores undergo a decrease in clay-rich shale. Reconstructed 3D CT images reveal the generation of large volumes of microscale pores and fractures, which leads to an increase in porosity of about 9%. The pore structure evolution in shale due to acid treatment is controlled by both mineralogy and microstructure. These findings demonstrate the promise of acid treatment for enhanced SRV and long-term productivity of shale oil and gas reservoirs in China.
Determination of ¹³⁷Cs, ⁴⁰K and ²¹⁰Po in water, bottom sediments and suspended matter of river systems of the Can Gio Biosphere Reserve, Vietnam was carried out. The average activity concentration of ¹³⁷Cs in waters of Ca Gau and Long Tau was 0.89 ± 0.14 and 1.08 ± 0.15 Bq m⁻³ and was comparable to the levels of this radioisotope in waters of the East Sea. The activity concentration of ¹³⁷Cs in bottom sediments was 2.23 ± 0.81 and 3.63 ± 1.24 Bq kg⁻¹. The activity concentration of ¹³⁷Cs in water and bottom sediments could be characterized as low. So, the water areas of the Ca Gau and Long Tau rivers could be attributed to areas with insignificant pollution by technogenic radionuclides. The ²¹⁰Ро activity concentration in bottom sediments of the Ca Gau and Long Tau rivers ranged from 9.2 ± 1.2 to 25.5 ± 2.1 Bq kg⁻¹, which is typical for river bottom sediments. Such values indicate the absence of anthropogenic enhancement of the entry of this radionuclide into the Can Gio river systems. The ⁴⁰K activity concentration varied within 467 ± 42–651 ± 39 Bq kg⁻¹ and represented typical values of potassium content in the bottom sediments of coastal water bodies, subject to a significant influence of the lithogenic component of suspended matter.
Nuclear power plant accidents typically lead to the contamination of large volumes of soils with radioactive cesium. This element is hard to desorb from soil, especially when it is bound to mica minerals, and aggressive and energy-consuming techniques are often required. In this study, we investigated the use of ultrasound with Mg²⁺ cation exchange for the removal of a¹³³Cs-contaminated vermiculite over a wide range of temperatures (20–200 °C). At room temperature, ultrasound was found to significantly accelerate Cs desorption but only reversibly adsorbed Cs species were removed. Under hydrothermal conditions and ultrasonic irradiation in contrast, the removal efficiency after 1 h was 50% at 100 °C and more than 95% at 200 °C, compared with only 50% without ultrasonication at 200 °C. Cs contamination can therefore be nearly totally removed, even from collapsed vermiculite sites where sorption is considered irreversible. Ultrasound waves and high temperatures both make trapped Cs more accessible by spreading the sheets and improving mass transfer. Acoustic noise spectra show that even at high pressure and temperature, cavitation bubbles form, oscillate and collapse, with the desired physical effects. These results demonstrate the potential of synergistic ultrasound and hydrothermal treatment for soil remediation.
Electron probe micro-analysis studies on individual particles (40-60 mesh) of weathered micas treated with solutions containing equivalent amounts of Rb and Sr showed partial segregation of these elements. Rb was concentrated at particle and step edges, at cracks, and, in the case of partially K-depleted biotite, at boundaries of vermiculite and mica zones ("wedge zones"). The scarcity of wedge zones in mica from which nearly all of the K had been removed reduced the overall selectivity for Rb. The restricted exchange of interlayer Mg ions from vermiculite-like zones by a mixed Rb-Sr solution was observed in earlier studies with these micas. The proposed explanation for these results was a closing down of the interlayer space at the edge of the particle due to Rb concentration in these positions. This explanation is confirmed by the present study.
The specificity of K adsorption on illite with respect to divalent cations can be explained by accepting the existence of three types of exchange sites, each of which shows a normal exchange process which may be described with the Gapon exchange equation. The K exchange on illite can be described mathematically as the sum of these three processes. Proof of the validity of the mathematical expression is presented. (Abstract retrieved from CAB Abstracts by CABI’s permission)
As agriculture is intensified in particular areas, the proportion of nutrients supplied by weathering becomes relatively small, compared to that added as fertilizers. Even in areas of well-developed agriculture, both in temperate and tropical regions, the weathering release of nutrient elements is of major importance in soil fertility and crop production. It is found that the inherent fertility of soils is related to their mineral content. As the weathering stage advances, soils gradually change, first toward increased productivity and finally, to extremely low productivity. The release of the major and minor elements can be correlated with mineral composition of the soil. In backward areas, agriculture has been directed toward maximum use of the native mineral source of nutrients by systems such as patch agriculture and paddy culture. Even in areas of intensive agriculture, it is clear that the addition of ground rocks (such as ground limestone) and minerals to soils is a reversal of the weathering scheme, with extremely beneficial effects on crop productivity. The chapter examines the processes and products of chemical weathering of minerals in soils from the standpoint of each of these phases and highlights the relative stability of minerals; weathering sequences and indexes. The chapter also lists out the factors affecting the reaction rates of chemical weathering and their distribution frequency.
