A systematic review of the sonophotocatalytic process for the decolorization of dyes in aqueous solution: Synergistic mechanisms, degradation pathways, and process optimization

Abstract Over the past few decades, the indiscriminate use of a large variety of synthetic dyes and their continuous input into aquatic environments has constituted a serious risk to human health and ecological systems. Among a variety of treatment methods for dye removal from the aqueous environment, sonophotocatalysis has recently received a lot of attention due to its green and environmental-friendly properties. The aim of this review is to present an in-depth study of the sonophotocatalytic process for the degradation of various dyes in aqueous matrices. In the present study, the fundamentals of sonolysis and photocatalysis for wastewater treatment and their merits and drawbacks are explained in detail, along with a detailed discussion on the benefits that can be obtained using the combined approach. Additionally, operational parameters, the pathway, and kinetics of the sonophotocatalytic dye degradation, the cost estimation as well as the application of sonophotocatalysis for the textile wastewater treatment are discussed based on recently published works. It is observed that sonophotocatalysis indeed results in the remarkable enhancement of the oxidation intensity and thus the degradation rates of dyes, however, its applicability in large-scale wastewater treatment is limited. According to findings, most of the sonophotocatalytic optimization processes can achieve high removal efficiency with low concentrations of dyes (i.e., 0.1–100 μM), low ultrasound frequencies (i.e., 20–200 kHz), and acidic pH values over a temperature range of 20–40 °C, though the exact recommended value would be dependent on the compound characteristics and the reactor geometry.