Key Laboratory of Arable Land Conservation (Middle and Lower Reaches of Yangtse River), Ministry of Agriculture and Rural Affairs, College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, China.
State Environmental Protection Key Laboratory of Soil Health and Green Remediation, Ministry of Ecology and Environment, Huazhong Agricultural University, Wuhan 430070, China.
Environ Sci Technol. 2021 Sep 7;55(17):11601-11611. doi: 10.1021/acs.est.0c06927. Epub 2021 Aug 9.
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 (ΔCd) for goethite (Goe) (-0.51 ± 0.04‰), hematite (Hem) (-0.54 ± 0.10‰), and ferrihydrite (Fh) (-0.55 ± 0.03‰). Neither the initial Cd 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] 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 ΔCd 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 applying Cd isotopes as environmental tracers.
镉 (Cd) 同位素在理解土壤和水生系统中 Cd 的地球化学循环方面具有巨大的潜力。铁 (oxyhydr) 氧化物可以通过吸附和同晶取代来固定 Cd，但这些相互作用如何影响 Cd 同位素分馏仍不清楚。在这里，我们表明，通过平衡分馏，吸附优先在铁 (oxyhydr) 氧化物表面富集较轻的 Cd 同位素，针铁矿 (Goe)、赤铁矿 (Hem) 和水铁矿 (Fh) 的分馏幅度 (ΔCd) 相似 (-0.51 ± 0.04‰、-0.54 ± 0.10‰ 和 -0.55 ± 0.03‰)。初始 Cd 浓度或离子强度以及 pH 值均不影响分馏幅度。轻同位素的富集归因于高度扭曲的 [CdO] 在固体上的吸附，这一点通过 Cd K 边扩展 X 射线吸收精细结构分析得到了证实。相比之下，当 Cd/Fe 摩尔比为 0.05 时，通过替代晶格 Fe 将 Cd 掺入 Goe 中，则优先固定重 Cd 同位素，分馏幅度为 0.22 ± 0.01‰。分馏可能发生在 Fh 通过溶解和再沉淀转化为 Goe 的过程中。这些结果提高了对地球关键带中受铁 (oxyhydr) 氧化物影响的 Cd 同位素分馏行为的理解，并表明与矿物的相互作用可能掩盖人为和自然 Cd 同位素特征，在将 Cd 同位素作为环境示踪剂应用时应谨慎考虑。
