## 3 Results and Discussion
### Effect of Antibiotics on the Decontamination Performance of BRC-MFC
#### 3.1.2 OFLX and TC Removal Performance  
The concentrations of OFLX in the influent and effluent in stage I and stage II are shown in Figure 3a. In stage I, the average removal rates of OFLX by R1-R4 were 93.91%, 98.98%, 99.52%, and 99.57%. In stage II, the average removal rates of OFLX by R1-R4 were 91.69%, 99.08%, 99.40%, and 99.41%. It can be seen that under OFLX stress of different concentrations, BRC-MFC has a good removal effect on OFLX, which can be maintained at more than 90%, and the removal rate is proportional to the concentration of OFLX. The degradation mechanism of OFLX in BRC-MFC mainly includes electrochemical reduction and biodegradation reaction. Antibiotics become a carbon source and electron donor in an MFC biological anode [51]. Electrogenerated microorganisms and antibiotic-degrading functional bacteria attach to the anode to form a biofilm, which can reduce the overpotential required for the degradation of antibiotics and their metabolites, thus promoting the degradation of antibiotics [52; 53]. The degradation of antibiotics at the bioanode relies mainly on anaerobic biodegradation and continuous electrical stimulation, and the anaerobic environment formed at the anode facilitates the mineralization of antibiotics, while continuous electrical stimulation can provide electrons to the environment, and through direct or indirect electron transfer to the bacterial cell, the stimulated microorganisms have enhanced metabolism and are able to rapidly metabolize antibiotics through secreted enzymes [54]. In addition, the cathode of MFC if modified with modified materials (Fe\({}^{0}\)/TiO\({}_{2}\)) can promote cathode production of -OH [55], which is extremely oxidizing, thus further enhancing the degradation of antibiotics. In terms of biodegradation, plant roots produce more secretions to decompose antibiotics under OFLX stress [56], but there is a limit to plant resistance to OFLX, and overly high concentrations (30-100 \(\mathrm{\SIUnitSymbolMicro g\cdot mL^{-1}}\)) of OFLX will cause a decrease in the content of antioxidant enzymes such as CAT, POD and SOD in plants, accompanied by the accumulation of reactive oxygen species such as O\({}_{2}\)', H\({}_{2}\)O\({}_{2}\) and MDA, which poison the plant itself [57].  
The concentrations of TC in the influent and effluent in stage II are shown in Figure 3b. The removal rates of TC by R1-R4 were 95.96%, 99.49%, 99.74%, and 99.60%, respectively. BRC-MFC also maintained more than 90% efficient removal rates for TC of different concentrations. The removal of OFLX and TC from the BRC-MFC reactor mainly depends on the adsorption of fillers and microbial degradation [58]. Since TC has stronger adsorption in the ambient medium [59], it can be seen that the removal rate of TC by BRC-MFC was always higher than that of OFLX. Each layer of fillers in BRC-MFC has a larger adsorption capacity for TC [16], and the maximum adsorption capacity of activated carbon in the electrode layer for TC reached 212.6 \(\mathrm{mg\cdot g^{-1}}\)[60]. In addition, the molecular structure of TC contains several hydroxyl groups (-OH) and an amino group (-NH\({}_{2}\)) [61], and these groups can coordinate with heavy metal ions to generate insoluble salts [62]. For example, if a small amount of iron filings is added to the submerged layer of BRC-MFC, TC will form a red complex with Fe\({}^{2+}\)[63]. In terms of microbial degradation, ammonia oxidation and nitrification reactions mainly occur in the surface layer of BRC-MFC, in which ammonia-oxidizing bacteria (AOB) convert ammonia-nitrogen to nitrate-nitrogen by ammonia monooxygenase (AMO), and some studies have found that AMO can also promote antibiotic degradation [64].  
Figure 3: (**a**) Removal performance of ofloxacin (OFLX). (**b**) Removal performance of tetracycline (TC).