Relationships between zinc and phosphate adsorption on montmorillonite and an iron oxyhydroxide

Abstract

A comparison is made between the behaviour of lepidocrocite and Ca-saturated montmorillonite in the adsorption of zinc and phosphate. It is shown that both adsorbents are sensitive to pH changes, so that the amounts of Zn adsorbed increase as pH increases in P-free systems, although the pH-dependence is much stronger in the case of the oxide. There seem to be two kinds of sites present on the clay surface, with one site showing higher preference for Zn over Ca. The number of high-preference sites for Zn increases as pH increases, and it is suggested that they are located on edge, variable charge regions. The amount of phosphate adsorbed by montmorillonite is nearly constant for pH values above 5, and the position of the plateau is independent of whether Zn is present in the system or not. This result contrasts with the known behaviour of oxides, and can be explained by assuming that P is adsorbed as H2PO4 H2PO-4 on surfaces with an electric potential of about -90 mV, which agrees with values of the electric surface potential calculated for montmorillonite in the literature. Adsorption of Zn or phosphate by the oxide is enhanced by the presence of the other adsorbate; however, when the adsorbent is montmorillonite, the amounts adsorbed are insensitive to the presence of the other adsorbate. It is suggested that, for the levels of Zn coverage studied here, adsorption of Zn and phosphate occurs on different kinds of sites on the clay mineral, while there seems to be a close relationship between the adsorbing sites for both adsorbates on the oxide.