Performance And Mechanism Of Nitrogen Removal And Electricity Generation In Pyrite-based Constructed Wetland-microbial Fuel Cell

Constructed wetlands-Microbial fuel cell（CW-MFC）is an ecological treatment technology used to transform organic pollutants into electricity.However,the performance improvement of CW-MFC is affected by the electrode material configuration.Pyrite is an emerging wetland substrate with good conductivity and high mechanical strength.In this study,pyrite was used as the electrode filler to construct five pyrite CW-MFC systems with different configurations.The coke CW-MFC system was used as the control.The nitrogen removal efficiency and electricity generation performance of these CW-MFC systems were measured under different aeration rates and COD loads.The contribution of pyrite to the performance of CW-MFC the effect of electrode spacing on the performance of CW-MFC,and the autotrophic denitrification potential of CW-MFC was explored in this study to evaluate the feasibility of pyrite application in CW-MFC and optimize the operating conditions.Besides,the microbiological mechanism of simultaneous nitrogen removal and electricity generation was investigated in order to provide data support and a theoretical basis for pyrite application in practical scenarios.（1）CW-MFC with pyrite as anode filler（System A）and coke-pyrite（volume ratio 1:1）CW-MFC as anode filler（System B）were constructed to investigate the nitrogen removal efficiency and electricity generation performance of CW-MFC under different COD loads（320 and 480 mg/L）and aeration rates（0.2 and 0.4 L/min）.The results showed that System A exhibited superior removal efficiency of PO₄^3--P（73.91±1.79%vs.68.21±1.27%）no matter how the conditions change,which was attributed to the abundant Fe ions in pyrite forming the chemical precipitation of PO₄^3--P.Under the aeration rate of 0.2 L/min,the removal efficiency of COD and NH₄⁺-N in System B was slightly higher than that in System A,which was due to the abundant porous structure of coke.With the increase of influent COD load,the autotrophic denitrification process with S^-and Fe²⁺ions in System A is strongly inhibited,indicating that the system could have the potential to treat low-carbon wastewater.When the aeration rate was increased to 0.4 L/min,the denitrification capacity and electricity generation performance of System A decreased by 11.89%and 19.64%,respectively.It might be because of the redundant O₂ which was diffused to the anode and damaged the original anaerobic environment,indicating that electrode spacing can affect the electrode function of pyrite in the system.Therefore,it is very important to adjust the balance between electrode spacing and aeration rate to achieve the best performance of pyrite-based CW-MFC that should be further studied.（2）Using pyrite as an anode filler,two CW-MFC systems with an electrode spacing of 10 cm（System A and D）and 18 cm（System C and E）were constructed respectively.Systems A and C were operated under high carbon loads（COD:300 and480 mg/L）.Systems D and E were operated under low carbon load（COD:50 and 180mg/L）.The effect of electrode spacing and aeration rates on the nitrogen removal efficiency and electricity generation performance of systems were explored.The results showed that System A and D exhibited higher removal performance of COD,NH₄⁺-N,NO₃^--N,and electricity generation than System C and E,respectively,at 0.2L/min of aeration rate.It was attributed to the fact that the smaller electrode spacing was conducive to the electron transfer between microorganisms,thus improving the removal of pollutants.With the increase of aeration rate,the removal efficiency of COD and NH₄⁺-N of the four systems was significantly improved,but the denitrification capacity of System C was higher than that of System A,indicating that electrode spacing and aeration rate synergically affected the denitrification performance of systems under higher carbon load.However,the denitrification capacity of System D was still higher than that of System E,indicating that electrode spacing is the key factor affecting denitrification performance under a lower carbon load.In addition,the increase in the aeration rate caused a lower electricity generation of System A and D compared to System C and E,respectively.In general,pyrite-based CW-MFC with 10 cm electrode spacing can achieve a stable performance of pollutant removal and electricity generation under a lower carbon load.（3）To explore the autotrophic denitrification potential of pyrite-based CW-MFC,a System F with the same configuration as System D was constructed and operated without organic carbon sources.The performance of System D and F was compared under different aeration rates（0.2 and 0.4 L/min）.The results showed that the NH₄⁺-N removal efficiency of System D（94.6±1.59%）was significantly lower than that of System F（96.26±1.64%）.This might be attributed to the higher reduction product of Fe³⁺in System F,causing a stronger anaerobic anammox coupled with iron reduction reaction with NH₄⁺-N.Also,the PO₄^3--P removal efficiency of System D was significantly higher than that of System F（86.67±0.94%vs.79.79±1.17%）,possibly due to the utilization of organic carbon in the biological phosphorus removal processes of System D.The NO₃^--N effluent concentration in System D was always lower than that in System F,resulting in a significantly higher TN removal efficiency compared to System F（61.54±5.96%vs.17.32±2.78%）.The denitrification and electricity generation performance of System D was higher than that of System F with a maximum open circuit voltage and power density achieved of 406 m V and 0.2m W/m³.Therefore,pyrite can play an advantage in the treatment of low-carbon and high-phosphorus wastewater.（4）The results of Alpha diversity analysis and non-metric multidimensional scaling analysis showed that the community composition of System A,B,and C was significantly different from that of System D,E,and F.Besides,the microbial community attached on the anode of each system with higher species richness and community diversity compared to the cathode.The dominant microbial community at the general level included Thiothrix,Candidatus-Competibacter,Thauera,norank＿o＿＿Run-SP154,and Bacteroidetes＿vadinha17.Canonical correlation analysis showed that System B and C exhibited higher denitrification capacity compared with System A,which was attributed to the higher abundance of Thiothrix and Candidatus＿Competibacter.High abundance electroactive microorganisms Hydrogenophaga and Trichloromonas are found in System A,B and C,which make a great contribution to the generation of electrical energy.Large numbers of Thauera denitrifying bacteria were detected in the cathode of System D,leading to a higher denitrification performance compared to System E and F.In System F,Rhodococcus,Gordonia,and Bacillus were the main denitrifying bacteria in the cathode that was conducive toautotrophic denitrification with iron and sulfur ions.It was also observed that higher NH₄⁺-N removal performance in System F was related to Nitrospira with a higher abundance.In conclusion,Thiothrix,Candidatus-Competibacter,Thauera,norank＿o＿＿Run-SP154,Hydrogenophaga and Trichloromonas were the main functional bacteria that can achieve synchronous nitrogen removal,phosphorus removal and electricity generation by CW-MFC of pyrites.