Zero-valent iron in phosphate removal: Unraveling the role of particle size and dissolved oxygen

Abstract

The effect of particle size and dissolved oxygen (DO) on zero-valent iron (ZVI) oxidation needs further study to propose strategies for improving phosphate (P) removal performance. In this study, batch and column experiments were conducted to investigate P removal by ZVI with five different particle sizes under aerobic and anaerobic conditions. The oxidation status of ZVI before and after the reaction was characterized. Under aerobic conditions, as ZVI size increased, the P removal rate initially increased and then decreased, while the difference in P removal was not obvious under anaerobic conditions. Furthermore, the P removal rate exhibited a significant positive correlation with the degree of ZVI oxidation (R2 = 0.877, p = 0.008), highlighting its pivotal role in P removal. The ZVI size and DO on ZVI oxidation primarily affect the types and quantities of iron corrosion products (FeCPs). Under aerobic conditions, the ZVI with small particle sizes (ZVIsps) (50 nm, 500 nm, and 5 μm) tended to form passivation layers composed of amorphous iron oxide (ap-FexOy) and FePO4, limiting ZVI oxidation and resulting in lower P removal rates (17.6%). Conversely, in situations with less passivation, amorphous iron hydroxides (ap-FexOHy) play a dominant role in the P removal process. The 50 μm ZVI eventually generated lepidocrocite (γ-FeOOH) under aerobic conditions, this sufficient oxidation was also the reason for its high P removal rate. Therefore, future research and applications of ZVI materials in P removal should concentrate on enhancing ZVI corrosion and regulating the formation of suitable FeCPs.