Arsenic removal from iron-containing groundwater by delayed aeration in dual-media sand filters.

Generally, abstracted groundwater is aerated, leading to iron (Fe) oxidation to Fe and precipitation as Fe-(hydr)oxide (HFO) flocs. This practice of passive groundwater treatment, however, is not considered a barrier for arsenic (As), as removal efficiencies vary widely (15-95%), depending on Fe/As ratio. This study hypothesizes that full utilization of the adsorption capacity of groundwater native-Fe based HFO flocs is hampered by rapid Fe oxidation-precipitation during aeration before or after storage. Therefore, delaying Fe oxidation by the introduction of an anoxic storage step before aeration-filtration was investigated for As(III) oxidation and removal in Rajshahi (Bangladesh) with natural groundwater containing 329(±0.05) µgAs/L. The results indicated that As(III) oxidation in the oxic storage was higher with complete and rapid Fe oxidation (2±0.01 mg/L) than in the anoxic storage system, where Fe oxidation was partial (1.03±0.32 mg/L), but the oxidized As(V)/Fe removal ratio was comparatively higher for the anoxic storage system. The low pH (6.9) and dissolved oxygen (DO) concentration (0.24 mg/L) in the anoxic storage limited the rapid oxidation of Fe and facilitated more As(V) removal. The groundwater native-Fe (2.33±0.03 mg/L) removed 61% of As in the oxic system (storage-aeration-filtration), whereas 92% As removal was achieved in the anoxic system.