Yin Hui, Yan Xinran, Zhu Chuanwei, Kwon Kideok D, Gu Xueyuan, Zhao Wei, Lanson Bruno, Li Wei, Ma Jingyuan, Li Yan, Li Jiangshan, Qiu Guohong, Feng Xionghan, Tan Wenfeng, Wen Hanjie, Huang Qiaoyun, Liu Fan
Hubei Key Laboratory of Soil Environment and Pollution Remediation, College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, China.
School of Earth Sciences and Resources, Chang'An University, Xi'an 710054, China.
Environ Sci Technol. 2024 Jul 18. doi: 10.1021/acs.est.4c03808.
Cadmium (Cd) geochemical behavior is strongly influenced by its adsorption onto natural phyllomanganates, which contain both layer edge sites and vacancies; however, Cd isotope fractionation mechanisms at these sites have not yet been addressed. In the present work, Cd isotope fractionation during adsorption onto hexagonal (containing both types of sites) and triclinic birnessite (almost only edge sites) was investigated using a combination of batch adsorption experiments, extended X-ray absorption fine structure (EXAFS) spectroscopy, surface complexation modeling, and density functional theory (DFT) calculations. Light Cd isotopes are preferentially enriched on solid surfaces, and the isotope fractionation induced by Cd adsorption on edge sites (ΔCd = -1.54 ± 0.11‰) is smaller than that on vacancies (ΔCd = -0.71 ± 0.21‰), independent of surface coverage or pH. Both Cd K-edge EXAFS and DFT results indicate the formation of double corner-sharing complexes on layer edge sites and mainly triple cornering-sharing complexes on vacancies. The distortion of both complexes results in the negative isotope fractionation onto the solids, and the slightly longer first Cd-O distances and a smaller number of nearest Mn atoms around Cd at edge sites probably account for the larger fractionation magnitude compared to that of vacancies. These results provide deep insights into Cd isotope fractionation mechanisms during interactions with phyllomanganates.
镉（Cd）的地球化学行为受其在天然水锰矿上的吸附作用强烈影响，天然水锰矿同时含有层边缘位点和空位；然而，这些位点的镉同位素分馏机制尚未得到研究。在本工作中，通过批量吸附实验、扩展X射线吸收精细结构（EXAFS）光谱、表面络合建模和密度泛函理论（DFT）计算相结合的方法，研究了镉在六方水锰矿（同时含有两种类型的位点）和三斜水钠锰矿（几乎只有边缘位点）上吸附过程中的同位素分馏。轻镉同位素优先富集在固体表面，镉在边缘位点吸附引起的同位素分馏（ΔCd = -1.54 ± 0.11‰）小于在空位上的分馏（ΔCd = -0.71 ± 0.21‰），与表面覆盖率或pH无关。镉K边EXAFS和DFT结果均表明，在层边缘位点形成了双角共享络合物，在空位上主要形成三角共享络合物。两种络合物的畸变导致了向固体的负同位素分馏，与空位相比，边缘位点处镉周围稍长的第一Cd-O距离和较少的最近邻锰原子数量可能是造成更大分馏幅度的原因。这些结果为与水锰矿相互作用过程中的镉同位素分馏机制提供了深入见解。
