New insights into the mechanism of Fered-Fenton treatment of industrial wastewater with high chloride content: Role of multiple reactive species

Abstract

Hydroxyl radical (OH) is considered the dominant reactive species in the electro-Fenton (EF) and Fered-Fenton (EF-Fere) processes for wastewater treatment. However, in chloride-rich media, this is arguable due to the obscure mechanisms for the oxidant speciation and pollutant degradation. Herein, the role of active chlorine and Fe(IV)-oxo species (FeIVO2+) as primary oxidizing agents in HClO-mediated Fered-Fenton (EF-Fere-HClO) process is discussed, along with the dependence of their contribution on the pollutant structure. HClO generated from anodic oxidation of Cl− can be consumed by added H2O2 to form singlet oxygen (1O2), which is detrimental because this species is quickly deactivated by water. The reaction between HClO and Fe2+ was proved to generate FeIVO2+, rather than OH or Cl suggested in the literature. The yield of FeIVO2+ species was proportional to the Cl− concentration and barely affected by solution pH. The long-lived HClO and FeIVO2+ can selectively react with electron-rich compounds, which occurs simultaneously to the non-selective attack of OH formed from Fenton's reaction. The FeIVO2+ and OH concentration profiles were successfully modelled. Although the accumulation of toxic chlorinated by-products from HClO-mediated oxidation might cause new environmental concerns, the toxicity of pesticide wastewater with 508 mM Cl− was halved upon EF-Fere-HClO treatment.