Influence of modified biochar supported sulfidation of nano-zero-valent-iron(S-nZVI/BC) on nitrate removal and greenhouse gas emission in constructed wetland

In this study,the biochar (BC) produced from sawdust,sludge,reed and walnut were used to support sulfidation of nano-zero-valent-iron (S-nZVI) to enhance nitrate NO_(3)^(-) removal and investigate the impact on greenhouse gas emissions.Batch experiment results showed the S-nZVI/BC(sawdust (2:1,500)),S-n ZVI/BC(sludge (2:1,900)),S-n ZVI/BC(reed (2:1,700)),and S-n ZVI/BC(walnut (2:1,700))respectively improved NO_(3)^(-) removal efficiencies by 22%,20%,3%and0.1%,and the selectivity toward N_(2)by 22%,25%,22%and 18%.S-nZVI uniformly loaded on BC provided electrons for the conversion of NO_(3)^(-) to N_(2)through Fe0.At the same time,FeSxlayer was formed on the outer layer of ZVI in the sulfidation process to prevent iron oxidation,so as to improve the electrons utilization efficiency After adding four kinds of S-nZVI/BC into constructed wetlands (CWs),the NO_(3)^(-) removal efficiencies could reach 100%and the N_(2)O emission fluxes were reduced by 24.17%-36.63%.And the average removal efficiencies of TN,COD,TP were increased by 21.9%,-16.5%,44.3%,repectively.The increasing relative abundances of denitrifying bacteria,such as Comamonas and Simplicispira,suggested that S-nZVI/BC could also improve the process of microbial denitrification.In addition,different S-nZVI/BC had different effects on denitrification functional genes (narG,nirk,nirS and nos Z genes),methanotrophs (pmoA) and methanogenesis (mcrA).This research provided an effective method to improve NO_(3)^(-) removal and reduce N_(2)O emission in CWs.