Non-electrostatic Surface Complexation Model Research Articles

Sorption of U(VI) on MX-80 bentonite, illite, shale and limestone in Na–Ca–Cl saline solutions

Uranium (U) has been identified as an element of interest for the safety assessment of a deep geological repository for used nuclear fuel. In this study, the sorption of U(VI) was studied in Na–Ca–Cl solutions at ionic strengths = 0.1–6 mol/kgw (m) in a CO2 free environment at pHm (molal H+ concentration where pHm = −log mH+) = 4–9 for MX-80 bentonite, illite and Queenston shale and at pHm = 5–9 for limestone. U(VI) sorption on MX-80 bentonite increased with pHm from pHm = 4 to 6, then decreased with pHm until pHm = 7, and then increased again with pHm to pHm = 9. U(VI) sorption on illite increased with pHm reaching a maximum at pHm = 7, and then decreased with further increases in pHm. The sorption behavior of U(VI) on shale was similar to that of illite, but the extent of decrease in the sorption coefficient (Rd) value with pHm was slightly more pronounced for the shale than observed for sorption on illite at pHm > 7. U(VI) sorption on limestone increased with pHm up to pHm = 8 and then seemed to be constant at pHm = 8–9. U(VI) sorption on all four solids was independent of ionic strength (0.1–6.0 m). The 2 site protolysis non-electrostatic surface complexation and cation exchange model successfully simulated the sorption of U(VI) onto MX-80 and illite, and the optimized values of surface complexation constants were estimated.

The adsorption of U(VI) on chlorite: batch, modeling and XPS study

Abstract A mechanistic modelling of the adsorption processes onto individual minerals presenting in the near- and far-fields can greatly enhance the credibility of long-term safety assessments of granite-based geological repositories. In this study, the titration and U(VI) adsorption characteristics of chlorite, one of the major minerals of rock fractures, have been studied. Potentiometric titration curves at two ionic strengths (0.1 and 0.4 mol/L NaCl) are successfully interpreted by considering protonation/deprotonation reactions on generic edge sites (≡SOH) in the framework of a non-electrostatic surface complexation model (SCM). The adsorption of U(VI) on chlorite was reached after 24 h, the adsorption kinetics can be described by a pseudo-second-order model. A non-electrostatic SCM with three surface complexes (≡SOUO2 +, ≡SO(UO2)3(OH)5 and ≡SO(UO2)3(OH)7 2−) was set up based on pH edges of U(VI) at adsorption equilibrium in the absence of CO2. Additional, experimental data measured as a function of U(VI) concentration, solid-to-liquid ratio and carbonate concentration were well reproduced by the proposed model. Finally, parallel experiments were conducted using X-ray photoelectron spectroscopy (XPS) to analyze the variation of U(VI) surface species speciation at different pH values. The good agreement between SCM prediction and XPS analysis demonstrates the reliability of the model in predicting and quantifying the radionuclides retention by chlorite.

Sorption of U(VI) on MX-80 Bentonite and Granite in Ca-Na-Cl Saline Solutions

Uranium has been identified as an element of interest for the safety assessment of a deep geological repository for used nuclear fuel. This paper examines the sorption behavior of U(VI) onto MX-80 bentonite and granite in Ca-Na-Cl solutions of varying ionic strengths [0.05 to 3 mol/kgw (m)] and across a pH range of 4 to 10. U(VI) sorption on MX-80 showed that U(VI) sorption gradually increased with pHm until pHm = 6, where it reached its maximum, and decreased slightly with pHm until pHm = 8, and then became constant. U(VI) sorption on granite increased along with pHm, reached the maximum around pHm = 7 to 8, and then slightly decreased with pHm. Both MX-80 and granite showed essentially no ionic strength dependence for sorption of U(VI). A nonelectrostatic surface complexation model successfully predicted sorption of U(VI) onto MX-80 and granite using the formation of an inner-sphere surface complex. Optimized values of surface complexation reaction constants (log K0) for the formation reactions of these surface species are proposed.

