Deciphering the degradation/chlorination mechanisms of maleic acid in the Fe(II)/peroxymonosulfate process: An often overlooked effect of chloride

In recent years, a significant effort has been devoted into investigating the effects of chloride on the degradation kinetics of aromatic pollutants. The impact of chloride on the decomposition of short-chain carboxylic acid intermediates from aromatics degradation has often been overlooked. In this study the roles of chloride in the oxidation of maleic acid (MA) in the Fe(II)/peroxymonosulfate (PMS) process was investigated. Degradation efficiency, reaction intermediates, adsorbable organic halogen (AOX) accumulation and mineralization were examined. The chloride ion (Cl⁻) was found to have an overall negative impact on MA degradation and mineralization in the Fe(II)/PMS system. The presence of Cl⁻ led to the formation of chlorinated by-products and a high production of AOX. The mineralization of MA was decreased with increasing Cl⁻ concentrations. Kinetic modeling demonstrated the impact of various radicals largely depended on the concentration of Cl⁻. The significance of Cl₂^•- or Cl₂ for MA destruction was enhanced with increasing Cl⁻ content, and overwhelmed that of SO₄^•- when the Cl⁻ concentration was over 5 mM. In the absence of Cl⁻, SO₄^•- was the primary radical responsible for MA oxidation. A possible degradation pathway is proposed (cis-trans isomerization, decarboxylation and halogenations processes). These results may help to understand the full oxidation pathways of refractory aromatic compounds and the mechanism of chlorinated by-products formation in industrial saline wastewater treatment.