Modelling and Experimental Investigation of Luminous Coupling in UVLED Driven Optical Fiber Reactors

Background: Photocatalytic oxidation is a promising tool for waste water treatment and decomposition of biologically non digestible substances. Immersed nanoscale catalyst particles from semiconductor materials such as TiO2 and ZnO can be excited by absorbed UV radiation, leading to hydroxyl-ion formation at the surface of the semiconductor and oxidative degradation of pollutants. Objective: This contribution deals with reactors equipped with catalyst coated light guides to combine the advantages of immobilized catalysts with nearly homogeneous irradiation. With experimental and theoretical methods the coupling and decoupling of radiation were investigated and the performance of catalyst coated light guides was tested by means of methylene-blue degradation. Methods: Radiation models, known from the recent literature, use single ray, parallel ray or multi ray models to approximate the light transmission. These models neglect Fresnel reflection and consider only coupling into the light guide. In this study, the LED was simulated as a Lambertian radiator using 10 4 rays with angle dependent intensities. This well-known model was extended with Fresnelreflection, which predicted the measured coupling efficiencies accurately. The simulations predict the decoupling and catalyst activation at the lateral surface of the light guide for two boundary cases, ideal matt and ideal reflective surfaces. To generate matt surfaces, the light guides were either scratched or coated with TiO2 p25 nanopowder. Sol-gel coating methods were used, to create reflective surfaces. Results: When using matt surfaces, the decoupling rate is very high: 80% of the radiant flux exits the light guide in less than 10 cm. If light guides with reflective surfaces are used, the radiant flux leaving the light guide is low: less than 10% of the radiation exited the light conductor in the first 10 cm. Methyleneblue degradation, seen as a model reaction, was used to determine the reactor performance by comparing the pseudo first order reaction coefficients. Due to the uniform light distribution along the length of the light guides and the resulting even formation of reactive radicals, the quantum yield was increased by a factor of 3, using sol-gel coated light guides, rather than powder coated light guides. Conclusion: The effectiveness of LED driven optical fiber reactors was intensified, if reflective surfaces are used instead of matt surfaces. These surfaces are achieved by sol gel chemistry. However, to use the complete amount of photons, which entered the optical fiber, very long light guides are needed.