This chapter focuses on soil potassium. In the soil-plant system, potassium behaves with extreme differences of solubility and mobility. Thus, its absorption from solutions and soils is highly efficient. Its movement through the plant is very rapid, yet its net retention in individual living cells may be quite strong and some of the bonds persist until death of the cell. Soil potassium ranges from the soluble state through states of decreasing mobility to that of an essential constituent of certain primary minerals from which it is released only by destruction of the crystal structure. In many soils, the entire range of known forms of potassium occurs; in other soils, some forms may be lacking, usually because of the absence of some types of primary or clay minerals. In this chapter, forms of soil potassium and their availability to plants are explained. A brief discussion on influence of other cations on forms of soil potassium is presented. Relationships of soil microorganisms to soil potassium are also elaborated.
The relation between.counter ion distribution, ion mobility and degree of contraction has been studied on hydrobiotite (2-5 /~ size), equilibrated with mixed RbCI-SrCle solutions. From X-ray data, isotopic exchange and chemical extraction, it appears that the counter ions of the contracted portions of the interlayers are not in equilibrium with the solution phase. The distribution of counter ions in the interlayer space follows a gradient from the margin of the mineral to the centre. This distribution is the result of the interruption of a diffusion process, caused by the contraction of layers during exchange. Isotopic exchange is rapid on the external surfaces and in the expanded portions of the interlayers, but very slow in the contracted part, where the rate of exchange amounts to only 0"002-0"02 m-eq/100 g/day, and decreases with time. On ex- change with MgCI 2 solutions, secondary expansion of the interlayer space occurs and some of the counter ions trapped in the contracted regions are released.
The relationships of particle size of biotite to K release, oxidation state of iron, cation‐exchange capacity, and total charge of the mineral were studied. The first stage of alteration, accomplished by leaching 15 minutes with 0.1 N NaCl, showed a marked relationship to particle size. At this point, the only K removed was that on the external surfaces of the particles, and consequently the fine particles lost a larger percentage of their total K at this stage than did the larger particles. However, by the last stage where 50% of the total K had been removed, the large particles generally had lost as much as the fine. Once the initially exchangeable K has been removed, the release of matrix K from the particles is suggested to be controlled by the concentration gradient and the geometry of the diffusion path available for the exchanging ions, Na and K. The equilibrium concentration of K in 0.1 N NaCl was shown to be proportional to the total K content of biotite which had been altered to different levels of total K. The oxidation state of iron in the biotite was shown to be a function of particle size and extent of alteration.
Two phlogopites depleted of potassium by extraction with sodium tetraphenyl boron released resorbed potassium more readily than the original micas, but the large difference in the exchange parameters of the two micas were paralleled by those of their potassium-saturated alteration products. The apparent basal spacings of the potassium-saturated alteration products were about 0·03Å larger than those of the corresponding untreated micas; their broader lines indicate structural irregularities.
The relation between.counter ion distribution, ion mobility and degree of contraction has been studied on hydrobiotite (2-5 /~ size), equilibrated with mixed RbCI-SrCle solutions. From X-ray data, isotopic exchange and chemical extraction, it appears that the counter ions of the contracted portions of the interlayers are not in equilibrium with the solution phase. The distribution of counter ions in the interlayer space follows a gradient from the margin of the mineral to the centre. This distribution is the result of the interruption of a diffusion process, caused by the contraction of layers during exchange. Isotopic exchange is rapid on the external surfaces and in the expanded portions of the interlayers, but very slow in the contracted part, where the rate of exchange amounts to only 0"002-0"02 m-eq/100 g/day, and decreases with time. On ex-change with MgCI 2 solutions, secondary expansion of the interlayer space occurs and some of the counter ions trapped in the contracted regions are released.
