Phosphate Recovery from Aqueous Solutions via Vivianite Crystallization: Interference of FeII Oxidation at Different DO Concentrations and pHs

Vivianite (Fe 3 (PO 4 ) 2 ·8H 2 O) crystallization has attracted increasing attention as a promising approach for removing and recovering P from wastewaters. However, Fe II is susceptible to oxygen with its oxidation inevitably influencing the crystallization of vivianite. In this study, the profile of vivianite crystallization in the presence of dissolved oxygen (DO) was investigated at pHs 5–7 in a continuous stirred-tank reactor. It is found that the influence of DO on vivianite crystallization was highly pH-related. At pH 5, the low rate of Fe II oxidation at all of the investigated DO of 0–5 mg/L and the low degree of vivianite supersaturation resulted in slow crystallization with the product being highly crystalline vivianite, but the P removal efficiency was only 30–40%. The removal of P from the solution was substantially more effective (to >90%) in the DO-removed reactors at pH 6 and 7, whereas the efficiencies of P removal and especially recovery decreased by 10–20% when Fe II oxidation became more severe at DO concentrations >2.5 mg/L (except at pH 6 with 2.5 mg/L DO). The elevated degree of vivianite supersaturation and enhanced rate and extent of Fe II oxidation at the higher pHs led to decreases in the size and homogeneity of the products. At the same pH, amorphous ferric oxyhydroxide (AFO)─the product of Fe II oxidation and Fe III hydrolysis─interferes with vivianite crystallization with the induction of aggregation of crystal fines by AFO, leading to increases in the size of the obtained solids.