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DIVISION S-2-SOIL CHEMISTRY

Adsorption of Mercury(II) by Kaolinite

^a Earth and Physical Sci. Div., Univ. of Texas, San Antonio, TX 78249-0663 USA
^b Dep. of Plant and Soil Sci., The Univ. of Tennessee, Knoxville, TN 37901-1071 USA
^c Dep. of Geological Sci., The Univ. of Tennessee, Knoxville, TN 37996-1410. Contribution from the Dep. of Plant and Soil Sciences, The Univ. of Tennessee USA

messington{at}utk.edu

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 pH₅₀ from 3.4 to 7 and 4.1, respectively. Nickel and Pb reduced the amount of Hg(II) adsorbed throughout the pH range examined. Ionic strength and the presence of SO₄ and PO₄ had relatively little impact on the Hg(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 ^-₂ and SiOHgCl⁰ or SiOHgOHCl^- (Cl system) inner-sphere species. The AlO^--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 AlOH sites on kaolinite should be considered to predict adequately Hg(II) retention.

Abbreviations: IS, ionic strength • TLM, triple layer model

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