Interaction of selenite with MX-80 bentonite: Effect of minor phases, pH, selenite loading, solution composition and compaction

Unlocking the bentonite microbial diversity and its implications in selenium bioreduction and biotransformation: Advances in deep geological repositories

Selenium, ⁷⁹Se, is one of the most critical radionuclides in radioactive waste disposed in future deep geological repositories (DGRs). Here, we investigate the impact of bentonite microbial communities on the allotropic transformation of Se(IV) bioreduction products under DGR relevant conditions. In addition, Se amendment-dependent shifts in the bentonite microbial populations are assessed. Microcosms of water-saturated bentonites were spiked with a bacterial consortium, treated with selenite and incubated anaerobically for six months. A combination of X-Ray Absorption Spectroscopy, Electron Microscopy, and Raman Spectroscopy was used to track the allotropic changes of the Se bioreduction products. Interestingly, the color of bentonite shifted from orange to black in the selenite-treated microcosms. In the orange layers, amorphous or monoclinic Se(0) were identified, whilst black precipitates consisted of stable trigonal Se(0) form. Illumina DNA sequencing indicated the distribution of strains with Se(IV) reducing and Se(0) allotropic biotransformation potential, like Pseudomonas, Stenotrophomonas, Desulfosporosinus, and unclassified-Desulfuromonadaceae. The archaea Methanosarcina decreased its abundance in the presence of Se(IV), probably caused by this oxyanion toxicity. These findings provide an understanding of the bentonite microbial strategies involved in the immobilization of Se(IV) by reduction processes, and prove their implication in the allotropic biotransformation from amorphous to trigonal Se(0) under DGR relevant conditions.

Investigation on the efficient separation and recovery of Se(IV) and Se(VI) from wastewater using Fe–OOH–bent

Decontamination of the toxic selenium compound, selenite (Se(IV)) and selenate (Se(VI)), from wastewater is imperative for environmental protection. Efficient approaches to remove Se(IV) and Se(VI) are in urgent needs. In this work, an accessible adsorbent Fe–OOH–bent was prepared and applied for the removal of Se(IV) and Se(VI) from wastewater. The batch experimental results demonstrate that Fe–OOH–bent exhibits high adsorption capacities of 5.01 × 10−4 and 2.28 × 10−4 mol/g for Se(IV) and Se(VI) respectively, which are higher than most of the reported bentonite based materials, especially in the case of Se(VI). Moreover, the Fe–OOH–bent displayed superior selectivity towards Se(IV) and Se(VI) even in the presence of excess competitive anions (Cl−, HCO3 −, NO3 −, SO4 2− and PO4 3−) and HA with concentrations of 1000 times higher than Se(IV) and Se(VI). By evaluating the adsorption ratio of Se(IV) and Se(VI), the reusability of Fe–OOH–bent was great through five adsorption-desorption cycles. For practical application, the column experiments were performed with simulated wastewater samples. The breakthrough and eluting curves of Se(IV) and Se(VI) were investigated through the columns packed with Fe–OOH–bent, and the results show that Se(IV) and Se(VI) can be successfully separated and recovered using 0.1 mol/L Na2SO4 (pH = 9.0) and 0.1 mol/L Na3PO4 (pH = 9.0), respectively. Our work provides a new approach for fractional separation as well as the recovery of Se(IV) and Se(VI) from wastewater.

Nickel Retention on Callovo-Oxfordian Clay: Applicability of Existing Adsorption Models for Dilute Systems to Real Compact Rock

In a waste management context, predicting the mobility of contaminants is essential. A key issue entails assessing the applicability of current knowledge on adsorption processes to natural systems. Such is the focus herein for nickel in interaction with Callovo-Oxfordian (COx) clay rock, a formation selected in France for possible radioactive waste disposal. The challenge is to link predictive modeling results with the experimental data characterizing the behavior of the labile and naturally occurring Ni fraction by implementing a new simple method. Retention studies on compact systems serve to complete this work. Combined electron microprobe and laser ablation high-resolution inductively coupled plasma mass spectrometry data show that natural Ni (∼39 mg kg^-1) is homogeneously distributed within the clay matrix, which corresponds to the main reservoir (∼70%). Data interpretation of desorption tests yields an in situ K_d value of ∼80 L kg^-1 and a labile Ni amount of ∼5 mg kg^-1, that is, ∼13% of the Ni inventory. Predictive modeling explains the sorption data in considering that only weak clay fraction sites take part in the adsorption. The role of the clay matrix in Ni retention is confirmed by analyzing the Ni-spiked compact COx samples, whereby an increase of the Ni content in the clay fraction is observed following the retention experiment.

