Effect and mechanism of phosphate enhanced sulfite activation with cobalt ion for effective iohexol abatement

Abstract

Abstract Sulfate radical (SO4•−) based advanced oxidation processes (AOPs) from sulfite activation has recently received attention for abatement of microorganic pollutants in the aquatic environments. Trace level Co(II) has been demonstrated to be effective for promoting sulfite activation (simplified as the Co(II)/sulfite system) and the corresponding radical formation, yet this process is challenged by the limited valence inter-transformation of Co(II)/Co(III). In order to enhance this valence inter-transformation, a novel Co(II)/HPO42−/sulfite system is developed in this work, in which complexation of Co(II) with HPO42− can regulate the electronic structure of Co(II) and promote valence inter-transformation of Co(II)/Co(III) during the sulfite activation process. The Co(II)/HPO42−/sulfite system exhibits superior iohexol abatement performance under circumneutral conditions. For pH 8.0 and Co(II) dose of 1 μM, the iohexol abatement efficiency is as high as 98%, which is considerably higher than that of the Co(II)/sulfite system (50%). SO4•− is identified as the predominant reactive radical contributing to iohexol abatement. The presence of HPO42− broadens the pH adaptability of the Co(II)/sulfite system for iohexol abatement. In addition, the coexisting Cl− exerts an inhibitory effect on iohexol abatement while the other cations and anions shows negligible effect. The Co(II)/HPO42−/sulfite system displays good reusability and adaptability towards various organic pollutants. This study highlights the important role of complexation of Co(II) with HPO42− in sulfite activation and provide a feasible idea for abatement of the microorganic pollutants.