Formation of Refractory Monomethylmercury Pools by Particle Adsorption

Adsorption of mercury (Hg) onto particles is known to limit Hg availability for important biogeochemical processes, including the methylation of Hg and biological uptake of monomethylmercury (MMHg). However, the extent to which particle adsorption forms refractory Hg pools, i.e., pools not readily available for desorption, remains largely unexplored. Here, we present novel data on the formation of refractory particle-bound MMHg in slurries prepared from environmental soils and waters representing a gradient from terrestrially influenced to marine systems. Refractory pools were quantified from differences in equilibrium partitioning between adsorption (up to 48 h) and desorption (up to 8 weeks) using an isotopically labeled MMHg tracer. Refractory particle-bound MMHg pools formed in all soil-water slurry combinations. The organic carbon content in soil and water increased and decreased partitioning coefficients, respectively, while effects on refractory particulate MMHg formation were less pronounced. These findings demonstrate the importance of considering refractory MMHg pools when predicting the fate and associated environmental risks of MMHg exported from terrestrial systems.