Surface complexation modeling of copper sorption by hydrous oxides of iron and aluminum

Surface complexation models were used to simulate adsorption (ADS) and coprecipitation (CPT) of copper (Cu) by hydrous oxides of iron (HFO) and aluminum (HAO) over a range of pH and surface-loading conditions. The generalized two-layer model was satisfactory for two very different conditions: (1) low sorbate/sorbent ratios where metal-oxide interaction is adequately described as Cu²⁺ coordination to surface functional groups and (2) under HFO-CPT conditions which result in extremely high adsorption site density (0.425 mol of sites/mol of Fe). As the sorbate/sorbent ratio is progressively increased, the models must account for metal hydrolysis and surface precipitate formation, and Cu interaction with both hydrous oxides could be fitted over a wide range of surface loadings using a comprehensive surface precipitation model. Similar mass law constants for sorption reactions were used for generalized two-layer and surface precipitation modeling, ADS and CPT conditions, and pH-edge and isotherm data. Corroborating sorption and spectroscopic evidence, modeling indicated that Cu precipitated on HAO, unlike HFO, has a markedly lower K(sp) than bulk precipitated Cu(OH)₂(s). Results also suggest that enhanced Cu removal by CPT was not simply a manifestation of higher surface area.