Analysis of Cadmium Retention Mechanisms by a Smectite Clay in the Presence of Carbonates.

Cadmium (Cd) is a toxic heavy metal with very low permissible exposure limits and is, thus, a very dangerous pollutant for the environment and public health and is considered by the World Health Organisation as one of the ten chemicals of major public concern. Adsorption onto solid phases and (co)precipitation processes are the most powerful mechanisms to retain pollutants and limit their migration; thus, the understanding of these processes is fundamental for assessing the risks of their presence in the environment. In this study, the immobilisation of Cd by smectite clay has been investigated by batch sorption tests, and the experimental data were interpreted with a thermodynamic model, including cation exchange and surface complexation processes. The model can describe the adsorption of Cd in smectite under a wide range of experimental conditions (pH, ionic strength, and Cd concentration). Under the conditions analysed in this study, the precipitation of otavite (CdCO3) is shown to have a limited contribution to Cd immobilisation.

Evaluation of component additive modelling approach for europium adsorption on 2:1 clays: Experimental, thermodynamic databases, and models.

This study evaluates the component additive approach for Eu adsorption on mixtures of smectite and illite, which are the most common clays used as barriers for contaminant retention in waste repositories. A thorough set of Eu adsorption data for Na-exchanged smectite and illite that encompasses a wide range of pH values, ionic strengths, and Eu concentrations was provided. This database is likely one of the largest sorption databases available for Eu in 2:1 clays, making it appropriate for sorption model calibration. The main adsorption mechanisms considered were surface complexation, on weak and strong clay edge sites, and cation exchange. Further, the role of principal ions, which are naturally leached from clays, as competitive factors for Eu retention, was evaluated in the modelling calculations. The main uncertainties related to the modelling procedures and the use of different thermodynamic data on sorption modelling were outlined. The reactions and parameters successful in modelling Eu adsorption on individual clays were used without any modification to model Eu adsorption on illite/smectite mixtures, wherein only the relative mineral proportions were considered. The fit of the sorption data in the mixed clay system was satisfactory, indicating that, in 2:1 clays, Eu sorption is an additive process, which stresses the predictive capacity of the component additive approach in these systems. This is an important support for assessing the performance of barrier materials for contaminant migration under different geochemical conditions.

Selenite Retention and Cation Coadsorption Effects under Alkaline Conditions Generated by Cementitious Materials: The Case of C-S-H Phases.

Contaminant migration is strongly controlled by sorption reactions; thus, the behavior of anions, which are (almost) not sorbing under alkaline conditions, is an issue of environmental concern. This is especially relevant in the frame of low and intermediate-low radioactive waste repositories, where the pH generated by cement-based materials is hyperalkaline. Selenite (SeO3 2-) sorption on calcium silicate hydrate (C-S-H) phases-the main cement sorbing minerals-has been investigated by batch experiments, ζ-potential measurements, and thermodynamic modeling to elucidate retention mechanisms and possible competitive/synergetic effects of cation coadsorption. Selenite sorption was shown to be nonlinear and slightly increasing with the C-S-H Ca/Si ratio; precipitation of CaSeO3(s) was observed for Se concentration higher than 2 × 10-3 M. Indeed, the presence of Ca is essential to enable selenite retention under alkaline conditions. Progressive additions of Na2SeO3 or NaCl salt to the phases produced a change in the C-S-H surface properties, that is, a decrease in the ζ-potential, in apparent agreement with anion adsorption. However, this effect had to be also correlated to Na coadsorption, as Cl showed null retention on the C-S-H phases. At the same time, anion adsorption had a clear effect on the retention of other cations (Ba) in the system. The distribution coefficient of Ba (at trace concentrations) suffered a moderate decrease by the presence of Na+ and Cl-, but it was improved by the presence of Na+ and SeO3 2-, indicating complex competitive/synergetic effects between anions and cations. All of the experimental data were satisfactorily modeled considering a classical double-layer approach.

