Arsenic removal in a sulfidogenic fixed-bed column bioreactor

In the present study, the bioremoval of arsenic from synthetic acidic wastewater containing arsenate (As5+) (0.5-20 mg/L), ferrous iron (Fe2+) (100-200 mg/L) and sulfate (2000 mg/L) was investigated in an ethanol fed (780-1560 mg/L chemical oxygen demand (COD)) anaerobic up-flow fixed bed column bioreactor at constant hydraulic retention time (HRT) of 9.6 h. Arsenic removal efficiency was low and averaged 8% in case iron was not supplemented to the synthetic wastewater. Neutral to slightly alkaline pH and high sulfide concentration in the bioreactor retarded the precipitation of arsenic. Addition of 100 mg/L Fe2+ increased arsenic removal efficiency to 63%. Further increase of influent Fe2+ concentration to 200 mg/L improved arsenic removal to 85%. Decrease of influent COD concentration to its half, 780 mg/L, resulted in further increase of As removal to 96% when Fe2+ and As5+ concentrations remained at 200 mg/L and 20 mg/L, respectively. As a result of the sulfidogenic activity in the bioreactor the effluent pH and alkalinity concentration averaged 7.4 ± 0.2 and 1736 ± 239 mg CaCO3/L respectively. Electron flow from ethanol to sulfate averaged 72 ± 10%. X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) analyses were carried out to identify the nature of the precipitate generated by sulfate reducing bacteria (SRB) activity. Precipitation of arsenic in the form of As2S3 (orpiment) and co-precipitation with ferrous sulfide (FeS), pyrite (FeS2) or arsenopyrite (FeAsS) were the main arsenic removal mechanisms.