Interlayer collapse affects on cesium adsorption onto illite

Eco-friendly natural mineral biotite as a cesium adsorbent: Utilizing low-concentration acid and hydrogen peroxide

Biotite, a phyllosilicate mineral, possesses significant potential for cesium (Cs) adsorption owing to its negative surface charge, specific surface area (SSA), and frayed edge sites (FES). Notably, FES are known to play an important role in the adsorption of Cs. The objectives of this study were to investigate the Cs adsorption capacity and behavior of artificially weathered biotite and identify mineralogical characteristics for the development of an eco-friendly geologically-based Cs adsorbent. Through various analyses, it was confirmed that the FES of biotite was mainly formed by mineral structural distortion during artificial weathering. The Cs adsorption capacity is improved by approximately 39% (from 20.53 to 28.63 mg g-1) when FES are formed in biotite through artificial weathering using a low-concentration acidic solution mixed with hydrogen peroxide (H2O2). Especially, the Cs selectivity in Cs-containing seawater, including high concentrations of cations and organic matter, was significantly enhanced from 203.2 to 1707.6 mL g-1, an increase in removal efficiency from 49.5 to 89.2%. These results indicate that FES of artificially weathered biotite play an essential role in Cs adsorption. Therefore, this simple and economical weathering method, which uses a low-concentration acidic solution mixed with H2O2, can be applied to natural minerals for use as Cs adsorbents.

Releasing mechanism of ammonium during clayey sediments compaction and its impact on groundwater environment

Compaction of clayey aquitard would release pore water containing high levels of ammonium to adjacent aquifers, potentially affecting the concentration of groundwater ammonium. However, the releasing process and impact effect of ammonium within clayey aquitard during compaction remain unknown. Four groups of pre-experiments and two groups of simulation experiments were conducted to reveal the releasing mechanism of ammonium during clayey sediment compaction. (1) The results of Experiment A simulating continuous sedimentation conditions showed that the sediment ammonium transferred into pore water sequentially through desorption of ion exchange form, degradation of organic matter, and simultaneous release of mineral-bound ammonium. The concentration of pore water ammonium was 3.54-8.20 mg N/L, with a significant periodical variation due to sediment ammonium transformation. The lower moisture content (<42.4 %) in the later stage of compaction inhibited the biological transformation of ammonium, and the change in mineral structure caused the isomorphic replacement of K to capture ammonium, resulting in a decrease in ammonium concentration in released pore water. (2) The results of Experiment B simulating artificial compaction conditions (such as land subsidence) showed that the pore water ammonium was primarily caused by desorption of ion exchange form ammonium due to changes in pore structure and moisture content. The ammonium concentration in pore water was 4.72-9.91 mg N/L, with a significant increase in response to a large change in pressure in the short term. (3) The estimate results in the Chen Lake wetland suggested that the contribution of clayey aquitard compaction to groundwater ammonium concentration in the adjacent aquifer would be 2.68-4.29 mg N/L, which accounted for a considerable portion of groundwater ammonium concentration and was far higher than that of advection and diffusion. The findings of this study reveal the releasing mechanism of ammonium during clayey sediments compaction, in which reaction products may affect adjacent aquifers.

Disproportionately High Contributions of 60 Year Old Weapons-137Cs Explain the Persistence of Radioactive Contamination in Bavarian Wild Boars

Radionuclides released from nuclear accidents or explosions pose long-term threats to ecosystem health. A prominent example is wild boar contamination in central Europe, which is notorious for its persistently high 137Cs levels. However, without reliable source identification, the origin of this decades old problem has been uncertain. Here, we target radiocesium contamination in wild boars from Bavaria. Our samples (2019-2021) range from 370 to 15,000 Bq·kg-1 137Cs, thus exceeding the regulatory limits (600 Bq·kg-1) by a factor of up to 25. Using an emerging nuclear forensic fingerprint, 135Cs/137Cs, we distinguished various radiocesium source legacies in their source composition. All samples exhibit signatures of mixing of Chornobyl and nuclear weapons fallout, with 135Cs/137Cs ratios ranging from 0.67 to 1.97. Although Chornobyl has been widely believed to be the prime source of 137Cs in wild boars, we find that "old" 137Cs from weapons fallout significantly contributes to the total level (10-68%) in those specimens that exceeded the regulatory limit. In some cases, weapons-137Cs alone can lead to exceedances of the regulatory limit, especially in samples with a relatively low total 137Cs level. Our findings demonstrate that the superposition of older and newer legacies of 137Cs can vastly surpass the impact of any singular yet dominant source and thus highlight the critical role of historical releases of 137Cs in current environmental pollution challenges.

