Ultrasonic irradiation of aqueous solutions

Ultrasonic irradiation of aqueous solutions of the chlorophenols was carried out with a Vibra Cell Model VC-250 direct immersion ultrasonic horn (Sonics Materials Newtown, CT) operated at a frequency of 20 kHz with a constant power output of 50 W (the actual insonation power at the solution was 49.5 W, and the power density was 52.1 W/cm2). Reactions were done in a glass sonication cell (4.4 cm i.d. by 10 cm), similar to the one described by Suslick (1988). The temporal course of the sonochemical processes was monitored by HPLC. 

The ultrasonic irradiation of aqueous solution of PNP was carried out with a Branson 200 sonifier that was operated at 20 KHz and with an output of electrical power of 84 W. Reactions were performed in a stainless-steel, continuous-flow reaction cell operated in the batch mode. All reactions were carried out with air-saturated solutions, and the concentration of p-benzo-quinone (p-BQ) was determined spectrophotometrically. Hydrogen peroxide... 

During ultrasonic irradiation of aqueous solutions, OH radicals are produced from dissociation of water vapor upon collapse of cavitation bubbles. A fraction of these radicals that are initially formed in the gas phase diffuse into solution. Cavitation is a dynamic phenomenon, and the number and location of bursting bubbles at any time cannot be predicted a priori. Nevertheless, the time scale for bubble collapse and rebound is orders of magnitude smaller than the time scale for the macroscopic effects of sonication on chemicals (2) (i.e., nanoseconds to microseconds versus minutes to hours). Therefore, a simplified approach for modeling the liquid-phase chemistry resulting from sonication of a well-mixed solution is to view the OH input into the aqueous phase as continuous and uniform. The implicit assumption in this approach is that the kinetics of the aqueous-phase chemistry are not controlled by diffusion limitations of the substrates reacting with OH. 

The ultrasonic irradiation of a solution induces acoustic cavitation, a transient process that promotes chemical activity. Acoustic cavitation is generated by the growth of preexisting nuclei during the alternating expansion and compression cycles of ultrasonic waves. For example, in aqueous liquid, temperatures as high as 4300 K and pressures over 1000 atm are estimated to exist within... 

The ultrasonic irradiation of aqueous sulfide solutions was conducted with a Branson 200 sonifier operating at 20 kHz by Kotronarou et al. (1992). Reactions were performed in a 50-mL water-jacketed, stainless-steel cell from Sonics Materials. The temperature inside the reaction vessel was kept constant at 25°C. All irradiations were carried out in air-saturated solutions at t = 0. A Haake A80 water circulating and temperature-controlling system was used for temperature control. Hydrogen peroxide was analyzed fluoro-metrically. Deionized water was used to prepare all the solutions. 

TA-NaBr-MRNi was prepared by the reported method [3]. RNi (W-1 type) was prepared from 1.9 g of Raney nickel alloy (Kawaken Fine Chemical Co., Ni/Al = 42/58). To wash out the excess base and aluminum salts, a sufficient amount of deionized water was used with ultrasonic irradiation. The modifying solution was prepared by dissolving of (R,R)-tartaric acid (1 g) and NaBr (6 g to 10 g) in 100 ml of water and adjusting the pH to 3.2 with IN NaOH aqueous solution. RNi was heated in the modifying solution at 100 C for 1 hour, washed with water (50 ml), methanol (50 ml, twice), and THF (10 ml). The TA-NaBr-MRNi obtained by this method was immediately used for the hydrogenation. 

Kotronarou A, Mills G, Hoffmann MR (1991) Ultrasonic irradiation of p-nitrophenol in aqueous solution. J Phys Chem 95 3630-3638... 

Zhu et al. [94] reported the synthesis of Sn02 semiconductor nanoparticles by ultrasonic irradiation of an aqueous solution of SnCLj and azodicarbonamide under ambient air. They found that the sonochemically synthesized Sn02 nanoparticles improved remarkably the performance of Li ion batteries such that there was about threefold increase (from 300 to 800 mAh/g) in the reversible capacity in the first lithiation to delithiation cycles. Similarly the irreversible capacity also increased by about 70% (from 800 to 1400 mA h/g). Wang et al. [95] reported the synthesis of positively charged tin porphyrin adsorbed onto the surface of silica and used as photochemically active templates to synthesise platinum and palladium shell and... 

