Sequential ferrous iron oxidation for arsenic removal from phosphate-containing groundwater

Groundwater arsenic (As) contamination is a severe drinking water quality problem and a threat to human health in Bangladesh and other countries. Appropriate arsenic removal technology has been a global challenge. Biological As(III) oxidation to As(V) and co-precipitation with groundwater-native Fe³⁺-precipitates is a promising approach, as it does not require adsorbents or chemicals. In order to optimize this process for application in Bangladesh, it is critical to prevent competition of phosphate with As(V) for removal with Fe³⁺-precipitates in groundwater. The study aimed to investigate delayed Fe²⁺ oxidation in solution towards more efficient As(III) and As(V) removal in phosphate-containing waters (2 mg/L). The research was conducted with 280 µg/L As(III) and 2.5 mg/L of Fe²⁺, with an initial pH of 7.0, being representative for the concentrations in the targeted village in rural Bangladesh. The experiments consisted of aerated single and multiple-step jar tests, with the multiple-steps jar test being the simulation of delayed Fe²⁺ oxidation through sequential dosing of 1.25 mg/L Fe²⁺. In the single-step jar test, the As(III) containing system with and without phosphate resulted in 24% and 47% As removal respectively, whereas the multiple-steps jar test, enhanced total As removal to 57% and 94% correspondingly. These results show that delayed Fe²⁺ aeration is beneficial to achieve more efficient As removal in the presence of phosphate.