| Antibiotics were widely used for human health, which have become a serious threat to the environment. After entering into water and soil environments, antibiotic resistant genes or bacteria can be induced after long-term antibiotic stress, which can make many new kinds of antibiotics invalidated, for resistant genes can be spread by means of horizontal gene transfer. It has attracted global concern for its enormous threat for human health. Conventional biological treatment had some disadvantages, such as inefficiency and low specialty, when used for wastewater containing antibiotics. So it was extremely important to establish a new rapid and efficient antibiotic wastewater pretreatment technology so as to eliminate antibiotic antimicrobial activity in this process. The researches of bio-electrochemical system(BES) cathodic pollutants reduction, such as nitrobenzene, chlorinated organic compounds and azo dyes have received widespread attention in recent years. The reduction products could achieve the goal of detoxification and decolorization especially in biocathode. However, antibiotics reduction in BES cathode were rarely reported. So in this study, five antibiotics, such as nitrofurazone(NFZ), furazolidone(FZD), metronidazole(MNZ), chloramphenicol(CAP) and florfenicol(FLO) were chosen for research, and they belong to nitrofurans,nitroimidazoles and chloramphenicols,respectively. To investigate the feasibility and pathway of their reduction and degradation in cathode, these five antibiotics were electrochemically reduced and degraded by poising at different cathode potential controlled with potentiostat. The results demonstrated that cathode potential had an significant influence on reduction rate and products formation, and cathodic effluent could lose toxicity and resistance in the process of reduction. It also revealed that nitrofurazone(NFZ) could be more efficiently reduced and degraded in biocathode than in abiotic cathode. Meanwhile, the applied voltage had an obvious effect on reduction rate and product formation of NFZ in biocathode in BES. The microbial community structure of cathodic biofilms changed obviously with different applied voltage. The CAP reduction results had shown that the inverted biocathode from bioanode still had a high efficiency in catalytic reduction compared to conventional biocathode. These results had provided the important basis for the development of an efficient degradation biotechnology for the antibiotics wastewater treatment.First, Cyclic voltammetry(CV) results showed that these antibiotics had different reduction peak current at different cathode potential, which implied these antibiotics could be reduced in cathode with corresponding potential. So, they were first electrochemically reduced in cathode by applying different potential controlled by potentiostat. Experimental results showed that the lower the cathode potential,the faster the reduction rate. When cathode potential was-1.25 V, NFZ and FZD were thoroughly ring-opened, while CAP and FLO were completely dehalogenated. Antibacterial activity assay results showed that reduction products of different cathode potential had lost their toxicity to Escherichia coli DH5α and Lactococcus lactis subsp.Applied voltage had an important influence on NFZ degradation in biocathode. The higher the applied voltage(the lower the cathodic potential), the faster the reduction rate of NFZ and formation of its product. NFZ and its main products(NFF and AMN) were more quickly degraded and formed at the supplied voltage of 0.8 V than 0.2 V and 0.5 V. Thus, the reduction and degradation rate k at 0.8 V was 1.46 and 2.63 times higher than abiotic and open-circuit control. When sodium bicarbonate served as carbon source rather than electron donor, the k was a little less than the conditions of glucose, which served as carbon source and electron donor. These results demonstrated that microbes attached to cathode electrode could completely rely on electrons supplied with electrode rather than glucose and other organic matter. The microbial community results based on Illumina 16 S r RNA gene sequencing analysis showed that different applied voltage could selectively enrich corresponding cathodic biofilm. Cathodic biofilm of supplied voltage of 0.8 V mostly enriched Enterococcus in bacterial genus,which was obviously different from 0.2 V and 0.5 V in which Klebsiella was predominant. These dominant bacteria in the three applied voltages also significantly different from open circuit control experiment, in which Pseudomonas was predominant. This showed that different applied voltage or cathode potential had led to different types of microbial community structure on the cathode electrode.Conventional biocathode could be achieved by pre-enrichment particular target contaminant degradation microbes and biofilm formation with cathode potential supplied. However, it was rarely reported that bioanode containing current producing bacteria reverse to biocathode for catalytic pollutants reduction research. Therefore, anti-chloramphenicol acclimation experiment of bioanode was studied by gradually increasing the concentration of CAP from 5 to 80 mg/L. After this bioanode was successfully started up, CAP was simultaneously reduced and degraded with bioanode coupled with abiotic cathode in the two-chamber reactors. After inversion biocathode from bioanode, it can produce electrons and degrade CAP, and this biocathode still had the ability of high reductive catalytic efficiency compared to conventional biocathode, whose reduction rate k was 1.63 times higher than abiotic cathode and sodium bicarbonate serving as carbon source, and 30.9% high than sodium acetate serving as carbon source and electron donor. It indicated that the cathodic biofilm can obtain electrons from cathodic electrode more efficiently after inversion and achieve CAP catalytic reduction and degradation. Based on the results of Illumina 16 S r RNA gene sequencing analysis, the microbial community in cathode had no significantly difference compared to bioanode before inversion. The dominant species of cathodic biofilm was Geobacter,a well-known current-producing microorganism, but the proportion changed from 52.64%(bioanode) to 67.54%(biocathode). It suggested that after electrode microenvironment changed, the anodophilic biofilm still had the ability of high efficient electron transfer to catalyze the reduction and degradation of CAP. |
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