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

Abstract

Generally, abstracted groundwater is aerated, leading to iron (Fe 2+ ) oxidation to Fe 3+ and precipitation as Fe 3+ -(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 2+ based HFO flocs is hampered by rapid Fe 2+ oxidation-precipitation during aeration before or after storage.Therefore, delaying Fe 2+ 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 2+ oxidation (2±0.01 mg/ L) than in the anoxic storage system, where Fe 2+ oxidation was partial (1.03±0.32mg/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 2+ and facilitated more As(V) removal.The groundwater native-Fe 2+ (2.33±0.03mg/L) removed 61% of As in the oxic system (storage-aeration-filtration), whereas 92% As removal was achieved in the anoxic system.