Photocatalytic oxidation of nitrite ion over carbon nitride

Abstract

Nitrite ion (NO2-) is a toxic inorganic contaminant, which is widely used in industry and agriculture as a food preservative and a fertilizing agent. One of the methods to reduce the toxicity of the NO2- is by oxidizing it into less hazardous compounds, such as nitrate ion (NO3-). In this study, we demonstrated that a simple and green photocatalytic process can be employed to oxidize the NO2- to NO3- over a metal free-carbon nitride photocatalyst under ultraviolet (UV) light irradiation. The carbon nitride was synthesized via pyrolysis of urea precursor by a thermal polymerization process at 823 K for 4 hours. The prepared carbon nitride was then characterized by using X-ray diffractometer (XRD), field emission scanning electron microscope (FESEM), diffuse reflectance UV-visible (DR UV-vis), fluorescence, and Fourier transform infrared (FTIR) spectrophotometers, as well as nitrogen adsorption-desorption isotherm analyzer. All the characterization results supported the successful synthesis of the carbon nitride. The carbon nitride was then used as the photocatalyst for oxidation of NO2- to NO3- under UV light irradiation for 3 h. The decrease of the NO2- and the formation of the NO3- were analyzed by using a high performance liquid chromatography (HPLC) equipped with Hypersil GoldTM PFP column. The mobile phase used was a mixture of methanol (MeOH) and water (H2O) with the ratio of MeOH:H2O was 30:70. The addition of orthophosphoric acid was required to set the pH at 2.5. The flow rate was fixed at 0.8 ml min-1 and the monitored wavelength was 220 nm. It was revealed that carbon nitride could oxidize NO2- to NO3- with a moderate conversion of 15%. Fluorescence quenching showed that there were good interactions between the emission sites of carbon nitride and the NO2- molecules. The good interactions would be one driving force for the carbon nitride to act as a good photocatalyst to oxidize the NO2- to NO3-. The oxidation pathway by the photogenerated species was also proposed.