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