Chlorella vulgaris enhance the photodegradation of chlortetracycline in aqueous solution via extracellular organic matters (EOMs): Role of triplet state EOMs.

Algae, which are ubiquitous in surface waters (e.g., lakes, ponds, wetlands) may play an important role in sunlight-driven transformation of compounds. This study is to investigate the role of algae (Chlorella Vulgaris) in chlortetracycline (CTC) photolysis and explore the photosensitive mechanism. The algae were found to significantly accelerate the photodegradation rate of CTC and extracellular organic matters (EOMs) were confirmed to be the major active substances. Triplet state EOMs (3EOMs*) were verified to be the dominant reactive species with 93% contribution to the indirect photodegradation rate of CTC, while ·OH and 1O2 contributed minor (7%). The steady-state concentration of 3EOMs* was determined by probe compounds (2,4,6-trimethylphenol) to be 3.50 × 10-14-1.88 × 10-13 M with the increase of EOMs from 2.0 to 8.0 mg L-1. The rate constant for reaction of 3EOMs* with CTC was calculated to be 3.17 × 109 M-1s-1. 3EOMs* were found to react with CTC mainly via electron transfer, on basis of susceptible reaction of 3EOMs* with the hydroxyl and amidogen groups in CTC. In addition, the energy transfer of 3EOMs* to CTC was possible according to the higher energy of 3EOMs* than that of triplet CTC.