Photocatalytic Conversion of Gaseous Nitrogen Trichloride into Available ChlorineExperimental and Modeling Study

In water, chlorine reacts with nitrogen-containing compounds to produce disinfection byproducts such as nitrogen trichloride which induces ocular and respiratory irritations in swimming pool workers. A technical solution has been used to reduce NCl₃ exposure to acceptable levels, by adding a stripping step to the water recycling loop. The pollutants extracted are currently rejected into the atmosphere without treatment. However, the physical properties of NCl₃ could be harnessed to induce its controlled degradation by direct or indirect light. This paper describes the way to transform NCl₃ into oxidizing chlorine by photocatalysis under laboratory conditions. Photocatalytic oxidation efficiently degrades gaseous nitrogen trichloride, producing compounds such as HClO. About 60% of NCl₃ decomposed was converted into HClO which could be used as a disinfection compound. A kinetic model is proposed for the photocatalytic process based on a convection/diffusion model. The Langmuir–Hinshelwood model was applied to the chemical part of the mechanism. The apparent quantum yield was also estimated to assess the optimal irradiance for NCl₃ transformation. The results show that photocatalysis performs much better than photolysis alone for NCl₃ removal, i.e. at least 25 times more efficient.