Emerging investigator series: Methylmercury speciation and dimethylmercury production in sulfidic solutions

Charlotte R. Kanzler; Peng Lian; Emma Leverich Trainer; Xiaoxuan Yang; Niranjan Govind; Jerry M. Parks; Andrew M. Graham

doi:10.1039/c7em00533d

Emerging investigator series: Methylmercury speciation and dimethylmercury production in sulfidic solutions

Charlotte R. Kanzler, Peng Lian, Emma Leverich Trainer, Xiaoxuan Yang, Niranjan Govind, Jerry M. Parks, Andrew M. Graham

Molecular Biophysics

Research output: Contribution to journal › Article › peer-review

24 Scopus citations

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 + H⁺ has a logK = 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. 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.

Original language	English
Pages (from-to)	584-594
Number of pages	11
Journal	Environmental Science: Processes and Impacts
Volume	20
Issue number	4
DOIs	https://doi.org/10.1039/c7em00533d
State	Published - Apr 2018

Funding

We thank So Jonsson and Scott Brooks for substantive comments on earlier versions of the manuscript, Ryne C. Johnston and Liyuan Liang for helpful discussions, and T. Andrew Mobley for insightful suggestions. This research was funded through the Grinnell College Mentored Advanced Project program. This work was also supported by the U.S. Department of Energy (DOE) Office of Science, Biological and Environmental Research, Subsurface Biogeochemical Research (SBR) Program through the Mercury Science Focus Area Program (SFA) at Oak Ridge National Laboratory (ORNL). ORNL is managed by UT-Battelle LLC for the U.S. DOE under contract number DE-AC05-00OR22725. This research also benetted from computational resources and expertise at the Environmental Molecular Sciences Laboratory (EMSL), a U. S. DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research (BER) and located at Pacic Northwest National Laboratory (PNNL), through Rapid Access award 50011 to JMP. PNNL is a multi-program national laboratory operated for the U.S. DOE by Battelle under contract DE-AC05-76RL01830.

Funders	Funder number
Office of Biological and Environmental Research
Office of Science, Biological and Environmental Research	SBR
U. S. DOE Office of Science User Facility
U.S. DOE	DE-AC05-00OR22725
UT-Battelle LLC
U.S. Department of Energy
Battelle	DE-AC05-76RL01830
Biological and Environmental Research	50011, PNNL
Oak Ridge National Laboratory	ORNL
Stephen F. Austin State University

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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 + H+ has a logK = 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. 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.",

author = "Kanzler, {Charlotte R.} and Peng Lian and Trainer, {Emma Leverich} and Xiaoxuan Yang and Niranjan Govind and Parks, {Jerry M.} and Graham, {Andrew M.}",

note = "Publisher Copyright: {\textcopyright} The Royal Society of Chemistry 2018.",

year = "2018",

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doi = "10.1039/c7em00533d",

language = "English",

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T2 - Methylmercury speciation and dimethylmercury production in sulfidic solutions

AU - Kanzler, Charlotte R.

AU - Lian, Peng

AU - Trainer, Emma Leverich

AU - Yang, Xiaoxuan

AU - Govind, Niranjan

AU - Parks, Jerry M.

AU - Graham, Andrew M.

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AB - 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 + H+ has a logK = 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. 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.

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Emerging investigator series: Methylmercury speciation and dimethylmercury production in sulfidic solutions

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