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Effects of pH, Cl−, and organic matter on Hg(II) adsorption from 10−7 M solution by 15 New Jersey soils were examined to understand the factors responsible for Hg partitioning to soils under different environmental conditions. Maximum adsorption ranged from 86 to 98% of added Hg and occurred at pH 3 to 5. Further increasing the pH significantly decreased adsorption of the added Hg, for example, from 89 ± 8% at pH 4.0 to 39 ± 11% at pH 8.5. An important factor was the complexation of Hg by dissolved organic matter whose concentration increased with increasing pH. When organic matter was removed from the soil samples, adsorption decreased under acidic conditions, but increased under alkaline conditions. The effect of Cl− on Hg(II) adsorption by soil depended on both pH and the soil organic matter content. At circumneutral and higher pH, addition of Cl− did not affect adsorption by any soil. The standard deviation of Hg adsorption for all soils was <4% for Cl− concentrations from about 1 × 10−6 to 1 × 10−2 M. At about pH 3, the effect of Cl− on Hg(II) adsorption by soil depended on the soil organic matter content. When the Cl− concentration increased from about 1 × 10−6 to 1 × 10−2 M, adsorption by the lowest organic matter soil (1.2 g C kg−1) decreased from 93 to 40%, whereas the measured adsorption by the largest organic matter soil (49.9 g C kg−1) decreased only from 95 to 91%.

Adsorption and desorption kinetics of Hg(II) on four soils at pH 6 were investigated to discern the mechanisms controlling the retention and release reaction rates of Hg(II) on soil. A stirred-flow method was employed to perform experiments. Apparent adsorption and desorption rate coefficients were determined by a one-site second-order kinetic model. Both adsorption and desorption were characterized by a biphasic pattern, a fast step followed by a slow step. After 2 min, the Hg(II) adsorbed for an 8 mg L -1 influent accounted for 4-38% of the total Hg(II) adsorbed within 5 h. Of the Hg(II) released within 8 h, 62-81% was desorbed during the first 100 min. Both adsorption and desorption rate coefficients were inversely correlated with the soil organic C content. Not all adsorbed Hg(II) was readily released. The greater the soil organic C content, the higher the fraction of Hg(II) that was resistant to desorption. The diffusion of Hg(II) through intraparticle micropores of soil organic matter may be the principal factor responsible for the observed irreversibility. In addition, the binding of Hg(II) to high affinity sites on soil organic matter, such as the S-containing (-S) groups, may also be important to Hg(II) persistence in soils.

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