The Crystallization of Arsenate-Contaminated Iron Hydroxide Solids at High pH

A potential alternative technology for wastewater treatment streams involves coprecipitation with colloidal iron hydroxide followed by aging at elevated temperatures and high pH to produce crystalline iron oxides. The resultant crystalline oxides, in addition to having reduced sludge volume, may chemically/physically incorporate metal ions and/or metalloid oxyanions. The increased stability of the crystalline solids may be advantageous in reducing the rate of release of the contaminants to groundwater. The rate of transformation of iron hydroxide model wastewater treatment solids contaminated with arsenate into crystalline products was measured at 60 and 70°C. The rate of transformation was found to be described by either a solid-state reaction equation for powdered compacts or a zero-order reaction controlled by desorption. X-ray diffraction demonstrates that, after crystallization has occurred, no solid solutions involving iron oxides and arsenate are formed.