Photocatalytic conversion of gaseous nitrogen trichloride into available chlorine – Experimental and modeling study

In water, chlorine reacts with nitrogen-containing compounds to produce disinfection by-products such as nitrogen trichloride which induces ocular and respiratory irritations in swimming pool workers. A technical solution has been used to reduce nitrogen trichloride exposure to acceptable levels, by adding a gas/liquid extraction 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. Photocatalytic oxidation efficiently degrades gaseous nitrogen trichloride, producing compounds such as HClO which can be recycled in the water treatment loop. 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 light intensity for NCl3 transformation. The results show that photocatalysis performs much better than photolysis alone for NCl3 conversion.