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 Cl or Cl for MA destruction was enhanced with increasing Cl content, and overwhelmed that of SO when the Cl concentration was over 5 mM. In the absence of Cl, SO 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.