Coupling microbial As(III) oxidation with Fe-sludge adsorption: A continuous dual-column approach for arsenic removal

Legacy acid mine drainage treatment generates Fe-rich sludge, a low-value waste despite its strong affinity for arsenic (As). Batch tests showed that As(III) oxidation by Thiomonas delicata Hö5 markedly enhanced subsequent As(V) sequestration by Fe-sludge. A continuous system for 100 mg/L As(III) at pH 3.5 was developed using immobilized cells on a porous silica carrier, achieving ∼90% As(III) oxidation at HRTs ≥4 h, whereas the un-inoculated control exhibited slower, unstable oxidation by an indigenous consortium. Connecting this oxidation column to a lower Fe-sludge bed created the separate-column configuration, which maintained >98% total As removal for 75 days at an HRT of 4 h and produced predominantly As(V) on solids. The un-inoculated system also achieved >98% removal when influent As(III) was lowered to 20 mg/L, indicating adaptability of indigenous oxidizers. A simplified mixed-column configuration also achieved >98% removal at HRTs of 20–4 h, though As was first sorbed as As(III) and oxidized later, resulting in weaker complexes and more leachable solids due to limited Fe-phase reorganization. Still, acid–thermal post-treatment reduced As leachability to <5.0 mg/L, highlighting that the mixed-column configuration remains practical, whereas the separate-column configuration provides the most reliable route to inherently stable As-bearing solids. • Microbial As(III) oxidation by Thiomonas delicata enables efficient As(V) generation. • Separate configuration maintains >98% removal and yields inherently stable As-bearing solids. • Mixed configuration also achieves >98% removal, with stability enhanced by post-treatment. • Fe-sludge provides strong As(V) adsorption via protonated Fe(III)-(oxyhydr)oxide surfaces. • Process offers a scalable arsenic treatment using low-cost, biosludge-derived media.