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Article Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Mercury Stable Isotope Fractionation during Abiotic Dark Oxidation in the Presence of Thiols and Natural Organic Matter Wang Zheng,*,†,# Jason D. Demers,‡ Xia Lu,† Bridget A. Bergquist,§ Ariel D. Anbar,#,∥ Joel D. Blum,‡ and Baohua Gu*,†,⊥ †
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 § Department of Earth Sciences, University of Toronto, 22 Russell Street, Toronto, Ontario M5S 3B1, Canada # School of Earth and Space Exploration and ∥School of Molecular Sciences, Arizona State University, Tempe, Arizona 85287, United States ⊥ Department of Biosystems Engineering and Soil Science, University of Tennessee, Knoxville, Tennessee 37996, United States
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S Supporting Information *
ABSTRACT: 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 massdependent 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 (ε202Hg and E199Hg) 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.
1. INTRODUCTION
Previous studies of Hg redox transformation and isotope fractionation have mostly focused on the reduction process, whereas the oxidation of Hg(0) is not as well-understood. In oxygenated surface aquatic environments, Hg(0) oxidation is largely driven by photochemical processes,7 whereas the dark oxidation of Hg(0) is found to be relatively slow in the absence of light.8 In subsurface anoxic and suboxic conditions where strong oxidants such as O2 are scarce, Hg(0) oxidation is often thought to be negligible. However, our studies have shown that Hg(0) can be oxidized abiotically in anoxic environments driven by free thiol compounds or thiol functional groups in naturally dissolved organic matter (DOM),9−11 and the rate of this dark oxidation pathway is comparable to that of the photooxidation of Hg(0) in surface waters.12 In addition, the dark oxidation of Hg(0) can be mediated by anaerobic bacteria in
Mercury (Hg) is a highly toxic metal and a global pollutant. It can be emitted into the atmosphere from both natural and anthropogenic sources and transported over long distances through atmospheric circulation.1−3 Once deposited to terrestrial and aquatic systems, Hg can be converted to neurotoxic methylmercury (MeHg) by certain anaerobic microbes and subsequently bioaccumulate and biomagnify in food webs,4,5 causing a serious health threat for humans and other animals feeding at high trophic levels. Chemical speciation of Hg, especially the redox transformation between elemental Hg(0) and oxidized Hg(II), plays an important role in mediating the atmospheric Hg deposition and re-emission from surface environments, which, in turn, affect Hg bioavailability and mobility in natural ecosystems. Elemental Hg(0) is highly volatile and is generally considered less reactive and less bioavailable than the oxidized Hg(II).6 It is thus crucial to understand the redox chemistry of Hg and its role in the biogeochemical transformation of Hg. © XXXX American Chemical Society
Received: Revised: Accepted: Published: A
September 7, 2018 October 25, 2018 October 29, 2018 October 29, 2018 DOI: 10.1021/acs.est.8b05047 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology water and sediments via biogenic thiol functional groups.13−17 Therefore, the dark oxidation of Hg(0) in anoxic conditions could be a ubiquitous and important biogeochemical process and play a major role in mediating the exchange of Hg between the surface and subsurface environments and regulating the size of the bioavailable Hg(II) pool in water and sediments. Stable isotope fractionation of Hg has been recognized as a powerful tool that provides insight into the transformation pathways of Hg.18−20 Mercury isotopes exhibit a wide range of mass-dependent fractionation (MDF) in chemical and biological reactions18,21 and at least three types of massindependent fractionation (MIF), including magnetic isotope effect, nuclear volume effect, and the MIF of even-massnumber isotopes.18,20,22,23 Detailed characterization of the MDF and MIF during various natural transformation processes of Hg is thus crucial for using Hg isotope fractionation as a tracer for the Hg biogeochemical cycle. Redox transformations of Hg that have been studied for isotope fractionation include aqueous-phase photoreduction and photodemethylation,24−29 gas-phase photooxidation,30 dark abiotic reduction,31 microbial reduction,32,33 and microbial demethylation.34,35 However, most of these studies focused on reduction or demethylation (i.e., conversion of Hg(II) or methylmercury to Hg(0)), whereas the isotope fractionation during oxidation of Hg(0) is much less well-understood, partly because the experiments for Hg(0) oxidation are more challenging to perform than Hg(II) reduction due to potential Hg(0) volatilization loss. In this study, we investigated Hg stable isotope fractionation and possible fractionation mechanisms during abiotic dark oxidation of dissolved Hg(0) in the presence of natural organic matter and low-molecular-weight (LMW) thiol compounds under anoxic conditions. While the kinetics of these reactions have been described previously in detail,9,10 the present study was undertaken to (1) determine Hg isotope enrichment factors during the dark abiotic oxidation of Hg(0) and (2) investigate the mechanism of Hg isotope fractionation during this process based on both experimental results and previously published theoretical calculations.36
grade BrCl, followed by a thorough rinsing with Millipore deionized water (MQ; 18.2 MΩ cm). All acids used were trace-metal-grade quality, and MQ water was used throughout the experiments. Oxidation by LMW Thiol Compounds. A total of 4 thiol compounds that are commonly observed in natural and biological systems were selected for the thiol experiments: Lcysteine (CYS), 2-mercaptopropionic acid (2-MPA), glutathione (GSH), and mercaptoacetic acid (MCA). A mixed solution of ∼60 nM (12 μg/L) Hg(0) and 5.0 μM thiol (thiol/ Hg = 80:1) was prepared in 9 mL glass vials containing 10 mM deoxygenated phosphate buffer (pH = 7), as previously used in studies of the reactions between Hg and natural organic matter and microorganisms, to maintain a constant pH during experiments.9,10,13,39 The thiol:Hg ratio was chosen based on our previous study to allow Hg(0) to be mostly oxidized within a reasonable time frame.9 The vials were tightly capped with no headspace to avoid volatilization loss of Hg(0), and were wrapped with aluminum foil to avoid light. A series of glass vials containing this solution was prepared for each thiol compound at the beginning of the experiment. At different time points, one vial was removed from the anoxic chamber, and the solution was immediately purged by ultrapure N2 for ∼10 min to remove the remaining Hg(0). The time required to completely remove residual Hg(0) was
