Study On The Performance And Mechanism Of G-C₃N₄-based Photo Bio-electro-Fenton For Removing Multi-pollutants In Swine Wastewater

Livestock and poultry wastewater contains lots of pollutants,such as antibiotics and heavy metals.When antibiotics and heavy metals coexist,they are easy to formcomplex and increase the difficulty of processing.Therefore,it is necessary to carry out the research of antibiotics,antibiotics/heavy metal complexes and multi-pollutants removal technology in practical wastewater.Bio-electro-Fenton（BEF）has been widely used for the removal of refractory organic compounds,such as antibiotics,due to its excellent degradation performance and chemical stability.However,the drawbacks of this process are its strict pH range of 2–3 and iron sludge limiting its large-scale application.Therefore,to expand the pH range of BEF,reduce the amount of iron sludge,improve the performance of BEF to remove new pollutants such as antibiotics and improve the adaptability of BEF system,a PBEF system was constructed by coupling photocatalysis with BEF,a new type of g-C₃N₄-based cathode material was prepared.Tetracycline and Cu²⁺with high concentration in livestock wastewater were used as target pollutants.The performance of PBEF systemfor antibiotics,antibiotics/heavy metal complexes and actual wastewater was studied,the degradation mechanism of pollutants was clarified,and the treatment efficiency and eco-toxicity of PBEF system were evaluated.The main conclusions are as follows:（1）Performance and mechanism of TC removal by PBEF system based on FeMn-LDH/g-C₃N₄（FM/CN）cathode1)FM/CN cathode was prepared by in-situ solvothermal method.The results of morphology and photoelectrochemical performance tests showed that FM/CN prepared at 180°C with 10%CN had higher crystallinity and more active sites,the band gap and the charge transfer resistance were the minimum,which can improve the efficiency of electron transfer and the catalytic performance.Therefore,FM/CN prepared at 180°C with 10%CN was selected for subsequent degradation experiments.2)The FM/CN cathode successfully widened the pH range of PBEF system（3-11）,and avoided the production of iron sludge.Under the optimum operating parameters,the maximum removal rate of TC was 75.95%,and the removal rate of TC remained above 70%after 5 cycles,the effective mineralization rate is 67.57%,which indicates that FM/CN cathode has excellent catalytic activity,reusability and mineralization performance.3)The PBEF system based on FM/CN cathode can produce many active substances,and the order of their contribution is:·OH>·O₂^–>h⁺>¹O₂.In three possible pathways of TC degradation,the electron-rich groups（amino group,phenol group and double bond）in TC molecule were attacked by many active species,and degraded H₂O,CO₂ and other small molecular substances through hydroxylation,dehydration,demethylation,deamination,deacylamino,dealkylation and epoxidation processes.In addition,QSAR toxicity prediction and acute toxicity test of luminescent bacteria showed that the ecotoxicity of TC degradation products were reduced.（2）Performance and mechanism of TC/Cu²⁺complex removal by PBEF system based on FeMn-LDH/g-C₃N₄/MoS₂（FM/CN/MS）cathode1)FM/CN/MS cathode was prepared by in-situ solvothermal method.The results of morphology characterization and photoelectrochemical performance tests showed that,compared with FM/CN,the active sites,such as graphite N,adsorbed oxygen,Mo⁴⁺and sulfur vacancy,were added in FM/CN/MS.FM/CN/MS（10%MS）had the smallest band gap and charge transfer resistance,which was beneficial to improve the electron transfer efficiency,promote the Fe（Ⅱ）/Fe（Ⅲ）cycle and·OH formation,enhance the catalytic performance.Therefore,FM/CN/MS（10%MS）was selected for subsequent degradation experiments.2)The doping of MS in FM/CN/MS cathode materials successfully broadened the pH range of PBEF systemto neutral,promoted the Fe（Ⅱ）/Fe（Ⅲ）cycle and increased the catalytic reaction rate,the removal rate constant of TC/Cu²⁺complex by FM/CN/MS increased by 2.7times.Under the optimum operating conditions,the maximum removal rate of TC/Cu²⁺complex was 80.12%.After five cycles,the removal rate of TC/Cu²⁺complex remained above76%,and the effective mineralization rate was 70.21%,the results showed that FM/CN/MS had excellent catalytic activity,reusability and mineralization properties.The coexisting substances,such as Cl^–、NO₃^–、SO₄^2–and HA had little inhibition on the removal of TC/Cu²⁺complex,and the removal rate of TC/Cu²⁺complex was still more than 60%under different water quality background,the results show that FM/CN/MS cathode has certain adaptability in practical wastewater treatment.3)The PBEF system based on FM/CN/MS cathode can produce a variety of active substances,and the order of their contribution is:·OH>h⁺>·O₂^–>¹O₂.In three possible pathways of TC degradation,the TC/Cu²⁺complex was broken under the attack of the active species,and after the collaterals,Cu（Ⅱ）got electron to be reduced to Cu（Ⅰ）,which acted as Fenton catalyst to catalyze H₂O₂ to produce·OH.The electron-rich groups（hydroxyl group,amino group and double bond）with higher electron density and lower bond energy in TC molecule were preferentially attacked,and degraded to H₂O,CO₂ and other small molecular substances through hydroxylation,demethylation,deamination,dehydration and epoxidation and other processes.In addition,QSAR toxicity prediction and acute toxicity test of luminescent bacteria showed that the ecotoxicity of TC/Cu²⁺degradation products were reduced.（3）Study on performance and mechanism of PBEF systemfor removal of multiple pollutants fromswine wastewater1)In PBEF system,the actual swine wastewater was served as nutrient source,and conventional pollutants,antibiotics and antibiotics resistant genes in swine wastewater were treated by biological oxidation and cathodic active species.The removal rates of COD,TOC,TN and ammonia nitrogen were 68.63%,63.70%,53.11%and 76.60%,respectively,and the removal rates of antibiotics were over 78%and the concentration of antibiotics in the effluent was lower（<1.5 ng/L）The elimination rate of resistance gene was more than 75%.Therefore,PBEF systemis mainly used to treat the refractory pollutants,such as antibiotics and resistant genes,and it can also be used as a pretreatment process of swine wastewater to reduce the organic load and reduce the treatment load of the subsequent treatment process.2)The mechanism of PBEF system treating swine wastewater included biodegradation and advanced oxidation.Biodegradation involved three processes:organic matter degradation,microbial growth and reproduction,and electricity generation.Bacteria,such as Firmicutes,Bacteroidetes,Proteobacteria and Actinomycetes played important roles in organic matter degradation and electricity production.In advanced oxidation process,FM/CN/MS cathode produced some active species,such as·OH,·O₂^–and h⁺in the cathode chamber,and further oxidized and degraded organic pollutants in swine wastewater.And PBEF systemcan significantly reduce the eco-toxicity of swine wastewater.3)The maximum voltage output,maximum power density and maximumcoulomb efficiency of swine wastewater were 347 mV,901.83 mW/m³ and 51.21%,respectively,and the net energy of swine wastewater was positive.The results showed PBEF systemcan utilize swine wastewater for bio-energy recovery,and realize swine wastewater resource.In conclusion,g-C₃N₄-based PBEF systemcan effectively remove multi-pollutants in swine wastewater and realize the reuse of swine wastewater,which provided theoretical support for the application of PBEF systemin practical engineering.