Photodegradation of methylene blue in a continuous-flow microchannel using titanium dioxide photocatalyst

Abstract

Recently, dye pollution in water sources, especially methylene blue, has significantly affected living beings and the environment. Research has been focused on the photocatalytic degradation of methylene blue because this technique is affordable, reusable, and environmentally sustainable. This involves employing a continuous microchannel reactor that exhibits a small diffusion distance and a uniform light transfer to the catalyst, which leads to enhancing methylene blue degradation efficiency. This research investigated three different types of titanium dioxide films with different morphologies: a compact film (CTiO2), a porous film produced by using 0.35 g of Pluronic P123 (3.95mM-MTiO2), and a porous film formed by adding 0.45 g of Pluronic P123 (5.08mM-MTiO2), undergoing increasing microchannel reactor widths of 1, 2, and 3 mm, respectively. The kinetics within the reactor were analyzed using a two-dimensional model based on the Langmuir-Hinshelwood equation in MATLAB. The model mainly analyzed two parameters: the reaction rate constant (k) and the adsorption equilibrium constant (K). Experimental results indicated that the 5.08mM-MTiO2 film had the highest methylene blue degradation performance. Kinetic investigation using the model showed that the 3.95mM-MTiO2 film exhibited the highest k value due to its superior specific surface area, whereas the 5.08mM-MTiO2 film exhibited the highest K value due to its largest pore size. This facilitates the transport of methylene blue molecules to the active sites, providing better degradation efficiency relative to other film types. However, when the channel width increased, the total degradation efficiency decreased. Analysis using the Damköhler number (Da) showed results over 1 in all studies, which strongly confirmed that mass transfer controls this microchannel reactor system. This limitation in methylene blue degradation upon increasing channel size is attributable not only to the chemical properties or adsorption capacity of the film surface, but also to limitations in the diffusion of methylene blue molecules from the solution to the photocatalyst surface.