Experiments were conducted to study the influence of stable Cs and K on ; the reactions of tracer quantities of Cs¹³ⷠand Kâ´Â² in soils and clay ; minerals, using an equilibrium batch method. Tracer quantities of both Cs/sup ; 137/ and Kâ´Â² were strongly sorbed in soils and clay minerals. Cesium was ; more strongly sorbed than K. Since the initial trace quantities of carrier-free ; Cs¹³ⷠwere very strongly sorbed, the sequence of the addition of the tracer ; radioisotope in relation to the addition of stable Cs and K determined the level ; at which equilibrium was attained. The absolute amount of Cs and K sorption ; depended on the kind of clay mineral and soil. Cesium sorption relative to K was ; greater in Ca-clay than in H-Al-clay. The concentration of the stable Cs and K ; present was also an important factor. When the tracer Cs¹³ⷠwas diluted to ; a negligible fraction of the total Cs in solution, only a negligible fraction of ; Cs¹³ⷠwas sorbed. In equimolar mixtures of Cs and K, the sorption of Cs ; relative to K decreased as the ionic concentration increased. As a corollary to ; this effect, under conditions of low ionic concentration, stable Cs was much more ; effective than K in releasing Cs¹³ⷠfrom soils. In high concentraltions, ; however, K may be, in certain soils, as effective as stable Cs. (auth)
Abstract- Completeness of exchange,of K from muscovite by Ba 2+ ions decreased with particle size below 20/~. Accompanying K exchange at 120~ using a repeated batch technique, was a marked loss of Si and the formation of boehmite,in the finer fractions. Several possible explanations for the unexpected high K retention of fine mica fractions are discussed. The formation of a diffusion-inhibiting surface "skin" is discounted, because equilibrium was obtained more rapidly with the fine fractions than with coarser fractions. The average degree of bending of unit mica layers due to peripheral expansion is probably greater in large particles. With bending, rotation of tetrahedra and shifting of adjacent layers with respect to each other may,induce greater release of K and a lower selectivity for this ion. Fault planes may,induce preferential expansion of individual layers and initiate interstrati- fication and splitting of particles. Splitting of particles may reduce bending and increase K selectivity.
A CONVENIENT optical method for studying the diffusion of water molecules in vermiculite is outlined below. The effect has similarities to that employed by Tiselius1 in his studies of diffusion in the zeolites; but, in this instance, it can be observed in reflected or transmitted light and is not a birefringence effect. Although magnesium vermiculite is referred to particularly, vermiculites saturated with cations other than magnesium behave in a similar (although not identical) manner.
The Rb/Sr selectivity coefficient of muscovite and biotite reached a maximum at an intermediate degree of K‐depletion of these micas. The Rb/Sr ratio of ions absorbed from a solution, initially containing equivalent amounts of Rb and Sr, was determined for the two micas altered to different degrees to Mg‐vermiculite. X‐ray diffraction of the Rb‐Sr treated vermiculites gave nonintegral peaks for the basal reflections, indicating random interstratification of Rb and Sr + Mg saturated layers. The results support previous proposals that selective sites for K are probably situated at wedge‐shaped zones occurring at frayed edges and deeper within the altered mica particles. As K is removed from mica, the number of such wedges should first increase and then decrease. Selectivity of K and other large monovalent ions should also follow this same trend.
Successively larger sorptions of Cs or K by most Ca‐saturated vermiculites collapsed their alternate layers, producing regularly interstratified mica‐vermiculite layer sequences. Additional sorption then collapsed the expanded layers within the interstratified mineral until the entire sample was collapsed to the mica structure. Sorption of Cs or K by Ca‐saturated montmorillonites, on the other hand, did not collapse their interlayers even when almost half of their exchange sites were occupied by Cs or K. Additional sorption then collapsed some layers producing random, not regular, interstratification.
The ease of collapse of interlayers and the subsequent regularity of interstratification was related to the layer charge density of the mineral. The large charge density in vermiculite collapses the layers readily on Cs or K sorption. The collapse in one layer prevents the entry of these cations in the adjacent layer, producing regularly interstratified mica‐vermiculite layer sequences. The lack of collapse and the random interstratification in montmorillonites, on the other hand, is attributed to their small charge density.
To observe the loci of cation exchange in natural soils, (16 cm ³ ) cubes of soil with such micromorphological features as silt and clay varves, isolated clay bodies in a silty matrix, and clay and silt skins on ped surfaces, were leached repeatedly with CsCl. After the soil cubes were saturated with Cs, they were dried and impregnated with a polyester resin.
X‐ray images and Cs activity measurements, determined on thin sections with the electron microprobe, demonstrated that Cs sorbed by microstructures enriched with clay was nearly twice that of silty matrices. These observations illustrate that the loci of cation exchange are heterogeneously distributed within the undisturbed soil horizon and are influenced by micromorphological features produced by geologic and pedogenic processes.
