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

Adsorption of monomethylmercury (MMHg) by soil from 2 X 10(-7) and 1 X 10(-6) M solutions was investigated to understand soil-water partitioning of MMHg. The effects of three factors-pH, Cl-, and Ca2+-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 X 10(-3) M Cl- decreased adsorption at low pH where CH3HgCl was dominant over CH3HgOH, and exhibited no effect on adsorption at high pH when CH3HgOH became predominant. The presence of a low concentration of Ca2+ (1.0 X 10(-4) M) had no effect on MMHg adsorption; however. a high concentration of Ca2+ (3.3 X 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 Ca2+ on the solubility of organic ligands.