Kazuma Kimura, Naoko Okibe
Fe-bearing waste materials generated during acid mine drainage treatment have potential for arsenic (As) immobilization. To clarify the influence of Fe(III) mineralogy, biogenic scorodite (FeAsO₄·2H₂O) crystallization mediated by Acidianus brierleyi was investigated using four representative Fe(III) minerals differing in sulfate content and crystallinity. Distinct crystallization pathways were observed depending on Fe(III) mineralogy. In the soluble Fe²⁺ system, scorodite crystallization proceeded via precursor formation followed by crystal growth. Among sulfate-free minerals, ferrihydrite initiated but did not sustain scorodite crystallization, whereas goethite suppressed microbial activity and inhibited pathway initiation. In contrast, scorodite crystallization was enhanced in sulfate-bearing mineral systems: jarosite exhibited a short induction period followed by rapid crystallization, whereas schwertmannite supported continuous scorodite crystallization without a distinct induction period. XRD and FTIR analyses confirmed earlier scorodite formation in sulfate-bearing systems. TCLP tests showed low As leachability for products formed with jarosite and schwertmannite (0.5-0.6 mg/L), whereas ferrihydrite yielded substantially higher TCLP values. These findings suggest that sulfate configuration influences the compatibility of Fe(III) minerals with scorodite precursor phases, whereas mineral crystallinity may influence the continuity of scorodite crystallization. This study provides a basis for predicting the performance of Fe-bearing waste materials and supports their reuse for efficient arsenic immobilization.