Cesium sorbed by weathered biotite sand grains was eight times greater than by clay bodies. The greater exchange capacity of the weathered biotite was attributed to the loss of interlayer potassium and conversion to a vermiculite‐like mineral. The smaller exchange capacity of the clay was attributed to the presence of illite, chlorite, and kaolinite, of low exchange capacity, to pedogenic weathering and aluminum interlayer formation in vermiculite and to the presence of 40 to 50% pore space. Although weathered biotite sand grains constituted only 1.6% of the whole soil, they contributed about 15% to the total exchange capacity of Charlton soil.
Potassium was introduced into expanded, i.e., K ⁺ ‐depleted, micas by (i) adding about 0.5 symmetry of KCl to a chromatographic column of expanded mica plus sand and (ii) by equilibrating suspensions of Ca ²⁺ or Mg ²⁺ ‐saturated expanded mica with 0.5, 1.0, 1.5, or 2.0 symmetry of KCl. Two expanded biotites readily formed a regular alternation of 10 and 14A spacings whereas two muscovites did not develop such a phase. A phologopite yielded this phase only at the lowest concentration of K ⁺ tested. Regular alternation developed most readily in the Ca ²⁺ ‐saturated minerals. Development of regular interstratification did not appear to be related to the gross surface‐charge‐density of the minerals, but did seem to be positively related to the content of octahedral Fe.
Cation exchange selectivity (CES) values for montmorillonite, vermiculite, biotite, muscovite, and micaceous vermiculitic clays from Triangle and Harpster soils were determined by equilibration of each exchanger with a mixed, equinormal solution containing two competing cations. The cation affinity of Na‐saturated materials increased in the order K < Mg < Ca < Sr < Ba for montmorillonite and the soil clays, Mg < K < Ca < Sr < Ba for biotite, Mg < Ca < Sr < K < Ba for muscovite, and K < Ba < Sr < Ca < Mg for vermiculite. Sodium‐saturated micaceous vermiculite gave a K/Ca CES value of 0.2, whereas K‐saturated, dried vermiculite gave a K/Ca CES value of 0.4, indicating that the large number of interlayer wedge sites produced by the structural collapse in K‐saturated and dried vermiculite increased the selectivity for K ions. Potassium‐saturated and dried muscovite having a CEC equal to the similarly treated vermiculite gave a K/Ca CES value of 4.0 compared to 0.4 for the vermiculite, suggesting that layer charge density has a large influence upon K selectivity. The higher CES values for muscovite relative to those for biotite are explained by the smaller ditrigonal holes and the oblique orientation of hydroxyl groups in the muscovite structure.
Energies with which cations adsorb water molecules are calulated and discussed. An amendment to the diffuse layer theory is developed which takes into consideration the hydration energy of the ions, position of the negative charge in the clay, and other steric factors. Calculations using this amended theory predict differences in the adsorption characteristics of Li ⁺ , Na, ⁺ and K ⁺ which appear to be in general agreement with experimental observations.
Calculations for the interacting diffuse layer case indicate that the fraction of the adsorbed ions residing in the first molecular layer of water (Stern Layer) is essentially independent of distance between the platelets when this distance is greater than 2.5A. Pushing the platelets closer together results in a arger portion of the adsorbed ions in the Stern Layer.
The various assumptions and estimates involved in the development of the theory and the calculations are discussed.
Interlayer K from biotite was extracted with sodium tetraphenylboron for varying time intervals. Then, the Cs sorbed from 10 ‐4 N and 10 ‐6 N CsCl solutions by the original biotite and its altered products was determined.
The increased removal of K successively increased the vermiculite component with expanded 14A lattice and a larger CEC and decreased biotite with a collapsed 10A lattice and a smaller CEC. Further, the greater the amount of vermiculite and larger the CEC, the more was the Cs sorbed. Obviously, the expanded component with a larger CEC sorbed more Cs than its collapsed counterpart. These sorptions by mixtures of biotite and vermiculite, thus, do not support the contention that Cs is sorbed selectively by minerals with a 10A lattice spacing.
The formation of mixed‐layer minerals occurred upon potassium sorption by sodium or magnesium vermiculites with a change in the interlayer cation composition from the homoionic state to one of varying K/Na or K/Mg proportions. The type of mixing occurring in the vermiculites, as deduced by examination of X‐ray diffraction patterns, was similar to that expected for regularly interstratified systems. The same conclusion could not be drawn for the Na‐ or Mg‐biotite material. The effect of particle size on this phenomenon is such that the tendency toward regularity of mixing was greater for the 50 to 74µ than for <2µ materials. It was concluded that it is difficult to distinguish regular from random interstratification in the trioctahedral mica minerals due to coincidence of all but the weakest peaks of the two systems.
