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

Abstract

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- = (CH3Hg)2S has a log K = 26.0. 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 x 10-6 h-1. Quantum chemical calculations suggest that the reaction proceeds by a novel mechanism involving rearrangement of the (CH3Hg)2Smore » 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. 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.Experimental and computational evidence show that dimethylmercury is produced from decomposition of bis(methylmercury(ii)) sulfide.« less

Authors:
 [1];  [2]; ORCiD logo [1];  [1]; ORCiD logo [3]; ORCiD logo [4]; ORCiD logo [1]
  1. Grinnell College Department of Chemistry; Grinnell; USA
  2. Biosciences Division; Oak Ridge National Laboratory; Oak Ridge; USA; University of Tennessee
  3. Environmental Molecular Sciences Laboratory; Pacific Northwest National Laboratory; Richland; USA
  4. Biosciences Division; Oak Ridge National Laboratory; Oak Ridge; USA
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Environmental Molecular Sciences Lab. (EMSL)
Sponsoring Org.:
USDOE
OSTI Identifier:
1439665
Report Number(s):
PNNL-SA-130738
Journal ID: ISSN 2050-7887; ESPICZ; 49868; KP1704020
DOE Contract Number:  
AC05-76RL01830
Resource Type:
Journal Article
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
Country of Publication:
United States
Language:
English
Subject:
Environmental Molecular Sciences Laboratory

Citation Formats

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., 2018. 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. https://doi.org/10.1039/C7EM00533D
Kanzler, Charlotte R., Lian, Peng, Trainer, Emma Leverich, Yang, Xiaoxuan, Govind, Niranjan, Parks, Jerry M., and Graham, Andrew M.. Mon . "Emerging investigator series: methylmercury speciation and dimethylmercury production in sulfidic solutions". United States. https://doi.org/10.1039/C7EM00533D.
@article{osti_1439665,
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- = (CH3Hg)2S has a log K = 26.0. 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 x 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. 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.Experimental and computational evidence show that dimethylmercury is produced from decomposition of bis(methylmercury(ii)) sulfide.},
doi = {10.1039/C7EM00533D},
url = {https://www.osti.gov/biblio/1439665}, journal = {Environmental Science: Processes & Impacts},
issn = {2050-7887},
number = 4,
volume = 20,
place = {United States},
year = {2018},
month = {1}
}

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