Interactions between micaceous minerals weathering and cesium adsorption

The environmental behavior of radioactive cesium (RCs) in contaminated areas is generally governed by soil and sediment components and natural weathering conditions. In this study, desorption tests and spectroscopic approaches were used to explore the interaction between the weathering of micaceous minerals (i.e., biotite and phlogopite) and the adsorption of Cs⁺ and the critical role of weathering in the environmental behavior of RCs. Results showed that the reaction sequence between weathering and Cs⁺ adsorption significantly affected the surface species of Cs⁺ and the structure of biotite and phlogopite. Regardless of whether it occurred before, after, or during Cs⁺ adsorption, weathering generated more high-affinity adsorption sites, namely, interlayer sites (ITs) and frayed edge sites (FESs), to different extents, and then facilitated the uptake of Cs⁺ at FESs and ITs on micaceous minerals in a poorly exchangeable state. Cs⁺ stabilized the micaceous mineral structure once it was absorbed within collapsed interlayers by hindering cation exchange and preventing further destruction during weathering. As important weathering factors, high temperature and Ca²⁺ content promoted the binding of Cs⁺ in the interlayers of biotite and phlogopite by enhancing interlayer cation exchange. These findings are beneficial for a better understanding of the environmental behaviors of RCs in the hydrosphere and pedosphere.

Influence of non-equilibrium and nonlinear sorption of 137Cs in soils. Study with stirred flow-through reactor experiments and quantification with a nonlinear equilibrium-kinetic model

This paper addresses the modelling of cesium sorption in non-equilibrium and nonlinear conditions with a two-site model. Compared to the classical K_d 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 K_d model to predict ¹³⁷Cs sorption in reactive transport conditions and supports an alternative non-equilibrium nonlinear approach.

Multi-cation exchanges involved in cesium and potassium sorption mechanisms on vermiculite and micaceous structures

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.

Adsorption behavior of Cs(I) on natural soils: Batch experiments and model-based quantification of different adsorption sites

Understanding the adsorption behavior of radiocesium (RCs) in natural soils is crucial for remediation and evaluation of radioactive contaminated sites. In this study, we investigated the adsorption behavior of Cs(I) onto natural soils collected in Beijing by batch adsorption experiments and sequential extraction. A multi-site adsorption model was built to quantitatively analyze the adsorption capacities of soil clay minerals and predict of Cs(I) adsorption ratio of different adsorption sites. Linear programming calculations show that illite/smectite (I/S) mixture and illite(I) are the mainly clay mineral composition. Batch adsorption experiment results show that soils adsorption of Cesium ions is an exothermic process, and the order of influence of competitive cations on the competitive adsorption strength of Cs(I) is:K⁺>Mg²⁺≈Ca²⁺>Na⁺. HA (Humic Acid)has little effect on soil adsorption. SEM-EDS analysis shows that Cs⁺ is mainly distributed on the surface (PS) of soil particles. Based on the above results, the adsorption of Cs(I) onto clay minerals in soils is well predicts in both linear programming calculations and a multi-site adsorption model. The multi-site adsorption model can quantitatively describe and predict the adsorption behavior of Cs(I) on different clay sites in the soils. Frayed edge sites (FES) in the soil can effectively fix trace RCs. The higher concentration of cesium ions is mainly adsorbed on the PS and TIIS. Sequential extraction experiment further proved the adsorption form of cesium in soil under trace and high concentration conditions.

Characteristic and remediation of radioactive soil in nuclear facility sites: a critical review

A huge amount of radioactive soil has been generated through decommissioning of nuclear facilities around the world. This review focuses on the difficulties and complexities associated with the remediation of radioactive soils at the site level; therefore, laboratory studies were excluded from this review. The problems faced while remediating radioactive soils using techniques based on strategies such as dry separation, soil washing, flotation separation, thermal desorption, electrokinetic remediation, and phytoremediation are discussed, along with appropriate examples. Various factors such as soil type, particle size, the fraction of fine particles, and radionuclide characteristics that strongly influence radioactive soil decontamination processes are highlighted. In this review, we also survey and compare the pool of available technologies currently being used for the remediation of radionuclide-contaminated soils, as well as the economic aspects of soil remediation using different techniques. This review demonstrates the importance of the integrated role of various factors in determining the effectiveness of the radioactive soil decontamination process.

