Factors to enable crystallization of environmentally stable bioscorodite from dilute As(III)-contaminated waters

Applicability of the bioscorodite method (use of the thermo-acidophilic Fe(II)-oxidizing archaeon Acidianus brierleyi for arsenic (As) oxidation and immobilization at 70 °C) was tested for synthetic copper refinery wastewaters of a wide range of dilute initial As(III) concentrations ([As(III)]_ini = 3.3-20 mM) with varying initial [Fe(II)]/[As(III)] molar ratios ([Fe(II)]_ini/[As(III)]_ini = 0.8-6.0). Crystallization of scorodite (FeAsO₄·2H₂O) tends to become increasingly challenging at more dilute As(III) solutions. Optimization of conditions such as initial pH, seed feeding and initial [Fe(II)]/[As(III)] molar ratio was found critical in improving final As removal and product stability: Whilst setting the initial pH at 1.2 resulted in an immediate single-stage precipitation of crystalline bioscorodite, the initial pH 1.5 led to a two-stage As precipitation (generation of brown amorphous precursors followed by whitish crystalline bioscorodite particles) with a greater final As removal. The formation process of bioscorodite precipitates differed significantly depending on the type of seed crystals fed (bio-versus chemical-scorodite seeds). Feeding the former was found effective not only in accelerating the reaction, but also in forming more recalcitrant bioscorodite products (0.59 mg/L, Toxicity Characteristic Leaching Procedure (TCLP) test). Under such favorable conditions, 94-99% of As was successfully removed as crystalline bioscorodite at all dilute As(III) concentrations tested by setting [Fe(II)]_ini/[As(III)]_ini at 1.4-2.0. Providing an excess Fe(II) (closer to [Fe(II)]_ini/[As(III)]_ini = 2.0) was found beneficial to improve the final As removal (up to 98-99%) especially from more dilute As(III) solutions.