Effects of chloride on PMS-based pollutant degradation: A substantial discrepancy between dyes and their common decomposition intermediate (phthalic acid)

A considerable effort has been devoted to elucidating the roles of chloride in oxidative degradation and chlorination of dyes. However, few investigations are available on kinetic analysis and transformation pathways of secondary degradation byproducts of dyes in saline wastewater treatment. Here the impact of chlorine on the degradation rate of phthalic acid, a typical dye degradation intermediate, by the Co²⁺/peroxymonosulfate (PMS) process was examined. Degradation efficiency, intermediate products, AOX (adsorbable organic halogen) formation and mineralization were considered. An overall negative impact was observed within the concentration of Cl⁻ up to 100 mM, differing from the dual effect of chloride on dye degradation process as previously observed. The presence of high levels of Cl⁻ led to a low production of AOX and a reduction of the formation of chlorinated by-products. The mineralization was also restrained when the Cl⁻ concentration was increased. Degradation pathways for these processes are proposed. These findings provide valuable information about the degradation pathways of dyes and about the formation mechanism of chlorinated by-products in industrial saline wastewater treatment.