Adsorption and coprecipitation of copper with the hydrous oxides of iron and aluminum

Hydrous oxides of iron (HFO) and aluminum (HAO) were studied for their ability to reduce Cu (initially 4.3 mg/L to microgram per liter levels typical of increasingly stringent wastewater discharge limits. Residual soluble Cu was compared following adsorption (ADS) where Cu was contacted with preformed oxide flocs, and coprecipitation (CPT), where Cu was added prior to HFO or HAO precipitation. For the HAO-ADS system, soluble Cu levels were markedly undersaturated with respect to homogeneous precipitation (PPT) of Cu(OH)₂(s) over the pH 6-9 range. In contrast, soluble Cu was lowered by HFO-ADS for pH <7.5 but comparable to PPT at higher pH. Isotherms suggest a Cu surface precipitate formed on HAO with solubility product lower than its HFO analogue. Compared to ADS, soluble Ca after CPT was similar for HAO but was dramatically lower for HFO. More Cu was incorporated internal to the HFO flocs during CPT, suggesting some Cu substitution into the HFO lattice. Removal of Cu was significantly influenced by oxide type and contact methodology.