Nitrofurazone Degradation Performance And Mechanism In The Bio-photoelectrochemical System Constructed With A G-C₃N₄/CdS Heterojunction Photocathode

With the extensive use of antibiotics in the treatment of human and animal diseases,many kinds of antibiotics are frequently detected in the environment,which have potential carcinogenic,teratogenic,mutagenic and other threats to human beings.Therefore,the effective removal of antibiotics in the water environment is particularly important.In this study,a bio-photoelectrochemical system（BPES）constructed with a bio-anode and a photocathode was developed to efficiently remove antibiotics（taking nitrofurazone（NFZ）as an example）from wastewater,on the basis of preparation and optimization of g-C₃N₄/Cd S heterojunction photocathode.The main research contents included:characterization and measurement of the physico-chemical properties and photoelectrochemical properties of g-C₃N₄/Cd S and corresponding photocathodes;evaluation of NFZ degradation performance in the BPES;identification of the intermediates produced during NFZ degradation,and accordingly,combining the microbial community analysis result,deduction of NFZ degradation pathway and mechanism;as well as the optimization of the BPES（including optimization of photocatalyst loading and NFZ concentration）and stability examination.The main conclusions are as follows:（1）Preliminary optimization of g-C₃N₄ result showed that the g-C₃N₄obtained with the calcination time of 2h exibited the best light absorption ability and photogenerated e^--h⁺pairs separation efficiency.Thus,this sample was used to synthesize g-C₃N₄/Cd S heterojunction photocatalyst（with mass ratios between g-C₃N₄ and Cd S of 1:9,5:5,and 7:3,respectively）.Meanwhile,the g-C₃N₄ photocathode,Cd S photocathode and g-C₃N₄/Cd S photocathodes were prepared.UV-vis DRS,PL and photocatalytic degradation of NFZ experiments showed that g-C₃N₄/Cd S（1:9）heterojunction photocatalyst possessed the best optical absorption performance and photocatalytic activity.EIS,LSV and i-t tests demonstrated that the g-C₃N₄/Cd S（1:9）photocathode exhibited the best photochemistry performance.In the BPES with a bioanode and a photocathode,the electroactive microorganisms could be gradually adapted to NFZ by gradually adding the NFZ,and reproducible and stable current output could be observed;（2）Among the BPESs constructed with different photocathodes（including g-C₃N₄ photocathode,Cd S photocathode and g-C₃N₄/Cd S photocathodes）,the one with g-C₃N₄/Cd S（1:9）photocathode had the best NFZ removal efficiency,that is,the removal reached 83.1%within the first 4h,much higher than those of mcirobial degradation（40.6%）,electrocatalytic degradation（4.3%）and photocatalytic degradation（7.7%）,showing obvious synergistic effect.Moreover,the TOC removal from the BPES with optimized g-C₃N₄/Cd S heterojunction photocathode was～90%,which was higher than that of the MEC（78%）without light irradiation.Besides,current output from this BPES was higher than the BPESs with other photocathodes.According to the intermediates detected by HPLC-MS,it could be inferred that the main reactions of NFZ degradation in this study were nitro reduction,C=N unsaturated bond reduction and C-N unsaturated bond and N-N bond cleavage causing by attack.After a series of reductive oxidation processes,two main straight-chain products were formed,and they were finally mineralized into H₂O and CO₂under the action of electrons or holes.On the overall characteristics of microbial community,the biodiversity of various samples followed the order:sample A（effluent from sewage treatment plant）>sample B（MEC anode biofilm）>sample C（BPES anode biofilm）>sample D（BPES cathode biofilm）.Proteobacteria accounted for the largest proportion of the four samples in the phylum analysis level,accounting for 66.53%,71.89%,74.67%and 57.31%,respectively.At the genus level,Pseudomnnas was the dominant bacteria in sample A,accounting for45.80%,while the proportion of Pseudomnnas in Sample B,Sample C and Sample D were significantly lower,which were 1.74%,2.92%and 6.25%,respectively.In addition,Geobacter was the most abundant bacteria in B,C and D samples,accounting for 31.64%,67.73%and 41.34%,respectively.Among the four samples,the amount of electrochemically active bacteria from the BPES anode was the highest,owing to the enrichment and acclimation of electroactive microorganisms.（3）Transient photocurrent response,EIS and LSV tests showed that among the photocathodes with different photocatalyst loadings（2.5mg/cm²,5mg/cm²,7.5mg/cm² and 10mg/cm²）,the g-C₃N₄/Cd S photocathode with a loading of 7.5mg/cm² displayed the best photoelectrochemical performance,and when being applied to the BPES,it showed the highest NFZ removal efficiency.When the concentration of NFZ ranged in 10 mg/L to 20 mg/L,NFZ removals from BPESs increased gradually;when further increased the NFZ concentration to 50mg/L,though the NFZ removals decreased gradually,but at least a removal of85%was achieved.Furthermore,NFZ removal only declined by 4%after 5 five consecutive runs;and current output at steady stage could be maintained at～8m A even after～40 d of operation,indicating that the stability of the system was quite good.Under the conditions of only light illumination and no applied voltage,～1.0 m A of current could be achieved from the self-biased BPES,demonstrating that this is a light-driven,self-biased and sustainable system;also,TOC removal of 63%could be obtained,suggesting desired NFZ degradation performance.