Mechanism of metal uptake by clay minerals ― X-ray spectroscopy and molecular modelling study
Clay minerals play essential roles in argillaceous rock formations since they are strong sorbents for transition and heavy metals. Iron is one of the most abundant redox-active element which may participate in redox interactions at the edge surface of clay minerals. It is expected to adsorb as various surface species on clay minerals, moreover, Fe can also be incorporated into the structure of the clay layer. Structural iron might participate in redox-reactions influencing the relative stability of the sorbed surface species. The accurate description of clay structures and the quantification of Fe2+, radionuclide uptake capacity and mechanism is essential to understand the fate of heavy metals and radioactive contaminants in subsurface environments.
In this thesis, several microanalytical techniques, mathematical methods and atomistic modelling approaches were combined to comprehend sorption processes of transition metals and radionuclides on clay minerals. The uptake capabilities of mineral phases present in argillaceous rock were estimated by applying multivariate methods (factor analysis, positive matrix factorization, cluster analysis) on combined micro-XRF (microscopic X-ray fluorescence) and SEM/EDX (scanning electron microscopy energy dispersive X-ray analysis) data sets. In Boda Claystone Formation (Hungary), the clayey matrix – containing phyllosilicates (mainly illite), fine grained albite, rutile and nanocrystalline hematite – was identified as mineral phases with the highest Nd uptake capability. Almost the same amount of Nd was found to bind to calcite, while the ion uptake capability of dolomite was orders of magnitude lower.
The structural properties of natural clay minerals, the sorption mechanism of Fe and UO22+ on them were studied by combining ab initio and XAS (X-ray absorption spectroscopy) approaches. Relax structure and molecular dynamics (MD) calculations based on density functional theory (DFT+U) were applied as basis to calculate EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near edge structure) spectra. A good agreement between the ab initio simulated and the measured XAS spectra demonstrates the robustness of the employed simulation approach.
The results suggest that iron in smectites is preferentially incorporated as Fe3+ (ferric iron) into the octahedral sheet and there is no preferential occupation of cis- or trans-sites. Natural smectites with similar structure originating from different mines have distinct arrangement of isomorphic substitutions (individual iron or Fe–Fe, Fe–Mg clusters). The structural differences might explain the distinct extent and efficiency of redox reactions happen at the clay edge surfaces. The relaxed structure calculation of several different clay structures with substituted Fe2+/Fe3+ in the bulk or at surface sites show that the energetically preferred position for ferrous iron is in the bulk, while for ferric iron is at the edge site. The spectroscopic calculations show that strong- and weak-site complexation has different characteristics. The quantitative interpretation of the atomistic modelling based XAS spectra measured on samples with distinct Fe loadings indicates particularly complex behaviour of iron.
The energy comparison of the relaxed structures of various bidentate uranyl surface complexes revealed that the structural parameters of the uranyl species are essentially determined by the placement of surface chemical groups (e.g. OH–) forming the adsorption site. Structural Fe3+ in the octahedral sheet of montmorillonite at the surface site increases the stability of bidentate uranium species.