Selenium(IV) Sorption Onto γ-Al2O3: A Consistent Description of the Surface Speciation by Spectroscopy and Thermodynamic Modeling.

The sorption processes of Se(IV) onto γ-Al2O3 were studied by in situ Infrared spectroscopy, batch sorption studies, zeta potential measurements and surface complexation modeling (SCM) in the pH range from 5 to 10. In situ attenuated total reflection fourier-transform infrared (ATR FT-IR) spectroscopy revealed the predominant formation of a single inner-sphere surface species at the alumina surface, supporting previously reported EXAFS results, irrespective of the presence or absence of atmospherically derived carbonate. The adsorption of Se(IV) decreased with increasing pH, and no impact of the ionic strength was observed in the range from 0.01 to 0.1 mol L-1 NaCl. Inner-sphere surface complexation was also suggested from the shift of the isoelectric point of γ-Al2O3 observed during zeta potential measurements when Se(IV) concentration was 10-4 mol L-1. Based on these qualitative findings, the acid-base surface properties of γ-Al2O3 and the Se(IV) adsorption edges were successfully described using a 1-pK CD-MUSIC model, considering one bidentate surface complex based on previous EXAFS results. The results of competitive sorption experiments suggested that the surface affinity of Se(IV) toward γ-Al2O3 is higher than that of dissolved inorganic carbon (DIC). Nevertheless, from the in situ experiments, we suggest that the presence of DIC might transiently impact the migration of Se(IV) by reducing the number of available sorption sites on mineral surfaces. Consequently, this should be taken into account in predicting the environmental fate of Se(IV).

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 SeO32-, 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.

Interlayer collapse affects on cesium adsorption onto illite.

Cesium adsorption onto Illite has been widely studied, because this clay is especially relevant for Cs migration-retention in the environment. The objective of this study is to analyze how Cs adsorption onto Illite is affected by structural changes produced by the presence of different exchangeable cations--and specifically interlayer collapse. Cs sorption isotherms were carried out with Illite previously exchanged with Na, K, or Ca, at a broad enough range of ionic strength, for the determination of the possible affect of the electrolyte on the structure of Illite. In the presence of Ca, the maximum sorbed Cs was unexpectedly high (900 mequiv · kg(-1)) given the cationic exchange capacity commonly accepted for Illite (near 200 mequiv · kg(-1)). This was explained by the expansion of Illite layers (decollapse) induced by large hydrated cations such as Ca(2+) that may facilitate cation uptake--especially Cs(+), which is a highly selective cation. In the presence of Ca (and most probably of other divalent cations), Cs accessibility to exchange positions is increased. Both experimental evidence and the modeling of Cs sorption onto Illite supported the hypothesis of decollapse. Our results demonstrate the requirement of accounting for Illite decollapse especially for high Cs loadings, because of the potential prediction errors for its migration. Ignoring the Illite decollapse could lead the biased estimation of selectivity coefficients and consequently the erroneous prediction of sorption/migration behavior of Cs, and possibly other contaminants, in the environment.

Predictions of TiO2-driven migration of Se(IV) based on an integrated study of TiO2 colloid stability and Se(IV) surface adsorption.

Colloids (ex. nanoparticles) may be relevant in contaminant migration in groundwater if the contaminant is strongly adsorbed onto colloids and they are stable in the environment. An integrated study of Se(IV) sorption and TiO2 colloid stability was performed in this work. This integrated study includes the experimental analysis and modelling of Se(IV) sorption onto TiO2 particle surface and the evaluation of the influence of adsorbed Se(IV) on TiO2 stability. Results indicated that low ionic strength and acid pH favour both high Se(IV) sorption and TiO2 colloid stability. However, at medium-high surface occupancy, sorbed Se(IV) highly affected TiO2 colloid stability. Anion adsorption decreases the positive surface charge shifting the TiO2 isoelectric point, thus promoting particle aggregation under acid pH. Nevertheless, the application of the single particle counting technique allowed determination of the presence of a significant fraction of TiO2 nanoparticles which are highly susceptible to migrate even under favourable conditions for aggregation. Therefore, although part of the TiO2 is disabled for migration by sorbed Se at acid conditions, still a considerable percentage that remains disaggregated could move together with Se. The developed Se(IV) sorption model can inform about the extent and type of sorption under specific geochemical conditions.

