Effects of pH, chloride and Calcium(II) on adsorption of monomethylmercury by soils

Adsorption of monomethylmercury (MMHg) by soil from 2 × 10 −7 and 1 × 10 −6 M solutions was investigated to understand soil‐water partitioning of MMHg. The effects of three factors—pH, Cl − , and Ca 2+ —on adsorption were examined. Adsorption as a function of pH followed a concave downward profile with the maximum adsorption occurring at pH between 5 and 6. Less adsorption at low pH values mainly resulted from the proton competition for surface sites and Cl − complexation for MMHg. Decreases in adsorption at high pH values are attributed to the complexation of MMHg with soluble organic ligands whose concentrations increase with pH. Addition of 1.0 × 10 −3 M CL decreased adsorption at low pH where CH 3 HgCl was dominant over CH 3 HgOH, and exhibited no effect on adsorption at high pH when CH 3 HgOH became predominant. The presence of a low concentration of Ca 2+ (1.0 × 10 −4 M) had no effect on MMHg adsorption; however, a high concentration of Ca 2+ (3.3 × 10 −3 M) increased adsorption at pH < 6 but had no effect at pH > 6. This is ascribed to the combined effects of pH and Ca 2+ on the solubility of organic ligands.