Copper uptake by silica and iron oxide under high surface coverage conditions: Surface charge and sorption equilibrium modeling

A sorption modeling approach based on surface complexation concepts was applied to predict copper uptake and its effects on the surface electrostatic potential of ferric oxide and silica colloids. Equilibrium modeling of copper uptake by ferric oxide using the traditional surface complexation model (SCM) was reasonably successful with some discrepancies especially in the acidic pH ranges and high colloid concentration cases. Good predictions of the ferric oxide charge reversals during uptake were obtained from the modeling. Based on the SCM predictions, copper removal from solution is due to the outer-sphere complexation of the first hydrolysis product, resulting in the surface-metal complex SO^-CuOH⁺. The SCM was found to be insufficient to describe copper uptake by silica particles. To address discrepancies between experimental data and SCM predictions, the SCM was modified to include attributes of the surface polymer model (SPM), which incorporates sorption of the dimeric copper species Cu₂(OH)₂²⁺. The continuum model (CM) was also studied as a second modification to the SCM to include formation of surface precipitates. Both the SPM and the CM were successful in modeling copper uptake and ζ potential variations as a function of pH at various solution conditions and colloid concentrations. From the SPM and CM predictions, it was concluded that for systems with high surface loadings, copper removal from solution occurs due to the formation of both monomeric and dimeric surface complexes, as well as through precipitation mechanisms.