Optimization of bioscorodite crystallization for treatment of as(III)-bearing wastewaters

Arsenic (As) is a major impurity contaminating metal refinery wastewaters. To immobilize As ions, we have previously reported microbial scorodite (FeAsO₄·2H₂O) crystallization using the thermo-acidophilic iron-oxidizing archaeon, Acidianus brierleyi. In order to extend the applicable range of As(III)-bearing metal refinery wastewaters (especially for dilute As(III) concentrations of 250–1500 ppm), this study investigated the effect of several factors possibly affecting the bioscorodite crystallization efficiency; (i) [Fe(II)]_ini/[As(III)]_ini molar ratio at different target As(III) concentrations, (ii) initial pH, and (iii) seed scorodite with different morphologies. The [Fe(II)]_ini/[As(III)]_ini molar ratio strongly affected the bioscorodite crystallization efficiency at each target As(III) concentration. Whilst the [Fe(II)]_ini/[As(III)]_ini molar ratio of 1.4 was most effective at 500–1500 ppm As(III), the optimal molar ratios for treating more dilute concentrations (< 500 ppm) were shown to be relatively higher. However, further increasing the [Fe(II)]_ini/[As(III)]_ini molar ratio resulted in formation of unwanted potassium jarosite (KFe₃(OH)₆(SO₄)₂) together with scorodite. Lowering the initial pH from 1.5 to 1.2 resulted in earlier scorodite nucleation, but lower overall As immobilization. Feeding chemical- and bio-scorodite seed crystals differently affected the reaction speed and the stability of newly-precipitated bioscorodite. The TCLP test indicated that scorodite formed on bioscorodite seeds is more stable than that formed on chemically-synthesized scorodite seeds.