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.