Sn(IV) Sorption onto Illite and Boom Clay: Effect of Carbonate and Dissolved Organic Matter

126Sn is a long-lived fission product and it is important to assess its sorption onto the host rocks surrounding a possible nuclear waste repository. Boom Clay (BC) is under investigation in Belgium as a potential host rock. To better understand Sn(IV) sorption onto the clay minerals constituting BC, sorption of Sn(IV) was here investigated on Illite du Puy (IdP), from pH 3 to 12. Sorption isotherms at pH ~8.4 were acquired in the presence and absence of carbonate, and in the presence and absence of BC dissolved organic matter (DOM). Sn(IV) strongly sorbed on IdP over the full range of the pHs and concentrations investigated. In the presence of carbonates, Sn(IV) sorption was slightly decreased, highlighting the Sn(IV)–carbonate complexation. DOM reduced the Sn(IV) sorption, confirming the strong complexation of Sn(IV) with DOM. The results were modelled with the 2-site protolysis non-electrostatic surface complexation model. The surface complexation constants and aqueous complexation constants with carbonate and DOM were optimized to describe the experimental data. The applicability of the component additivity approach (CAA) was also tested to describe the experimental Sn(IV) sorption isotherm acquired on BC in BC pore water. The CAA did not allow accurate prediction of Sn(IV) sorption on BC, highlighting the high sensitivity of the model to the Sn(IV)-DOM complexation.

Reduction of U(VI) on Chemically Reduced Montmorillonite and Surface Complexation Modeling of Adsorbed U(IV).

Adsorption and subsequent reduction of U(VI) on Fe(II)-bearing clay minerals can control the mobility of uranium in subsurface environments. Clays such as montmorillonite provide substantial amounts of the reactive surface area in many subsurface environments, and montmorillonite-containing materials are used in the storage of spent nuclear fuel. We investigated the extent of reduction of U(VI) by Fe(II)-bearing montmorillonite at different pH values and sodium concentrations using X-ray absorption spectroscopy and chemical extractions. Nearly complete reduction of U(VI) to U(IV) occurred at a low sodium concentration at both pH 3 and 6. At pH 6 and a high sodium concentration, which inhibits U(VI) binding at cation-exchange sites, the extent of U(VI) reduction was only 70%. Surface-bound U(VI) on unreduced montmorillonite was more easily extracted into solution with bicarbonate than surface-bound U(IV) generated by reduction of U(VI) on Fe(II)-bearing montmorillonite. We developed a nonelectrostatic surface complexation model to interpret the equilibrium adsorption of U(IV) on Fe(II)-bearing montmorillonite as a function of pH and sodium concentration. These findings establish the potential importance of structural Fe(II) in low iron content smectites in controlling uranium mobility in subsurface environments.

Mg-Fe LDH-coated biochars for metal(loid) removal: Surface complexation modeling and structural change investigations

Although an increasing number of studies involving layered double hydroxide/biochar composites (LDH/BC) have been published recently, development of advanced modeling approaches of adsorption onto the composites and/or their testing and use in real soils and waters are rare. In this study, Zn(II) and As(V) sorption onto Mg-Fe LDH/BC composites was quantified using bulk sorption experiments, and the observed adsorption behavior was modeled using a non-electrostatic surface complexation model (NEM). In general, it was found that LDH is the main phase causing the removal of metal(loid)s from solution. In contrast, the contribution of BC was evident only in systems containing the most effective type of BC. Inner-sphere surface complexation was found to be the dominant adsorption mechanism at low Zn(II) and As(V) concentrations. In contrast, precipitation of Zn and formation of amorphous ferric arsenates contributed to the total metal removal at higher metal(loid) loadings. Surprisingly, increasing As(V) concentrations led to enhanced Mg leaching from the LDH/BC composites and to the destruction of the LDH structure. Nevertheless, all composite materials were stable at environmentally relevant metal(loid) concentrations. Our study is the first to use a surface complexation component additivity (CA) modeling approach to account for metal(loid) adsorption behavior onto LDH/BC composites, and we find that although site blocking does occur to some extent, the CA approach provides a reasonable estimate of the adsorption behavior across a wide range of pH conditions. Comprehensive studies such as this one are necessary in order to understand and optimize the behavior of novel composite materials before their subsequent use in environmental technologies.

