One-Step Treatment of Phosphite-Laden Wastewater: A Single Electrochemical Reactor Integrating Superoxide Radical-Induced Oxidation and Electrocoagulation

Elimination of phosphite from water is more difficult than elimination of phosphate owing to its higher solubility and resistance to biotransformation. Herein, we report an efficient, facile, electrochemical method integrating electrooxdiation (EO) and electrocoagulation (EC) to treat phosphite-laden wastewater. The mechanistic studies demonstrate that in-situ-generated Fe2+ at an Fe anode can react with in-situ-generated O-2 at a mixed metal oxide (MMO) anode, leading to formation of O-center dot(2)-, a reactive species predominantly responsible for oxidation of phosphite to phosphate. The phosphate is immediately coagulated by Fe hydroxides that are formed due to the production of OH- at a stainless-steel cathode. The integrated EO/EC system enables a phosphite removal efficiency of 74.25% (MMO anode, 100 mA; Fe anode, 100 mA; reaction time, 60 min), a significantly higher efficiency rate than the rate obtained in the control experiments in the absence of an MMO anode (<23.41%) and the rate obtained with the chemical coagulation process (<5.03%). The quenching experiments with scavengers and electron spin resonance tests verify the pivotal role of O-center dot(2)- in transformation of phosphite. Tests carried out with nickel-plating wastewater further demonstrate the superiority of this integrated system, as evidenced by efficient removal of phosphite and nickel from the solution.