高岭石对重金属离子的吸附机理及其溶液的pH条件

Mechanisms of Adsorption of Heavy Metal Ions on Kaolinite and Their Solution as a Function of pH

高岭石对Cu2 + ,Pb2 + 离子的吸附实验及高岭石的溶解实验表明,高岭石对重金属离子的吸附有别于石英单一表面配位模式,离子交换和表面配位模式并存,并随溶液pH由酸性往碱性的变化发生规律性的演替:pH <6 .5时主要表现为外圈层配位的离子交换吸附,且在pH <4时由于受到高岭石表层中铝的高溶出及溶液中较高离子强度的影响,高岭石对Cu2 + ,Pb2 + 离子的吸附率较低,pH为5～6时由于高岭石端面的荷电性为近中性,吸附率则有明显的提升并且表现为一个吸附平台;pH >6 .5时离子交换和表面配位均为重要吸附机制,pH再升高时沉淀机制则起着重要作用。研究表明,pH调控高岭石-水界面溶解与质子化-去质子化反应过程,并影响着Cu2 + ,Pb2 + 离子的吸附行为。最后采用Sverjensky (1993)表面配位的物理模型对吸附结果作了描述。

The adsorption behavior of heavy metal ions ( Cu^(2+) and Pb^(2+)) to kaolinite was examined and the kaolinite dissolution was conducted in batch experiments as a function of pH and ion strength (electrolyte concentration). The pH_(PNZPC) of kaolinite in NaNO_3 was determined as 5.2, but the aluminol groups (>AlOH ) were deprotonated at pH >6.5. The adsorption experimental results showed that the modes in adsorption of Cu (II) and Pb (II) ions onto kaolinite were quite different from that to quartz. The adsorption mechanisms could be well interpreted in term of two interfacial adsorption modes: an ion-exchange and a surface complexation coexisting with the adsorption processes of Cu (II) and Pb (II) in the case of kaolinite. However, only single surface complexation for quartz was detected. The succession of adsorption models of Cu (II) and Pb (II) to the kaolinite with increasing pH is as follows:(1) The adsorption isotherms at pH<6.5 could be elucidated mainly by ion-exchange at the permanently-charged site on the silanol faces, suggesting that ion-exchange model was major contributor to the adsorption of Cu (II) and Pb (II) adsorbed to kaolinite. Being affected by both the aluminum ion dissolved from kaolinite and the ion strength (too high) under condition of acidic pH (<4), the sorption percentage of heavy metal ions to kaolinite was very low; (2) An adsorption plateau around pH= 5～6 characterizes adsorption edges of Cu (II) and Pb(II) ions, because the kaolinite crystal edges almost carried neutral charge in this pH value; (3) Besides the ion exchange model, the surface complexation model in which the Cu (II) and Pb (II) ions are complexed to aluminol and perhaps silanol groups that occur in particular at the kaolinite crystal edges, also regulates the adsorption behavior of Cu (II) and Pb (II) to kaolinite at pH >6.5.(4) In addition, the surface precipitation model becomes major contributor to the uptake of the Cu (II) and Pb (II) cations to the above-mentioned two minerals at higher pH;As a consequence of the reaction processes of protonation-deprotonation and dissolution at kaolinite-water interface regulated by pH, the medium pH controls the transport and the fate of Cu (II) and Pb (II) ions in soil and sedimentation environment. Finally the experimental adsorption results were described in this paper by the Physical Surface-(Complexation) Model of Sverjensky D A (1993).