Evaluation of centrifugal ultrafilters for size fractionation of total mercury and methylmercury in freshwaters

Environmental context Although mercury associated with colloids is an important part of the aquatic Hg cycle, there is currently no fast and reliable method to separate complexes smaller than traditional filter pore sizes. We test commercially available centrifugal ultrafilters for their applicability to size fractionation of total Hg and methylmercury in freshwaters. Sorption of Hg onto the filters precludes their use for fractionation of inorganic Hg, the approach proved to be very suitable for methylmercury fractionation regardless of sample organic matter content. Abstract Amicon Ultra-15 centrifugal filters with nominal molecular weight cut-offs of 100, 30 and 3kDa, were tested for separating Hg complexes in freshwaters. Experiments used Hg-contaminated water from East Fork Poplar Creek (EFPC) and laboratory-prepared Hg solutions containing Suwannee River natural organic matter (SR-NOM). Investigations focussed on Hg and dissolved organic carbon blank levels, Hg sorption and leaching, Hg mass balance closure and spike recoveries of inorganic and methylmercury (MeHg). Hg spike recoveries for EFPC samples were low (57±16%, n≤30) due to sorption. MeHg recovery averaged 87±9% (n≤15) suggesting it was less affected by sorptive losses. SR-NOM samples yielded similar dissolved organic matter (DOM) and MeHg size fractionation patterns with ∼20% of the MeHg found in the less than 3-kDa fraction. Overall, the distribution of MeHg followed a pattern similar to the DOM, indicating the importance of both sample DOM quantity and quality for MeHg partitioning in aquatic systems. Although the use of these ultrafilters for inorganic Hg in freshwater samples is not recommended, they were successfully used for MeHg in EFPC where the majority of MeHg was found to be either dissolved or associated with phases smaller than 3kDa.