Biochar and hematite amendments suppress emission of CH4 and NO2 in constructed wetlands

Constructed wetlands (CWs) are considered a widely used cost-effective technology for pollutant removal. However, greenhouse gas emissions are a non-negligible problem in CWs. In this study, four laboratory-scale CWs were established to evaluate the effects of gravel (CWB), hematite (CWFe), biochar (CWC), and hematite + biochar (CWFe-C) as substrates on pollutants removal, greenhouse gas emissions, and associated microbial characteristics. The results showed that the biochar-amended CWs (CWC and CWFe-C) enhanced the removal efficiency of pollutants, with 92.53 % and 93.66 % of COD and 65.73 % and 64.41 % of TN removal, respectively. Both single and combined inputs of biochar and hematite significantly reduced CH4 and N2O fluxes, with the lowest average of CH4 flux obtained in CWC (5.99 +/- 0.78 mg CH4 m-2 h-1) and the least N2O flux in CWFe-C (287.57 +/- 44.84 mu g N2O m-2 h-1). The substantial reduction of global warming potentials (GWP) was obtained in the applications of CWC (80.25 %) and CWFe-C (79.5 %) in biochar-amended CWs. The presence of biochar and hematite mitigated CH4 and N2O emissions by modifying microbial communities with higher ratios of pmoA/mcrA and nosZ genes abundances, as well as increasing the abundance of denitrifying bacteria (Dechloromona, Thauera and Azospira). This study demonstrated that biochar and the combined use of biochar and hematite could be the potential candidates as functional substrates for the efficient removal of pollutants and simultaneously reducing GWP emissions in the constructed wetlands.

Automated summary

Constructed wetlands (CWs) with biochar and hematite as substrates enhanced pollutant removal and reduced greenhouse gas emissions. The biochar-amended CWs achieved high removal efficiencies of COD and TN, and significantly lowered CH4 and N2O fluxes. The application of biochar and hematite also decreased the global warming potentials (GWP) and modified microbial communities, promoting denitrifying bacteria abundance. This study highlights the potential of biochar and the combined use of biochar and hematite as functional substrates for efficient pollutant removal and reduction of GWP emissions in CWs.