Influence Of Modified Biochar Supported Nano Iron Composites On Selective Nitrate Removal And Greenhouse Gas Emission Reduction In Constructed Wetlands

Constructed wetland,known as a new clean ecological technology,has been widely used in sewage treatment.The application of constructed wetland has been restricted by the low nitrate(NO₃^-)removal efficiency in surface water with low C/N ratio and the discharge of greenhouse gases,such as carbon dioxide(CO₂),methane(CH₄)and nitrous oxide(N₂O).Improving NO₃^-removal and reducing greenhouse gas emission in constructed wetlands has become the focus of research.In this study,the modified biochar(BC)supported Fe-Cu/polyvinylpyrrolidone(Fe-Cu/PVP/BC)was prepared to improve NO₃^-removal and selectivity towards nitrogen in constructed wetland.The influences of Fe-Cu/PVP/BC composite on microbial community structure and diversity were investigated.In addition,BC supported sulfidated nano zerovalent iron(S-n ZVI/BC)was prepared to enhance NO₃^-removal and greenhouse gas(CO₂,CH₄ and N₂O)emission reduction in constructed wetland.In this study,the mechanism of NO₃^-removal and greenhouse gas emission reduction using S-n ZVI/BC composite were elucidated.The main research conclusions are as follows:(1)The optimal preparation methods of Fe-Cu/PVP/BC and S-n ZVI/BC and the effects of ratio,dosage,pyrolysis temperature and biochar types on the materials were analyzed.The ratio of Fe:Cu:BC and dosage of Fe-Cu/PVP/BC have a significant influence o in NO₃^-removal and selectivity for N₂.The optimum selectivity of NO₃^-to N₂(31%)was observed at Fe:Cu:BC ratio of 1:2:3 and Fe-Cu/PVP/BC dosage of 1mg·L^-1;The ratio of S-n ZVI:BC and pyrolysis temperature of BC have a significant influence o in NO₃^-removal.The optimal S-n ZVI:Bc ratio is 2:1,and the optimal pyrolysis temperatures of sawdust BC,sludge BC,reed and walnut shell BC are 500?,900?and 700?,respectively.(2)The removal effect of Fe-Cu/PVP/BC and S-n ZVI/BC on NO₃^-and other pollutants in constructed wetland and the removal mechanism of pollutants by materials was clarified.Fe-Cu/PVP/BC and S-n ZVI/BC can provide electron donors for denitrifying microorganisms to improve the ability of denitrification and NO₃^-removal in constructed wetland.The NO₃^-removal efficiency of constructed wetland which add Fe-Cu/PVP/BC and S-n ZVI/BC was much higher than that of blank group(46.5±6.5%).In addition,Fe-Cu/PVP/BC and S-n ZVI/BC can not only provde Fe³⁺and Cu²⁺to adsorb phosphorus,but also transformed to metallic oxides on the BC surface as active sorption sites which caused precipitation,adsorption and flocculation of phosphorus.The phosphorus removal efficiency of constructed wetland which add Fe-Cu/PVP/BC and S-n ZVI/BC was 5 times higher than that of the blank group.(3)The change of greenhouse gas emission in constructed wetland after S-n ZVI/BC addition and the influence mechanism of S-n ZVI/BC on the greenhouse gas emission in constructed wetland was mastered.The N₂O emission fluxes of the experimental group adding S-n ZVI/BC_sawdust,S-n ZVI/BC_sludge,S-n ZVI/BC_reed and S-n ZVI/BC_{walnut shell}(0.96 mg m^-2 h^-1,1.04 mg m^-2 h^-1,0.92 mg m^-2 h^-1and 0.94 mg m^-2 h^-1)and CO₂ emission fluxes(793.7 mg m^-2 h^-1,791.8 mg m^-2 h^-1,744.0 mg m^-2 h^-1and745.0 mg m^-2 h^-1)were significantly lower than those of blank experimental group(793.7 mg m^-2 h^-1,791.8 mg m^-2 h^-1,744.0 mg m^-2 h^-1and 745.0 mg m^-2 h^-1).In addition to S-n ZVI/BC_sawdust,the CH₄ flux of S-n ZVI/BC_sludge,S-n ZVI/BC_reed and S-n ZVI/BC_{walnut shell}(4.1mg m^-2 h^-1,3.9 mg m^-2 h^-1 and 4.0mg m^-2 h^-1)were all lower than that of blank group(4.4 mg m^-2 h^-1).(4)The effects of Fe-Cu/PVP/BC and S-n ZVI/BC on microbial abundance and community structure in constructed wetlands were analyzed.Fe-Cu/PVP/BC enhanced the abundance of Azospira and Thauera which related to anaerobic ammonia oxidation and denitrification,and made the biological denitrification ability of the constructed wetland supplemented with Fe-Cu/PVP/BC higher than that of the blank group.The results of NO₃^-concentration change and denitrification function genes(nar G,nark,nir S and nos Z)in constructed wetland effluent further verified the above results.The addition of S-n ZVI/BC enhanced the abundance of Comamonas,Simplicispir,Azonexus,Hydrogenophaga,Dechloromonas,Azospira and Thauera related to denitrification and NO₃^-removal in the constructed wetlands.The increase in abundance of bacteria related to denitrification and NO₃^-removal was also different with different BC species.Except for the experimental group adding S-n ZVI/BC_sawdust,the abundance of bacteria genus related to production of methane(Lentimicrobiaceae)in the experimental group(add S-n ZVI/BC_sludge,S-n ZVI/BC_reed,S-n ZVI/BC_{walnut shell})were significantly lower than that in the blank group.Therefore,the CH₄ production capacity of constructed wetland(add S-n ZVI/BC_sludge,S-n ZVI/BC_reed and S-n ZVI/BC_{walnut shell})was lower than that of blank group.This is consistent with NO₃^-concentration and CH₄ emission flux of effluent from constructed wetland.The experimental results show that BC supported nano iron composites can achieve NO₃^-removal and greenhouse gas emission reduction in constructed wetland through their material properties and influence on microbial community structure.This provide a new scheme and theoretical basis for the enhancement of NO₃^-removal performance and greenhouse gas emission reduction of constructed wetlands...