Multiple-pathway arsenic oxidation and removal from wastewater by a novel manganese-oxidizing aerobic granular sludge

Aerobic granular sludge (AGS) is a powerful biotechnology to remove various heavy metal(loid)s from wastewater, but not including arsenic (As). In this study, a novel manganese-oxidizing aerobic granular sludge (Mn-AGS) was developed to remove As from organic wastewater. Eight sequencing batch reactors (SBRs) were operated in duplicate to investigate the feasibility of As removal by Mn-AGS. The immobilized As in the granular sludge was characterized by sequencing extraction, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), and Raman spectroscopy. Oxidation pathways for As and their contributions in Mn-AGS were evaluated by seven batch experiments under different conditions. The results indicated that As removal efficiency was much higher by Mn-AGS than by AGS. In Mn-AGS, As(III) was efficiently oxidized into As(V) (74.6%-82.6%) and then mostly bound on amorphous ferrihydrite and biogenic Mn oxides (bio-MnO_x) (56.2%-65.0%), while metal arsenates, such as ferric arsenate, were not detected. Importantly, As removal was greatly improved by a small addition of Fe(II) in Mn-AGS. This might be primarily caused by Fenton reactions, because this improvement was removed when H₂O₂, self-generated in Mn-AGS, was scavenged by exogenous catalase (CAT). This study provided a novel extension of the traditional AGS technology to treat As in organic wastewater with an acceptable degree of efficiency.