Study on the Use of CTAB-Treated Illite as an Alternative Filler for Natural Rubber

Fillers are indispensable for rubber composites. Carbon black as an efficient reinforcing filler is most widely used in the rubber industry. However, the utilization of nonrenewable feedstock, energy consumption, and footprint for making carbon black lead to the seeking of alternative substitutes for carbon black, which is of great significance. Here in this work, the possibility of illite, a most common mineral in sedimentary rocks, as an alternative filler for natural rubber (NR) is determined. It is found that pristine illite slows the curing rate and decreases the cross-linking density of NR, which results in the inferior performance of NR. This is associated with the weak filler-rubber interaction, which is a vital factor in deciding the performance of rubber composites. Therefore, illite has been modified using hexadecyl trimethyl ammonium bromide (CTAB), a commonly used cation surfactant, for improving the filler-rubber interaction. The thus obtained C-illite is confirmed to be efficient for (i) enhancing the illite-NR interaction, (ii) improving the dispersion of illite in the NR matrix, and (iii) accelerating the curing process of NR with increased cross-linking density. All of these lead to significantly improved mechanical properties and wear resistance of the C-illite/NR composites, e.g., a 71.88% increase of the modulus at 300% strain compared to the pure NR and a 23.79% reduction of the DIN abrasion volume compared to the NR filled with 40 phr pristine illite. This illustrates the high possibility of CTAB-modified illite with an optimal particle size as a promising alternative filler of carbon black for reinforcing rubbers.

Key factors controlling radiocesium sorption and fixation in river sediments around the Fukushima Daiichi Nuclear Power Plant. Part 2: Sorption and fixation behaviors and their relationship to sediment properties

We systematically investigated the sorption and fixation behaviors of radiocesium (¹³⁷Cs) for sediments taken from the rivers of Ukedo and Odaka around the Fukushima Daiichi Nuclear Power Plant. By comparing the Cs sorption and sequential desorption results at various Cs concentrations, across a range of sediment properties, we were able to understand the different contributions at frayed edge sites (FESs) and regular exchange sites (RESs) of the clay minerals, and their relationships with the Cs concentrations and the contents of organic matter (OM). The Cs sorption and fixation were dominated by FESs at trace Cs concentrations, and by ion exchange at RES and the collapse of interlayers at higher Cs concentrations. The Cs sorption at lower Cs concentration was strongly related to radiocesium interception potential (RIP); however, Cs fixation was more related to clay mineralogy (i.e. contents of mica, vermiculite and hydroxy-interlayered vermiculite) rather than the RIP. The first-order kinetic constants for time-dependent Cs sorption at low Cs concentrations were correlated negatively to the ratio between the total organic carbon and RIP values. This implies that Cs access to FESs requires a relatively long duration that is dependent on the contents of the OM. From these results, the sorption and fixation mechanisms were confirmed to be significantly different at different Cs concentrations. Then, the prediction of Cs transport should be based on the key mechanisms that are dominant at the actual trace levels of Cs. A significant difference between the Cs fixation behaviors at the Ukedo River and Odaka River may be understood by considering the differences in their clay mineralogy, due to the different geological settings and weathering stages of both catchments.

Selective separation of Cs-contaminated clay from soil using polyethylenimine-coated magnetic nanoparticles

We evaluated the feasibility of using magnetic nanoparticles (MNPs) coated with polyethylenimine (PEI), a cationic polymer, to remediate radioactive contaminated soil by separating Cs-contaminated clay from the soil. The influences of the solution pH, PEI-to-MNPs mass ratio, and the PEI-MNPs dose on the magnetic separation performance were systematically examined. The highest SE% of illite from solution through electrostatic attraction was approximately 100% at a mass ratio of 0.04 g-PEI-MNPs/g-clay. The PEI coating clearly enhanced the adhesion between MNPs and clay minerals by increasing the quantity of functional amine groups available for adsorbing negatively charged clay minerals. In separation experiments using a soil mixture, the PEI-coated MNPs selectively separated clay- and silt-sized fine particles smaller than 0.038 mm even in the presence of a large amount of sand when used at a low dose (mass ratio of 0.05 g-PEI-MNPs/g-clay) and without pH control. We also used the PEI-MNPs to separate ¹³⁷Cs-contaminated illite from soil under an external magnetic field. After magnetic separation, the highest removal efficiency achieved for ¹³⁷Cs removal from the treated soil was 81.7% at a low nanoparticle dosage, which resulted in satisfying the reduction of radioactivity and waste volume. The results clearly demonstrate that the selective separation of Cs-contaminated clay using PEI-coated MNPs is a promising technique for remediating radioactive soil.

