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/PCwalnut ((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 N-2 through Fe-0. At the same time, FeS x 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. (C) 2022 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.

Automated summary

This study used biochar to support the sulfidation process of nano-zero-valent-iron, aiming to enhance nitrate removal and reduce greenhouse gas emissions. The results showed that the addition of S-nZVI/BC improved nitrate removal efficiency and reduced N2O emissions in constructed wetlands. The research also revealed the impact of S-nZVI/BC on microbial denitrification processes and functional genes.