Kinetics and Mechanism of the Sonolytic Degradation of CCl_4: Intermediates and Byproducts

Abstract

The sonolytic degradation of aqueous carbon tetrachloride is investigated at a sound frequency of 20 kHz and 135 W (112.5 W cm^(-2)) of power. The observed first-order degradation rate constant in an Ar-saturated solution is 3.3 × 10^(-3) s^(-1) when the initial CCl_4 concentration, [CCl_4]_i, is 1.95 × 10^(-4) mol L^(-1) and increases slightly to 3.9 × 10^(-3) s^(-1) when [CCl_4]_i = 1.95 × 10^(-5) mol L^(-1). Low concentrations (10^(-8)−10^(-7) mol L^(-1)) of the organic byproducts, hexachloroethane and tetrachloroethylene, are detected, as well as the inorganic products chloride ion and hypochlorous acid. The chlorine mass balance after sonolysis is determined to be >70%. The reactive intermediate, dichlorocarbene, is identified and quantified by means of trapping with 2,3-dimethyl-2-butene. The presence of ozone in the sonicated solution does not significantly effect the rate of degradation of carbon tetrachloride; however, O_3 inhibits the accumulation of hexachloroethane and tetrachloroethylene. Ultrasonic irradiation of an aqueous mixture of p-nitrophenol (p-NP) and carbon tetrachloride results in the acceleration of the sonochemical degradation of p-NP. The sonolytic rate of degradation of p-NP appears to be enhanced by the presence of hypochlorous acid, which results from the sonolysis of CCl_4.