(165d) Development of Dispersed-Type Sonophotocatalysis Using Piezoelectric Effect Caused by Ultrasonic Resonance

Recently, degradation of persistent organic pollutants (POPs) with low biodegradability in the environment and in industrial and municipal wastewaters has gained importance. Therefore, various chemical, physical, and biological processes such as filtration separation, advanced oxidation processes (AOPs), electric degradation, and plant integration have been attempted for the effective degradation. This study focuses on sonophotocatalysis (SP), which is a combination of sonolysis (ultrasonic degradation) and photocatalysis. SP has not been put to practical use thus far, since optimization of the operation conditions to achieve a high synergetic effect has not been carried out. In addition, SP cannot be used to treat large quantities of wastewater since power consumption by the UV lamp used in this technique is high owing to the low efficiency of energy conversion.

In this study, a dispersed-type SP process with dispersed light sources, has been proposed for the effective and energy-efficient degradation of POPs. In this method, the piezoelectric effect caused by ultrasonic resonance in a piezoelectric element is used for producing luminescence in a LED. A luminescent device composed of multiple UV-LEDs and a piezoelectric element was designed for dispersion of UV light in water; this device was confirmed to show luminescence under ultrasonic irradiation. One of the major advantages of this method is the decrease in operation cost, because electric power need not be supplied to the LEDs directly. Since UV-LEDs used here have a long lifetime, the exchange frequency is decreased to a considerable extent. Since the luminescent device can be easily decreased in size and dispersed in the reactor, the photocatalytic reaction can possibly occur throughout the bulk of the reactor. Because of this, the efficiency of photocatalysis is increased even during the treatment of wastewater with low transparency.

Sonophotocatalytic degradation experiments of methylene blue in water were carried out using several such devices, and the results were compared with those obtained in sonolysis, photocatalysis, and fixed type SP. The comparison showed that the degradation rate constants in fixed-type and dispersed-type SP were larger than the sum of the rate constants obtained for sonolysis and photocatalysis; further, the synergetic effect caused by the combination of sonolysis and photocatalysis was 7.5% and 18% in fixed-type and dispersed-type SP, respectively.