Photo-generated hydrated electrons for selective reduction of nitrate to dinitrogen: Selectivity and mechanism

Abstract

The hydrated electron (eaq−) is one of the most admired short-lived reactive species (RSs) for selective reduction of nitrate (NO3−) to nitrogen (N2). However, eaq− donor often exhibits reducibility, which may react with intermediate products, affecting the final product composition of NO3− reduction. Herein, Fe2+, SO32− and the combination of Fe2+ and SO32− were selected as eaq− donors for NO3− photo-reduction to explore the roles of eaq− donors in the product of NO3− reduction. The N2 selectivity achieved through Fe2+/UV, SO32−/UV and Fe2+/SO32−/UV systems was found to be 17.03 %, 45.45 %, and 97.53 % respectively. The main products of NO3− reduction in Fe2+/UV system were nitrite (NO2−) and nitrogen oxide (NOX), while in SO32−/UV system were NO2− and hydroxylamine disulfonate (HON(SO3)22−). Notably, in the Fe2+/SO32−/UV system, various short-lived reactive species (eaq−, SO3•−, etc.) were generated, and eaq− was the main reactive species for NO3− reduction. The generated NO2− and NOX intermediates could be quickly converted to N-S compounds, such as HON(SO3)22− by SO32− and SO3•−, while the generated HON(SO3)22− could be further reduced to N2 by the combined action of Fe3+ from photooxidation of Fe2+, UV and eaq−. Significantly, the Fe2+/SO32−/UV system showed high selectivity and efficiency for removal of NO3− from secondary effluent of municipal domestic sewage. This study helps to design new photo-reduction systems for NO3− removal from nitrogen-containing wastewater/water.