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Sustain. Environ. Res., 20(4), 245-250 (2010) 245 (Formerly, J. Environ. Eng. Manage.)
pH-AFFECTING SONOCHEMICAL FORMATION OF HYDROXYL RADICALS UNDER 20 KHz ULTRASONIC IRRADIATION
Ting-Nien Wu1,* and Meng-Chun Shi1,2
1Department of Environmental Engineering Kun Shan University
Tainan 710, Taiwan 2Sustainable Environment Research Center National Cheng Kung University Tainan 709, Taiwan
Key Words: Ultrasonic, sonolysis, radical, pH ABSTRACT
Advanced oxidation processes have drawn much attention in their application to wastewater treatment recently. However, it is seldom to utilize sonochemical degradation as the tool of environmental remediation. This study utilized 20 kHz ultrasonic irradiation as the mechanism of radical production. The production rate of radicals serves as the evaluation tool of sonolysis efficiency under the control of different pHs. Three schemes of detecting radicals were employed, including Fricke measurement, hydrogen peroxide indirect measurement, and 4-chlorobenzoic acid probe measurement. The experimental results were confirmed through different measurement methods. The threshold of ultrasonic intensity was also examined to form radicals under the acidic, neutral, or basic surroundings.
Ultrasound has been applied to physical disper- sion process and chemical oxidation reaction for more than a half century. Its broad applications embrace many fields such as emulsification, homogenization, extraction, degreasing, filtration, medical image, cell destruction, surface cleaning, dissolution, drying, de- gasification, etc. Ultrasound also can generate hy- droxyl radicals to destruct or mineralize organic com- pounds, the so-called sonolysis. Many advanced oxi- dation processes (AOPs) are able to produce hydroxyl radicals, such as photocatalysis, Fenton reactions, ozone, UV-peroxide, radiolysis, electron beam irradia- tion and sonolysis. AOPs are highly promising for remediating wastewater because of rapid reaction, easy control, and no secondary pollution. However, ultrasound is not often used as a pollutant remedy in the field of environmental engineering .
Under an extreme condition, ultrasonic waves can cause the effect of acoustic cavitation through the formation, growth, and implosive collapse of micro- bubbles in a liquid. Cavitation responsible for sonoly- sis serves as a means of concentrating the diffuse en- ergy of sound. Microbubble collapse induced by cavi-
tation intensifies local heating to roughly 5000 oC and high pressure up to 50.5 MPa, and these hot spots are cooling fast at the rate above 109 K s-1 . Such an ex- treme condition is able to break the chemical bonds, induce liquid-phase burning, initiate radical reactions, and degrade most persistent organic pollutants. In other words, sonolysis of organic pollutants is mainly ascribed to the sonochemical generation of radicals and thermal decomposition in the cavitation micro- bubbles.
A growing number of lab studies have demon- strated that ultrasound irradiation results in a rapid, safe and effective decomposition on aromatics , phenols , chlorinated compounds , and pesti- cides [6,7] in aqueous solution. Several factors includ- ing ultrasonic frequency, strength of ultrasounds, sol- ute temperature, cavitation gases, and solute pH may influence the sonochemical degradation of organic contaminants. The pH plays an important role in the sonochemical system because the physical properties of chemicals can be modified depending upon aque- ous pH. The sonochemical degradation rate of 4- nitrophenol has been reported to decrease with in- creasing pH, but the disappearance of aniline is found favourable in the alkaline solution by sonolysis .
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