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.
Key Laboratory of Surficial Geochemistry, Ministry of Education, School of Earth Sciences and Engineering, Nanjing University, Nanjing, Jiangsu 210023, China.
Environ Sci Technol. 2023 Apr 25;57(16):6636-6646. doi: 10.1021/acs.est.2c08028. Epub 2023 Apr 12.
The Zn isotope fingerprint is widely used as a proxy of various environmental geochemical processes, so it is crucial to determine which are the mechanisms responsible for isotopic fractionation. Iron (Fe) (hydr)oxides greatly control the cycling and fate and thus isotope fractionation factors of Zn in terrestrial environments. Here, Zn isotope fractionation and related mechanisms during adsorption on and substitution in three FeOOH polymorphs are explored. Results demonstrate that heavy Zn isotopes are preferentially enriched onto solids, with almost similar isotopic offsets (ΔZn = 0.25-0.36‰) for goethite, lepidocrocite, and feroxyhyte. This is consistent with the same average Zn-O bond lengths for adsorbed Zn on these solids as revealed by Zn K-edge X-ray absorption fine structure spectroscopy. In contrast, at an initial Zn/Fe molar ratio of 0.02, incorporation of Zn into goethite and lepidocrocite by substituting for lattice Fe preferentially sequesters light Zn isotopes with ΔZn of -1.52 ± 0.09‰ and -1.18 ± 0.15‰, while Zn-substituted feroxyhyte (0.06 ± 0.11‰) indicates almost no isotope fractionation. This is closely related to the different crystal nucleation and growth rates during the Zn-doped FeOOH formation processes. These results provide direct experimental evidence of incorporation of isotopically light Zn into Fe (hydr)oxides and improve our understanding of Zn isotope fractionation mechanisms during mineral-solution interface processes.
锌同位素指纹广泛用作各种环境地球化学过程的示踪剂，因此确定导致同位素分馏的机制至关重要。铁（氢）氧化物极大地控制着锌在陆地环境中的循环和归宿，从而控制着其同位素分馏因子。在这里，研究了三种 FeOOH 多晶型物上吸附和取代过程中的锌同位素分馏及相关机制。结果表明，重锌同位素优先富集在固体上，针铁矿、纤铁矿和羟高铁矿的同位素偏移（ΔZn=0.25-0.36‰）几乎相同。这与锌 K 边 X 射线吸收精细结构光谱揭示的这些固体上吸附锌的平均 Zn-O 键长度相同是一致的。相比之下，在初始 Zn/Fe 摩尔比为 0.02 时，通过取代晶格 Fe 将 Zn 掺入针铁矿和纤铁矿中，优先隔离轻的 Zn 同位素，其 ΔZn 分别为-1.52±0.09‰和-1.18±0.15‰，而 Zn 取代的羟高铁矿（0.06±0.11‰）则表明几乎没有同位素分馏。这与 Zn 掺杂 FeOOH 形成过程中不同的晶体成核和生长速率密切相关。这些结果为将同位素轻的 Zn 掺入 Fe（氢）氧化物提供了直接的实验证据，并提高了我们对矿物-溶液界面过程中 Zn 同位素分馏机制的理解。
