A comparative study of sulfite activation using different transition metal ions for the degradation of bisphenol A

Transition metal ions have been considered as important catalysts for activating sulfite, while hardly any comparative study could be found so far. Herein, the performance and mechanism of Fe3+, Fe2+, Ce4+, Co2+, Mn2+ and Cu2+ to activate sulfite were compared for the abatement of a typical emerging contaminant (EC), bisphenol A (BPA), under natural reoxygenation condition. Results demonstrated that solution pH was crucial in determining the catalytic efficiency. Acidic conditions were more favorable for sulfite activation for Fe3+, Fe2+, Mn2+ and Ce4+, while Co2+/S(IV) and Cu2+/S(IV) processes demonstrated better performance under alkaline conditions. The Fe2+ and Ce4+ showed stronger adaptability to wide pH range than the other metals. The highest BPA degradation efficiency was achieved in Ce4+/S(IV) process at the optimum pH of 4.26, while effective removal of BPA was difficult in Mn2+/sulfite and Cu2+/sulfite processes. The formation of metal-SO3 complex was important for Fe3+, Fe2+, Mn2+, Co2+ and Cu2+, and direct electron transfer initiated the reaction of Ce4+. The SO3•- was identified as the primary radical species, and O2 participated in the evolution of other reactive oxygen species (ROS). Competitive quenching experiments showed that SO4•- and •OH dominated the degradation of BPA. Besides, Ce4+ also contributed to BPA degradation, while Cu(I) mainly acted as the radical scavenger that inhibited BPA degradation. The result of this study may help understand the mechanism of different Mn+/S(IV) processes and choose the appropriate process for the removal of EC.