Strontium migration in a crystalline medium: effects of the presence of bentonite colloids.

The effects of bentonite colloids on strontium migration in fractured crystalline medium were investigated. We analyzed first the transport behaviour of bentonite colloids alone at different flow rates; then we compared the transport behaviour of strontium as solute and of strontium previously adsorbed onto stable bentonite colloids at a water velocity of approximately 7.1·10(-6)m/s-224m/yr. Experiments with bentonite colloids alone showed that - at the lowest water flow rate used in our experiments (7.1·10(-6)m/s) - approximately 70% of the initially injected colloids were retained in the fracture. Nevertheless, the mobile colloidal fraction, moved through the fracture without retardation, at any flow rate. Bentonite colloids deposited over the fracture surface were identified during post-mortem analyses. The breakthrough curve of strontium as a solute, presented a retardation factor, R(f)~6, in agreement with its sorption onto the granite fracture surface. The breakthrough curve of strontium in the presence of bentonite colloids was much more complex, suggesting additional contributions of colloids to strontium transport. A very small fraction of strontium adsorbed on mobile colloids moved un-retarded (R(f)=1) and this fraction was much lower than the expected, considering the quantity of strontium initially adsorbed onto colloids (90%). This behaviour suggests the hypothesis of strontium sorption reversibility from colloids. On the other hand, bentonite colloids retained within the granite fracture played a major role, contributing to a slower strontium transport in comparison with strontium as a solute. This was shown by a clear peak in the breakthrough curve corresponding to a retardation factor of approximately 20.

Surface reactions kinetics between nanocrystalline magnetite and uranyl.

Magnetite is the most important end member of iron corrosion products under reducing environment, which is the condition expected in a deep geological high level radioactive waste disposal. Nanocrystalline magnetite was synthesized in the laboratory and its physicochemical properties were analyzed in detail. The kinetics of the adsorption of U(VI) and the kinetics of the actinide reduction to a lower oxidation state, in presence of the oxide, were studied by means of batch sorption techniques and X-ray photoelectron spectroscopy (XPS) analysis. The results showed that the uranium sorption and reduction processes on the magnetite surface have very fast kinetics (hours), the reduction process being triggered by sorption. XPS measurements showed that the speciation of uranium at the surface does not show significant changes with time (from 1 day to 3 months), as well as the quantity of uranium detected at the surface. The surface speciation depended on the initial pH of the contact solution. Considering that the Eh of equilibrium between magnetite and the solution, under our experimental conditions, is slightly positive (50-100 mV), the uranium reduction would also be thermodynamically possible within the liquid phase. However, the kinetics of reduction in the liquid occur at a much slower rate which, in turn, has to depend on the attainment of the magnetite/solution equilibrium. The decrease of uranium in solution, observed after the uranyl adsorption stage, and particularly at acidic pH, is most probably due to the precipitation of U(IV) formed in the solution.

Experimental and modeling study of the uranium (VI) sorption on goethite.

Acicular goethite was synthesized in the laboratory and its main physicochemical properties (composition, microstructure, surface area, and surface charge) were analyzed as a previous step to sorption experiments. The stability of the oxide, under the conditions used in sorption studies, was also investigated. The sorption of U(VI) onto goethite was studied under O(2)- and CO(2)-free atmosphere and in a wide range of experimental conditions (pH, ionic strength, radionuclide, and solid concentration), in order to assess the validity of different surface complexation models available for the interpretation of sorption data. Three different models were used to fit the experimental data. The first two models were based on the diffuse double layer concept. The first one (Model 1) considered two different monodentate complexes with the goethite surface and the second (Model 2) a single binuclear bidentate complex. A nonelectrostatic (NE) approach was used as a third model and, in that case, the same species considered in Model 1 were used. The results showed that all the models are able to describe the sorption behavior fairly well as a function of pH, electrolyte concentration, and U(VI) concentration. However, Model 2 fails in the description of the uranium sorption behavior as a function of the sorbent concentration. This demonstrates the importance of checking the validity of any surface complexation model under the widest possible range of experimental conditions.