An approach for the efficient immobilization of 79Se using Fe-OOH modified GMZ bentonite

Because of high mobility, the immobilization of long-lived fission product 79Se (often existed as 79Se(IV) and 79Se(VI) anions) is a critical consideration in the repository of high-level radioactive waste. In this work, a Fe-OOH modified bentonite (Fe-OOH-bent) was synthesized as a potential filling material in the repository site for effective adsorption and present the migration of different species of 79Se. The adsorbent was characterized using FT-IR, XRD, XFS, zeta potential and BET to clarify its physical properties, compositions and structures. A good thermal and radiation stabilities of Fe-OOH-bent was confirmed by its stable uptake ratio for Se(IV) and Se(VI) compared to original samples. The batch experimental results show that Se(IV) and Se(VI) can be efficiently removed from aqueous by Fe-OOH-bent within 60 min with maximum adsorption capacities of 68.45 mg/g for Se(IV) and 40.47 mg/g for Se(VI) in the optimal conditions, indicating its high potential application in consideration of its simple synthesis process, low cost and high adsorption capacity in view of immobilization of 79Se. The surface species and variation of oxide state of Fe as well as Se(IV) and Se(VI) onto Fe-OOH-bent were investigated by XPS analysis. The values of relative area of Se(IV)–O and Se(VI)–O in XPS spectra followed the same tendency as their adsorption ratio with the variation of system pH, suggesting that the formation of complexes between selenium species and Fe-OOH-bent surface.

Analysis of the improvement of selenite retention in smectite by adding alumina nanoparticles

Smectite clay is used as barrier for hazardous waste retention and confinement. It is a powerful material to retain cations, but less effective for retaining anionic species like selenite. This study shows that the addition of a small percentage of γ-Al₂O₃ nanoparticles to smectite significantly improves selenite sorption. γ-Al₂O₃ nanoparticles provide high surface area and positively charged surface sites within a wide range of pH, since their point of zero charge is at pH8-9. An addition of 20wt% of γ-Al₂O₃ to smectite is sufficient to approach the sorption capacity of pure alumina. To analyze the sorption behavior of the smectite/oxide mixtures, a nonelectrostatic surface complexation model was considered, accounting for the surface complexation of HSeO₃^- and SeO₃^2-, the anion competition, and the formation of surface ternary complexes with major cations present in the solution. Selenite sorption in mixtures was satisfactorily described with the surface parameters and complexation constants defined for the pure systems, accounting only for the mixture weight fractions. Sorption in mixtures was additive despite the particle heteroaggregation observed in previous stability studies carried out on smectite/γ-Al₂O₃ mixtures.

Bentonite modification with hexadecylpyridinium and aluminum polyoxy cations and its effectiveness in Se(IV) removal

Usage of bentonite as a buffer material is suggested in radioactive waste repositories. Although bentonites have higher sorption ability to cations, they cannot adsorp anions due to negative surface charge. Nowadays, ongoing researches focus on increasing anion adsorption ability of the bentonites with modification. Organic-pillared bentonite (OPBent) was produced by modification of sodium bentonite with aluminum polyoxy and hexadecylpyridinium cations in this study. Variation in structure after modification was demonstrated by using different characterization techniques. Se removal efficiency of OPBent is investigated by using (75)Se, since selenium (Se) is one of the important long lived fission products found in radioactive waste and has toxic anionic species in an aqueous environment. The effect of reaction time, solid/liquid ratio, pH and concentration on the adsorption performance were examined. Se speciation and its effect onto adsorption were also investigated by measuring Eh-pH values under certain experimental conditions. Additionally, importance of the amount of Al-polyoxy cations used in modification was investigated by comparing these results with the results of other organic-pillared bentonite produced in our previous research. Experimental results confirmed that both cations were successfully placed into the bentonite interlayer and significant change in the host structure leads to increase Se adsorption. Consequently, bentonite modification improves its Se adsorption ability and further investigations are needed related to the usage of this adsorbent in other remediation studies especially in sorption of other anionic pollutants.

