Enhanced Biodegradation Of Tetracycline And Its Fate And Behavior In Natural Environment

Conventional biological water treatment process could not remove PPCPs effectively for their high biotoxicity and low concentration in aquatic environment. Therefore, it is necessary to develop new biological technology to remove PPCPs. Moreover, PPCPs exhibit a "persistent existence" state, and it is an urgent research subject to investigate the fate and behavior of PPCPs in natural environment. This work aimed to realize the enhanced biological removal of antibiotic, and to explore the migration and transformation of antibiotic in aquatic environment. This kind of knowledge is help for the development of new technology to deal with PPCPs. The main results and conclusions are listed as following:(1) We investigated the fate and behavior of tetracycline (TC) in nitrifying sludge system, as well as the effects of TC dosage on sludge performance. TC could be effectively removed via initial adsorption and subsequent biodegradation, while biodegradation was the primary mechanism in this study. Compared to RII, no significant negative effects were found on dehydrogenase activity under higher TC stress in RI. It is interesting that RI showed better nitrification performance than RII, especially higher nitrite oxidation capacity. Moreover, exposure to TC also promoted the formation of aggregation and affected the composition of nitrifying bacteria. It indicated that larger aggregations were discovered in reactors with high TC stress. The aggregation might lead to multilayer structure of sludge to protect microorganism inside, which would explain the higher relative abundance of NOB in reactors with high TC stress.(2) The biodegradation and behavior of TC at high concentrations were explored in an enriched mixed culture system. TC at high concentrations could be effectively removed in the enriched mixed culture, and biodegradation was the primary mechanism. TC exposure could decrease the microbial diversity and enrich the dominant bacteria. These effects were more prominent with higher TC concentration. The TC exposure affected the microbial communities of activated sludge, resulting in a decrease in microbial diversity and the enrichment of dominant bacteria.(3) In this study, the reaction mechanisms between EPS and tetracycline, which were typical representatives of antibiotics, were explored and the proteins were identified as the active constituents in EPS during the reaction.The quenching mechanism of EPS on tetracycline was a static quenching process, suggesting the formation of complex. There is one binding site during the interaction and the process is spontaneous in which electrostatic forces play a major role.The changes of hydroxyl, carboxyl and amino were observed in the interaction, especially hydroxyl groups in tetracycline. It would damage or weaken the activity of antibiotics and affect the biotransformation and fate of antibiotics in aquatic environment.(4) The effect mechanisms of humic acid substances on photochemical behavior of tetracycline were also evaluated in this work. TA and GA were selected as NOM surrogates to systematically investigative their roles on the environmental fate of tetracycline. Humic acid substances could inhibit direct photolysis of tetracycline completely through competing photon with tetracycline and binding with tetracycline via hydrophobic interaction and van der waals force. Also, humic acid substances could generate 3NOM* and enhance-OH productivity to promote the indirect photolysis of tetracycline. However, the photolysis path was not change in the presence of humic acid. Moreover, tetracycline amounts in cell increased and the hydroxyl groups in tetracycline were changed in presence of humic acid substances. These results might explain why humic acid substances improved the biotoxicity of tetracycline.