Role of surface precipitation in copper sorption by the hydrous oxides of iron and aluminum

Isotherms were developed at pH 6.9 for adsorption (ADS) and coprecipitation (CPT) of Cu by hydrous oxides of Fe (HFO) and Al (HAO) to study the role o f sorbate/sorbent ratio in metal cation removal. For low sorbate/sorbent co nditions, HFO had a higher Cu retention capacity than HAO regardless of con tact methodology. For either oxide, CPT was consistently more effective tha n ADS in removing Cn from solution. At high sorbate/sorbent ratios, surface precipitation dominates and the oxide's net cation retention capacity depe nds on the nature and solubility of the precipitate formed at the oxide-wat er interface. X-ray diffraction patterns and isotherms of HAO for both ADS and CPT suggest formation of a solid solution [e.g., CuAl2O4(s)] with drama tically lower solubility than eu(OH)(2)(s) precipitated in bulk solution. I n contrast, Cu precipitated on the HFO surface exhibited a solubility compa rable to the bulk precipitated Cu(OH)(2)(s). Therefore, at high sorbate/sor bent ratios, HAO has a higher Cu "apparent" sorption capacity than HFO. The relative utility of these oxides as metal scavengers thus depends markedly on sorbate/sorbent conditions. (C) 1999 Academic Press.