Radionuclide behaviour in the near-field of a geological repository for spent nuclear fuel

Even though chemical processes related to the corrosion of spent nuclear fuel in a deep geological repository are of complex nature, knowledge on underlying mechanisms has very much improved over the last years. As a major result of numerous studies it turns out that alteration of irradiated fuel is significantly inhibited under the strongly reducing conditions induced by container corrosion and consecutive H2 production. In contrast to earlier results, radiolysis driven fuel corrosion and oxidative dissolution appears to be less relevant for most repository concepts. The protective hydrogen effect on corrosion of irradiated fuel has been evidenced in many experiments. Still, open questions remain related to the exact mechanism and the impact of potentially interfering naturally occurring groundwater trace components. Container corrosion products are known to offer considerable reactive surface area in addition to engineered buffer and backfill material. In combination, waste form, container corrosion products and backfill material represent strong barriers for radionuclide retention and retardation and thus attenuate radionuclide release from the repository near-field.

Comparative study of Se oxyanions retention on three argillaceous rocks: Upper Toarcian (Tournemire, France), Black Shales (Tournemire, France) and Opalinus Clay (Mont Terri, Switzerland)

A comparative study of selenium oxyanion sorption was carried out by means of batch sorption experiments on three argillaceous rocks that differ in their mineralogical compositions and textural properties. The results show no selenate (Se(VI)) sorption onto the argillaceous rocks after 60 days, but clear sorption of selenite (Se(IV)), the extent being closely related to the initial Se(IV) concentration. At the lowest concentration ([Se(IV)]eq < 10(-8) mol L(-1)), the ranking of rock affinity for Se(IV) is Black Shales > Opalinus Clay (OPA) > Upper Toarcian, with Rd values of 910 ± 70, 600 ± 65 and 470 ± 70 mL g(-1) respectively. The Se(IV) sorption isotherms acquired for the three argillaceous rocks can be reproduced well by means of Langmuir formalism, particularly with a two-site Langmuir model. The comparison of the Se(IV) sorption isotherms obtained for these three rocks led to identification of pyrite associated with natural organic matter (NOM) as one of the main phases involved in selenium retention. While the desorption results suggested a significant Se(IV) reduction in the Upper Toarcian samples, the reversible sorption shown on the Black Shales and OPA samples was correlated with a sulfate increase, symptomatic of surface oxidation of pyrite which could limit the Se(IV) reduction in favor of sorption.

Temperature effect on U(VI) sorption onto Na-bentonite

U(VI) sorption on a purified Na-bentonite was investigated from 298±2 to 353±2 K by using a batch experimental method as a function of pH, U(VI) concentration, carbonate concentration and solid-to-liquid ratio (m/V). The data at 298±2 K could be well described by a surface complexation model (SCM) with a complex located on layer sites (X2UO2) and three complexes located on edge sites (≡SOUO2 +, ≡SO(UO2)3(OH)5, and ≡SO(UO2)3(OH)7 2−). The intrinsic equilibrium constants (K int) of the surface reactions at 333 ± 2 and 353±2 K were obtained by fitting U(VI) sorption curves vs. pH on the Na-bentonite. The model enables U(VI) sorption in the presence of carbonate (p CO2 =10−3.58 atm) to be described without considering any ternary surface complexes involving carbonate, except for underestimation around pH 7 (6<pH<7.5). The standard enthalpy changes (Δr H m θ) of the surface reactions were evaluated from the K int values obtained at three temperatures (298±2, 333±2 and 353±2 K) via the van´t Hoff equation. The proposed SCM and Δr H m θ of the surface reactions enable U(VI) sorption on the Na-bentonite at other temperatures to be predicted.

Experimental study and modelling of selenite sorption onto illite and smectite clays

This study provides a large set of experimental selenite sorption data for pure smectite and illite. Similar sorption behavior existed in both clays: linear within a large range of the Se concentrations investigated (from 1x10(-10) to 1x10(-3) M); and independent of ionic strength. Selenite sorption was also analysed in the illite/smectite system with the clays mixed in two different proportions, as follows: (a) 30% illite-70% smectite and (b) 43% illite-57% smectite. The objective of the study was to provide the simplest model possible to fit the experimental data, a model also capable of describing selenite sorption in binary illite/smectite clay systems. Selenite sorption data, separately obtained in the single mineral systems, were modeled using both a one- and a two-site non-electrostatic model that took into account the formation of two complexes at the edge sites of the clay. Although the use of a two-site model slightly improved the fit of data at a pH below 4, the simpler one-site model reproduced satisfactorily all the sorption data from pH 3 to 8. The complexation constants obtained by fitting sorption data of the individual minerals were incorporated into a model to predict the adsorption of selenium in the illite/smectite mixtures; the model's predictions were consistent with the experimental adsorption data.