Testing of a 4-fold UV-LED photoreactor for the degradation of methylene blue

This study investigated the photocatalytic degradation of methylene blue (MB) using four commercial TiO₂ photocatalysts (P25, P90, PC105, PC500) in suspension under 365 nm UV-LED irradiation, with the reaction monitored by UV-Vis spectrometry. Among these, P25 showed the highest activity and was subsequently immobilised on metal plates. While suspended P25 achieved complete MB degradation, immobilisation on plates led to reduced activity. Photocatalyst films prepared via the sol-gel method were homogeneous and mechanically stable. In batch experiments with immobilised P25, MB degradation increased with decreasing lamp distance and smaller solution volumes, whereas in custom photoreactors higher flow rates and a larger number of catalyst plates enhanced degradation. In the 4-fold photoreactor, nearly complete MB degradation (99.1%) was achieved at 3.0 mL s^-1 (108 liquid cycles). The improved performance is attributed to the combination of a large catalyst surface area, thin solution layers, enhanced mass transport, and efficient irradiation. A scale-up analysis highlights the necessity of compact photoreactor designs with optimised plate arrangements to enable practical large-scale water treatment.