Sonophotocatalytic reactors for wastewater treatment: a critical review

Abstract

The common optimum operating conditions for sonochemical and photocatalytic oxidation coupled with the similarity in the mechanism of destruction leading to a possible synergism and the possible elimination of some of disadvantages observed for individual processing techniques due to the effects of the other technique has prompted the development of sonophotocatalytic reactors in recent years. The present work aims at critically evaluating the status of the sonophotocalytic reactors with a special focus on wastewater treatment applications. Initially, the discussion has been concentrated on the individual techniques focusing on the recommendations for the optimum operating parameters, novel reactor designs for large scale operations and overview of the literature illustrations in the recent years. The discussion about hybrid technique starts with the mechanism of the expected synergism between sonochemistry and photocatalysis. Different reactor configurations used for the hybrid technique have been analyzed and recommendations have been made for the design of optimum configuration. An optimum set of operating parameters has also been recommended based on the critical analysis of the available literature. It has been observed that sonophotocatalytic oxidation indeed results in significant enhancement of the oxidation intensity and thus the rates of degradation, but its application on a larger scale of operation is hampered, perhaps due to high cost and the lack of suitable design strategies associated with the sonochemical reactors. Some recommendations have been also made for the future work required to eliminate the drawbacks associated with the sonochemical and/or sonophotocatalytic reactors.