Behavior of sulfate ions during biogenic scorodite crystallization from dilute As(III)-bearing acidic waters

This study revealed the importance of SO42− ions during biogenic scorodite crystallization via a two-stage As-removal process, using a combination of liquid and solid analyses (chemical digestion, FT-IR, SEM, TG-DTA, particle distribution). The first-stage As-removal was induced by microbial oxidation of Fe2+ and As(III), precipitating SO42−-bearing amorphous precursors composed of basic ferric sulfate (MFex(SO4)y(OH)z) and ferric arsenate (FeAsO4·(2 + n)H2O). This was followed by an induction period (a period of constant concentration), where dissolution-recrystallization of unstable amorphous precursors proceeded: Re-dissolved metal ions became locally concentrated on the surface of precursors, which gave the driving force for the second-stage As-removal as secondary layers of crystalline biogenic scorodite (Fe(AsO4)0.94(SO4)0.08·1.69H2O) out of even more dilute and seeded solution. This phase transformation process was also accompanied by continuous dehydration. This two-stage As-removal via SO42−-mediated phase transformation was shown to be key to promote biogenic scorodite formation with greater final As-removal from dilute As(III)-bearing solutions.