胡敏酸对高岭石吸附铜离子的强化作用

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

考察酸性条件特别是在近中性 pH范围内胡敏酸对高岭石吸附铜离子的强化作用。研究表明 ,胡敏酸的加入可以提高高岭石对铜离子的吸附率 ,甚至在pH 5～ 6附近高岭石对铜离子的吸附率也从约 5 0 %提高到约 6 5 %。当 pH <4时 ,由于高岭石表面铝的高溶出或胡敏酸阴离子基团离解程度降低等因素 ,使其表面对胡敏酸的吸附率有所降低 ,但与高岭石样品相比 ,胡敏酸高岭石复合体对铜离子的吸附仍然有明显的增加。胡敏酸对高岭石吸附铜离子的强化机制是 ,高岭石端面形成了Al—HA—Cu三元配合物 (B型 ) ,与传统的诸如pH、离子强度与离子初始浓度等介质条件影响不同。在 pH >7时高岭石端面及腐殖质基团去质子化增强 ,因而静电排斥降低了高岭石对胡敏酸的吸附 ,从而使得胡敏酸对铜离子在高岭石表面上的吸附作用有所减弱 ,此时可能出现胡敏酸铜及氢氧化铜的沉淀 。

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 pH<4 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 pH>7 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 pH>7 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.