Trace metal adsorption/coprecipitation on hydrous ferric oxide under realistic conditions: The role of humic substances

Abstract

Adsorption of Cd, Cu and Ni (and possibly Pb) onto hydrous ferric oxide is significantly modified in the presence of humic substances. Employing realistic concentrations in a synthetic freshwater, it is found that adsorption/coprecipitation of Cd and Ni (and less certainly Pb) is enhanced in the presence of humics. Cu adsorption may also be enhanced, but any effect is masked by a strong competitive complexation of Cu with soluble humics. The pH dependency of the adsorption process is modified in a manner consistent with “ligand like” metal adsorption. The data are considered in terms of four models which might account for the enhanced adsorption. The most plausible one involves a complexation of metals with adsorbed humics, which is stronger than that with soluble humics.A series of experiments using natural waters of widely differing composition demonstrates the generality of the observations made with synthetic freshwaters. Cu adsorption is dominated by the competitive formation of soluble Cu-humic complexes. It is also found to be independent of alkalinity, indicating that neither Ca/Mg nor HCO3/CO3 ions influence the adsorption or complexation reactions. Cd adsorption, on the other hand, is essentially independent of the humic concentration, but dependent upon alkalinity. As alkalinity increases so Cd adsorption decreases, a phenomenon probably due to competition from Ca/Mg ions. These findings have important implications for the development of realistic speciation models.