Contribution of the Excited Triplet State of Humic Acid and Superoxide Radical Anion to Generation and Elimination of Phenoxyl Radical

Contributions of excited triplet state of humic acid ((3)HA*) and superoxide radical anion (O-2(center dot-)), which is mainly generated via the reaction of O-2 with HA-derived reducing intermediates (HA(center dot-)), to phenol transformation were revealed using acetaminophen, 2,4,6-trimethylphenol and tyrosine as probe molecules. Phenol transformation was initiated by (3)HA*, leading to the formation of the phenoxyl radical (PhO center dot), but the distribution of transformation intermediates was codetermined by (3)HA* and HA(center dot-). The influence of HA(center dot-) essentially resulted from the production of O-2(center dot-), which affected the fate of PhO center dot. PhO center dot could undergo dimerization, or react with O-2(center dot-), leading to either phenol peroxide formation (radical addition) or phenol regeneration (electron transfer). In addition, PhO center dot could bind to HA or react with HA radicals, particularly in the absence of O-2 and O-2(center dot-). These PhO center dot reactions were dependent on the reduction potential and structure of PhO center dot. This study also proved that the reaction of phenol with O-1(2) and the reaction of PhO center dot with O-2(center dot-) lead to the same oxidation product. The contributions of (3)HA* and its generated O-1(2) , HA(center dot-) and its generated O-2(center dot-) to phenol transformation were pH-dependent.