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

This study revealed the importance of SO_4^ 2− 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 Fe^2+ and As(III), precipitating SO_4^ 2− -bearing amorphous precursors composed of basic ferric sulfate (MFe_x(SO_4)_y(OH)_z) and ferric arsenate (FeAsO_4·(2+n)H_2O). 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(AsO_4)_0.94(SO4)_0.08·1.69H_2O) out of even more dilute and seeded solution. This phase transformation process was also accompanied by continuous dehydration. This two-stage As-removal via SO_4^ 2- -mediated phase transformation was shown to be key to promote biogenic scorodite formation with greater final As-removal from dilute As(III)-bearing solutions.