Kinetic controls on the complexation between mercury and dissolved organic matter in a contaminated environment

The interaction of mercury (Hg) with dissolved natural organic matter (NOM) under equilibrium conditions is the focus of many studies but the kinetic controls on Hg-NOM complexation in aquatic systems have often been overlooked. We examined the rates of Hg-NOM complexation both in a contaminated Upper East Fork Poplar Creek (UEFPC) in Oak Ridge, Tennessee, and in controlled laboratory experiments using reducible Hg (HgR) measurements and C₁₈ solid phase extraction techniques. Of the filterable Hg at the headwaters of UEFPC, >90% was present as Hg_R and this fraction decreased downstream but remained >29% of the filterable Hg at all sites. The presence of higher Hg_R concentrations than would be predicted under equilibrium conditions in UEFPC and in experiments with a NOM isolate suggests that kinetic reactions are controlling the complexation between Hg and NOM. The slow formation of Hg-NOM complexes is attributed to competitive ligand exchange among various moieties and functional groups in NOM with a range of binding strengths and configurations. This study demonstrates the need to consider the effects of Hg-NOM complexation kinetics on processes such as Hg methylation and solid phase partitioning.