Uranium (VI) sorption on illite under varying carbonate concentrations: Batch experiments, modeling, and cryogenic time-resolved laser fluorescence spectroscopy study

Dissolved inorganic carbon (DIC) present in groundwaters can affect both the aqueous and surface species of hexavalent uranium (U(VI)) owing to its strong complexation ability with U(VI). However, the effect of DIC on U(VI) sorption on illite, which is a critical component of argillaceous rocks, is still unclear. In this study, we investigated the sorption of U(VI) on conditioned illite du Puy with varying DIC concentrations (up to 250 mM DIC) as a function of pH through batch sorption experiments, surface complexation modeling, and cryogenic time-resolved laser fluorescence spectroscopy (TRLFS). The distribution coefficients of U(VI) decreased with an increase in the DIC concentration. At relatively high DIC concentrations with respect to experimentally investigated range (≥100 mM DIC), no U(VI) sorption was observed. The U(VI) sorption on illite was modeled using the two-site protolysis non-electrostatic surface complexation and cation exchange model. Two ternary surface complexation reactions with carbonate were needed to depict the experimental sorption data in addition to binary and ternary hydroxo surface complexation reactions used to describe the U(VI) sorption to illite without carbonate. Cryogenic TRLFS data revealed that U(VI) did sorb to illite at high DIC concentrations (up to 10 mM DIC). The spectra were unchanged with varying DIC concentrations (2.0, 5.0, and 10 mM DIC) at pH ∼8.5, indicating that the surface speciation of U(VI) remained the same. The decay curves were biexponential, which further indicates that at least two surface species were responsible for the sorption. These results could improve the performance assessment of a deep geological disposal system in a sedimentary host rock.

Sorption of Se(-II) on illite, MX-80 bentonite, shale, and limestone in Na–Ca–Cl solutions

Selenium has been identified as an element of interest for the safety assessment of a deep geological repository (DGR) for used nuclear fuel. In Canada, groundwaters at DGR depths in sedimentary rocks have been observed to have a high ionic strength. This paper examines the sorption behavior of Se(-II) onto illite, MX-80 bentonite, Queenston shale, and argillaceous limestone in Na–Ca–Cl solutions of varying ionic strength (0.1–6 mol/kgw (m)) and across a pH range of 4–9. Little ionic strength dependence for Se(-II) sorption onto all solids was observed except that sorption at high ionic strength (6 m) was generally slightly lower than sorption at low ionic strength (0.1 m). Illite and MX-80 exhibited the expected results for anion sorption, while shale and limestone exhibited more constant sorption across the pH range tested. A non-electrostatic surface complexation model successfully predicted sorption of Se(-II) onto illite and MX-80 using the formation of an inner-sphere surface complex and an outer-sphere surface complex. Optimized values for the formation reactions of these surface species were proposed.

