Ultrasonic Induced Dehalogenation and Oxidation of 2-, 3-, and 4-Chlorophenol in Air-Equilibrated Aqueous Media. Similarities with Irradiated Semiconductor Particulates

Article abstract of DOI:10.1021/j100061a021

Three chlorophenols <2-chlorophenol (2-CPOH), 3-chlorophenol (3-CPOH), and 4-chlorophenol (4-CPOH)> were examined under pulsed sonolytic conditions (frequency, 20 kHz; power, 50 W) in air-equilibrated aqueous media.These phenols aer totally transformed to dechlorinated, hydroxylated intermediate products via first-order kinetics in about 10 h for 2-CPOH and 3-CPOH and about 15 h for 4-CPOH; rate constants for the disappearance of these phenols are (4.8 +/- 0.4) x 10^-3 min^-1, (4.4 +/- 0.5) x 10^-3 min^-1, and (3.3 +/- 0.2) x 10^-3 min^-1, respectively, for approximately 80 μM initial concentration.Dechlorination is nearly quantitative and occurs soon after initiation of the disappearance of the initial substrate.Comparison of the intermediate products formed by the sonochemical technique with those reported earlier from oxidation of these substrates by direct photolysis, UV/peroxide, and irradiated semiconductor (SC) particulates (TiO2 and ZnO) suggests that the sonochemical oxidation process finds strong similarities with and physically mirrors the heterogeneous photocatalytic process with SC particulates, in particular, where k's are 1-2 orders of magnitude greater; this infers the need for the substrate to diffuse to the bubble/liquid interface in contrast to preadsorbed substrates on the semiconductor particles.The kinetics show two regimes: a low-concentration regime where the rate is zero order in i, and a second regime at higher concentrations where the rate displays saturation-type kinetics reminiscent of Langmuirian type behavior in solid/gas systems.The relevant mechanistic significance is that the reaction takes place in the solution bulk at low concentrations of chlorophenol, while at higher concentrations the reaction occurs predominantly at the gas bubble/liquid interface.