Simultaneous adsorption of cadmium, zinc, and lead on hydroxyaluminum- and hydroxyaluminosilicate-montmorillonite complexes

We examined the adsorption behavior of Cd, Zn, and Pb from mixed solutions on synthetically prepared Hydroxyaluminum (HyA)and Hydroxyaluminosilicate (HAS)-montmorillonite (Mt) complexes within the pH range 4 to 8. Initial concentrations of each metal (Me-1) of 1 x 10(-6) M in binary systems and 1 x 10(-6), 2 x 10(-5), 5 x 10(-5) M in ternary systems were used in a background electrolyte of 0.01 M NaClO4. The presence of HyA and HAS polymers on Mt greatly increased the adsorption of all three metals. Lead appeared to have the strongest affinity for adsorption on both Mt and on the complexes. The adsorption affinity sequence of Pb > Cd > Zn on Mt did not change with Me-1 and could be explained with hard-soft acid-base (HSAB) theory. On the complexes, the affinity sequences of Zn > Cd in binary systems and Pb much greater than Zn greater than or equal to Cd in most ternary systems were consistent with predictions based on the first hydrolysis constants. We propose that HyA-Mt and HAS-Mt are composed of nonuniform metal adsorption sites. For a given type of site, metal selectivity was predominantly determined by the metal properties, softness and ease of hydrolysis, while ionic potential had a limited predictive index for metal adsorption. The overall adsorption behavior of the metals along with their extraction patterns showed that the strength of adsorption followed the order of Pb much greater than Zn > Cd among the metals and of HyA-Mt greater than or equal to HAS-Mt much greater than Mt among the adsorbents.