Boosting photocatalytic reduction of nitrate to ammonia enabled by perovskite/biochar nanocomposites with oxygen defects and O-containing functional groups.

Photocatalytic ammonia synthesis from waste nitrate has emerged as a promising strategy in water treatment; however, the conversion and selectivity still remain a great challenge. Herein, recyclable magnetic perovskite (LaFeO3)/biochar nanocomposites were successfully synthesized by the co-pyrolysis of the lotus biomass and Fe/La salts without extra organic complexants. Results showed that the lotus interacted with the iron ions (Fe3+) and the lanthanum ions (La3+) changing the surface and structural characteristics of catalysts. Oxygen defects of LaFeO3 were enhanced due to biomass introduction, which accelerated the separation of electron-hole pairs. On the other hand, Fe/La salts participated in the modification process of the biochar surface during the carbonization, which promoted the exposure of oxygen-containing functional groups and aromatic structures facilitating the nitrate adsorption. Notably, the redox-active quinone/phenol groups on the biochar surface contributed to the photogenerated electrons exchange favoring the ammonium ion (NH4+) selectivity as direct electron donor. Nitrate conversion reached 98% and ammonia selectivity reached 97% over the LaFeO3/biochar photocatalyst under visible light irradiation, when the mass ratio of lotus and Fe/La salts was optimized. Our findings may potentially provide a green and cost-effective way for ammonia recovery from nitrate contaminants.