Removal of toxic recalcitrant organic compounds in wastewater has become a matter of considerable interest. There has been a rapid development in a wide range of treatment methods to solve this problem. Many efforts have been devoted to improve the degradation efficiency and possible mineralization of the organic pollutants, especially in the case of hydrophilic compounds. Heterogeneous sonocatalytic degradation process involving different types of catalyst particularly titanium dioxide (TiO2) appears to be the most promising technology. To further improve the degradation efficiency, some researchers focus on the enhancement of the catalyst by modifying it through metal ion doping or the use of composite semiconductor materials to prohibit rapid recombination of electron/hole pairs and backward reactions. Effects of the addition of chemical additives e.g. hydrogen peroxide, Fenton reagent, various types of salts and ozone have also been elucidated. Different methods for enhancing free hydroxyl radical (OH) generation which lead to higher oxidation rates have been investigated. This paper critically reviews up-to-date technologies to be applied in conjunction with sonochemical degradation process for enhancing degradation of recalcitrant organic compounds. The addition of catalysts and chemical additives with ultrasonic irradiation are particularly reviewed as effective methods to increase the generation of free OH. © 2011 Elsevier B.V.
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