Enhanced phosphate removal by zero valent iron activated through oxidants from water: batch and breakthrough experiments

Abstract

In this study, oxidants including hydrogen peroxide (H₂O₂), hypochlorite (ClO^-) and persulfate (S₂O₈ ^2-) were employed to promote zero-valent iron (ZVI) corrosion and enhance phosphate (P) removal from water through batch and breakthrough experiments. Characterization results indicated that the addition of oxidant can cause large-scale corrosion of the iron surface. This subsequently generates more iron ions and active minerals, resulting in a large number of reaction-adsorption sites for P removal. Therefore, compared with the ZVI alone system (29.4%), the removal efficiency of P by oxidant/ZVI system (H₂O₂ : ClO^- : S₂O₈ ^2- = 33.2% : 54% : 67.1%) was improved. For the oxidant/ZVI system, H₂O₂ can promote the corrosion of ZVI to a certain extent. However, the solution pH could be increased during the corrosion process. This leads to inhibition of P removal performance by the H₂O₂/ZVI system, which only increased by 12.9% to 33.2%. The reaction between NaClO and ZVI consumes less H⁺, and the reaction product Cl^- can pierce the passivation layer on the surface of the ZVI through the pitting effect. As such, the NaClO/ZVI system attained a 54% P removal rate. Compared with H₂O₂ and NaClO, a better P removal effect of about 67.1% can be achieved by using Na₂S₂O₈, since the oxidation corrosion process of Na₂S₂O₈ does not consume H⁺, and it also has the strongest oxidizing properties. Furthermore, an appropriate increase in oxidant dosing (0.1-2 mM) could improve the efficiency at which of P is removed. Five batch cycle experiments showed that the oxidant/ZVI system has a higher removal capacity and longer life-span. In the long-term column running, the P removal capacity and operation life of the NaClO/ZVI column are 9.6 times and 3.2 times higher than that of the ZVI column, respectively. This work demonstrates that an oxidant/ZVI system can be an efficient method for P removal in water, which also provides a new idea for solving the problem of ZVI corrosion passivation.