The aqueous photodegradation of the widely used antibiotic chlortetracycline(CTC) was investigated under simulated sunlight.The quantum yield of photodegradation increased from 3.3 × 10-4 to 8.5 × 10-3 within the pH range of 6.0 to 9.0.The presence of Ca2+,Fe3+,and NO3-enhanced the photodegradation of CTC,whereas Mg2+,Mn2+,and Zn2+ inhibited the degradation with the order Mn2+ > Zn2+ > Mg2+ at pH 7.3.The monovalent cations(Na+ and K+) had negligible effect on the photolysis of CTC.Fulvic acid(FA) decreased the photodegradation of CTC due to light screening effect.Hydrogen peroxide(H2O2) was formed concurrently with direct photodegradation of CTC.The generation rate of H2O2 increased from 0.027 to 0.086 μmol/(L.min) when the pH ranged from 6.0 to 9.0.The CTC solution was about three-fold more toxic to the Photobacterium phosphoreum bacteria after irradiation,suggesting that the photoproducts and H2O2 formed in the CTC solution exhibited high risk on the bacteria.By LC-ESI(+)-MS,the photoproducts of CTC were identified.The direct photodegradation of CTC was involved in hydroxylation and N-demethyl/dedismethyl processes.The main photoproducts included the iso-CTC analog containing hydroxyl groups(m/z 511.4 and 495.4),and the N-demethyl/dedismethyl products of the photoproduct m/z 495.4(m/z 481.3 and 467.4).In addition,the photochemical dechlorination of CTC led to tetracycline(m/z 445.5).
Yong ChenHua LiZongping WangTao TaoDongbin WeiChun Hu
Tetracyclines constitute one of the most important antibiotic families and represent a classic example of phototoxicity.The photoproducts of tetracyclines and their parent compounds have potentially adverse effects on natural ecosystem.In this study,the self-sensitized oxidation products of tetracycline (TC) and oxytetracycline (OTC) were determined and the effects of Ca2+ and Mg2+ on self-sensitized degradation were investigated.The Ca2+ and Mg2+ in the natural water sample accounted for enhancement (pH 7.3) and inhibition (pH 9.0) of photodegradation of TC and OTC due to the formation of metal-ions complexes.The formation of Mg2+ complexes was unfavorable for the photodegradation of the tetracyclines at both pH values.In contrast,the Ca2+ complexes facilitated the attack of singlet oxygen (1O2) arising from self-sensitization at pH 7.3 and enhanced TC photodegradation.For the first time,selfsensitized oxidation products of TC and OTC were verified by quenching experiments and detected by LC/ESI-DAD-MS.The products had a nominal mass 14 Da higher than the parent drugs (designated M+14),which resulted from the 1O2 attack of the dimethylamino group on the C-4 atom of the tetracyclines.The presence of Ca2+ and Mg2+ also affected the generation of M+14 due to the formation of metal-ions complexes with TC and OTC.The findings suggest that the metal-ion complexation has significant impact on the selfsensitized oxidation processes and the photoproducts of tetracyclines.
