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Title: Emerging investigator series: methylmercury speciation and dimethylmercury production in sulfidic solutions

Alkylated mercury species (monomethylmercury, MeHg, and dimethylmercury, DMeHg) exhibit significant bioaccumulation, and pose significant risks to ecosystems and human health. Although decades of research have been devoted to understanding MeHg formation and degradation, little is known about the DMeHg formation in aquatic systems. Here, we combine complementary experimental and computational approaches to examine MeHg speciation and DMeHg formation in sulfidic aqueous solutions, with an emphasis on the formation and decomposition of the binuclear bis(methylmercuric(II)) sulfide complex (CH 3Hg) 2S. Experimental data indicate that the reaction 2CH 3Hg + + HS- $$→\atop{←}$$ (CH 3Hg) 2S + H + has a log K = 26.0 ± 0.2. Thus, the binuclear (CH 3Hg) 2S complex is likely to be the dominant MeHg species under high MeHg concentrations typically used in experimental investigations of MeHg degradation by sulfate-reducing bacteria (SRB). Our finding of a significant abiotic removal mechanism for MeHg in sulfidic solutions through the formation of relatively insoluble (CH 3Hg) 2S suggests careful reexamination of reported “oxidative demethylation” of MeHg by SRB and perhaps other obligate anaerobes. We provide evidence for slow decomposition of (CH 3Hg) 2S to DMeHg and HgS, with a first-order rate constant k = 1.5 ± 0.4 × 10 -6 h -1. Quantum chemical calculations suggest that the reaction proceeds by a novel mechanism involving rearrangement of the (CH 3Hg) 2S complex facilitated by strong Hg–Hg interactions that activate a methyl group for intramolecular transfer. Predictions of DMeHg formation rates under a variety of field and laboratory conditions indicate that this pathway for DMeHg formation will be significant in laboratory experiments utilizing high MeHg concentrations, favoring (CH 3Hg) 2S formation. Finally, in natural systems with relatively high MeHg/[H 2S] T ratios (the oxic/anoxic interface, for example), DMeHg production may be observed, and warrants further investigation.
Authors:
 [1] ;  [2] ; ORCiD logo [1] ;  [1] ; ORCiD logo [3] ; ORCiD logo [4] ; ORCiD logo [1]
  1. Grinnell College, Grinnell, IA (United States). Dept. of Chemistry
  2. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Biosciences Division; Univ. of Tennessee, Knoxville, TN (United States). Dept. of Biochemistry and Cellular and Molecular Biology
  3. Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Environmental Molecular Sciences Lab. (EMSL)
  4. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Biosciences Division
Publication Date:
Grant/Contract Number:
AC05-00OR22725; AC05-76RL01830
Type:
Accepted Manuscript
Journal Name:
Environmental Science: Processes & Impacts
Additional Journal Information:
Journal Volume: 20; Journal Issue: 4; Journal ID: ISSN 2050-7887
Publisher:
Royal Society of Chemistry
Research Org:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Sponsoring Org:
USDOE Office of Science (SC), Biological and Environmental Research (BER) (SC-23)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; 54 ENVIRONMENTAL SCIENCES
OSTI Identifier:
1458372

Kanzler, Charlotte R., Lian, Peng, Trainer, Emma Leverich, Yang, Xiaoxuan, Govind, Niranjan, Parks, Jerry M., and Graham, Andrew M.. Emerging investigator series: methylmercury speciation and dimethylmercury production in sulfidic solutions. United States: N. p., Web. doi:10.1039/C7EM00533D.
Kanzler, Charlotte R., Lian, Peng, Trainer, Emma Leverich, Yang, Xiaoxuan, Govind, Niranjan, Parks, Jerry M., & Graham, Andrew M.. Emerging investigator series: methylmercury speciation and dimethylmercury production in sulfidic solutions. United States. doi:10.1039/C7EM00533D.
Kanzler, Charlotte R., Lian, Peng, Trainer, Emma Leverich, Yang, Xiaoxuan, Govind, Niranjan, Parks, Jerry M., and Graham, Andrew M.. 2018. "Emerging investigator series: methylmercury speciation and dimethylmercury production in sulfidic solutions". United States. doi:10.1039/C7EM00533D.
@article{osti_1458372,
title = {Emerging investigator series: methylmercury speciation and dimethylmercury production in sulfidic solutions},
author = {Kanzler, Charlotte R. and Lian, Peng and Trainer, Emma Leverich and Yang, Xiaoxuan and Govind, Niranjan and Parks, Jerry M. and Graham, Andrew M.},
abstractNote = {Alkylated mercury species (monomethylmercury, MeHg, and dimethylmercury, DMeHg) exhibit significant bioaccumulation, and pose significant risks to ecosystems and human health. Although decades of research have been devoted to understanding MeHg formation and degradation, little is known about the DMeHg formation in aquatic systems. Here, we combine complementary experimental and computational approaches to examine MeHg speciation and DMeHg formation in sulfidic aqueous solutions, with an emphasis on the formation and decomposition of the binuclear bis(methylmercuric(II)) sulfide complex (CH3Hg)2S. Experimental data indicate that the reaction 2CH3Hg+ + HS- $→\atop{←}$ (CH3Hg)2S + H+ has a log K = 26.0 ± 0.2. Thus, the binuclear (CH3Hg)2S complex is likely to be the dominant MeHg species under high MeHg concentrations typically used in experimental investigations of MeHg degradation by sulfate-reducing bacteria (SRB). Our finding of a significant abiotic removal mechanism for MeHg in sulfidic solutions through the formation of relatively insoluble (CH3Hg)2S suggests careful reexamination of reported “oxidative demethylation” of MeHg by SRB and perhaps other obligate anaerobes. We provide evidence for slow decomposition of (CH3Hg)2S to DMeHg and HgS, with a first-order rate constant k = 1.5 ± 0.4 × 10-6 h-1. Quantum chemical calculations suggest that the reaction proceeds by a novel mechanism involving rearrangement of the (CH3Hg)2S complex facilitated by strong Hg–Hg interactions that activate a methyl group for intramolecular transfer. Predictions of DMeHg formation rates under a variety of field and laboratory conditions indicate that this pathway for DMeHg formation will be significant in laboratory experiments utilizing high MeHg concentrations, favoring (CH3Hg)2S formation. Finally, in natural systems with relatively high MeHg/[H2S]T ratios (the oxic/anoxic interface, for example), DMeHg production may be observed, and warrants further investigation.},
doi = {10.1039/C7EM00533D},
journal = {Environmental Science: Processes & Impacts},
number = 4,
volume = 20,
place = {United States},
year = {2018},
month = {1}
}

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