Aspects of kaolinite characterization and retention of Pb and Cd

Abstract

This study illustrates the complimentary nature of selected techniques for characterizing kaolinite. For the particular type of kaolinte studied, scanning electron micrographs revealed the presence of crystallites and mostly larger than clay-sized particles. These findings were in agreement with the low cation exchange capacity, low specific surface area, low zero point of charge and high purity that were determined for the kaolinite. Batch equilibrium tests were conducted on kaolinite suspensions that had been adjusted to pH 4 and pH 6. These suspensions were spiked with varying concentrations of Pb (as PbCl 2) and Cd (as CdCl 2). At equilibrium, the metal uptake was greater for the pH 6 suspensions than for the pH 4 suspensions, the metal uptake was generally greater when greater concentrations of metal had been used to spike the suspensions, and there was a reduction in suspension pH that accompanied the metal uptake. A comparison of the equilibrium curves of both metal retention and suspension pH, as a function of the initial metal concentration, combined with an analysis of metal speciation, provided evidence for the adsorption of both divalent (Pb 2+ and Cd 2+) and monovalent (PbCl + and CdCl +) species by the kaolinite. Cation exchange was a primary retention mechanism and the order of selectivity for the pH 4 and pH 6 suspensions of kaolinite appeared to be Pb 2+>H +>Cd 2+. At higher pH and higher metal concentrations, there was an increase in Cd uptake relative to Pb uptake. This may have been a result of less competition between H + ions and metal ions for adsorption sites, and because CdCl + forms at a lower salt concentration than PbCl +, and so a greater proportion of monovalent Cd was adsorbed.