[Influences of Biochar on Pollutant Removal Efficiencies and Nitrous Oxide Emissions in a Subsurface Flow Constructed Wetland].

Biochar, pyrolyzed from agricultural biomass wastes, has been widely used as an improver in wastewater treatment to regulate the oxygen distributions and microbial communities because of its extended surface area and porous structure. In addition, biochar has been shown to play a role in enhancing the porosity, adsorbing ammonium (NH4+-N), and reducing nitrous oxide (N2O) emissions. In this paper, five groups of constructed microcosm wetlands (CW) were built in a greenhouse with different biochar doses of 40%, 30%, 20%, 10%, and 0% (named as BW-40, BW-30, BW-20, BW-10, and CW-K, respectively) to investigate the influences of biochar on pollutant removal efficiencies and N2O emissions. The results showed that the concentration of effluent dissolved oxygen (DO) was less than 0.5 mg·L-1, and the pH was stable at around 7.2 in every CW. Additionally, the effluent oxidation-reduction potential (ORP) was found to have moderately increased with the increases in the quantity of biochar, and the conductivity (Cond) test results showed the opposite trend. However, the effects of biochar on DO, pH, ORP, and Cond were not significant (P>0.05). The chemical oxygen demand (COD) removal rates were up to 90% in all CWs. On the other hand, significantly higher removal efficiencies for NH4+-N and total nitrogen (TN) were found in CWs filled with biochar (P<0.05). The average NH4+-N removal rates were (57.96±10.63)%, (51.12±11.74)%, (48.55±8.75)%, (43.95±9.74)%, and (34.76±14.16)% in BW-40, BW-30, BW-20, BW-10, and CW-K, respectively, while the total nitrogen (TN) average removal rates were (80.21±10.63)%, (78.48±5.73)%, (76.80±4.20)%, (75.88±5.85)%, and (70.92±5.68)%, respectively. Nitrate (NO3--N) was not detected in the CWs for there were sufficient carbon sources and suitable denitrification environments. Moreover, the average fluxes of N2O ranged from 13.53 mg·(m2·d)-1 to 45.30 mg·(m2·d)-1 in the experimental systems. Compared with the control, the reduction rates of N2O in the BW-40, BW30, BW20, and BW10 were 70.13%, 68.26%, 50.83%, and 37.90%, respectively, and the ratios of N2O emissions to the removed nitrogen in CWs with biochar were significantly lower than those in the CW without biochar. Positive correlations were observed between the N2O fluxes and nitrite (NO2--N) concentrations, and the lower N2O emissions could be attributed to the higher oxygen transfer and lower NO2--N accumulation rates in response to the biochar addition. These results demonstrate that biochar could be used as an amendment to strengthen the nitrogen removal and reduce the N2O emissions in CWs.