Sonozonation (sonication/ozonation) for the degradation of organic contaminants - A review

Ozonation (OZ) is an important advanced oxidation process to purify water and wastewater. Because of the lower solubility and instability of ozone (O-3), selective oxidation and dependence on pH value, the industrial applications of OZ have been hindered by the following disadvantages: incomplete removal of pollutants, lower mineralization efficiency and the formation of toxic by-products. Meanwhile, OZ seems to have higher processing costs than other technologies. To improve the treatment efficiency and O-3 utilization, several combined processes, such as H2O2/O-3, UV/O-3, and Cavitation/O-3, have been explored, while the combined method of ultrasonication (US) with OZ is a promising treatment technology with a complex physicochemical mechanism. In US alone, the sonolysis of water molecules can produce more powerful unselective oxidant hydroxyl radicals ((OH)-O-center dot), and directly cause the sonochemical pyrolysis of volatile pollutants. In US/OZ, US can promote the mass transfer of O-3, and also drive the chemical conversion of O-3 to enhance the formation of (OH)-O-center dot. Various layouts of US/OZ devices and the interactive effects of US/OZ (synergism or antagonism) on the degradation of various organics are illustrated in this review. The main factors, including US frequency, pH value, and radical scavengers, significantly affect the mass transfer and decomposition of O-3, the formation of (OH)-O-center dot and H2O2, the degradation rates of organics and the removal efficiencies of COD and TOC (mineralization). As a result, US can significantly increase the yield of (OH)-O-center dot, thereby improving the degradation efficiency and mineralization of refractory organics. However, US also enhances the decomposition of ozone, thereby reducing the concentration of O-3 in water and impairing the efficiency of selective oxidation with O-3 molecules.