Sulfate radical mediated degradation of 5-halogenosalicylic acids: Phenoxyl radical transformation pathways

In the present study, we investigated the degradation kinetics and transformation pathways of two 5-halogen-osalicylic acids (5XSAs), namely, 5-chlorosalicylic acid (5ClSA) and 5-bromosalicylic acid (5BrSA) by sulfate radical (SO4 center dot-) in a thermo-activated persulfate system. The reaction pathways and mechanisms were proposed based on laser flash photolysis (LFP) techniques, HPLC-HRMS and molecular orbital calculations. Our results revealed that efficient removal of 5XSAs could be achieved by thermo-activated persulfate, and phenoxyl radicals were found to play key roles in the primary oxidation pathways. The subsequent transformation of phenoxyl radicals included hydroxylation and coupling processes. The resulting coupling products could undergo secondary reactions with sulfate radical, including dehalogenation, decarboxylation and hydroxylation. Hydroxylated products were in turn oxidized by SO4 center dot-, leading to the ring opening and the formation of a series of small molecular carbonyl byproducts. These processes could be responsible for the mineralization and the release of Br- or Cl-. In addition, the degradation rate constants of 5XSAs increased appreciably with increasing temperature, and higher efficiency of oxidation was observed around neutral initial pH. Moreover, degradation kinetics were found to be hardly affected by dissolved oxygen (DO), showing the possibility of applying SR-AOPs under environmental realistic conditions, not only for surface waters, but also for oxygen-deficient underground waters. The present work could increase our understanding on the reactivity and pathways of halogen phenols widely present in natural waters.