Enhanced activation of sulfite by the Co(II)-bicarbonate complexes for iohexol abatement: Role of free radicals versus high valent cobalt species

Abstract

Sulfite activation with Co(II) (simplified as Co(II)/sulfite process) has been widely studied for the abatement of organic contaminants, yet the efficiency in the Co(II)/sulfite process is compromised by the slow redox cycling of Co(II)/Co(III). Regarding that the complexation effect between Co(II) and bicarbonate (HCO3−) can enhance the conversion between Co(II) and Co(III) in the advanced oxidation processes, a novel Co(II)/HCO3−/sulfite process is constructed in this study. The iohexol abatement efficiency in the Co(II)/HCO3−/sulfite process is >92 % within 10 min of reaction at trace Co(II) dosage, which is significantly higher that of Co(II)/sulfite process. Apart from the conventional radical generation in the Co(II)/sulfite process, it is unexpectedly found that high valence cobalt species (Co(IV)) are generated. HCO3− can enhance the generation of Co(IV), sulfate radical (SO4•−) and hydroxyl radical (HO•), which play the important role in iohexol abatement. The identification of Co(IV) and its contribution to iohexol abatement is elucidated in the Co(II)/HCO3−/sulfite process. The enhancement can be attributed to the complexation effect between Co(II) and HCO3−, in which the CoHCO3+ and CoCO3 species exhibit excellent ability on sulfite activation. According to electrochemical characterization and theoretical calculation, the presence of HCO3− can reduce the energy barrier in the reaction process, accelerating the Co(II)/Co(III) redox cycling. The Co(II)/HCO3−/sulfite process exhibits excellent practical application potential in the abatement of various pollutants in the water and wastewater treatment.