Generation and stability of bentonite colloids at the bentonite/granite interface of a deep geological radioactive waste repository.

The possible mechanisms of colloid generation at the near field/far field interface of a radioactive repository have been investigated by means of novel column experiments simulating the granite/bentonite boundary, both in dynamic and in quasi-static water flow conditions. It has been shown that solid particles and colloids can be detached from the bulk and mobilised by the water flow. The higher the flow rate, the higher the concentration of particles found in the water, according to an erosion process. However, the gel formation and the intrinsic tactoid structure of the clay play an important role in the submicron particle generation even in the compacted clay and in a confined system. In fact, once a bentonite gel is formed, in the regions where the clay is contacted with water, clay colloids can be formed even in quasi-static flow conditions. The potential relevance of these colloids in radionuclide transport has been studied by evaluating their stability in different chemical environments. The coagulation kinetics of natural bentonite colloids was experimentally studied as a function of the ionic strength and pH, by means of time-resolved light scattering techniques. It has been shown that these colloids are very stable in low saline (approximately 1 x 10(-3) M) and alkaline (pH > or = 8) waters.

Simultaneous estimation of effective and apparent diffusion coefficients in compacted bentonite.

Effective diffusion coefficients (D(e)) are usually measured by means of "through-diffusion" experiments in which steady state is reached, and the "time-lag" methods are used to estimate the apparent diffusion coefficient (D(a)). For sorbing radionuclides (as caesium), the time needed to reach steady-state conditions is very large, and the precision in D(a) determinations is not satisfactory. In this paper, a method that allows determining simultaneously effective and apparent diffusion coefficients in compacted bentonite without reaching steady-state conditions is described. Basically, this method consists of an "in-diffusion" experiment in which the concentration profile in the bentonite sample is used to estimate D(a), and the temporal evolution of the solute concentration in the reservoir is used to estimate D(e). This method has several advantages over the typical "through-diffusion" experiments, in particular: (a) experiment duration is significantly shorter, (b) D(a) values are measured with greater precision and (c) it is not necessary to maintain a constant solute concentration in the reservoir. This new method has been used to estimate the effective and apparent diffusion coefficients for caesium in FEBEX bentonite and in order to validate it, the results have been compared with results previously obtained with standard methods. Experimental results have been satisfactorily modelled using a simple model of diffusion in porewater and the measured value of D(e)(Cs) is very similar to D(e)(HTO) in the same bentonite. There is no evidence of "surface diffusion" in FEBEX bentonite for caesium.

Study of the contaminant transport into granite microfractures using nuclear ion beam techniques.

Hydrated bentonite is a very plastic material and it is expected to enter in the rock microfractures at the granite/bentonite boundary of a deep geological high-level waste repository. This process is enhanced by the high swelling pressure of the clay. Since bentonite has a very good sorption capability for many radionuclides, the displacement of the clay might lead to a "clay-mediated" contaminant transport into the rock. The aim of this work is to study the contaminant transport into granite microfractures using nuclear ion beam techniques, and to determine to what extent the clay can favour it. To do so, bentonite previously doped with uranium, cesium and europium was put in contact with the surface of granite sheets. Granite sheets contacted with non-doped bentonite and with radionuclide solutions were also prepared as references. This allowed analysing the differences in the diffusion behaviour of the three systems: clay, radionuclides and clay plus radionuclides. A combination of Rutherford backscattering spectrometry (RBS) and other nuclear ion-beam techniques such as particle-induced X-ray emission (PIXE) and microPIXE was used to study the depth and lateral distribution of clay and contaminants inside granite. It was also tried to evaluate not only the diffusion depth and diffusion coefficients but also the different areas of the granite where the diffusants have a preferential access.