Radiocesium interaction with clay minerals: Theory and simulation advances Post-Fukushima

Insights at the microscopic level of the process of radiocesium adsorption and interaction with clay mineral particles have improved substantially over the past several years, triggered by pressing social issues such as management of huge amounts of waste soil accumulated after the Fukushima Dai-ichi nuclear power plant accident. In particular, computer-based molecular modeling supported by advanced hardware and algorithms has proven to be a powerful approach. Its application can now generally encompass the full complexity of clay particle adsorption sites from basal surfaces to interlayers with inserted water molecules, to edges including fresh and weathered frayed ones. On the other hand, its methodological schemes are now varied from traditional force-field molecular dynamics on large-scale realizations composed of many thousands of atoms including water molecules to first-principles methods on smaller models in rather exacting fashion. In this article, we overview new understanding enabled by simulations across methodological variations, focusing on recent insights that connect with experimental observations, namely: 1) the energy scale for cesium adsorption on the basal surface, 2) progress in understanding the structure of clay edges, which is difficult to probe experimentally, 3) cesium adsorption properties at hydrated interlayer sites, 4) the importance of the size relationship between the ionic radius of cesium and the interlayer distance at frayed edge sites, 5) the migration of cesium into deep interlayer sites, and 6) the effects of nuclear decay of radiocesium. Key experimental observations that motivate these simulation advances are also summarized. Furthermore, some directions toward future solutions of waste soil management are discussed based on the obtained microscopic insights.

Radionuclides in surface waters around the damaged Fukushima Daiichi NPP one month after the accident: Evidence of significant tritium release into the environment

Following the Fukushima nuclear accident (2011), radionuclides mostly of volatile elements (e.g., ¹³¹I, ^134,137Cs, ¹³²Te) have been investigated frequently for their presence in the atmosphere, pedosphere, biosphere, and the Pacific Ocean. Smaller releases of radionuclides with intermediate volatility, (e.g., ⁹⁰Sr), have been reported for soil. However, few reports have been published which targeted the contamination of surface (fresh) waters in Japan soon after the accident. In the present study, 10 surface water samples (collected on April 10, 2011) have been screened for their radionuclide content (³H, ⁹⁰Sr, ¹²⁹I, ¹³⁴Cs, and ¹³⁷Cs), revealing partly unusually high contamination levels. Especially high tritium levels (184 ± 2 Bq·L^-1; the highest levels ever reported in scientific literature after Fukushima) were found in a puddle water sample from close to the Fukushima Daiichi nuclear power plant. The ratios between paddy/puddle water from one location only a few meters apart vary around 1% for ¹³⁴Cs, 12% for ¹²⁹I (¹³¹I), and around 40% for both ³H and ⁹⁰Sr. This illustrates the adsorption of radiocesium on natural minerals and radioiodine on organic substances (in the rice paddy), whereas the concentration differences of ³H and ⁹⁰Sr between the two waters are mainly dilution driven.

An EXAFS study for characterizing the time-dependent adsorption of cesium on bentonite

Bentonite is considered for use as a buffer material in the final disposal repositories of radioactive waste. Long-lived 135Cs with a half-life of 2.3 × 106 years is a key radionuclide in high-level waste, and lots of 137Cs with a half-life of 30.2 years exists in low-level waste. Therefore, the adsorption of Cs on bentonite is a critical issue in evaluating the long-term safety of radioactive waste disposal. In this study, EXAFS techniques were used to characterize the time-dependent process from the beginning of adsorption to equilibrium. From the results of this study, we found changes including to the Cs adsorption sites, the Cs-O distance between Cs and the oxygen atom, and that the adsorption of Cs ions occurred before the reaction reached equilibrium. The fraction of OS complexes when Cs was adsorbed on bentonite can refer to the CN (Cs-O1st)/CN (Cs-O2nd) ratio of coordination numbers, and this study found that the OS complex was the major adsorption species when Cs adsorbed onto bentonite. In addition to the ratio CN (Cs-O1st)/CN (Cs-O2nd) providing information on the adsorption site, we also discussed the change of Cs-O1st and Cs-O2nd bond distances to identify the adsorption sites at different times. Comparing the XRD patterns of montmorillonite and bentonite, we found that the interlayer collapsed after Cs was adsorbed onto montmorillonite, but it expanded after Cs was adsorbed onto bentonite. From the results of EXAFS fitting, we found that the movement of Cs ions was from regular interlayer sites to expanded interlayer sites, which caused the interatomic distance of Cs-O2nd to decrease with an increase in time. It was revealed that the adsorption of Cs on bentonite occurred in two steps. The first step includes the rapid uptake of Cs by attachment to the oxygen atoms of the H2O molecules at the regular interlayer sites, especially for the OS complexes. The second step includes a slower process where dehydrated Cs ions move from the regular interlayer sites to the expanded interlayer sites. In this study, Cs L3-edge EXAFS spectroscopy was conducted for the Cs adsorbed on bentonite to identify the Cs adsorption sites over time, as this is important in evaluating the mobility of Cs in the environment. These results are beneficial in finding the process of Cs adsorption on bentonite, which could be used for the design of the final disposal of spent nuclear fuel.

