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

Accurate, direct measurement of aqueous monomethylmercury (MeHg) is challenging, and there remains a need for a simple, standardized passive sampling system for MeHg. Several challenges in the development of an equilibrium passive sampler for MeHg within waters include convincingly demonstrating reversible equilibrium with a given sampler and the establishment of partitioning coefficients (Kps) for environmentally relevant MeHg species. Here we describe the validation, with specific experiments designed to improve our ability to meet those key challenges, of a novel polymeric equilibrium passive sampler comprised of agarose gel with embedded activated carbon particles (ag+AC) to estimate MeHg concentrations. We conducted isotherm bottle experiments with ag+AC polymers to constrain partitioning of various environmentally relevant species of MeHg bound to dissolved organic matter (MeHgDOM) across a range of DOM sizes and sources, including small organic thiols using cysteine as a model ligand, as MeHgDOM is expected to be the dominant MeHg species across a wide range of aquatic environments. Sampler-water partitioning coefficients ranged from log(Kps) = 1.98±0.09 for MeHg bound to Suwannee River Humic Acid to log(Kps) = 3.01±0.23 for MeHg complexed with cysteine. Reversible equilibrium was demonstrated for MeHgDOM species in a series of isotopically labeled exchange experiments, in which we also compared the kinetics of adsorption and desorption of MeHg species to evaluate the potential use of stable isotope-labeled MeHg as performance reference compounds. 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. 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.

Authors: Spencer Washburn, Cynthia Gilmour, Upal Ghosh, Jada Damond, James Sanders