Zn uptake by illite and argillaceous rocks

The uptake of Zn by Illite du Puy (IdP) and two argillaceous rocks, Opalinus Clay (OPA) and Boda Claystone (BODA) was investigated. The uptake of Zn by illite was studied in 0.1 M NaCl at near-neutral pH and Zn loadings varying from 2.1 to 42 mmol/kg. The Zn uptake by the two argillaceous rock systems was carried out in 0.1 M NaCl at pH 7.2 and in their respective porewaters at pH 8.0 to evaluate the influence of porewater composition. The Zn loadings varied from 1.8 to 86 mmol/kg and 1.7 to 60 mmol/kg for OPA and BODA, respectively.The simplified “bottom-up approach” to predict the uptake of Zn to natural rocks with different clay mineral contents and porewater compositions was tested. This approach was found to be applicable in the argillaceous rock systems at low Zn equilibrium ([Zn]eql) concentrations below 10−7 M. However, at [Zn]eql above 10−7 M, the model calculations clearly underpredicted the sorption data.The extended X-ray absorption fine structure (EXAFS) results support findings based on wet chemistry calculations using the 2 site protolysis non-electrostatic surface complexation and cation exchange (2SPNE SC/CE) sorption model. In the IdP system with prolonged reaction times of up to two years, formation of Zn precipitates was not observed. This indicates that the Zn surface complexes are stable over a time period of two years, and dissolution and recrystallization processes might not play a significant role in the Zn-IdP uptake process. In addition, the EXAFS results on illite corroborate previous findings on the existence of weak and strong sites types in dioctahedral clay minerals, as assumed in the 2SPNE SC/CE model. In the OPA and BODA systems, precipitation processes could start to begin at metal loadings of 16 mmol/kg and 24 mmol/kg, respectively. The differences between argillaceous rock samples prepared in NaCl and porewater are modest indicating that in the case of Zn the clay minerals play a predominant role in the uptake process, and that the effects originating from the porewater are minor. Furthermore, despite the differences in mineralogy, both argillaceous rocks show the same uptake behaviour, indicating that the clay minerals are predominantly responsible for the sorption at low and the precipitation processes at higher Zn concentrations. The study demonstrates how results on “pure” systems such as IdP can be transferred to predict the uptake by argillaceous rocks at low sorbate concentrations.

Sorption of Np(IV) on MX-80 in Ca-Na-Cl Type Reference Water of Crystalline Rock

The pH dependence of sorption distribution coefficient (Kd) of Np(IV) on MX-80 in Ca-Na-Cl type solution with the ionic strength of 0.3 M, which was similar to one of the reference groundwaters in crystalline rock, was experimentally investigated under the reducing conditions. The overall trend of Kd on MX-80 was independent of pH at 5 ≤ pH ≤ 10 but increased as pH increased at pH ≤ 5. The 2-site protolysis non-electrostatic surface complexation and cation exchange model was applied to the experimentally measured pH dependence of Kd and the optimized surface complexation constants of Np(IV) sorption on MX-80 were estimated. The values of surface complexation constants in this work agreed relatively well with those in the Na-Ca-Cl solution previously evaluated, suggesting that compared to Na+, the competition of Ca2+ with Np(IV) for surface complexation on MX-80 was not much strong in Ca-Na-Cl solution. The sorption model well predicted the pH dependence of Kd values but slightly overestimated the sorption at the low pH region.

Surface reactivity of the cyanobacterium Synechocystis sp. PCC 6803 – Implications for trace metals transport to the oceans

Cyanobacteria are abundant in nearly every surface environment on Earth. Understanding their chemical reactivity and metal binding capacity with varying ionic strength (IS) is paramount to understanding trace metal cycling in natural environments. We conducted an investigation on the cell surface reactivity of the freshwater cyanobacterium Synechocystis sp. PCC 6803 at freshwater (0.01 M NaCl) and marine (0.56 M NaCl) IS. Potentiometric titration data were used to develop a multiple discrete site, non-electrostatic surface complexation model (SCM), and corresponding cell surface functional group identities were verified using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. Synechocystis cells were best modeled in FITEQL 4.0 using a non-electrostatic 2-site protonation model. Cadmium (Cd) adsorption experiments paired with SCM was utilized to calculate the binding constants of Cd. Synechocystis surface functional groups demonstrated a stronger affinity for Cd across the entire pH range studied (3–9) at freshwater IS, with the greatest difference at circumneutral pH (6–8) where Cd adsorption in freshwater IS was 60% greater than at marine IS. These data combined with the ubiquitous distribution of Synechocystis in freshwater and brackish environments suggest that these organisms could play an important role in trace metal cycling in environments with large salinity gradients, such as estuaries and deltas, and could act as a transport mechanism for trace metals from terrestrial to marine settings.