The temperature at which vermiculite loses or gains an interlayer of water molecules was compared for a number of interlayer ion saturations. Results indicate the difference between contracting and expanding forces in vermiculite to be dependent upon interlayer ion charge, distance, and hydration energy. Important interlayer forces therefore must be electrostatic attraction between the negatively charged layers and the positive interlayer ions, and expansion due to ion hydration. Basically, ion fixation appears to occur when electrostatic forces of attraction exceed those of hydration
Transvaal vermiculite flakes were weathered in solutions of salts or acids as well as in nutrient solutions inoculated with water extracts of soils from hemlock ( Tsuga canadensis ) and tulip poplar ( Liridendron tulipifera ) forest stands. Morphological changes were studied with a microscope and the structural changes were determined by X‐ray diffraction analysis, while the changes in chemical composition were estimated by an electron microprobe.
Weathering of flakes proceeded from edges inward and produced a weathered zone parallel to the flake edges. However, the changes produced by salt solutions were different from those by acids and biological agents.
Treatment with salt solution produced the expanded weathered zone with a 14A c‐axis spacing as compared to 10A spacing of the unaltered zone. Electron microprobe analysis revealed that the salt treatment replaced the interlayer K ions by the hydrated cations of the salt. Treatment with acids or biological agents, on the other hand, produced a bleached and fragile weathered border that was amorphous to X‐rays. Electron microprobe analysis showed that the bleached zone was depleted of Fe, Mg, and Al, besides the interlayer K ions; concentration of these cations decreased towards the extremity of the edge. The amorphous nature of the weathered zone is attributed to the loss of the unit cell periodicity resulting from the removal of a portion of the octahedral and tetrahedral cations.
The release of K from K‐Ca‐illites by repeated extraction with 0.5 N solutions of various salts was determined. From comparison between the action of NH 4 ions and Na ions it could be deduced that the rapidly exchangeable K occupies two types of sites, which were named “planar” sites and “edge‐interlattice” sites, respectively. The very high preference for K of the edge‐interlattice sites explains why, e.g., Na ions and Ca ions are very ineffective in removing K from these sites as long as the released K ions are not removed from the system. NH 4 ions are, in contrast, highly competitive for the edge‐interlattice sites, but their action is kept within bounds by the fact that the mineral lattice remains fairly well closed during the penetration of NH 4 ions. An analysis of the exchange curve against Na ions yielded an estimate of the distribution and properties of the different types of sites, viz about 500 µeq. per g. rapidly exchangeable sites, of which 96% is situated on the external planar side of the lattice (preference factor for K about 2 in comparison to Ca ions), and 4% is located on the edges (preference factor for K about 500 in comparison to Na ions). In addition to this the illite studied contained about 700 µeq. K per g. in interlattice positions, which will lead to a very slow release of K if the chemical potential of K in solution is maintained at a low level.
A mathematical expression that describes the release of interlayer K from mica particles in NaCl‐NaTPB solutions was derived. The mica particles were assumed to be circular discs and the rate determining process for the release of K was considered to be the simultaneous diffusion of K and Na within the particle. Therefore, this derivation involved the solution of a two‐dimensional radial diffusion problem in which K diffuses from a receding weathering front to the periphery of the particle.
The validity of these assumptions and the resulting expression was evaluated with experimentally determined K release data. The experimental data were obtained with different size fractions of biotite and muscovite placed in NaCl‐NaTPB solutions for different periods of time. The results obtained with each size fraction were accurately described by the theoretical expression. In the case of a 50 to 62 µ biotite fraction, this was true even though 90% of the K was released in a period of 113 hours. On the other hand, the expression did not account for the effect of particle size. A modification of the expression, based on the experimental results, however, accounted for the effects of both time and particle size.
Column leaching experiments with 0.001 N CsCl with 1 N KCl or CaCl 2 showed Cs sorption by a number of clays to be much larger from Ca than from K solutions. The sorption affinity of Cs relative to the complementary ion corresponded with that found by others for the Cs‐K ion pair, but in Cs‐Ca systems the Cs was sorbed to a larger extent than would have been expected from the results of experiments where Cs‐Ca ratios in the equilibrium solution were not so small.
While all except a very small proportion of the sorbed Cs was displaced on leaching with 1 N KCl, some 36 times as much remained after leaching with equal volumes of 1 N CaCl 2 .
