Bismuth microsphere for photo-assisted nitrate removal: Experimental and theoretical investigations

Abstract

• The nitrate was effectively and selectively removed by bismuth microspheres. • The photo activity of bismuth was attributed to electron transfer. • The effects of adsorption on the orbitals of nitrogen-compounds were analyzed. • Theoretical calculation reveals the process of electron transfer. Nitrate is a common pollutant in both natural and engineered water environments; nitrate removal has thus been a long-standing challenge. In this study, inert bismuth microspheres were developed for the efficient removal of nitrate from water under ultraviolet irradiation. Based on the experimental conditions of a 25 mg dosage of bismuth microspheres and a reaction time of 60 min, the nitrate could be removed with an efficiency as high as 98.2%, with a total nitrogen (TN) removal of 93.7% and negligible accumulation of nitrite. This indicated the high activity and selectivity of the bismuth microspheres for nitrate removal. Combined with the properties of bismuth and the morphology of the samples, the optical properties of bismuth microspheres were mainly due to the inter-band transition, and the electron transfer in the inter-band transition resulted in the reduction of nitrate. The detailed electron transfer mechanism was elucidated by density functional theory calculations, confirming that both direct and indirect electron transfer occurred between bismuth and nitrate. In conclusion, this study provides a proof-of-concept demonstration of the use of bismuth metal for more effective nitrate removal in water treatment.