Methyl phenyl sulfoxide (PMSO) as a quenching agent for high-valent metal-oxo species in peroxymonosulfate based processes should be reconsidered

Nowadays, the possible appearance of high-valent metal-oxo species (like Co(IV)) in transition metals activated peroxymonosulfate (PMS) processes was explored by sulfoxides type quenching agent (e.g. methyl phenyl sulfoxide (PMSO)), as corresponding sulfones (e.g. methyl phenyl sulfone (PMSO2)) would be selectively produced by the attack of high-valent metal-oxo species on PMSO. However, the confirmed reactive oxygen species like hydroxyl radical (•OH), sulfate radical (SO4•−), single oxygen (1O2) or superoxide (O2•−) in PMS based processes shows highly aggressive and non-selective characteristics towards organic compounds. We therefore designed this work to examine whether PMSO conversion to PMSO2 would be interfered by those reactive oxygen species. By the comparison of PMSO degradation and PMSO2 formation in heat/PMS and Co2+/PMS systems, we found PMSO was easily converted to PMSO2 without Co2+. Scavenging experiments demonstrated that 1O2 or O2•− directly resulted in PMSO to PMSO2 while •OH and SO4•− also made contributions as they helped generate 1O2 and O2•−. Four common products of PMSO were found in heat/PMS and Co2+/PMS systems and more hydroxylated products were formed in the latter. Theoretical calculations further proved that 1O2 played a leading role in the conversion of PMSO to PMSO2. All these results indicated that in PMS based processes, PMSO was not an appropriate quenching agent for high-valent metal-oxo species like Co(IV). This work hoped to deepen the understanding of the formation and transformation mechanisms of reactive species in PMS based processes.