New insight of zero-valent iron activation S(IV) in absence of external hydrogenation ions for decomposing micropollutants: Two radical generation pathways depending on O2

As one of the most promising advanced oxidation processes, the activation of S(IV) by iron-based materials to produce SO4•− has become a hot technology for the degradation of organic pollutants. However, the mechanism of radical chain reactions has not been clearly explored. Herein, we established a novel AOP system based on the combination of zero-valent iron (ZVI) and potassium pyrosulfite (PPS). Combined with the degradation trends of CBZ and the change of DO concentration, the system was proposed to be divided into a slow initiation stage in 5 min and a rapid degradation stage in 5 − 10 min. Through a series of quenching experiments, electron paramagnetic resonance tests and several probe experiments, the mechanisms of two radical chain reactions were proposed between ZVI and PPS: the first one produced O2•− and HO•, and the other produced SO3•−, SO5•−, SO4•− and HO•. The two-stage reaction and two radical production paths of the ZVI/PPS system were proposed for the first time, which provided theoretical guidance for the practical application of the ZVI/PPS system. The ZVI/PPS system has been found to have excellent environmental adaptability in the treatment of industrial wastewater.