Carbon nanotubes and plant diversity reduce greenhouse gas emissions and improve nitrogen removal efficiency of constructed wetlands

Abstract

Adding carbon nanotubes (CNTs) and manipulating plant diversity are ways to improve the functioning of constructed wetlands (CWs), such as by maintaining the high nitrogen (N) removal efficiency and low emission of gaseous pollutants from CWs used for wastewater treatment. The conversion of biomass improved by diversity to bioenergy can further reduce greenhouse gas emissions. We established 108 microcosms to simulate CWs to explore the effects of adding CNTs to wastewater and assembling plant diversity on ammonia (NH3), nitrous oxide (N2O), and methane (CH4) emissions from CWs. The results showed that (1) plant species mixtures with CNTs lowered NH3 emissions and NH3 emissions per unit N removal (NH3/NR) to 5 and 10%, respectively, compared with monocultures without CNTs; (2) mixtures with CNTs decreased global warming potential integrating N2O and CH4 emissions (GWPnon-CO2) and GWP non-CO2 per unit N removal (GWPnon-CO2/NR) to 28 and 32%, respectively, compared with monocultures without CNTs; (3) CNTs decreased the biomass production in some monocultures, while the mixture with CNTs doubled the biomass production compared with monocultures without CNTs; and (4) the presence of Arundo donax (high mass species) in communities with CNTs decreased GWPnon-CO2/NR to 23% and NH3/NR to 9% of that under monoculture without CNTs. This study suggests that adding CNTs and increasing high species richness or assembling mixtures with high-biomass species are more effective and cleaner for maintaining high N removal and bioenergy production while reducing N2O, CH4, and NH3 emissions in CWs.