Sequential Fe²⁺ oxidation to mitigate the inhibiting effect of phosphate and silicate on arsenic removal

Abstract

Sequential iron (as Fe2+) oxidation has been found to yield improved arsenic (as As(III)) uptake than the single-step oxidation. The objective of this study was to gain a better understanding of interactions with phosphate (PO43−) and silicate (SiO42−) during sequential Fe2+ and As(III) oxidation and removal, as these are typically found in groundwater and known to interfere with As removal. The laboratory experiments were performed using single and multi-step jar tests with an initial As(III/V), Fe2+, PO43−, SiO42− concentrations, and pH of 200 μg/L, 2.5 mg/L, 2 mg/L, 16 mg/L and 7.0, respectively representing the targeted natural groundwater in Rajshahi district, Bangladesh. The sequential Fe2+ and As(III) oxidation in the multi-step jar tests indicated that the PO43− hindrance on As removal in the first Fe2+ oxidation step was compensated for in the second. Moreover, smaller Fe flocs (<0.45 μm) were observed in the presence of SiO42−, potentially providing more surface area during the second Fe2+ oxidation step leading to better overall As removal. Altogether it may be concluded that controlling the As(III) and Fe2+ oxidation sequence is beneficial for As removal compared to single-step Fe2+ oxidation, both in the presence and absence of PO43− and/or SiO42−.