Effect of borate buffer on organics degradation with unactivated peroxymonosulfate: Influencing factors and mechanisms

Considering that borate buffer is frequently used to maintain the pH in the peroxymonosulfate (PMS)-based advanced oxidation process and several oxygenated anions (OH-, PO43- , and CO32- ) can stimulate PMS, the efficient degradation of organic pollutants through a PMS/borate system was systematically reported in this work. The presence of borate significantly enhanced the degradation of acid orange 7 (AO7) by PMS at all PMS concentrations, but not by persulfate or hydrogen peroxide. Additionally, concentrations of borate from 0.04 to 0.6 M accelerated the degradation of AO7 by PMS, while the corresponding Icobs values increased and then decreased, reaching a maximum of 0.0284 min-1 at 0.1 M borate. The decline at higher borate concentrations resulted from the generation of unreactive intermediate peroxoboric acid (HOOB(OH)2) between HSO5- and H3BO3, which consumed a portion of PMS. The removal of AO7 was stimulated by the presence of borate at lower pH = 7.0-9.0 but was suppressed at a higher of pH 9.5 and 10.0. Based on scavenging experiments and electron paramagnetic resonance tests, it can be inferred that 1O2 together with SO4 center dot- and center dot OH were responsible for AO7 removal in the PMS/borate system. TOC removal was also enhanced by PMS/borate system compared to PMS alone. PMS/borate system was also effective in the degradation of other organic contaminants and preferentially oxidized the electron-rich moieties. Consequently, more attention should be paid to the use of borate buffer in the studies of PMS-based advanced oxidation processes, and this work can provide a reference for future research.