Total and methylmercury partitioning between colloids and true solution: From case studies in sediment overlying and porewaters to a generalized model.

Tangential flow ultrafiltration was used to determine the partitioning of total mercury (THg) and monomethylmercury (MMHg) between colloids and true solution in sediment overlying and porewaters collected in Lake Geneva (Switzerland and France), Venice Lagoon (Italy), and Baihua Reservoir (China). Overlying water and porewater spanned different ranges of THg and MMHg concentrations, redox conditions, and salinity. Total Hg, MMHg, and dissolved organic carbon (DOC) concentrations were measured in filter-passing (<0.45 μm), colloidal (3 kDa-0.45 μm), and truly dissolved (<3 kDa) fractions. The percentages of filterable Hg and MMHg associated with colloids (arithmetic means ±1 standard deviation [SD]) were 29 ± 11% for THg (range, 4-60%) and 44 ± 17% for MMHg (range, 15-65%). Ultrafiltration DOC mass balances were often not satisfactory. However, this was apparently without consequences on THg/MMHg fractionation, suggesting that only a part of total DOC controlled THg/MMHg partitioning in overlying water and porewater. Linear relationships existed between filter passing and truly dissolved concentrations of THg and MMHg, suggesting that mechanisms controlling their partitioning are, at least partly, similar across aquatic systems. These linear relationships could be extended to data from published studies and ultrafilterable concentrations often could be predicted, within a factor of 2, from the measurement of filter-passing ones. The possibility to easily model THg/MMHg partitioning across aquatic systems will facilitate its consideration in general biogeochemical THg/MMHg models.