ULTRASONIC INDUCED DEHALOGENATION AND OXIDATION OF 2-CHLOROPHENOL, 3-CHLOROPHENOL, AND 4-CHLOROPHENOL IN AIR-EQUILIBRATED AQUEOUS-MEDIA - SIMILARITIES WITH IRRADIATED SEMICONDUCTOR PARTICULATES

Three chlorophenols {2-chlorophenol (2-CPOH), 3-chlorophenol (3-CPOH), and 4-chlorophenol (4-CPOH)} were examined under pulsed sonolytic con ditions (frequency, 20 kHz; power, 50 W) in air-equilibrated aqueous m edia. These phenols are totally transformed to dechlorinated, hydroxyl ated 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 di sappearance 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), respect ively, for approximately 80 mu M initial concentration. Dechlorination is nearly quantitative and occurs soon after initiation of the disapp earance of the initial substrate. Comparison of the intermediate produ cts formed by the sonochemical technique with those reported earlier f rom oxidation of these substrates by direct photolysis, flash photolys is, 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 photocatal ytic process with SC particulates, in particular, where k's are 1-2 or ders of magnitude greater; this infers the need for the substrate to d iffuse to the bubble/liquid interface in contrast to preadsorbed subst rates on the semiconductor particles. The kinetics show two regimes: a low-concentration regime where the rate is zero order in [CPOH](i), a nd a second regime at higher concentrations where the rate displays sa turation-type kinetics reminiscent of Langmuirian type behavior in sol id/gas systems. The relevant mechanistic significance is that the reac tion takes place in the solution bulk at low concentrations of chlorop henol, while at the higher concentrations the reaction occurs predomin antly at the gas bubble/liquid interface.