Kinetics of Mercury(II) Adsorption and Desorption on Soil

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.