Environmental Sciences Division , Oak Ridge National Laboratory , Oak Ridge , Tennessee 37830 , United States.
Department of Earth and Environmental Sciences , University of Michigan , Ann Arbor , Michigan 48109 , United States.
Environ Sci Technol. 2019 Feb 19;53(4):1853-1862. doi: 10.1021/acs.est.8b05047. Epub 2018 Nov 27.
Mercury (Hg) stable isotope fractionation has been widely used to trace Hg sources and transformations in the environment, although many important fractionation processes remain unknown. Here, we describe Hg isotope fractionation during the abiotic dark oxidation of dissolved elemental Hg(0) in the presence of thiol compounds and natural humic acid. We observe equilibrium mass-dependent fractionation (MDF) with enrichment of heavier isotopes in the oxidized Hg(II) and a small negative mass-independent fractionation (MIF) owing to nuclear volume effects. The measured enrichment factors for MDF and MIF (εHg and EHg) ranged from 1.10‰ to 1.56‰ and from -0.16‰ to -0.18‰, respectively, and agreed well with theoretically predicted values for equilibrium fractionation between Hg(0) and thiol-bound Hg(II). We suggest that the observed equilibrium fractionation was likely controlled by isotope exchange between Hg(0) and Hg(II) following the production of the Hg(II)-thiol complex. However, significantly attenuated isotope fractionation was observed during the initial stage of Hg(0) oxidation by humic acid and attributed to the kinetic isotope effect (KIE). This research provides additional experimental constraints on interpreting Hg isotope signatures with important implications for the use of Hg isotope fractionation as a tracer of the Hg biogeochemical cycle.
汞（Hg）稳定同位素分馏已被广泛用于追踪环境中 Hg 的来源和转化过程，但许多重要的分馏过程仍不清楚。在此，我们描述了在硫醇化合物和天然腐殖酸存在的情况下，溶解元素 Hg(0)的非生物暗氧化过程中 Hg 同位素分馏。我们观察到了平衡质量分馏（MDF），即氧化后的 Hg(II)中较重同位素的富集，以及由于核体积效应导致的较小的负质量独立分馏（MIF）。实测的 MDF 和 MIF 的富集因子（εHg 和 EHg）分别在 1.10‰至 1.56‰和-0.16‰至-0.18‰之间，与 Hg(0)和硫醇结合的 Hg(II)之间平衡分馏的理论预测值吻合良好。我们认为，观察到的平衡分馏可能是由 Hg(0)和 Hg(II)之间的同位素交换控制的，这是在 Hg(II)-硫醇配合物生成后发生的。然而，在腐殖酸氧化 Hg(0)的初始阶段，观察到了明显减弱的同位素分馏，这归因于动力学同位素效应（KIE）。这项研究为解释 Hg 同位素特征提供了额外的实验约束，对将 Hg 同位素分馏作为 Hg 生物地球化学循环示踪剂的应用具有重要意义。