Waste Brick Dust as Potential Sorbent of Lead and Cesium from Contaminated Water

Adsorption properties of waste brick dust (WBD) were studied by the removing of Pb^II and Cs^I from an aqueous system. For adsorption experiments, 0.1 M and 0.5 M aqueous solutions of Cs⁺ and Pb²⁺ and two WBD (Libochovice-LB, and Tyn nad Vltavou-TN) in the fraction below 125 µm were used. The structural and surface properties of WBD were characterized by X-ray diffraction (XRD) in combination with solid-state nuclear magnetic resonance (NMR), supplemented by scanning electron microscopy (SEM), specific surface area (S_BET), total pore volume and zero point of charge (pH_ZPC). LB was a more amorphous material showing a better adsorption condition than that of TN. The adsorption process indicated better results for Pb²⁺, due to the inner-sphere surface complexation in all Pb²⁺ systems, supported by the formation of insoluble Pb(OH)₂ precipitation on the sorbent surface. A weak adsorption of Cs⁺ on WBD corresponded to the non-Langmuir adsorption run followed by the outer-sphere surface complexation. The leachability of Pb²⁺ from saturated WBDs varied from 0.001% to 0.3%, while in the case of Cs⁺, 4% to 12% of the initial amount was leached. Both LB and TN met the standards for Pb^II adsorption, yet completely failed for any Cs^I removal from water systems.

Which types of clay minerals fix cesium ions effectively? the "cavity-charge matching effect"

How can radioactive Cs+ ions be removed from aqueous solution? From this perspective, the adsorption of Cs+ was investigated by using five types of clay minerals possessing different charge exchange capacities. The fixation ability for Cs+ depended on the charge exchange capacity of the clay minerals. Phlogopite and vermiculite, where the number of charges is almost equal to half the number of siloxane ditrigonal cavities in the structure, exhibited a strong Cs+ fixation ability among these clay minerals. In these clay minerals, effective interlayer collapse, which leads to quasi-irreversible adsorption of Cs+, is expected from the introduction of Cs+ into the layer space. This is named the "cavity-charge matching effect". This study clarifies why only phlogopite and vermiculite can fix Cs+ quite strongly among various types of clay minerals. These findings are beneficial for removing radioactive Cs+ ions from the environment using clay minerals through the cavity-charge matching effect.

Selective adsorption and irreversible fixation behavior of cesium onto 2:1 layered clay mineral: A mini review

In this study, we reviewed the selective adsorption and irreversible fixation of cesium (Cs⁺) on clay minerals. The selective adsorption of Cs⁺ results from reactions with frayed edge sites (FES) of clay minerals. The content of FES is about 0.1-2.0% of the total cation exchange capacity (CEC). The fractionation of Cs⁺ in actual accident sites mainly exists as a residue, which is important because it is closely related to the strong binding between Cs⁺ and soils. Cs⁺ adsorbed onto FES can move into the deeper interlayer via weathering processes, thereby Cs⁺ can be irreversibly fixed in the interlayer of non-expanding 2:1 layered clay mineral. Additionally, Cs⁺ can be adsorbed in the interlayer of the expanding clay mineral and can be fixed by interlayer collapse. For this reason, Cs⁺ adsorption onto FES is defined as 'selective adsorption' subsequent sorption in the interlayer as 'irreversible fixation'. Furthermore, the extended X-ray absorption fine structure (EXAFS) analysis can confirm that Cs⁺ bound to illite is coordinated with the outer surface (O_OS) and interlayer surface oxygens (O_IS) through FES or interlayer sites. Through these processes, Cs⁺ is adsorbed selectively onto FES, while Cs⁺ can subsequently move into the interlayer and become more strongly fixed.