Vermiculite sorbed very large amounts of Cs (about 1/3 of exchange capacity) from 0.001 N CsCl‐1 N CaCl 2 . The sorbed Cs was largely “fixed” against exchange with 1 N CaCl 2 . This suggests that Cs contained in interlayer spaces of vermiculite results in interplanar distances which will admit K but not Ca.
Rate of release of fixed K from vermiculite with 0.1 N NaCl was studied to attempt to characterize the ratelimiting process. Of the processes considered, film diffusion satisfied most criteria as rate limiting. The activation energy of the release of K by 0.1 N NaCl in the system studied was about 3,550 cal./mole, a value of the order frequently obtained for diffusion processes.
Electron probe micro-analysis studies on individual particles (40–60 mesh) of weathered micas treated with solutions containing equivalent amounts of Rb and Sr showed partial segregation of these elements. Rb was concentrated at particle and step edges, at cracks, and, in the case of partially K-depleted biotite, at boundaries of vermiculite and mica zones (“wedge zones”). The scarcity of wedge zones in mica from which nearly all of the K had been removed reduced the overall selectivity for Rb. The restricted exchange of interlayer Mg ions from vermiculite-like zones by a mixed Rb-Sr solution was observed in earlier studies with these micas. The proposed explanation for these results was a closing down of the interlayer space at the edge of the particle due to Rb concentration in these positions. This explanation is confirmed by the present study.
A study of the surface morphology of vermiculites from several different sources by the electron microscope has indicated that, unlike the smooth surfaces of micas, the vermiculite surfaces show micromorphological structural variations such as small humps, prominent crystallographic steps on the basal cleavage planes, marginal rolling of the layers and layer buckling. The production of layer buckling may result from the inhomogeneity of interlayer population caused by the introduction of hydrated cations replacing potassium during weathering. The (110) steps on the surface of the cleavage can be observed and these are visualized as nuclei for weathering in addition to the particle edges.
The surface morphology of vermiculites is closely related to the nature of the interlayer cation. When treated with potassium or ammonium salts, the surfaces assume smooth morphology, the K and NH 4 ions occupying interlayer sites and collapsing the mineral. When treated with Na or Li salts, the complex surface morphology persists, and in some cases is enhanced, possibly by the slow exchange of residual potassium from the interlayer. On this basis, the presence of unweathered mica cores have been identified in natural vermiculites which show varying amounts of residual K 2 O content. It is possible to offer an explanation from structural considerations for the complex morphology of the vermiculite surfaces including the curling of the layers on depletion of potassium. The morphological features of vermiculites have been produced on biotite mica by leaching with salt solutions.
Vermiculite (Libby, Mont.) was ground in a Waring blender in a 1 m NaCl solution and, after removal of excess electrolyte, the clay fraction was separated by sedimentation. The clay was predominantly vermiculite: X-ray diffraction patterns of Ca-saturated and oriented specimens showed an intense and sharp 15 Å and a weak 25 Å diffraction maxima and their integral orders. The intensity of the 25 Å reflection, attributed to regularly interstratified layers of vermiculite (15 Å) and mica (10 Å), was less than 20% of the 15 Å peak.
Additions of varying amounts of potassium or cesium, ranging from 10% to 100% of exchange capacity, to Ca-saturated clay showed that the collapse of the vermiculite lattice proceeds through a 1:1 regular interstratification of a 15 Å and a 10 Å lattice. Successive additions increased the 25 Å diffraction peak at the expense of the 15 Å reflection until the entire sample was interstratified. Further additions of K (or Cs) reduced the intensity of the 25 Å reflection and produced a 10 Å reflection until the entire sample was collapsed to 10 Å and no 25 Å reflection was recorded. These observations point out that under certain environmental conditions, the diagenetic formation of micas from vermiculite may proceed through an interstratification of the two in a manner analogous to weathering of biotite to vermiculite through an interstratified stage.
One-dimensional Fourier synthesis from the intensities of the 00 ℓ diffraction maxima of the interstratified mixture was carried out. In addition, a mechanism for the formation of the interstratified mixture was postulated: the replacement of Ca by K (or Cs) in one layer reduces the effective negative charge on the adjacent layer. Consequently, the K cannot replace the Ca in this but replaces the Ca in the next layer forming the interstratified mixture.
