An often-overestimated adverse effect of halides in heat/persulfate-based degradation of wastewater contaminants

Halides (X⁻) in the industrial wastewater are usually thought to adversely affect the degradation kinetics and mineralization rates in several SO₄^[rad]−-based advanced oxidation processes. However, their unfavorable effects might be overestimated, particularly the heat/persulfate (PS) system as tested in the present study. Here the degradation of phenol, benzoic acid, coumarin and acid orange 7 (AO7) was examined with the presence of chloride or bromide in a heat/PS process. Cl⁻ was found to have a dual effect (inhibition followed by enhancement) on the decomposition rates of organic pollutants, whereas the effects of Br⁻ are insignificant within the tested concentration (0–0.2 mM). However, some chlorinated or brominated compounds were still identified in this heat/PS system. Unexpectedly, the mineralization rates of AO7, phenol, benzoic acid and coumarin were not apparently inhibited. In addition, the formation of adsorbable organic halogen (AOX) in the heat/PS system was much less than those in the peroxymonosulfate (PMS)/Cl⁻ or PMS/Br⁻ systems. According to the results of kinetic modeling, SO₄^[rad]− was the dominating radical for AO7 degradation without Cl⁻ or Br⁻, but Cl₂^[rad]− was the main oxidant in the presence of Cl⁻, SO₄^[rad]−, Br^[rad] and Br₂-^[rad] were responsible for the oxidation of AO7 in the presence of Br⁻. The present study assumes that X₂/HOX, rather than halogen radicals, is responsible for the enhanced formation of organohalogens. These findings are meaningful to evaluate the PS-based technologies for the high-salinity wastewater and to develop useful strategies for mitigating the negative effects of halides in advanced oxidation processes (AOPs).