Synergistic reduction and oxidation system using 4,7-dihydroxycoumarin as a green photocatalyst for efficient nitrobenzene degradation

Abstract

In this study, a synergistic reduction–oxidation process was proposed using natural organic matters (NOMs) as an organic photocatalyst for complete degradation of nitrobenzene (NB) that was difficult to oxidative decomposition directly. It has been verified by electron spin resonance (ESR) that both reductive organic free radical (4H-chromene-2,4,7-triol radical, S2) and oxidative OH radicals could be generated sustainably by using 4,7-dihydroxycoumarin (4,7-DHC) as a green photocatalyst under UV light irradiation, and 95.07% of NB degradation efficiency was achieved after 4-h irradiation with 40 µM 4,7-DHC. The formation of aniline as a reductive product was convenient for the attack of OH radicals, making it easier for the complete degradation of NB. In addition, 4,7-DHC showed high photocatalytic activity in a wide range of pH from 1 to 13, and the best photocatalytic activity was achieved with a pH value of 4.56. Besides, the effect of common coexisted ions (SO42−, Cl− and NO3−) on the NB degradation was discussed, and the highest k value was obtained with SO42− (80 µM), NO3− (120 µM) or Cl− (20 µM). Moreover, 4,7-DHC exhibited extensive degradability to the frequently coexisted pollutants in NB-contaminated wastewater including humic acid (HA), aniline (AN) and m-dinitrobenzene (m-DNB), which might be extended into a potential approach for environmental remediation of NB-contaminated water due to its low cost and accessibility of 4,7-DHC as NOMs.