Surface complexation of Ca and competitive sorption of divalent cations on montmorillonite under alkaline conditions

In the geological disposal system of high-level radioactive and transuranic wastes, an increase in Ca concentration (arising from the alteration of cementitious materials) could affect the retention of other radionuclides due to competitive sorption. Batch sorption experiments were performed to investigate the sorption behavior of Ca and its competition with other divalent cations (Sr and Ni) on the edge sites of Kunipia F montmorillonite under alkaline conditions. Ca sorption increased with pH (when pH > 8) and it was more pronounced under high ionic strength conditions, indicating that Ca formed a surface complexation with the edge sites. The sorption behavior of Sr was similar to that of Ca. Meanwhile, the Ni sorption increased with pH, when this was >6 (for lower pH values than in the case of Ca and Sr), Ni tends to form surface complexes more readily than Ca and Sr. The results of a fitting analysis conducted based on the 2-site protolysis non-electrostatic surface complexation and cation exchange (2SPNE SC/CE) model indicate that Ca and Sr sorb onto the ≡SW2OH sites and Ni sorbs onto the ≡SSOH sites. Additionally, the sorption of the CaOH+ and SrOH+ ionic pair species by cation exchange was implied at pH ~ 12. Competitive sorption experiments were also carried out to evaluate the effect of Ca on the sorption of Sr and Ni. Sr sorption decreased with Ca concentration in the alkaline pH region, whereas Ni sorption was not affected by Ca concentration. Overall, these results indicate that Ca and Sr sorb onto the same sites, while Ca and Ni sorb onto different sites; additionally, the competitive sorption seemed to depend on chemical similarities (e.g., hydrolysis behavior). The sorption model parameters obtained from the single-element batch sorption experiments successfully reproduced the results of the competitive sorption experiments.

Sorption of Pd on illite, MX-80 bentonite and shale in Na–Ca–Cl solutions

This paper examines sorption of Pd(II) onto illite, MX-80 bentonite, and Queenston shale in Na–Ca–Cl solutions of varying ionic strength (IS) from 0.01 to 6.0 mol/L (M) and pHc ranging from 3 to 9 under atmospheric conditions. A 2-site protolysis non-electrostatic surface complexation and cation exchange model was applied to the Pd sorption onto illite and MX-80 using PHREEQC, and the model results were compared to the experimental ones obtained in this work. Surface complexation and cation exchange constants were estimated for both illite and MX-80 through the optimization process to bring the predicted distribution coefficients from the model into alignment with the experimentally derived values. These optimized surface complexation constants were compared to existing linear free energy relationships (LFER).

Sorption of rare earth elements on schwertmannite and their mobility in acid mine drainage treatments

Rare Earth Elements (REE) are nowadays considered critical raw materials due to their increasing use in modern industry and their shortage of supply. Acid mine drainage (AMD) contains REE concentrations several orders of magnitude higher than the rest of continental and marine waters, and the sludge from its treatment may become a supplementary source of REE. Schwertmannite, a Fe(III)-sulfate-hydroxide is the most common mineral precipitated from AMD and a main constituent of the neutralization sludge. The objective of this work is to study the mechanism of REE retention in synthetic schwertmannite and to predict the REE behavior in a column experiment mimicking an AMD passive remediation system.Suspensions of synthetic schwertmannite in sulfate solutions show that Y and the lanthanides are effectively sorbed at pH values higher than 4.5, and sorption is complete at pH values higher than 6.5. The experimental partition coefficients clearly show a preferential enrichment of heavy REE in the solid phase. Unlike the rest of the REE, Sc sorption occurred at a lower pH, from 3 to 5. The experimental results have been described with a non-electrostatic surface complexation model in which the aqueous complex MSO4+ exchanges with two H+ from the surface of schwertmannite, forming a bidentate surface complex, (XO)2MSO4-. Scandium sorption was also accurately predicted with the addition of a second bidentate surface complex, (XO)2MOH.The equilibrium constants for REE sorption on schwertmannite calculated in the present work, together with those for REE sorption on basaluminite (Lozano et al., 2019, GCA, 258, 50–62) were used to model the behavior of different REE observed in the pore water and solid of a column experiment. Schwertmannite and basaluminite were the main solid phases formed due to the progression of the neutralization. First schwertmannite precipitated at pH below 4 and then basaluminite precipitated at pH above 4. Both minerals can sorb REE in a similar pH range. However, since Y and the lanthanides sorbed at pH values higher than 5, their sorption only occurred on basaluminite. In contrast, the Sc sorption edge extended from pH 3 to 5 and Sc partially sorbed on schwertmannite. As a practical consequence, REE preferentially accumulated in the basaluminite residue of AMD neutralization systems, but a minor fraction of Sc can be retained in the schwertmannite waste.

