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The Speciation of Cd in Cd–Fe Coprecipitates: Does Cd Substitute for Fe in Goethite Structure?

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journal contribution
posted on 19.08.2019, 18:04 by Lu Liu, Xiaoming Wang, Mengqiang Zhu, Jingyuan Ma, Jing Zhang, Wenfeng Tan, Xionghan Feng, Hui Yin, Fan Liu
Anthropogenic inputs of Cd into environments through various industrial and agricultural activities cause wide concerns around the world. The fate and toxicity of Cd in soils is substantially affected by its association with various iron (hydr)­oxides via adsorption, isomorphous substitution, and/or coprecipitation mechanisms but the Cd speciation in Cd–Fe coprecipitates during Fe (hydr)­oxides formation is rarely studied. In the present study, Cd–Fe coprecipitates during goethite formation were obtained and sequentially treated with acidic ammonium oxalate and nitric acid, and then the Cd speciation in these treated samples were determined by a combination of acidic dissolution kinetics, powder X-ray diffraction, transmission electron microscopy, Fe and Cd K-edge extended X-ray absorption fine structure spectroscopy, and the atomic pair distribution function analysis. The acidic ammonium oxalate-treated samples (NCd_o, N = 1–6) contain some ferrihydrite besides goethite, and the amount of ferrihydrite increases with increasing Cd content. Accordingly, more Cd is accumulated in ferrihydrite rather than goethite, and minor Cd­(OH)2 precipitate probably exists in the NCd_o samples with high Cd content. In the NCd_o sample with the highest Cd content (6.9 ± 0.2 mol %), ferrihydrite proportion can be as high as one-third and retains almost one-half of the total Cd. Further treatment of these NCd_o samples with nitric acid solution almost totally removes the 2-line ferrihydrite, finite goethite particles and the Cd associated with them. The thus obtained Cd–Fe coprecipitates (NCd_n, N = 1–6) are probably Cd-substituted goethites with Cd uniformly distributed in the mineral structure with a slightly enrichment in interior regions. These results help to have a better understanding of the Cd speciation in iron (hydr)­oxides and the transformation and fate of Cd in polluted soils enriched in iron oxide minerals and also provide a useful strategy to investigate the speciations of heavy metals in complex matrixes.