Effect of seawater and dissolved cations on scorodite synthesis via iron coprecipitation

In recent years, one of the more effective developments in the fixation of arsenic from nonferrous waste has been the synthesis of scorodite via the co-precipitation of arsenic with iron during Fe(II) oxidation. This work examines the impact of cation impurities on the process with the aim of widening the application of this approach to incorporate the use of seawater and the treatment of ferrous metallurgy waste. Scorodite formation was promoted by the presence of cations in the order of Cu2+ >> Zn2+ > Al3+ > Ca2+ ∼ Mg2+, with differences in precipitation behaviour, yield and kinetics due to specific cation effects on iron oxidation. A grain size increase was observed with Cu, while the opposite was found with Al, attributed to crystallisation from solution over epitaxial growth. The use of seawater decreased As precipitation by <4 % without affecting grain size, showing the potential to be utilised as reaction medium. While the presence of Na+ hinders As precipitation, the impact appears less pronounced with seawater due to competing effects from other cations. Since scorodite precipitation is slowed down by high Fe(III) in the feed, a pre-reduction approach with Al, Cu and Zn base metals was examined for conversion of Fe(III) in metallurgical waste to Fe(II) prior to co-precipitation with As. While this produced stable scorodite precipitates, the yield and presence of unstable by-products were impacted by the method of reductant addition.