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

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 pH6.5 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).