Bio-attenuation of arsenic and iron coupled with nitrate remediation in multi-oxyanionic system: Batch and column studies

Highlights: • Nitrate, arsenic, sulphate and iron removal was seen in multi-oxyanionic system. • Iron removal was solely as iron sulphides and depended on sulphate concentration. • Arsenic and iron were precipitated as biogenic orpiment and mackinawite. • Removal proceeded as an order of nitrate, arsenate and sulphate. • Column reactor met WHO guidelines for arsenic and iron contaminated bore-well water. -- Abstract: Co-occurrence of arsenic and iron along with nitrate in groundwater makes the trio an onerous combination both in terms of potability and treatment. To meet drinking water guidelines, batch and column laboratory trials were conducted on simulated and bore-well water for attenuation of arsenic (1000 μg/L), iron (5 mg/L) and nitrate (150 mg/L). Increment in sulphate showed a direct individual impact on iron removal, meeting WHO guidelines. The bio-kinetic parameters were in the range of: μ_max = 0.079–0.551/d, K_s = 116.18–645.19 mg/L, K_d = 0.0009–0.0077/d and Y = 0.034–0.094 mg MLVSS/mg COD. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) confirmed orpiment precipitation and/or co-precipitation with mackinawite are the key mechanisms for arsenic and iron attenuation. Column experiments were conducted by charging simulated groundwater containing arsenic (500 μg/L), nitrate (50 mg/L), sulphate (25 mg/L) and iron (3 mg/L) in an acetate (105 mg/L as COD) fed flow-through bioreactor at constant empty bed contact time of 60 min. Profile sampling illustrated segregation of different terminal electron accepting zones following thermodynamic yield for sequential removal of different oxyanions. This study showed the importance of considering microbially mediated terminal electron-accepting processes (TEAP) for multi-oxyanion removal in engineered systems.