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

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 ± 0.03 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 d⁻¹) than at sites with identified Hg source (𝑘_𝑑 = 1.8 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.