The Impact of pH and Ion Exchange on 133Cs Adsorption on Vermiculite

Many studies have shown that the adsorption of ions like K and Cs on 2:1 clay minerals can prompt the collapse of their interlayers and render the adsorbing ions nonexchangeable. This study sought to better understand this unique adsorption mechanism through the generation of an adsorption envelope for Cs adsorption on vermiculite and the exploration of the kinetics of interlayer collapse. The collapse of the vermiculite interlayer was confirmed via X-ray diffraction (XRD), and the timing of interlayer collapse was determined by placing Cs in competition with K at different time intervals. The adsorption envelope for Cs on vermiculite showed that although H competition does affect the adsorption of Cs on vermiculite, the effect of this competition is quite limited, even at very low pH values. This hypothesis is supported by the fact that XRD demonstrated a significant decrease in interlayer dimension after Cs adsorption. Finally, kinetics experiments showed that the irreversible adsorption of K and the collapse of the interlayer may take place on a much longer time scale than previously considered.

Radiocesium interaction with clay minerals: Theory and simulation advances Post-Fukushima

Insights at the microscopic level of the process of radiocesium adsorption and interaction with clay mineral particles have improved substantially over the past several years, triggered by pressing social issues such as management of huge amounts of waste soil accumulated after the Fukushima Dai-ichi nuclear power plant accident. In particular, computer-based molecular modeling supported by advanced hardware and algorithms has proven to be a powerful approach. Its application can now generally encompass the full complexity of clay particle adsorption sites from basal surfaces to interlayers with inserted water molecules, to edges including fresh and weathered frayed ones. On the other hand, its methodological schemes are now varied from traditional force-field molecular dynamics on large-scale realizations composed of many thousands of atoms including water molecules to first-principles methods on smaller models in rather exacting fashion. In this article, we overview new understanding enabled by simulations across methodological variations, focusing on recent insights that connect with experimental observations, namely: 1) the energy scale for cesium adsorption on the basal surface, 2) progress in understanding the structure of clay edges, which is difficult to probe experimentally, 3) cesium adsorption properties at hydrated interlayer sites, 4) the importance of the size relationship between the ionic radius of cesium and the interlayer distance at frayed edge sites, 5) the migration of cesium into deep interlayer sites, and 6) the effects of nuclear decay of radiocesium. Key experimental observations that motivate these simulation advances are also summarized. Furthermore, some directions toward future solutions of waste soil management are discussed based on the obtained microscopic insights.

Fate of 137Cs, 90Sr and 239+240Pu in soil profiles at a water recharge site in Basel, Switzerland

An important process in the production of drinking water is the recharge of the withdrawn ground water with river water at protected recharge fields. While it is well known that undisturbed soils are efficiently filtering and adsorbing radionuclides, the goal of this study was to investigate their behaviour in an artificial recharge site that may receive rapid and additional input of radionuclides by river water (particularly when draining a catchment including nuclear power plants (NPP)). Soil profiles of recharge sites were drilled and analysed for radionuclides, specifically radiocesium (¹³⁷Cs), radiostrontium (⁹⁰Sr) and plutonium (^239+240Pu). The distribution of the analysed radionuclides were compared with an uncultivated reference soil outside the recharge site. The main activity of ¹³⁷Cs was located in the top soil (4.5-7.5 cm) and reached down to a depth of 84 cm and 48 cm for the recharge and the reference site, respectively. The found activities of ^239+240Pu originate from the global fallout after 1950. ^239+240Pu appeared to be strongly adsorbed onto soil particles. The shape of the depth profile was similar to ¹³⁷Cs, but also similar between the recharge and the reference site. In contrast, ⁹⁰Sr showed a uniform distribution over the entire depth of the recharge and reference profiles indicating that ⁹⁰Sr already entered the gravel zone and the ground water. Elevated inventories of the radionuclides were observed for the recharge site. The soil of the recharge field exhibited a threefold higher activity of ¹³⁷Cs compared to the reference soil. Also for ^239+240Pu higher inventories where observed for the recharge sites (40%). ⁹⁰Sr behaved differently, showing similar inventories between reference and recharge site. We estimate that 75-89% of the total inventory of ¹³⁷Cs in the soil at the recharge site (7.000 Bq/m²) originated from the fallout of the Chernobyl accident and from emissions of Swiss NPPs. This estimate is based on the actual activity ratio of ¹³⁷Cs/^239+240Pu of 22 for global fallout. The investigations identified radiostrontium as potential threat to the ground water.