Contribution of Ternary Reaction to Pd Sorption on MX-80 in Na-Ca-Cl Solution at High Ionic Strength

In this study, we first examined the sorption of Pd on MX-80 in Na-Ca-ClO4 solution as a function of pHc (3–9) and ionic strength (0.1 M–4 M) and confirmed that the experimentally derived Kd values could be fitted by a 2-site protolysis nonelectrostatic surface complexation and cation exchange (2SPNE SC/CE) model using three binary surface complexation constants previously estimated. Then, we investigated the sorption of Pd on MX-80 in Na-Ca-Cl-ClO4 solution as a function of pHc (3–9) and Cl-/ClO4- molar concentration ratio (0–∞) at the ionic strength = 4 M. We found that the sorption of Pd on MX-80 in Na-Ca-Cl-ClO4 solution could be simulated only by the three binary and one ternary surface complexations (S-OH+Pd2++4Cl-↔S-OPdCl43-+H+). This suggests that the contribution of other ternary surface complexations such as ≡S-OH + Pd2++xCl-↔ ≡S-OPdClxx-1-+H+ (x = 1, 2 and 3) to Pd sorption in Na-Ca-Cl-ClO4 solution with ionic strength = 4 M was negligibly small.

Determination of complex formation constants of neptunium(V) with propionate and lactate in 0.5–2.6 m NaCl solutions at 22–60°C using a solvent extraction technique

Abstract Natural clay rocks like Opalinus (OPA) and Callovo-Oxfordian (COx) clay rock are considered as potential host rocks for deep geological disposal of nuclear waste. However, small organic molecules such as propionate and lactate exist in clay rock pore water and might enhance Np mobility through a complexation process. Therefore, reliable complex formation data are required in the frame of the Safety Case for a nuclear waste repository. A solvent extraction technique was applied for the determination of NpO 2 + ${\rm{NpO}}_2^ + $ complexation with propionate and lactate. Extraction was conducted from isoamyl alcohol solution containing 10−3 M TTA and 5 · 10−4 M 1,10-phenanthroline. Experiments were performed in 0.5–2.6 m NaCl solutions at temperatures ranging from 22 to 60 °C. Formation of 1:1 Np(V) complexes for propionate and lactate was found under the studied conditions. The SIT approach was applied to calculate equilibrium constants β°(T) at zero ionic strength from the experimental data. Log β°(T) is found linearly correlated to 1/T for propionate and lactate, evidencing that heat capacity change is near 0. Molal reaction enthalpy and entropy ( Δ r H m ∘ ${\Delta _{\rm{r}}}H_{\rm{m}}^ \circ $ and Δ r S m ∘ ${\Delta _{\rm{r}}}S_{\rm{m}}^ \circ $ ) could therefore be derived from the integrated van’t Hoff equation. Data for log β° (298.15 K) are in agreement with literature values for propionate and lactate. Np(V) speciation was calculated for concentrations of acetate, propionate and lactate measured in clay pore waters of COx. In addition, the two site protolysis non-electrostatic surface complexation and cation exchange (2SPNE SC/CE) model was applied to quantitatively describe the influence of Np(V) complexation on its uptake on Na-illite, a relevant clay mineral of OPA and COx.

