Cadmium Isotope Fractionation during Adsorption and Substitution with Iron (Oxyhydr)oxides

作者：Xinran Yan, Mengqiang Zhu, Wei Li, Caroline L. Peacock, Jingyuan Ma, Hanjie Wen, Fan Liu, Zhengbing Zhou, Chuanwei Zhu, Hui Yan Yin · 发表于：Environmental Science & Technology · 年份：2021 · DOI：10.1021/acs.est.0c06927 · 被引用次数：135 · 研究领域：Radioactive element chemistry and processing、Extraction and Separation Processes、Catalytic Processes in Materials Science

Cadmium (Cd) isotopes have great potential for understanding Cd geochemical cycling in soil and aquatic systems. Iron (oxyhydr)oxides can sequester Cd via adsorption and isomorphous substitution, but how these interactions affect Cd isotope fractionation remains unknown. Here, we show that adsorption preferentially enriches lighter Cd isotopes on iron (oxyhydr)oxide surfaces through equilibrium fractionation, with a similar fractionation magnitude (Δ 114/110 Cd solid-solution ) for goethite (Goe) (−0.51 ± 0.04‰), hematite (Hem) (−0.54 ± 0.10‰), and ferrihydrite (Fh) (−0.55 ± 0.03‰). Neither the initial Cd 2+ concentration or ionic strength nor the pH influence the fractionation magnitude. The enrichment of the light isotope is attributed to the adsorption of highly distorted [CdO 6 ] on solids, as indicated by Cd K-edge extended X-ray absorption fine-structure analysis. In contrast, Cd incorporation into Goe by substitution for lattice Fe at a Cd/Fe molar ratio of 0.05 preferentially sequesters heavy Cd isotopes, with a Δ 114/110 Cd solid-solution of 0.22 ± 0.01‰. The fractionation probably occurs during the transformation of Fh into Goe via dissolution and reprecipitation. These results improve the understanding of the Cd isotope fractionation behavior being affected by iron (oxyhydr)oxides in Earth’s critical zone and demonstrate that interactions with minerals can obscure anthropogenic and natural Cd isotope characteristics, which should be carefully considered when applyi...