Uptake Mechanisms of a Novel, Activated Carbon-Based Equilibrium Passive Sampler for Estimating Porewater Methylmercury

Abstract

Here we describe the validation of a novel polymeric equilibrium passive sampler comprised of agarose gel with embedded activated carbon particles (ag+AC), to estimate aqueous monomethylmercury (MeHg) concentrations. Sampler behavior was tested using a combination of idealized media and realistic sediment microcosms. Isotherm bottle experiments with ag+AC polymers were conducted to constrain partitioning to these materials by various environmentally relevant species of MeHg bound to dissolved organic matter (MeHgDOM) across a range of sizes and character. Partitioning coefficients ranged from log(Kps) = 1.98±0.09 for MeHg bound to Suwannee River Humic Acid to log(Kps) = 3.15±0.0.52 for MeHg complexed with Upper Mississippi River Natural Organic Matter. Reversible equilibrium exchange of environmentally relevant MeHg species was demonstrated through a series of dual isotope labeled exchange experiments. Isotopically labeled MeHgDOM species approached equilibrium in the samplers over 14 d while mass balance was maintained, providing strong evidence the ag+AC polymer material is capable of equilibrium measurements of environmentally relevant MeHg species within a reasonable deployment time frame. Samplers deployed across the sediment-water interface of sediment microcosms estimated both overlying water and porewater MeHg concentrations within a factor of 2 to 4 of measured values, based on the average measured Kps values for species of MeHg bound to natural organic matter (MeHgNOM) in the isotherm experiments. Taken together, our results indicate that ag+AC polymers, used as equilibrium samplers, can provide accurate MeHg estimations across many site chemistries, with a simple back calculation based on a standardized Kps.