Adsorption of mercury(II) by kaolinite

Adsorption of Hg(II) by kaolinite was investigated as a function of solution pH, ionic strength, and the competitive or complexation effects of ligands (Cl, SO₄, PO₄) and metals (Ni and Pb). Mercury(II) adsorption from a 0.6 μM Hg(II) solution was primarily influenced by pH. The Hg(II) adsorption edge was described by a pH₅₀(pH where 50% adsorption occurs) of 3.4 and a pH_max(pH where maximum adsorption occurs) of 4.4. At pH values above the pH_max, Hg(II) retention decreased with increasing pH. Chloride and Ni shifted ph₅₀from 3.4 to 7 and 4.1, respectively. Nickel and Pb reduced the amount of Hg(II) adsorbed througout the pH range examined. Ionic strength and the presence of SO₄ and PO₄ had relatively little impact on the I Ig(II) adsorption envelope. The adsorption of Hg(II) was predicted through the application of the triple layer model (TLM) by assuming that the kaolinite surface was composed of equal proportions of silanol and aluminol groups. The TLM model suggests that the silanol group was responsible for retaining the bulk of the adsorbed Hg(II), through the formation of the ≡SiO^-.HgOH⁺ outer-sphere, and the ≡SiOHg (OH)₂^- and ≡SiOHgCl⁰ or ≡SiOHgOHCl^- (CI system) inner-sphere species. The ≡AIO^-HgOH⁺ outer-sphere complex accounted for a small percentage (<15-35 %) of the adsorbed Hg(II). The TLM results suggested that Hg(II) adsorption by both ≡SiOH and ≡AIOH sites on kaolinite should be considered to predict adequately Hg(II) retention.