Interlayering of 2: 1 layer silicates varies as a function of chemical weathering from the simple, homogeneous K or Na interlayers of micas to the heterogeneous systems of mica intercalated with expanded 2: 1 layer silicates. “Frayed edge” type of weathering at dislocation planes of mica is collated with K release and preferential cation-exchange uptake of K relative to Ca by such expansible layer silicate systems; mica islands maintain alignment of the silica sheet cavities, which facilitates recapture of lattice K. Intercalation of the expanded 2: 1 layer silicates with alumina interlayers appears to be a characteristic function of chemical weathering in soils, with the formation of 2: 1–2: 2 intergrades not only of 14 Å spacing but also of swelling 18 Å types that give small 12, 14, 18 Å and higher spacing peaks (along with the 10 Å peak) at 550°C. Interlayer precipitates appear to be characteristic of soil clays, contrasting with “pure” minerals of deposits developed in less “open” environments than those of soils. The “2: 2 lattice building” phenomenon in expansible 2: 1 layer silicates relates to layer charge density and crystal size, and frequently tends to inhibit the formation of free gibbsite in soil chemical weathering so long as there are expansible layer silicates present to become intercalated with aluminum hydroxide—a weathering phenomenon that may be called an “antigibbsite effect”. Accumulation of alumina (possibly with some iron, magnesium, and allophane) as interlayers in 2: 1 minerals of soils is seen as a genetic stage in the 2: 2 → 1: 1 weathering sequence through which kaolinite and halloysite develop in soils.
Di- and trioctahedral micas were altered to vermiculite-like minerals by extracting the K with sodium tetraphenylboron. Chemical analysis of original and altered micas shows that loss of K is accompanied by an increased loss on ignition, oxidation of some Fe ²⁺ to Fe ³⁺ , loss of divalent octahedral cations, mainly Mg ²⁺ , loss of OH ⁻ (or sorption of H ⁺ ) and decrease in net negative charge.
The following reactions are suggested to explain these changes: (1) replacement of K by Na at interlayer sites; (2) release of structural OH ⁻ ions exposed by replacement of K, which decreases the negative charge and allows the structure to expand and more K to be replaced; (3) oxidation of Fe ²⁺ ions by the reaction
4Fe ²⁺ + 4 structural (OH ⁻ ) + O 2 → 4Fe ³⁺ + 4 structural (O ²⁻ ) + 2H 2 O;
and (4) release of divalent octahedral ions, possibly through some of the holes left when structural OH ⁻ is lost.
The wide implications of these proposals are discussed.
Interlayer K in eleven trioctahedml micas, with compositions ranging from fluorphlogopite to biotite, was replaced by treating cut flakes with aqueous solutions of Na salts at controlled pH values. Replacement in natural micas was faster at pH 4.5 than at pH 9, and there is a co-operative action or synergy of H + and Na + in replacing K +. The synergetic effect of H + tends to be greatest in micas that lose most net negative charge when K is replaced,and increases when Fe2+ in biotite is oxidized by heating. Possible relationships between the synergetic effect and the chemical com- position of micas are discussed in relation to the structure and chemical changes in the aluminosilicate layers when K is exchanged, and it is concluded that incorporation of protons into the structure is responsible for the synergetic effect.
The amounts of potassium retained, after exchange with normal ammonium acetate solution at pH 7, were measured with six montmorillonite- beidellite minerals, whose interlayer charge ranged 0-72-1.00 M+/SisO ~ and where tetrahedral substitution of A1 for Si ranged 0.14--0.93 M+/SisO ~. The amount of potassium retained increased with increasing interlayer charge, but there was no correlation between the amount of potassium retained and the tetrahedral charge. The retention of potassium by expanding minerals is important to agriculture in connection with the fixation of potassium by soil clays. There have been several attempts to find how fixation is related to the composition and charge distribution of expanding 2:1 lay silicates. Wear & White (1951) suggested that the amount of potassium retained in these minerals after exchange varied directly with the proportion of negative charge caused by tetrahedral substitution of A1 for Si. Barshad (1954) considered that the total charge controls expansion and hence the removal of potassium, and is thus more important than the type of substitution. However, Weaver (1958) and Mackenzie (1963) considered that potassium fixation is related to the amount of tetrahedral charge. Because of the difficulty of obtaining pure mineral samples of suitable composition, the relative importance of total charge and type of substitution has been tested only for a small range of composition. This paper gives the results obtained with six montmorillonites of different charge and isomorphous substitution. MATERIALS The samples studied were beidellite (U.S.N.M. R4762) from the Black Jack Mine, Idaho, obtained from the U.S. National Museum; beidellite* from Unterrupsroth, Germany, obtained from Professor Dr U. Hofmann; montmorillonite from Wyoming, U.S.A., obtained from F. W. Berk and Co; montmorillonite from Redhill, England, obtained from the Fullers Earth Union; montmorillonite from Camp Berteau,