Thallium Adsorption onto Illite

We investigated the adsorption of Tl⁺ onto purified Illite du Puy (IdP). Distribution coefficients (K_d) for trace Tl adsorption indicated a moderate pH-dependence from pH 2.5 to 11. Adsorption isotherms measured at Tl⁺ concentrations from 10^-9 to 10^-2 M at near-neutral pH on illite saturated with Na⁺ (100 mM), K⁺ (1 and 10 mM), NH₄⁺ (10 mM) or Ca²⁺ (5 mM) revealed a high adsorption affinity of Tl⁺ in Na⁺- and Ca²⁺-electrolytes and strong competition with K⁺ and NH₄⁺. Cation exchange selectivity coefficients for Tl⁺ with respect to Na⁺, K⁺, NH₄⁺, and Ca²⁺ were derived using a 3-site sorption model. They confirmed the strong adsorption of Tl⁺ at the frayed edges of illite, with Tl selectivity coefficients between those reported for Rb⁺ and Cs⁺. X-ray absorption spectra of Tl adsorbed onto Na-exchanged IdP indicated a shift from adsorption of (dehydrated) Tl⁺ at the frayed edges at low loadings to adsorption of (hydrated) Tl⁺ on planar sites at the highest loadings. Our results suggest that illite is an important adsorbent for Tl in soils and sediments, considering its often high abundance and its stability relative to other potential adsorbents and the selective nature of Tl⁺ uptake by illite.

Cesium sorption reversibility and kinetics on illite, montmorillonite, and kaolinite

Understanding sorption and desorption processes is essential to predicting the mobility of radionuclides in the environment. We investigate adsorption/desorption of cesium in both binary (Cs+one mineral) and ternary (Cs+two minerals) experiments to study component additivity and sorption reversibility over long time periods (500days). Binary Cs sorption experiments were performed with illite, montmorillonite, and kaolinite in a 5mM NaCl/0.7mM NaHCO3 solution (pH8) and Cs concentration range of 10^-3 to 10^-11M. The binary sorption experiments were followed by batch desorption experiments. The sorption behavior was modeled with the FIT4FD code and the results used to predict desorption behavior. Sorption to montmorillonite and kaolinite was linear over the entire concentration range but sorption to illite was non-linear, indicating the presence of multiple sorption sites. Based on the 14day batch desorption data, cesium sorption appeared irreversible at high surface loadings in the case of illite but reversible at all concentrations for montmorillonite and kaolinite. A novel experimental approach, using a dialysis membrane, was adopted in the ternary experiments, allowing investigation of the effect of a second mineral on Cs desorption from the original mineral. Cs was first sorbed to illite, montmorillonite or kaolinite, then a 3.5-5kDalton Float-A-Lyzer® dialysis bag with 0.3g of illite was introduced to each experiment inducing desorption. Nearly complete Cs desorption from kaolinite and montmorillonite was observed over the experiment, consistent with our equilibrium model, indicating complete Cs desorption from these minerals. Results from the long-term ternary experiments show significantly greater Cs desorption compared to the binary desorption experiments. Approximately ~45% of Cs desorbed from illite. However, our equilibrium model predicted ~65% desorption. Importantly, the data imply that in some cases, slow desorption kinetics rather than permanent fixation may play an important role in apparent irreversible Cs sorption.

Effects of NH4+, K+, Mg2+, and Ca2+ on the Cesium Adsorption/Desorption in Binding Sites of Vermiculitized Biotite

