Electric energy saving in photocatalytic removal of crystal violet dye through the simultaneous use of long-persistent blue phosphors, nitrogen-doped TiO2 and UV-light emitting diodes

The aim of this work is to minimize the consumption of electricity associated with the light source in a photocatalytic process for the removal of pollutants from wastewater. For this purpose, an optimized visible light active nitrogen-doped TiO2 was used. To reduce electric energy consumption, nitrogen-doped TiO2 was coupled with long-afterglow blue phosphors (chemical formula: Sr2.90Eu0.03Dy0.07Al4SiO11) able to emit visible light when excited with UV light. Specifically, the experimental tests were performed using crystal violet dye as a model pollutant at 10 mg L−1 initial concentration. The experimental campaign has been carried out with the aim of assessing whether the photocatalytic activity of the nitrogen-doped TiO2 catalyst is still present in dark conditions, thanks to the light emitted by phosphors and consequently to determine the energy saving in terms of electrical consumption and related costs. After having established the optimal content of nitrogen-doped TiO2 in the physical mixture with phosphors, tests were carried out by alternating lighting periods with dark periods in order to identify the optimal irradiation time to obtain not only the total crystal violet degradation, but also the highest energy savings. In particular, the simultaneous presence of long-afterglow photoluminescent materials, nitrogen-doped TiO2 photocatalyst and UV light emitting diodes resulted in an economic and energy saving of about 40% thanks to the possibility of alternating “light” and “dark” periods.