Analyzed specimens of muscovite, rose muscovite, margarite, phlogopite, lepidomelane, biotite, and a mica-vermiculite were used. Elements released upon treatment with dilute hydrochloric acid and water were determined. The relationships of K, Na, Ca, Mg, Al, Fe, and monomeric silicic acid in solution to final pH were studied. The release of K, Na, and Ca was pH-independent over a considerable range. The pH values attained were to low to account for the K, Na, or Ca by Donnan hydrolysis alone. Some polymeric or highly colloidal silicates or alumino-silicate anion is presumed to exist in solution. For Mg, Al, and Fe the curves were compared with the solubility-product relationships of the hydroxides. No correspondence was found; the mica curves gave much smaller slopes. Mg, Al, and Fe were reincorporated into the silicate structures as the pH rose. The curves for monomeric silica released did not resemble that for amorphous silica; hence silica was also re-incorporated. The effects of order of treatments upon the exchange properties of mica surfaces were found to be small. /Author/
Potassium-sensitive cationic electrodes have been used to study the rates of release of potassium from five illitic soil clays. This release was complete within a few minutes for clay-size particles and was probably controlled by diffusion. The electrode response time of 15 sec to 0.001 pK unit was too slow to permit accurate measurement of the rate of release, as the process was more than 90% complete in this time. As expected, the chemical reaction was too fast to be measured independently of diffusion. The electrodes were also used to calculate stepwise changes in Gapon constants as potassium (or sodium) was adsorbed by the clay in competition with calcium ions. The Gapon constants for potassium decreased greatly as potassium was adsorbed, but did not reach the low values expected for external exchange sites until comparatively large amounts of potassium were taken up. Potassium ions were held much more tightly thm sodium ions until the external sites began to be filled. The continuous change of Gapon constant with potassium content precluded a sharp distinction between interlayer and external exchangeable potassium. Values of the constants corresponding to the equilibrium solubilities measured in earlier work showed that the clays had a strong preference for potassium over calcium at the potassium contents in these clay preparations, and this showed that the solubility equilibria refer to potassium in the interlayer regions of the clay minerals.
The potassium equilibrium solubility and the potassium extracted from five illitic soil clays by various solutions have been measured. The solutes were chlorides of inorganic monovalent and divalent ions, of amines, and of heterocyclic nitrogen bases; and in addition other salts of sodium and calcium, several amino acids, and a few non-ionic compounds were used. The amounts of potassium displaced from these clay preparations depended on the size and shape of the displacing cations-generally the smaller the cation the more readily did it release potassium-and this effect continued to control release down to quite low levels of potassium solubility. This confirms the previous view that the potassium left in these clays after extensive treatment with sodium and calcium chloride lies in the interlayer spaces of the illite minerals, and provides a partial verification of the original hypothesis that potassium release from these interlayer spaces is a complex reaction involving hydrogen ions. There is some evidence that the potassium removed comes from a limited number of interlayers which show decreasing accessibility to larger ions. There are no positive indications that chemical attack of the clay mineral structure is responsible for potassium release when the clay is not heated or dried. Amino acids are able to displace potassium with an effectiveness which decreases at greater molecular weights and approaches that of primary amines of similar size in spite of the presence of the negatively charged carboxyl groups. Otherwise the release of potassium depends almost entirely on displacement by other cations and is unaffected by anions or non-ionic compounds.
The equilibrium solubility of potassium from the lattice of illites in four soil clays has been measured. This solubility is related to the availability of this potassium and, when account is taken of the fineness of the clay particles, it is found to decrease as the illite structure becomes more degraded by loss of lattice potassium. The solubility depends on the pH of the system in the same way for all the clays, and a minimum value is found near to pH 11. Above this pH, potassium release is due to breakdown of the clay mineral. At lower pH values the release is due either to the instability of the clay mineral structure towards hydrogen ions or to displacement of potassium by other cations acting together with hydrogen ions.
The swelling of montmorillonite, in salt solutions and at various water contents, was studied using low angle X-ray diffraction methods. The initial swelling is crystalline and dependent on the hydration energy of the interlayer cation. With monovalent cations interlayer swelling proceeds to the region of macroscopic swelling where the mineral is liquid crystalline. Preliminary calculations, from the experimental data, of the attractive force between neighbouring sheets shows that this force decreases at a lower rate, with respect to distance, than would be expected from van der Waals' forces.