Adsorption Behavior of Cadmium, Zinc, and Lead on Hydroxyaluminum– and Hydroxyaluminosilicate–Montmorillonite Complexes

Abstract

The current imperfect understanding about the adsorption behavior of heavy metals on hydroxyaluminum (HyA)‐ and hydroxyaluminosilicate (HAS)‐interlayered phyllosilicates led us to conduct this study. We examined the adsorption behavior of Cd, Zn, and Pb on synthetically prepared HyA– and HAS–montmorillonite (Mt) complexes in comparison with that on untreated Mt. A very dilute initial metal concentration of 10−6 M in 0.01 M NaClO4 background was used in all the adsorption systems. The presence of HyA and HAS polymers on Mt greatly promoted the adsorption of all three metals. Such promoting effects of HyA and HAS polymers on the metal adsorption were, however, not very different from each other. The observed adsorption selectivity sequences of Pb > Zn > Cd on Mt as well as Pb ≫ Zn ≥ Cd on the complexes resemble the reported metal selectivity sequences on amorphous Fe and Al hydroxides. At different pHs, partitioning the adsorbed metals into strongly and weakly held fractions indicated that specific adsorption rather than nonspecific adsorption might have largely controlled the metal selectivity, particularly on the complexes. This led us to assume a predominant involvement of interlayered HyA or HAS polymers in metal adsorption from such dilute solutions. On Mt, the metals were predominantly adsorbed on the permanent charge sites in an easily replaceable state. However, a substantial involvement of the edge OH− groups of Mt in specific adsorption of the metals was also evident, especially at higher pH. Obviously, on Mt and on the complexes, the relative abundance of each type of site and their affinity to heavy metals were substantially different.