Effects And Mechanism Of Algae On Chlortetracycline Photodegradation Behaviors In Aqueous Environment

Antibiotics as an important group of pharmaceuticals are extensively consumed in human and veterinary medicines.Recently,their frequently detection in the environment is of great concerns because of their potential hazardous to ecosystem and human health.Photochemical decay has been confirmed to be a significant degradation process for many antibiotics in aqueous environment due to their sensitivity to light.Algae,which are ubiquitous in surface waters?e.g.,lakes,ponds,wetlands?may play an important role in sunlight-driven transformation of antibiotics.The objectives of this study are to reveal the role of algae in CTC photolysis and explore the photosensitive mechanisms.Firstly,the effects of algae on the photodegradation kinetic and pathway of CTC were investigated.It was found that algae could accelerate the photodegradation of CTC significantly,the rate constant of CTC was 0.0246 min^-1 in the presence of algae,which was almost 2 times higher than that in absence of algae.Meanwhile,in the presence of algae,the kinds of CTC degradation products and pathway increased obviously.Therefore,algae not only accelerate the photodegradation of CTC in water environment,but also have an important impact on the photodegradation pathways of CTC.The role of oxygen secretion,chlorophyll,enzyme and extracellular organic matters?EOMs?in the process of algae mediated CTC photodegradation was further explored.It was founded that EOMs could significantly enhance the photodegradation of CTC,confirming EOMs were the dominant photosensitive substance in the process of CTC photodegradation mediated by algae.³EOMs*and ¹O₂ were found to be the main active species produced by EOMs sensitization via EPR and chemical probes.Here,³EOMs*was confirmed to be the dominant reactive species in the indirect photodegradation of CTC induced by algae,contributing more than 90%to the indirect photodegradation of CTC.³EOMs*could react with CTC via electron transfer and energy transfer with second order reaction constants 2.653×10⁹ M^-1s^-1.The photosensitization efficiency and chemical structure characteristics of different EOMs were investigated.All of CV-EOMs,SM-EOMs,MA-EOMs and NOM could enhance the photodegradation of CTC via inducing indirect photodegradation,but the photosensitization efficiency was different.In order to quantify their difference in photosensitization efficiency,the³EOMs*yields of CV-EOMs,SM-EOMs,MA-EOMs and NOM were determined to be139.89±5.46,125.35±4.69,91.76±3.53 and 72.84±4.45?L/mol-photon?,respectively.The photochemical characteristics and molecular weight distribution of EOMs and NOM were characterized,and the structure-activity relationship between the chemical structure characteristics of EOMs/NOM and their sensitization efficiency was further constructed.The effective photosensitive substances of EOMs/NOM are mainly concentrated in the components with molecular weight less than 10k Da,and the E2/E3 index has a good positive correlation with sensitization efficiency.Notably,there was an excellent positive correlation between the carbonyl content and the sensitization efficiency?R²=0.9835?,indicating that the small molecule carbonyl substance was the main photosensitive component in EOMs.Finally,the effect of coexisting components in water on the sensitization process of EOMs was discussed.Cl^-was found could enhance the sensitization efficiency of EOMs by reducing the quenching rate of ³EOMs*.When NOM and EOMs coexist,the sensitization process of EOMs was inhabited by NOM,and it was confirmed the inhibition of NOM on the sensitivity of EOMs was due to the reduction of oxidation intermediates CTC by phenols in NOM.In addition,the photodegradation rate of CTC in the effluent with algae was 1.4 times higher than that in the effluent without algae.It can be predicted that the enhancement effects of algae on the photodegradation of CTC is significant in the real water,even if the concentration of algae is relatively low.