Clays and Clay Minerals, Vol. 28, No. 5,321-327, 1980. SELECTIVE ADSORPTION OF ZINC ON HALLOYSITE

Abstract--Zn- and Ca-adsorption equilibria of five Ca-saturated halloysite samples were measured at equi-librium Zn concentrations up to 10-2 M in 10 4 to 10-z M CaCI2. The results were interpreted on K zn vs. [Zn]/CEC (%) plots, where KZ ~ is the selectivity coefficient (=[Zn][Ca]/[Ca][Zn]), Zn and Ca represent the adsorbed species, and CEC is the cation-exchange capacity. All Zn adsorption occurred at cation-exchange sites, and 0.77 to 36.0 meq Zn/100 gclay, which constitutes 9 to 83 % of the CEC, was adsorbed with "high selectivities " (KZ ~> 10). The higher values were found for two spherical and one "filmy" halloysites, whereas the lower values were found for two tubular halloysites. The magnitude of their 001 intensity, hydration in interlayer space, CEC, and "free " iron oxide content did not correlate with the selective Zn adsorption, but a good correlation was found between the proportion of "high selectivity" sites for Zn and proportion of "high affinity " sites for H +. The adsorption of Zn at the "high selectivity" sites was not completely reversible, and KZ~, values> 1000 were recorded in 0.5 M CaC12 for Zn which occupied 10-40 % of the exchange sites. Selective Zn adsorption decreased with decreasing pH, and all adsorbed Zn was extracted with 0.1 M HC1