Adsorption behavior of cadmium, zinc, and lead on hydroxyaluminum- and hydroxyaluminosilicate-montmorillonite complexes

The current imperfect understanding about the adsorption behavior of heavy metals on hydroxyaluminum (HyA)- and hydroxyaluminosilicate (HAS)-interlaye red phyllosilicates led us to conduct this study. We examined the adsorptio n behavior of Cd, Zn, and Ph on synthetically prepared HyA- and HAS-montmor illonite (Mt) complexes in comparison with that on untreated Mt, A very dil ute initial metal concentration of 106 M in 0.01 M NsClO(4) 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 eff ects of HyA and HAS polymers on the metal adsorption were, however, not ver y different from each other. The observed adsorption selectivity sequences of Pb > Zn > Cd on Mt as well as Pb much greater than Zn greater than or eq ual to Cd on the complexes resemble the reported metal selectivity sequence s on amorphous Fe and Al hydroxides, At different pHs, partitioning the ads orbed metals into strongly and weakly held fractions indicated that specifi c adsorption rather than nonspecific adsorption might have largely controll ed the metal selectivity, particularly on the complexes. This led us to ass ume 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, How ever, a substantial involvement of the edge OH- groups of Mt in specific ad sorption 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 a nd their affinity to heavy metals were substantially different.