Performance and mechanism study of g-C3N4/rGO heterojunction enhanced NO3− reduction by nZVI under visible light irradiation

Abstract

Zero valent iron (ZVI) is a low-cost, earth-abundant reactive metal, and it has a strong reducing ability with a standard redox potential of − 0.44 V. Therefore, it is an effective and widely used reducing agent for pollutant degradation. Since nanoscale zero valent iron (nZVI) shows a larger specific surface area which leads to the faster reaction rate and higher conversion efficiency, it has been widely used in NO3− nitrogen reduction reaction, however, it shows disadvantages of poor chemical stability, narrow applicable pH, and low total nitrogen (TN) removal efficiency. To solve these problems, in this paper, the nZVI nanoparticles were deposited on the g-C3N4/rGO composite, and its NO3− removal efficiency under visible light irradiation was investigated. The effects of nZVI loading, initial pH of solution, catalyst dosage and initial NO3− concentration on NO3− reduction performance were studied. The results showed that the g-C3N4/rGO/nZVI material achieved 100% of NO3− reduction efficiency in 60 min. And the TN removal efficiency of g-C3N4/rGO/nZVI was 82.8%, superior to nZVI and g-C3N4/nZVI (13.3% and 38.2%, respectively). Finally, the denitrification mechanism of g-C3N4/rGO/nZVI was proposed as follows: Under light irradiation, the photogenerated electrons on g-C3N4 transferred via rGO to the nZVI surface, thus improving the reactivity and stability of nZVI. The photogenerated electrons and active electrons on the surface of nZVI reduced NO3− to N2, NH4+, and other products, while the photogenerated holes further oxidized part of NH4+ to N2 and other gaseous products. During this process, the rGO material could inhibit the oxidation of nZVI by the photogenerated holes from g-C3N4.