Fe(II) uptake on natural montmorillonites. II. Surface complexation modeling.

Fe(II) sorption edges and isotherms have been measured on low structural Fe-content montmorillonite (STx) and high structural Fe-content montmorillonite (SWy) under anoxic (O2 < 0.1 ppm) and strongly reducing conditions (Eh = -0.64 V). Under anoxic conditions Fe(II) sorption on SWy was significantly higher than on STx, whereas the sorption under reducing conditions was essentially the same. The uptake behavior of Fe(II) on STx under all redox conditions (Eh = +0.28 to -0.64 V) and SWy under reducing conditions (Eh = -0.64 V) was consistent with previous measurements made on other divalent transition metals. All of the sorption data could be modeled with the two-site protolysis nonelectrostatic surface complexation and cation exchange (2SPNE SC/CE) sorption model including an additional surface complexation reaction for Fe(II) which involved the surface oxidation of ferrous iron surface complexes (≡S(S,W)OFe(+)) to ferric iron surface complexes (≡S(S,W)OFe(2+)) on both the strong and weak sites. The electron acceptor sites on the montmorillonite are postulated to be the structural Fe(III).

Fe(II) sorption on a synthetic montmorillonite. A combined macroscopic and spectroscopic study.

Extended X-ray absorption fine structure (EXAFS) and Mössbauer spectroscopy combined with macroscopic sorption experiments were employed to investigate the sorption mechanism of Fe(II) on an iron-free synthetic montmorillonite (Na-IFM). Batch sorption experiments were performed to measure the Fe(II) uptake on Na-IFM at trace concentrations as a function of pH and as a function of sorbate concentration at pH 6.2 and 6.7 under anoxic conditions (O2 < 0.1 ppm). A two-site protolysis nonelectrostatic surface complexation and cation exchange sorption model was used to quantitatively describe the uptake of Fe(II) on Na-IFM. Two types of clay surface binding sites were required to model the Fe(II) sorption, the so-called strong (≡S(S)OH) and weak (≡S(W)OH) sites. EXAFS data show spectroscopic differences between Fe sorbed at low and medium absorber concentrations that were chosen to be characteristic for sorption on strong and weak sites, respectively. Data analysis indicates that Fe is located in the continuity of the octahedral sheet at trans-symmetric sites. Mössbauer spectroscopy measurements confirmed that iron sorbed on the weak edge sites is predominantly present as Fe(II), whereas a significant part of surface-bound Fe(III) was produced on the strong sites (∼12% vs ∼37% Fe(III) species to total sorbed Fe).

A spectroscopic characterization and quantification of m(III)/clay mineral outer-sphere complexes.

For the long-term safety assessment of deep radioactive waste repositories an understanding of the interactions of actinides with mineral surfaces at a molecular level is necessary. The retention/mobility of the released radionuclides is strongly dependent on sorption/desorption reactions at mineral surfaces. Thus, a quantitative understanding of the uptake mechanisms of actinides on clay minerals will make an important contribution to long-term safety assessments. Using time-resolved laser fluorescence spectroscopy (TRLFS), it was possible to differentiate between nonsorbed aquo ions and outer-sphere sorbed Cm(III) onto different montmorillonites. In addition, Cm(III)/clay outer-sphere complexation at different ionic strengths using NaCI as the background electrolyte is quantified. Finally, the results are verified by sorption model calculations.

Diffusion of 22Na and 85Sr in montmorillonite: evidence of interlayer diffusion being the dominant pathway at high compaction.

A mechanistic understanding of transport phenomena in compacted clays is essential for the use of such materials as engineered barrier systems for the safe geological disposal of radioactive wastes. The present contribution is a first step in the development of an integrative treatment of the properties of tracer cations in compacted bentonites with respect to diffusion and sorption. The diffusion of 22Na and 85Sr in highly compacted montmorillonite and kaolinite is investigated as a function of the "external salt concentration" (NaClO4), i.e., of the solution in equilibrium with the clay. Consistent results were obtained from through-diffusion experiments and tracer profile analysis. Knowledge of genuine diffusion coefficients of the filter plates turned out to be crucial in cases where the diffusive resistance of the filter plates was similar to that of the clay. Diffusion coefficients formally calculated on the basis of the tracer concentration gradient in the external aqueous phase, and the sorption distribution ratios were found to decrease with increasing external salt concentration in the case of montmorillonite. In a logarithmic representation of these data, a slope of -1 was obtained for the monovalent 22Na, whereas the slope was -2 for the divalent 85Sr. In the case of kaolinite, diffusion coefficients were independent of the external salt concentration. It is postulated that the diffusion of the tracer cation through the interlayer water is the dominant pathway in compacted swelling clays under the experimental conditions tested. Effective diffusion coefficients, based on a tracer concentration gradient in the interlayer water of the clay, were found to be independent of the composition of the external aqueous phase. The latter gradient is assumed to be a function of the external salt concentration, according to a calculated distribution of the tracer cation between free pore water and the interlayer water via cation exchange.

