Application of multiple stable mercury isotopes to determine the adsorption and desorption dynamics of Hg(II) and MeHg to sediments

Enriched stable isotopes were added to sediment suspensions of a freshwater lake to study the kinetics of Hg(II) and methylmercury partitioning between the dissolved and solid phase. 200Hg(II) and Me199Hg were spiked to lake sediment suspensions and concentrations of dissolved and solid phase mercury species were measured in regular intervals over a 2-day period. Equilibrium concentrations were established for the added 200Hg(II) and Me199Hg within the first 60 min with average partitioning constants (logKd) for total Hg and methylmercury reaching values of 4.3 and 3.9, respectively. Desorption was instantaneous (<30 min). No secondary adsorption and desorption was observed during the duration of the experiment. Spiking with different concentrations of mercury species revealed and inhomogeneous distribution of binding sites. The adsorption process was described using the Freundlich isotherm showing a reduced sorption capacity at lower equilibrium concentrations, suggesting a limited capacity of strong binding sites. Me200Hg formation was observed during the experiment and as much as 3.5% of the added 200Hg(II) tracer was methylated. The results suggest that the vast majority of dissolved mercury species quickly equilibrate with the solid phase.