Sonolysis phenol

In the experiment involving oxidative enzyme HRP (EC 1.11.1.7, RZ 1.9, 240 purpuro gallin (units/mg)) [89] for the enzymatic treatment and ultrasonic waves of 423 kHz and 5.5 W, the phenol degradation rate was found to increase. The ultrasound assisted biodegradation method has been found to be more efficient method than the sonolysis and enzyme treatment when operated individually. 

The majority of systems studied have been aqueous solutions of either aromatic compounds or halogenated hydrocarbons. Such materials represent models for the major classes of organic pollutants in waste and ground water. The primary products resulting from the sonochemical treatment of phenol at 541 kHz (27 °C with bubbled air) are hydroquinone and catechol [22]. These compounds are easy to monitor and are clearly seen to be intermediates which disappear as the reaction progresses (Fig. 4.1). Similarly the sonolysis of aqueous 4-chlorophenol leads to products mainly characteristic of oxidation by OH radical e. g. 4-chlorocatechol but in both cases the final organic products are CO, CO2 and HCOOH (Scheme 4.2) [22-25]. 

The occurrence of an optimum frequency at 200 kHz was explained through a two step reaction pathway. In the first step water sonolysis produces radicals within the bubble. In step two the radicals must migrate to the bubble interface or into the bulk aqueous medium to form peroxide or react with the phenolic substrate. The authors suggest that the lower frequencies are the most efficient for the decomposition of molecules inside the bubble but a proportion of the radicals recombine inside the bubble at high temperature to form water thereby reducing the overall yield of H2O2 (Eqs.4.1 and 4.2). 

Sonolysis provokes electron-transfer reactions in which hindered phenols act as donors (Aleksandrov et al. 1995). Steric hindrance does not allow a donor and an acceptor to come closer and thus, prevents or significantly hampers overlapping of the corresponding orbitals. Acoustic field effect helps in overcoming this hindrance. 

Ultrasonic irradiation (-50 W/cm2) of a 100-mL air-equilibrated aqueous solution of 4-chlorophenol resulted in the first-order disappearance of the phenol, accompanied after a 1-hr delay by the first-order growth of CT. The pH of the isonated solution dropped gradually from the initial value of 5.1 to 3.5 after 11 hr. Sonolysis of the aqueous solution of 3-chlorophenol showed an induction period of -90 min following which its concentration decreased via first-order kinetics. The pH of the insonated solution of 2-chlorophenol decreased at first to 4.9 and then recovered to its near initial value until 9 hr of insonation, when it dropped abruptly to pH 4.4 and remained constant. The initial drop in pH that occurred during the induction period was also observed for the first-order disappearance of the phenol. It therefore suggests that there are various possible sites where reactions may occur in sonochem-istry. 

Monochlorophenols represent an important class of environmental water pollutants of moderate toxicity to mammalian and aquatic life they possess relatively strong organoleptic effects, with taste thresholds of 0.1 pg/L (ppb). Their principal sources are the natural degradation of chlorinated herbicides (e.g., chlorophenoxyacetic acids), chlorination of phenolic substances in waste effluents, and chlorine treatment of drinking water. Of the three chlo-rophenols examined here, 3-chlorophenol exhibited the greatest resistance to sonolysis. 

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