The reversibility of cesium adsorption in contaminated soil is largely dependent on its interaction with micaceous minerals, which may be greatly influenced by various cations. Herein, we systematically investigated the effects of NH₄⁺, K⁺, Mg²⁺, and Ca²⁺ on the adsorption/desorption of Cs⁺ into different binding sites of vermiculitized biotite (VB). Original VB was initially saturated by NH₄⁺, K⁺, or Mg²⁺; we then evaluated the adsorption of Cs⁺ on three treated VBs, and the desorption by extraction with NH₄⁺, K⁺, Mg²⁺, or Ca²⁺ was further evaluated. Our structural analysis and Cs⁺ extractability determinations showed that NH₄⁺ and K⁺ both collapsed the interlayers of VB, resulting in the dominant adsorption of Cs⁺ to external surface sites on which Cs⁺ was readily extracted by NH₄⁺, K⁺, Mg²⁺, or Ca²⁺ irrespective of their species, whereas Mg²⁺ maintained the VB with expanded interlayers, leading to the overwhelming adsorption of Cs⁺ in collapsed interlayer sites on which the Cs⁺ desorption was difficult and varied significantly by the cations used in extraction. The order of Cs⁺ extraction ability from the collapsed interlayers was K⁺ ≫ Mg²⁺ ≈ Ca²⁺ ≫ NH₄⁺. These results could provide important insights into Cs migration in soil and its decontamination for soil remediation.

Enhanced desorption of cesium from collapsed interlayer regions in vermiculite by hydrothermal treatment with divalent cations

Adsorption of cesium (Cs) on phyllosilicates has been intensively investigated because natural soils have strong ability of immobilizing Cs within clay minerals resulting in difficulty of decontamination. The objectives of present study are to clarify how Cs fixation on vermiculite is influenced by structure change caused by Cs sorption at different loading levels and how Cs desorption is affected by various replacing cations induced at different treating temperature. As a result, more than 80% of Cs was readily desorbed from vermiculite with loading amount of 2% saturated Cs (5.49×10^-3mmolg^-1) after four cycles of treatment of 0.01M Mg²⁺/Ca²⁺ at room temperature, but less than 20% of Cs was desorbed from saturated vermiculite. These distinct desorption patterns were attributed to inhibition of Cs desorption by interlayer collapse of vermiculite, especially at high Cs loadings. In contrast, elevated temperature significantly facilitated divalent cations to efficiently desorb Cs from collapsed regions. After five cycles of treatment at 250°C with 0.01M Mg²⁺, ∼100% removal of saturated Cs was achieved. X-ray diffraction analysis results suggested that Cs desorption was completed through enhanced diffusion of Mg²⁺ cations into collapsed interlayer space under hydrothermal condition resulting in subsequent interlayer decollapse and readily release of Cs⁺.

Direct Exchange Mechanism for Interlayer Ions in Non-Swelling Clays

The mobility of radiocesium in the environment is largely mediated by cation exchange in micaceous clays, in particular Illite-a non-swelling clay mineral that naturally contains interlayer K⁺ and has high affinity for Cs⁺. Although exchange of interlayer K⁺ for Cs⁺ is nearly thermodynamically nonselective, recent experiments show that direct, anhydrous Cs⁺-K⁺ exchange is kinetically viable and leads to the formation of phase-separated interlayers through a mechanism that remains unclear. Here, using classical atomistic simulations and density functional theory calculations, we identify a molecular-scale positive feedback mechanism in which exchange of the larger Cs⁺ for the smaller K⁺ significantly lowers the migration barrier of neighboring K⁺, allowing exchange to propagate rapidly once initiated at the clay edge. Barrier lowering upon slight increase in layer spacing (∼0.7 Å) during Cs⁺ exchange is an example of "chemical-mechanical coupling" that likely explains the observed sharp exchange fronts leading to interstratification. Interestingly, we find that these features are thermodynamically favored even in the absence of a heterogeneous layer charge distribution.

Desorption kinetics of cesium from Fukushima soils

Understanding the behaviors of Cs(+) in soils is crucial for evaluation of the impacts of disposal of soils contaminated by radiocesium, (137)Cs. The desorption rate of Cs(+) evaluated in relatively short periods of time may not be adequate for such a purpose. In this study, we investigated long-term desorption kinetics of (137)Cs and (133)Cs from soils collected in Fukushima Prefecture by batch desorption experiments in the presence of cation exchange resin as a sorbent. The sorbent can keep the concentration of Cs(+) in the aqueous phase low and prevent re-sorption of desorbed Cs(+). Up to 60% of (137)Cs was desorbed after 139 d in dilute KCl media, which was larger than the desorption by conventional short-term extraction with 1 M ammonium acetate. Desorption of (137)Cs continued even after this period. It was also found that high concentration of K(+) prevented desorption of Cs(+) in the initial stage of desorption, but the effect was alleviated with time. The desorbed fraction of stable Cs was smaller than that of (137)Cs. This indicated that (137)Cs may gradually move to more stable states in soils. The half-life of (137)Cs desorption from the slowest sorption site was estimated to be at least two years by a three-site desorption model.