An XAFS study of zinc and cadmium sorption mechanisms on montmorillonite and hydrous ferric oxide over extended reaction times

The purpose of this study was to understand sorption reactions and identify sorption species and the coordination environment of Zn and Cd sorbed on montmorillonite or hydrous ferric oxide (HFO). Because the fate of metals in the soil environment depends on speciation and the final form of sorption products, it is important to identify the sorption product to design and implement clean-up processes. Both macroscopic bulk sorption experiments and microscopic x-ray absorption spectroscopic experiments were conducted under different condition of metal loading, aging time, and including or excluding of atmospheric carbon dioxide.; Extended x-ray absorption fine structure (EXAFS) analysis of zinc sorption on montmorillonite showed dependence of the concentration and the reaction time that different types of sorption products were formed. At low concentration, Zn formed mononuclear outer-sphere surface complexes on the surface of montmorillonite over reaction times up to six months. At high concentration, zinc coordinated as octahedrally with about six oxygen atoms for all aging times. Zn-Zn contribution in the second shell indicated formation of multinuclear surface complexes or surface precipitates for the samples aged up to 11 days. Zn-Zn and Zn-Al/Si contributions in the second shell suggested formation of mixed metal coprecipitates such as a Zn phyllosilicate-like phase or a Zn-Al hydrotalcite-like phase for the samples aged 20 days and more.; The sorption species and coordination environment of zinc sorbed on to HFO did not change for aging times up to six months. In the low concentration samples, Zn formed inner-sphere surface complexes on the surface of HFO. Zn was tetrahedrally coordinated with oxygen atoms. Zn-Fe worked well for second shell contributions. For the higher concentration samples, both inner-sphere surface complexes and polynuclear complexes were feasible sorption products. The first shell was tetrahedrally coordinated. The second shell could be contributed from either Zn-Fe or Zn-Zn correlations with almost the same bond distance.; Compared to the results from Zn sorption on montmorillonite, HFO had a higher sorption capacity and Zn sorption was faster. As a long-term sink, however montmorillonite would be more stable for Zn uptake than HFO at higher zinc concentration because mixed-metal precipitates such as a Zn phyllosilicate-like phase or a Zn-Al hydrotalcite-like phase would be more stable than inner-sphere complexes or polynuclear complexes.; The structure and coordination environments of the sorbed cadmium ions on montmorillonite or HFO were identical and consistent regardless of cadmium loadings, aging time, or the presence of atmospheric carbon dioxide. Cd formed mononuclear outer-sphere surface complexes on the surface of montmorillonite or HFO. Cadmium was physically sorbed to the surfaces of montmorillonite or HFO, indicating Cd sorbed on montmorillonite or HFO could be easily remobilizable and transportable.