Enhanced Photolysis and Photocatalysis Assisted by Hydrodynamic Cavitation for the Removal of Methylene Blue Dye

Abstract

This study investigates the synergistic integration of hydrodynamic cavitation (HC) with photolysis and photocatalysis to enhance the degradation of synthetic dyes in aqueous systems. Methylene blue (MB) is selected as a model pollutant, and tap water is used as the working fluid. HC and UVC photolysis are evaluated independently as control experiments, resulting in four treatment configurations: hydrodynamic vaporous cavitation, dynamic flow-UVC photolysis, HC-UVC photolysis, and HC-UVC-ZnO photocatalysis. In the last two treatments, air flow is introduced at the Venturi throat, generating an aerated turbulent flow. The proposed recirculating HC-UVC platform combines controlled gas injection with independent residence-time control, providing a well-defined experimental basis for comparative evaluation and future scale-up. Dye degradation is monitored by UV–vis spectroscopy and supported by measurements of physicochemical water quality parameters, dissolved oxygen, and residual photocatalyst detection. Apparent kinetics analysis and electrical energy per order (EEO) are used to quantitatively compare process performance. Strong synergistic effects are observed when HC is coupled with photonic and catalytic processes. In particular, HC coupled with photocatalysis achieved rapid degradation kinetics and the lowest EEO, reaching 98% MB removal within less than 1 h. Despite its superior performance, photoluminescence analysis revealed residual ZnO powder below the detection limits of electrochemical and absorption techniques, indicating a potential limitation associated with downstream photocatalyst recovery. In contrast, HC coupled with photolysis achieved comparable degradation efficiencies at longer processing times while avoiding the use of solid catalysts. These results demonstrate that optimal process selection should balance degradation efficiency, energy consumption, system complexity, and post-treatment requirements, highlighting HC-UVC photolysis as a robust alternative for practical water treatment applications where simplicity and long-term operability are prioritized.