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

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⁻.