Anisotropic diffusion in layered argillaceous rocks: a case study with Opalinus Clay.

Anisotropic diffusion was studied in Opalinus Clay, a potential host rock for disposal of spent fuel, vitrified high-level waste, and long-lived intermediate-level waste in Switzerland. Diffusion parallel to the bedding was measured using a radial through-diffusion technique and diffusion perpendicular to the bedding bythe classical (planar) through-diffusion technique. The samples used were from Mont Terri (MT) and from Benken (BE). Diffusion of HTO, 36Cl-, and 22Na+ parallel and perpendicular to the bedding was studied under confining pressures of 7 MPa (MT) and 14 MPa (BE). The results indicate that diffusion parallel to the bedding is faster than diffusion perpendicular to the bedding by a factor of 4-6 for the three radionuclides, indicating that the Opalinus Clay is anisotropic. This might be explained by smaller path lengths (tortuosity) for species diffusing parallel to the fabric. The degree of anisotropy is slightly smaller for Opalinus Clay from Mont Terri than from Benken. This is due to the lower overburden pressure in Mont Terri resulting in a lower preferential orientation of the clay platelets.

Porewater chemistry in compacted re-saturated MX-80 bentonite.

Bentonites of various types are being investigated in many countries as backfill materials in high-level radioactive waste disposal concepts. Being able to understand the chemistry of the porewater in compacted bentonite is very important since it is critical to predicting radionuclide solubilities and to the synthesis of sorption data bases, and hence to repository safety studies. In this paper, porewater compositions in compacted bentonites are calculated, taking into consideration such factors as montmorillonite swelling, semi-permeable membrane effects, very low "free water" volumes, and the highly effective buffering characteristics of the exchangeable cations and the amphoteric edge sites. The former buffer the cation concentrations and the latter fix the pH in the porewater of a re-saturated bentonite. The above considerations are used in conjunction with previously measured physico-chemical characterisation data on MX-80 powder to calculate porewater compositions in compacted bentonites. For the MX-80 material specified, the porewaters calculated for initial dry densities between 1200 and 1600 kg m(-3) had relatively high ionic strengths (I approximately 0.3 M), similar cation concentrations and a pH equal to 8.0. The porewaters changed from being Na(2)SO(4)-rich at 1200 kg m(-3) to a NaCl/Na(2)SO(4) type water at 1600 kg m(-3).

Th uptake on montmorillonite: a powder and polarized extended X-ray absorption fine structure (EXAFS) study.

The uptake process of Th(IV) onto montmorillonite was studied using powder and polarized-EXAFS (P-EXAFS) spectroscopy. Sorption samples were prepared in 0.1 M NaClO(4) solutions either undersaturated (pH 2 and 3, [Th](initial): 2.7x10(-6) to 4x10(-4) M) or supersatured (pH 5, [Th](initial): 4.3x10(-5) to 4x10(-4) M) with respect to amorphous ThO(2). Th loading varied between 1-157 micromol/g at pH 3 and 14-166 micromol/g at pH 5 and equaled 41 micromol/g at pH 2. At pH 5 and high surface loading the EXAFS spectrum resembled that of amorphous Th(OH)(4), suggesting the precipitation of a Th hydrous hydroxide. At low and intermediate surface coverage two O coordination shells at approximately 2.24 and approximately 2.48 A, and one Si shell at 3.81-3.88 A, were systematically observed regardless of pH. The formation of Th nucleation products and Th-Si solution complexes and the sorption of Th on a silica precipitate were excluded from the EXAFS spectra analysis and solution chemistry. In these conditions, Th was shown to bond the montmorillonite surface by sharing double corners with Si tetrahedra. This structural interpretation is consistent with surface coverage calculations which showed that the edge sites were saturated in the two highest concentrated samples (34 and 157 micromol/g) at pH 3.