Effective degradation of refractory nitrobenzene in water by the natural 4-hydroxycoumarin under solar illumination

In this study, nitrobenzene (NB) as typical refractory organic pollutants was effectively degraded by a new green approach, which was achieved by the chemical effect of natural organic matters (NOMs) under solar illumination as a potential natural degradation process. 4-hydroxycoumarin (4HC), which is natural compound and can be extracted from many plants, was found as an efficient photosensitizer to promote the photoreduction of NB to 4-aminophenol under the solar illumination. The reaction products were definitely identified by LC-MS/MS and ¹H NMR. The response spectrum of 4HC excited state (4H-chromene-2,4-diol radical, S₁) as key intermediate was also obtained by transient absorption spectroscopy (TAS) measurements, which showed that the decay time of S₁ was around 250 ps. Then, the measurements of electron paramagnetic resonance (EPR) confirmed the existence of OH. As a result, the reaction mechanism between 4HC and NB was proposed. In addition, the influence parameters such as light sources, gas surroundings, solvents, pH values were investigated to further reveal the reaction process.