Pettier et al. (1992) studied the sonochemical degradation of pentachlorophenol in aqueous solutions saturated with different gases at 24 °C. Ultrasonic irradiation of solutions saturated with air or oxygen resulted in the liberation of chloride ions and mineralization of the parent compound to carbon dioxide. When the solution was saturated with argon, pentachlorophenol completely degraded to carbon monoxide and chloride ions, in aqueous solution, pentachlorophenol was degraded by ozone at a reaction rate of >3.0 x 10 /M-sec at pH 2.0 (Hoigne and Bader, 1983). 

We recently reported (3) that ultrasonic irradiation of alkaline oxic aqueous solutions of bivalent sulfur, S(-II), at 20 kHz resulted in the rapid oxidation of S(-II). The observed distribution of the oxidation products was similar to that reported for y-radiolysis of S(-II). The ultrasound-induced oxidation of S(-II) in alkaline solutions was attributed to the reaction of HS" with OH radicals (3). These radicals form during ultrasonic irradiation of water as a result of the high-temperature decomposition of water vapor inside the hot cavitation bubbles (1,2). Although the experimental results were qualitatively consistent with our proposed mechanism, some questions remained unanswered concerning the amount of OH released into the aqueous phase and the existence and relative importance of additional oxidants. 

Some surfactants possessing two hydrocarbon chains attached to a single head group, e.g. the dialkyldimethylammonium chlorides, form lamellar structures when dispersed in aqueous solution. When such turbid solutions are subjected to ultrasonic irradiation, optically clear solutions are formed in which the surfactant is dispersed in the form of closed vesicles [104] similar in structure to the liposomes formed by phospholipids. The use of these totally synthetic bilayer vesicles as model membranes and as potential drug delivery systems is currently under investigation. 

Reduction Mechanism of Metal Ions in Aqueous Solution Under Ultrasonic Irradiation... 

To synthesize metal nanoparticles in an aqueous solution, the reduction reactions of the corresponding metal ions are generally performed. Gutierrez et al. [21] reported the reduction of A11CI4 and Ag+ ions in an aqueous solution by ultrasonic irradiation under H2-Ar mixed atmosphere. They found that the optimum condition of these reductions was under the 20 vol% H2 and 80 vol% Ar atmosphere. Following this study, many papers reported the sonochemical reduction of noble metal ions under pure Ar atmosphere to produce the corresponding metal nanoparticles [22-28],... 

It has been reported that the sonochemical reduction of Au(III) reduction in an aqueous solution is strongly affected by the types and concentration of organic additives. Nagata et al. reported that organic additives with an appropriate hydro-phobic property enhance the rate of Au(III) reduction. For example, alcohols, ketones, surfactants and water-soluble polymers act as accelerators for the reduction of Au(III) under ultrasonic irradiation [24]. Grieser and coworkers [25] also reported the effects of alcohol additives on the reduction of Au(III). They suggested that the rate of the sonochemical reduction of Au(III) is related to the Gibbs surface excess concentration of the alcohol additives. 

Based on the above results, ultrasonic irradiation to ion-exchanged [Pd(NH3)4]2+-zeolite powders was performed in an aqueous solution containing 2-propanol. The reduction of [Pd(NH3)4]2+-zeolite to Pd°-zeolite was confirmed by XPS analyses. However, from XPS depth analyses of the prepared samples, it was suggested that the [Pd(NH3)4]2+ complexes in the zeolite pore were not sufficiently reduced even in the presence of 2-propanol. Presumably, the reductants formed from 2-propanol sonolysis could not easily diffuse into the zeolite nano-pore (size 1.2 nm) and/or reductants undergo recombination reactions and quenching reactions with the walls. In addition, the results of XPS spectral analyses of the sonochemically prepared Pd-zeolite powders indicated that the average size of the Pd clusters on the zeolite surface is roughly estimated to be less than 1 nm and composed of several dozen Pd atoms. 

Kotranarou A, Mills G, Hoffman MR (1992) Oxidation of hydrogen sulfide in aqueous solution by ultrasonic irradiation. Environ Sci Technol 26 2420-2428... 

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