The role of humic acids in the enhancement of copper (II) adsorption on kaolinite

The enhancement of copper (Ⅱ) adsorption on kaolinite in the presence of humic acids under acidic conditions, in particular near the neutral pH range of pH 5～6, was investigated. In the experiment, the addition of humic acids increased the adsorption percentage of copper (Ⅱ) on kaolinite, and even around pH 5～6 the adsorption percentage of copper (Ⅱ) increased from ca. 50% to ca. 65% in the presence of humic acids. As the high Al/Si dissolution ratio of kaolinite and the low dissociation extent of negative ligand of humic acids at pH4 were not favorable for humic acid adsorption, a decrease in the adsorption percentage of humic acid on kaolinite surface was observed. However, in comparison with kaolinite, the adsorption percentage of copper on humic-kaolinite complexes increased obviously to some extent. The enhancement mechanism of copper adsorption on kaolinite surface was the formation of ternary complexes of (Al)()()-HA-Cu (B-type) at the edge of kaolinite, unlike the effects of the traditionally-considered factors such as pH, electrolyte concentration and initial ion concentration. As simultaneous deprotonation of the groups of both the humic acid and the kaolinite edge at pH7 was intensified, and electrostatic repulsion among the groups reduced the adsorption percentage of humic acid on kaolinite, a decrease in copper (Ⅱ) adsorption on kaolinite occurred in the presence of humic acids. Nevertheless, at pH7 the precipitation of humic-copper (Ⅱ) complexes and Cu(OH)_2 probably took place, and the apparent sorption percentage of copper (Ⅱ) on kaolinite only changed insignificantly.