The mechanism of cadmium surface complexation on iron oxyhydroxide minerals |
| |
| Institution: | 1. Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China;2. University of Chinese Academy of Sciences, Beijing 100049, China;3. Department of Ecosystem Science and Management, University of Wyoming, Laramie, WY 82071, United States;4. Nanjing Institute of Environmental Sciences, Ministry of Environmental Protection of the People''s Republic of China, Nanjing 210008, China;5. State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210023, China |
| |
| Abstract: | Many sediment and soil systems have become significantly contaminated with cadmium, and earth scientists are now required to make increasingly accurate predictions of the risks that this contamination poses. This necessitates an improved understanding of the processes that control the mobility and bioavailability of cadmium in the environment. With this in mind, we have studied the composition and structure of aqueous cadmium sorption complexes on the iron oxyhydroxide minerals goethite (α-FeOOH), lepidocrocite (γ-FeOOH), akaganeite (β-FeOOH), and schwertmannite (Fe8O8(OH)6SO4) using extended X-ray adsorption fine structure spectroscopy. The results show that adsorption to all of the studied minerals occurs via inner sphere adsorption over a wide range of pH and cadmium concentrations. The bonding mechanism varies between minerals and appears to be governed by the availability of different types of adsorption site at the mineral surface. The geometry and relative stability of cadmium adsorption complexes on the goethite surface was predicted with ab initio quantum mechanical modelling. The modelling results, used in combination with the extended X-ray adsorption fine structure data, allow an unambiguous determination of the mechanism by which cadmium bonds to goethite.Cadmium adsorbs to goethite by the formation of bidentate surface complexes at corner sharing sites on the predominant (110) crystallographic surface. There is no evidence for significant cadmium adsorption to goethite at the supposedly more reactive edge sharing sites. This is probably because the edge sharing sites are only available on the (021) crystallographic surface, which comprises just ～2% of the total mineral surface area. Conversely, cadmium adsorption on lepidocrocite occurs predominately by the formation of surface complexes at bi- and/or tridentate edge sharing sites. We explain the difference in extended X-ray adsorption fine structure results for cadmium adsorption on goethite and lepidocrocite by the greater availability of reactive edge sharing sites on lepidocrocite than on goethite. The structures of cadmium adsorption complexes on goethite and lepidocrocite appear to be unaffected by changes in pH and surface loading. There is no support for cadmium sorption to any of the studied minerals via the formation of an ordered precipitate, even at high pH and high cadmium concentration. Cadmium adsorption on akaganeite and schwertmannite also occurs via inner sphere bonding, but the mechanism(s) by which this occurs remains ambiguous. |
| |
| Keywords: | |
| 本文献已被 ScienceDirect 等数据库收录！ |
|
