The formation of mineral arsenates in wastewaters

Abstract

Recently determined thermodynamic constants (1) suggest that most arsenate minerals may be more insoluble than previously reported, with tin arsenate found to be the least soluble of all the metal arsenates studied (K sp °=1.48x10 -48 ). It is conceivable that the precipitation of arsenate minerals in wastewater plays an important role in the removal of both arsenate and heavy metals from municipal wastewater systems. In this study, K sp ° was used to determine whether or not the formation of solid-phase metal arsenates would occur in wastewater by calculating ion concentration product (ICP) and saturation index (SI). ICP was calculated using the following equations: ICP=[Me 2+ ] 3 [AsO 4 3- ] 2 or ICP=[Me 2+ ][HAsO 4 2- ], where Me represents the metal of interest. SI was then used to determine whether the mineral would dissolve or precipitate: SI=ICP/K sp °. Metal and arsenic concentrations used in calculations were from five studies of raw wastewater. Based on thermodynamic calculations, it does not appear that precipitation of metal arsenate occurs in these wastewater treatment plants (WWTPs) when the pH is neutral. The removal of arsenate from water and wastewater by tin treatment is also discussed.