Study On Decontamination Performance And Strengthening Mechanism Of Pyrite-Based Bioretention System Coupled With MFC

Bioretention system（BRS）is one of the common low-impact development measures in sponge city（SPC）construction,which can achieve the dual goals of runoff control and pollutant reduction.However,the removal of nitrogen-containing pollutants by traditional BRS is unstable,since it is difficult to achieve efficient removal of nitrate nitrogen（NO₃^--N）,and even the phenomenon of leaching may occur.At the same time,as a dynamic plant-soil ecosystem,BRS will also produce a certain amount of greenhouse gases（GHG）while dealing with runoff pollution.Especially,it may cause a surge of GHG while strengthening the nitrogen removal performance,which makes it difficult for the construction of SPC to achieve the carbon emission reduction target and even become the source of GHG emission.At present,there are few studies on the synergistic emission reduction of BRS-enhanced nitrogen removal and GHG,so it is necessary to explore an optimization measure that can enhance the performance of pollution removal and realize the emission reduction of GHG.In this study,microbial fuel cells（MFCS）coupled BRS（M-BRS）,biochar based BRS（B-BRS）,MFCS coupled biochar based BRS（BM-BRS）,MFCS coupled pyrite/biochar composite BRS（PBM-BRS1:10%biochar+10%pyrite;PBM-BRS2:10%biochar+20%pyrite）,MFC-coupled pyrite based BRS（PM-BRS）,and traditional BRS（C-BRS）were constructed.The removal performance,bioelectricity generation capacity,and GHG emission characteristics of different systems were investigated,and the system construction method and influencing factors of enhanced pollution removal and GHG collaborative emission reduction were investigated.Through scanning electron microscopy（SEM）,X-ray energy spectrum（XPS）,Fourier transforms infrared spectroscopy（FTIR）and other characterization methods,combined with microbial and denitrification potential analysis,the relevant mechanism of intensive nitrogen removal and GHG emission reduction was revealed from microbial and electrochemical perspectives.All of these provide a theoretical basis and technical support for the design and optimization of nitrogen removal performance of BRS under the background of dual carbon and try to promote the integration of PCy construction and carbon emission reduction actions.The main research conclusions are as follows:（1）The enhanced BRS has better pollution removal performance and GHG emission reduction capacity than C-BRS on the whole,especially the MFC coupled system helps to improve the system’s pollution removal performance,in which the pollutant removal rate of PBM-BRS2 increases by 5%～30%.In particular,the removal rate of NO₃^--N and TN was significantly higher than that of B-BRS and C-BRS（P<0.05）.The output voltage of PBM-BRS2 can reach 104.06 m V.However,the total GHG emission flux of PM-BRS was the lowest（971.20±277.54 mg CO₂/m²·d）,which decreased by 49.56%～60.75%compared with C-BRS,followed by PBM-BRS2（1049.53±779.17 mg CO₂/m²·d）.The results showed that the addition of biochar could promote the denitrification process and alleviate the release of N₂O,but it could enhance the release of CH₄and CO₂,increasing the total emission flux of GHG.Coupled MFC can significantly reduce the promoting effect of biochar on GHG emission,which shows the application potential of MFC-coupled systems in GHG emission reduction.Combining the performance of decontamination and GHG emission reduction,PBM-BRS2 is the best system that can achieve both BRS-enhanced decontamination and GHG emission reduction.（2）With the increase of influent C/N,the removal of COD in each system increases first and then decreases,while the removal of NH₄⁺-N,NO₃^--N,and TN all increases,and the accumulation of NO₂^--N also weakens gradually.At the same time,the output voltage and power density increased with the increase of influent C/N,while the emission fluxes of CO₂,N₂O,and CH₄decreased significantly（P<0.05）.When the influent C/N was 26.7,the total GHG emission flux of each system was the lowest（178.63±212.79～1886.52±258.77 mg CO₂/m²·d）.In particular,the total emission fluxes of PM-BRS and PBM-BRS2 decreased by 87.48%and 72.15%,respectively,when C/N=6.7.With the increase of C/N value,the removal process of COD and NO₃^--N in each system has obvious fluctuation.（3）Compared with C-BRS,enhanced BRS can increase microbial diversity,and the MFC coupling system can also significantly affect the community structure of BRS.In particular,it can enrich electrochemically active bacteria（such as Proteobacteria）through electrochemical interaction,and promote denitrification through electron transfer.The system with pyrite addition has a higher abundance of denitrifying bacteria and electric-producing bacteria.The electric-producing bacteria Geobacteraceae also has the function of dissimilating the reduction of Fe（Ⅲ）.Desulfobacterota’s coexistence with pyrite demonstrates that its use as an anode can further enhance denitrification capacity by promoting the cycle of iron and sulfur.At the same time,PBM-BRS2 had a higher abundance of denitrification enzymes（nar G,nir S,nor B,and nos Z）and a stronger denitrification potential（17.88μg NO₃^--N/kg·h）,which showed a better ability to enhance nitrogen removal and GHG synergistic emission reduction.（4）After the reaction,the relative contents of Fe and S in pyrite showed a decreasing trend,which confirmed that autotrophic denitrification with pyrite as an electron donor occurred in the pyrite addition system,thus reducing the emission flux of N₂O.At the same time,Fe²⁺/Fe³⁺produced in the oxidation process of pyrite can be attached to biological carbon for in-situ modification（Fe@BC）to enhance the electron transfer efficiency and promote the denitrification process.The addition of pyrite coupled with the MFC system can give the electrogenic bacteria an advantage in the competition of organic matter by forming an efficient current transport process,and weakening the metabolic activity of methanogens.The electrochemical action can also decrease the O/C value of biochar（from 1.264 to 0.878）,significantly increase the peak strength of C=C and C=O bonds,and appear obvious peaks formed by Fe-O bond stretching vibration,effectively enhance the electron transport capacity of biochar,promote the denitrification process and inhibit the respiration and metabolism of methanogens.The production of N₂O and CH₄is further reduced,indicating that PM-BRS and PBM-BRS2 have better GHG emission reduction ability.