Advantages of aeration in arsenic removal and arsenite oxidation by structural Fe(II) hydroxides in aqueous solution

Structural Fe(II) Hydroxide (SFH) was used to remove As(III) and As(V) from aerated synthetic wastewater with initially 30 mg/L As at neutral pH values. Batch experiments were performed to investigate the influence of O₂, pH and coexisting HPO₄²⁻ on As removal by 0.5 g (Fe)/L SFH. The results suggested that removal efficiency for As(III) and As(V) were ∼60% and ∼50% under anoxic conditions at initial pH 7.0 and 9.0, respectively. However, the efficiency increased to over 90% for both As(III) and As(V) under oxic conditions. The in situ mineral transformation of SFH to newly formed Fe(II/III) and FeOOH was responsible for the robust removal. As(III) removal was nearly unaffected by HPO₄²⁻ (1.0 mM), while obviously inhibition was observed in the case of As(V). The microstructure of fresh and reacted SFH was characterized by FTIR and XPS and these results indicate that As was removed via inner-sphere complexation and precipitation. XPS studies showed that As(III) was converted to As(V) after 2 h of reaction in the pH range of 7.0–9.0, which is attributable to enhanced formation of Fe(IV) by the Fenton reaction during mineral transformation. In practice, results of this study suggest that aeration is advantageous to significantly enhancement of As(III) oxidation and removal with highly reactive SFH.