Evaluation of the sono-assisted photolysis method for the mineralization of toxic pollutants

Persistent organic pollutants (POPs), which are highly toxic and potential health hazards, are discharged in wastewater and cannot be mineralized by conventional biological and physicochemical methods of wastewater treatment. Advanced oxidation processes (AOPs) such as electrochemical, sonochemical, photolysis, Fenton, and Fenton like processes seem to be good and promising wastewater treatment technology alternatives. However, flaws limit their use in industrial applications. Recently, combined AOPs have gained attention among researchers to overcome these flaws of single AOPs. In this review, we studied the combined sonolysis and photolysis techniques for the degradation of the POPs. The efficiency of sono-photolysis has been examined by considering the single strategies, i.e., sonolysis and photolysis process, and the effect of various parameters of sono-photoreactor on the kinetics of degradation. Combined irradiation of ultrasound and ultraviolet light has been reported to give the synergistic effect under different mechanisms. The physical feature of ultrasound provides the intense mixing of the pollutant, which supports UV light to deeply penetrate the medium and accelerate the hydroxylation/oxidation reaction where toxic pollutants generated due to photolysis were degraded. H2O2 generated during the transient collapse of microbubble experienced a photocleavage to produce additional •OH radicals, which increases the degradation of the pollutants. A significant synergistic effect has been reported during combined treatment due to the increase in •OH radicals generation and photolysis of H2O2 either added externally or formed by the US, which also avoids the secondary pollution. Critical analysis of the geometry of various types (batch and continuous) of the sono-photo reactor used during treatment have also been evaluated. The kinetics of the degradation process has been reviewed in detail. This review article demonstrates a thorough and critical analysis (1998–2020) of sono-photolysis techniques, which improve the understanding of combined treatment and provides an outline direction for future research.