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. [Display omitted] •An overall adverse impact of Cl− on MA degradation process is found.•Formation of chlorinated by-products and enhanced AOX level are observed.•Concentration profiles of chlorine radicals in the Fe(II)/PMS system are modelled.•Possible degradation pathways of MA are proposed.