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.

Influence of modified biochar supported Fe-Cu/polyvinylpyrrolidone on nitrate removal and high selectivity towards nitrogen in constructed wetlands.

In this study, the biochar (BC) supported Fe-Cu bimetallic stabilized by PVP (Fe-Cu/PVP/BC) were prepared and utilized to enhance the nitrate (NO3-) removal and the selectivity toward nitrogen (N2). Results showed the optimum Fe:Cu:BC ratio and the dosage of the BC (pyrolysis at 700 °C) supported Fe-Cu bimetallic stabilized by polyvinylpyrrolidone (PVP) (Fe-Cu/PVP/BC700) were respectively 1:2:3 and 1 mg L-1 with the selectivity toward N2 of 31 %. This was mainly due to the synergy among Fe0, Cu0 and BC in the Fe-Cu/PVP/BC. The addition of Fe0 could reduce the NO3- through providing electron. The Cu0 and BC improved the selectivity of NO3- to N2 through forming [Cu-NO2-ads] and adjusting redox potential. The addition of Fe-Cu/PVP/BC could supply electrons for denitrification and enhance the relative abundances of Azospira and Thauera related to denitrification to improve NO3- removal. This result was further confirmed by the variations of denitrifying functional genes (narG, nirK, nirS and nosZ). This research provided an effective method to improve NO3- removal during surface water treatment in constructed wetlands (CWs) by adding Fe-Cu/PVP/BC.

Effects of substrate type on enhancing pollutant removal performance and reducing greenhouse gas emission in vertical subsurface flow constructed wetland.

Constructed wetlands (CWs), known as an alternative clean technology, have been widely used for sewage treatment. However, greenhouse gas (N2O, CH4 and CO2) emissions are the accompanying problem in CWs. To mitigate the net global warming potential (GWP) with the constant removal efficiency for contaminants is attracting wide attention recently. In this study, four CWs were established to explore the effects of substrate types (gravel, walnut shell, manganese ore and activated alumina) on contaminant removal and greenhouse gas emissions. CWs using manganese ore substrate with function of electronic exchange showed high removal efficiencies on COD (90.1%), TN (65.1%), TP (97.1%) and low greenhouse gas flux. The emission fluxes of N2O, CH4 and CO2 were 0.07-0.20, 2.00-252.30 and 337.54-782.57 mg m-2 h-1, respectively. Especially, the lowest average CH4 emission flux in the manganese ore CW was only 2.00 mg m-2 h-1 while those of N2O in walnut shell CW was only 0.07 mg m-2 h-1, which will make a significant contribution on the mitigation of GWP of CWs. High-throughput sequencing results indicated that microbial community diversity and richness changed significantly among different substrates. The high pmoA and low mcrA, caused by the introduction of manganese ore as substrate, also explained why there was little CH4 emission in CWs. Our study provided new insights into GWP mitigation and contaminant removal enhancement in CWs using optimal substrate.