Interactions of divalent cations with the surface of pyrite

Abstract

Interactions between pyrite and dissolved Ca 2+, Mn 2+, Co 2+, Ni 2+, Cd 2+, and Zn 2+ were studied as a function of pH, ionic strength, and adsorbate concentration in NaCl solutions and in seawater. Apparent adsorption constants were calculated using an ion-exchange surface complexation model, and second-order rate constants for the sorption of Co 2+, Ni 2+, Cd 2+, and Zn 2+ were calculated from their uptake kinetics. Apparent adsorption constants generally varied by less than an order of magnitude, except for Ca 2+ which exhibited complex behavior. The apparent adsorption constants decreased in the order Co 2+ > Cd 2+ > Mn 2+ > Ni 2+ > Ca 2+ > Zn 2+. The ranking of the apparent adsorption constants does not correlate with the first hydrolysis constants of the adsorbates or the solubility products of their sulfide minerals. The behavior of the second-order forward and reverse rate constants in different solutions indicates that adsorption is more complex than the second-order reaction model described by the ion exchange surface complexation model. The forward sorption reaction rate constants increased with increasing pH, and decreased with increasing ionic strength or initial adsorbate concentration. The reverse sorption reaction rate constants increased with increasing pH or initial adsorbate concentration, and were unaffected by ionic strength. The effect of pH on the forward and reverse rate constants may have been due to, respectively, a surface-induced hydrolysis reaction mechanism and a rate-limiting step prior to the surface complex-hydrogen ion reaction. The effect of ionic strength on the forward rate constants may have been due to site competition with Na +, while the effect of initial adsorbate concentration on the forward and reverse rate constants may have been due to, respectively, the assumption of a constant concentration of surface sites available for reaction and the presence of heterogeneous surface sites.