Microheterogeneous Triplet Oxidation of Hydrophobic Organic Contaminants in Dissolved Black Carbon Solutions under Simulated Solar Irradiation

Dissolved black carbon (DBC) is proven to accelerate the triplet-mediated photodegradation of hydrophobic organic contaminants (HOCs). However, its photosensitization mechanisms are not clear. In this study, five HOCs including 2,4,6-trimethylphenol, N,N-dimethylaniline, 17β-estradiol, 17α-ethinylestradiol, and bisphenol A were selected as model compounds to explore the triplet-mediated phototransformation of HOCs in illuminated DBC solutions. All five HOCs presented high organic carbon-water partition coefficient (KOC) values in DBC solutions, indicating the strong sorption capacity of DBC for HOCs. When reaching sorption equilibrium, the apparent pseudo-first-order rate constants of HOCs vs log[DBC] were well fitted with a sorption-enhanced phototransformation model (R2 > 0.98). Using the sorption-enhanced phototransformation model, the degradation rates of HOCs determined at intra-DBC (kDBC,HOCs') were 1-2 orders of magnitude higher than those observed in aqueous bulk solution (kHOCsaq). Moreover, typical triplet quenchers (2,4,6-trimethylphenol and oxygen) exhibited a microheterogeneous quenching effect on the triplet-mediated photodegradation of 17β-estradiol. Therefore, our results suggested that HOCs underwent a microheterogeneous photooxidative degradation process in DBC solutions. Furthermore, a sorption-enhanced phototransformation mechanism was proposed to elucidate the microheterogeneous photooxidative behavior of HOCs in DBC solutions. This study provides new insights into the fate and transport of HOCs in aquatic environments.