February 16, 2018

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Clarifying Rates of Methylmercury Production

New model provides more accurate rate constant estimates for mercury methylation and demethylation.

The Science

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.

The Impact

Simulations of methylmercury production and transport demonstrate that methylmercury production is likely significantly larger than estimated by currently used models.

Summary

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 mercury through the action of microorganisms in a process called mercury methylation. Because of its toxicity, there have been many attempts to measure Hg methylation and MMHg demthylation 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 mercury and MMHg during methylation or demethylation assays, and they re-analyzed previous assays. Then they used a multisite 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 and 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 that projected by single-rate, first-order models.

Principal Investigator

Scott Brooks
Oak Ridge National Laboratory
brookssc@ornl.gov

Program Manager

Paul Bayer
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
paul.bayer@science.doe.gov

Funding

This work was funded by the Subsurface Biogeochemical Research program of the Office of Biological and Environmental Research (BER), within the U.S. Department of Energy (DOE) Office of Science, and is a product of the Mercury Science Focus Area (SFA) at Oak Ridge National Laboratory. The isotopes used in this research were supplied by the DOE Office of Science through the Isotope Program of the DOE Office of Nuclear Physics.

References

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). http://doi.org/10.1021/acs.est.7b05152.