Neptunium sorption and redox speciation at the illite surface under highly saline conditions

Neptunium (Np) uptake on illite is investigated in 1 and 3.2 molal (m) NaCl solutions under inert (Ar) atmosphere for 4<pHm<10 (pHm=−log mH+) and 5×10−8<[Np(V)]tot<3×10−4M. In agreement with a previous study in 0.1m NaCl solutions (Marsac et al., 2015a), Np(V) is the prevailing oxidation state in the aqueous solution, but Np uptake by illite is affected by surface induced reduction. The extent of Np(V) reduction to Np(IV) follows the measured redox potential (or the pe=−log ae−), which is influenced by the introduced Np(V) amount, because of the low redox capacity of the illite. The presence of Np(IV) on the solid phase is verified by X-ray Absorption Near Edge Spectroscopy (XANES). We can conclude that Np uptake by illite is not significantly affected by the variation of mNaCl from 0.1 to 3.2m and thus is in agreement with reports on tetravalent actinide and Np(V) sorption to clays at high ionic strength. The combination of (i) the two site protolysis non-electrostatic surface complexation and cation exchange model, (ii) the specific ion interaction theory to calculate activity coefficients for dissolved species and (iii) by accounting for redox equilibria and the stability of surface Np species, the overall Np uptake by illite can be simulated as a function of pHm, pe and mNaCl using a single set of parameters. The present experimental and modeling results are particularly important in the context of deep geological nuclear waste disposal since many sedimentary rocks or clay formations that are deemed suitable for this purpose exhibit highly saline porewaters.

Sorption and redox speciation of plutonium at the illite surface under highly saline conditions

Natural groundwater may contain high salt concentrations, such as those occurring at several potential deep geological nuclear waste repository sites. Actinide sorption to clays (e.g. illite) under saline conditions has, however, been rarely studied. Furthermore, both illite surface and ionic strength may affect redox speciation of actinides like plutonium. In the present study, Pu sorption to illite is investigated under anaerobic conditions for 3<pHm (=−log mH+)<10andmNaCl=1.0and3.2 molal (m). Results are compared with previous data for mNaCl=0.1m. According to redox potential measurements and based on Eu(III)-illite sorption data (taken as analogue of Pu(III)), the strong effect of mNaCl on overall Pu uptake observed for pHm<6 is mainly attributed to the presence of Pu(III) and its competition with Na+ for ion exchange sites. For pHm>6, overall Pu uptake is largely insensitive to mNaCl due to the prevalence of strongly adsorbed Pu(IV). By applying appropriate corrections to the activity coefficients of dissolved ions and using the 2-site protolysis non-electrostatic surface complexation and cation exchange (2 SPNE SC/CE) model, experimental data on Pu sorption to illite as a function of pH, Eh and mNaCl can be very well reproduced.

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.

Sorption behavior of thorium onto granite and its constituent minerals

ABSTRACTThe sorption behavior of thorium (Th) onto granite and its major constituent minerals, feldspar, quartz and mica were investigated by batch sorption experiments. Experiments were carried out under variable pH and carbonate concentrations. Distribution coefficients (Kd) decreased with increased carbonate concentrations and showed the minimal value at around pH 10. This sorption tendency was seen for all the rock and mineral samples, and it was due to the formation of aqueous hydroxo–carbonate complexes of Th in the solutions. The order of sorbability for Th was mica > feldspar > quartz ≈ granite. The sorption behaviors of Th were analyzed by the non-electrostatic surface complexation model with the Visual Minteq computer program. The Kd for granite were calculated by component additivity approaches. The model calculations were able to explain the experimental results reasonably well. It was shown that the sorption behavior of Th onto granite can be explained by the complexation with the surface sites of mainly biotite and feldspar.