Binding of mercury(II) to reduced sulfur in soil organic matter along upland-peat soil transects

The binding of Hg²⁺ in organic matter of soils and waters controls the transport and transformations of Hg in terrestrial and aquatic ecosystems. We developed a competitive complexation method using the strong complexation of Hg²⁺ by Br^- for determining the Hg²⁺ binding strength in organic soils at native and elevated Hg concentrations. The distribution coefficients determined in KBr suspensions for sorption of native Hg²⁺ to soil organic carbon (SOC) (K(soc)) are in the range of 10²² to 10²³. The K(soc) significantly decreased with increased additions of Hg²⁺ and with decreasing pH. Using data for reduced organic S concentrations determined by x-ray absorption near-edge structure spectroscopy (XANES), we calculated surface complex formation constants on the order of 10³² for a model site having acidity constants of mercaptoacetic acid. This value is in fair agreement with the tabulated value of 10^34.5 for Hg²⁺ binding in mercaptoacetic acid. At native Hg concentrations, formation constants and K(soc) values were similar for different types of soil organic matter along transects from uplands into wetlands, despite varying concentrations of Hg and reduced organic S. Our adsorption data are consistent with the conclusions from our previous extended x-ray absorption fine structure spectroscopy (EXAFS) study that in a humic acid and soil, Hg²⁺ ions bond in two-fold coordination involving one reduced S and one O or N.