Modeling kinetics of copper uptake by inorganic colloids under high surface coverage conditions

Copper uptake by ferric oxide and silica particles is studied through batch kinetic experiments. Copper uptake rates are found to be strongly dependent on pH and on the sorbate/sorbent molar concentration ratio. Dramatic changes to the zeta potential of both colloids from baseline values are observed. Modeling of copper uptake and zeta potential charge reversals using the surface complexation model (SCM) yields poor descriptions under high surface coverage conditions. The conventional SCM, modified in the recent literature to (i) the surface polymer model (SPM), which additionally incorporates uptake of dimeric copper species; and (ii) the continuum model (CM), which includes formation of surface precipitates, is extended here to model uptake kinetics. Both the SPM and the CM are successful in modeling copper uptake rates as well as zeta potential variations over a wide range of solution conditions. For systems with high surface loadings, copper removal from solution appears to result from the formation of monomeric and dimeric surface complexes, as well as through precipitation mechanisms. It is further concluded that a kinetic model incorporating diffusion through the surface film of sorbed and precipitated copper species as the rate-limiting process, in association with the SPM and CM, successfully describes the effect of pH and colloid concentration on copper uptake and oxide particle zeta potential histories.