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 (NO3--N) removal and investigate the impact on greenhouse gas emissions. Batch experiment results showed the S-nZVI/BCsawdust (2:1, 500), S-nZVI/BCsludge (2:1, 900), S-nZVI/BCreed (2:1, 700), and S-nZVI/BC walnut (2:1, 700) respectively improved NO3--N removal efficiencies by 22%, 20%, 3% and 0.1%, and the selectivity toward N2 by 22%, 25%, 22% and 18%. S-nZVI uniformly loaded on BC provided electrons for the conversion of NO3--N to N2 through Fe0. At the same time, FeSx layer 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 NO3--N removal efficiencies could reach 100% and the N2O 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 nosZ genes), methanotrophs (pmoA) and methanogenesis (mcrA). This research provided an effective method to improve NO3--N removal and reduce N2O emission in CWs.