The effect of heterogeneous seed crystals on arsenite removal as biogenic scorodite

With the aim to effectively oxidize and remove highly toxic As(III) from acidic metal-refinery wastewaters, the seeding effect of different heterogeneous minerals was investigated on the formation of biogenic scorodite (FeAsO₄·2H₂O), using the Fe²⁺/As(III)-oxidizing thermo-acidophilic archaeon Acidianus brierleyi. Heterogeneous hematite-seeds exhibited even greater As-removal efficiency relative to homogeneous scorodite-seeds. While the effect of magnetite-seeds was mostly comparable to scorodite-seeds, feeding goethite or ferrihydrite negatively affected the speed of As precipitation, forming jarosite or jarosite/scorodite mixture, respectively, instead of scorodite. Similarly to those formed on scorodite-seeds (TCLP As leachability; 0.49 mg/L), the final scorodite products formed on hematite-seeds or magnetite-seeds were also highly stable (0.51 mg/L or 0.39 mg/L, respectively), well below the US standard of 5 mg/L. The effectiveness of hematite seeding was also demonstrated in the lower-temperature scorodite crystallization reaction (45°C), where Fe²⁺-oxidizing moderately-thermophilic acidophilic bacterium Acidimicrobium ferrooxidans was employed, after the complete As(III) oxidation by Thiomonas cuprina. The overall results suggested that the effectiveness of hematite was, at least partly, attributed to its highly-positive surface charge. This effect was retained even when cells attached onto the hematite surface. This made the mineral an effective absorbent for anionic As(V) and SO₄ ²¹, consequently speeding up the reaction by shortening the steady-state induction period between the two As-removal stages, during the biogenic scorodite crystallization process.