Demethylation or Sorption? The Fate of Methylmercury in the Presence of Manganese Dioxide

Manganese oxide (MnO2) amendments to mercury (Hg)-contaminated sediment have been shown to decrease methylmercury (MeHg) concentrations in sediment porewater. In the absence of solid-phase MeHg measurements, it is unclear whether the decreased porewater concentrations are due to inhibition of methylation, adsorption of the MeHg to the MnO2, demethylation of MeHg catalyzed by the MnO2, or some combination of these mechanisms. We conducted controlled laboratory experiments to determine whether MeHg losses from solution in the presence of amorphous MnO2 are due to sorption or demethylation. We quantified MeHg in the dissolved phase and solid phase and found that no demethylation occurred. We used batch and isotherm experiments to determine MeHg sorption efficiency to MnO2 in a variety of solutions with varying ionic strength and dissolved organic matter (DOM) concentration. Isotherms over an initial concentration range of 5-500 ng/L of MeHg showed no saturation effects. Increasing ionic strength from 0.012 M to 0.1 M produced relatively minor decreases in MeHg sorption. Increasing DOM concentrations from 0.64 to 16.4 mg C/L yielded substantial decreases in MeHg sorption. Under the experimental conditions, MnO2 is a less efficient MeHg sorbent compared to other sorbent materials, such as Thiol-SAMMS®, activated carbon, and biochar.