Critical review of mercury methylation and methylmercury demethylation rate constants in aquatic sediments for biogeochemical modeling

Abstract

Mercury is a toxin that causes neurological impairments in adults, is particularly harmful for fetuses and children, and is deadly in severe cases, making it a worldwide health concern. Methylmercury (MeHg) is the environmentally relevant form of mercury (Hg) because it biomagnifies along the food chain. Methylmercury is mainly produced in aquatic sediments via methylation of inorganic Hg (Hg(II)) and transformed back via demethylation. Because transformation rates determine MeHg concentrations, quantification of methylation and demethylation rates is needed to inform management of MeHg. Published rate constants for Hg(II) methylation ($k_m$) and MeHg demethylation ($k_d$) vary greatly, stemming partly from differences in experimental methods. We conducted a comprehensive review of rate laws, evaluated published rate constants, and performed biogeochemical simulations to assess variability in reported $k_m$ and $k_d.$ Based on selected studies employing the same pseudo-first-order rate law and similar experimental methods, we found that $k_m=0.04\pm 0.03\mathrm{ }{\mathrm{d}}^{-1}$ is a reasonable range for wetland sediments. Over a number of environments, maximum $k_d$ was smaller at sites without Hg source ($k_d=0.5\mathrm{ }{\mathrm{d}}^{-1}$) than at sites with identified Hg source ($k_d=1.8\mathrm{ }{\mathrm{d}}^{-1}$). Larger variability and higher uncertainty in $k_d$ compared to $k_m$ highlight the need for more research on MeHg demethylation rates. This critical review: (a) aids the design of future experimental studies of $k_m$ and $k_d;$ (b) provides guidance for comparing rate constants from different studies; (c) presents a biogeochemical reaction model to assess rate constants; and (d) informs selection of $k_m$ and $k_d$ values from the literature for use in model simulations.

Graphical abstract

Keywords:

• Biogeochemical simulation
• kinetics
• mercury
• mercury isotope tracer
• methylmercury
• rate law

HANDLING EDITORS:

• Jörg Rinklebe and Lena Ma

Acknowledgments

Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the Delta Stewardship Council. This journal article has been peer reviewed and approved for publication consistent with USGS Fundamental Science Practices (https://pubs.usgs.gov/circ/1367/). We thank C. Gilmour and G. Schwartz for providing experimental data. We thank C. Gilmour, M. Marvin-DiPasquale, and three anonymous reviewers for their helpful reviews of this manuscript. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. government.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This material is based upon work supported by the Delta Stewardship Council Delta Science Fellows Program and the Delta Science Program in partnership with the California Department of Fish and Wildlife (Contract #18208).