Arsenic co-precipitation with iron oxidation products and retention during precipitate aging: Effects of phosphate, silicate and calcium

Arsenic (As) co-precipitates with ferric iron (Fe(III)) precipitates formed by the oxidation of ferrous iron (Fe(II)) in water. This process is of great importance in the removal of As from drinking water and for the fate of As in environmental systems. We studied the effects of phosphate (P), silicate (Si) and calcium (Ca) on the co-precipitation of arsenate (As(V)) with fresh Fe(II)-derived Fe(III)-precipitates as well as the retention or release of As(V) during precipitate aging. The structure of the Fe oxidation products and their transformation over time are greatly affected by P, Si and Ca, with substantial consequences for the fate of co-transformed As(V). Both Si and Ca favorably impact on As(V) removal and retention: Si inhibits the formation of crystalline Fe(III)-solids with lower sorption capacity, and Ca enhances P removal in more stable Ca-Fe(III)-phosphate and thereby reduces the adverse effect of P on As(V) removal. Our results emphasize that the structural diversity of Fe oxidation products caused by variations in water chemistry and resulting variations in precipitate reactivity must be taken into account when assessing the effects of Fe(III)-precipitate formation and transformation on As.