Simultaneous adsorption of zinc and phosphate on synthetic lepidocrocite

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Abstract
The joint adsorption of zinc and phosphate on an iron oxide is studied as a function of concentration of each adsorbate and pH. For a given P concentration, the amount of Zn adsorbed increases with pH and, at pH values above 6, more than 99% of total Zn in solution is taken up by the oxide. For a given pH value, increasing levels of P adsorbed cause strong increases in Zn adsorption, but precipitation of Zn phosphate can be ruled out, as the solution compositions invariably correspond to systems that are undersaturated with respect to that compound. From the plot of log(Zn2+) against pH, it is concluded that equilibria between soil solutions and what has been termed 'soil Zn' can be accounted for by the presence of oxide surfaces with phosphate adsorbed. Indirect estimations of the variations of the net charge conveyed to the surface by the adsorbed species suggest that Zn is adsorbed as ZnOH+.