New model provides more accurate rate constant estimates for mercury methylation and demethylation.
Using new experiments and re-analyses of previous experiments, a new two-site reversible sorption model was developed to describe the production of methylmercury over time. The new model takes into account competing processes and results in faster rates of production than previously estimated.
Simulations of methylmercury production and transport demonstrate that methylmercury production is likely significantly larger than estimated by currently used models.
Mercury (Hg) is a toxic element that occurs naturally and as an anthropogenic pollutant in the environment. The neurotoxin monomethylmercury (MMHg) is a particular concern because it biomagnifies in aquatic environments and has adverse development effects on young children and developing embryos. MMHg is formed in the environment from inorganic Hg through the action of microorganisms in a process called Hg methylation. Because of its toxicity, there have been many attempts to measure Hg methylation and MMHg demethylation rates in various environmental settings with differing results. Even in laboratory experiments, rates for the methylation of Hg to MMHg often exhibit kinetics that are inconsistent with first-order kinetic models. In a new study, scientists from Oak Ridge National Laboratory used time-resolved measurements of filter-passing Hg and MMHg during methylation/demethylation assays, and they re-analyzed previous assays. Then they used a multi-site kinetic sorption model to show that competing kinetic sorption reactions can lead to apparent non-first-order kinetics in Hg methylation and MMHg demethylation. The new model can describe the range of behaviors for time-resolved methylation/demethylation data reported in the literature including those that exhibit non first-order kinetics. Additionally, the team showed that neglecting competing sorption processes can confound analyses of methylation/demethylation assays, resulting in rate constant estimates that are systematically biased low. Simulations of MMHg production and transport in a hypothetical periphyton biofilm bed illustrate the implications of the new model and demonstrate that methylmercury production may be significantly different than projected by single-rate first-order models.
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This work was funded by the U.S. Department of Energy, Office of Science, Biological and Environmental Research, Subsurface Biogeochemical Research Program and is a product of the Science Focus Area (SFA) at ORNL. The isotopes used in this research were supplied by the United States Department of Energy Office of Science by the Isotope Program in the Office of Nuclear Physics.
Publications Olsen, T.A., K. A. Muller, S. L. Painter, and S. C. Brooks. "Kinetics of Mercury Methylation Revisited" Environmental Science & Technology 52(4), 2063-2070 (2018). [DOI:10.1021/acs.est.7b05152]
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