Yoshio Nagata

Sonochemical degradation of chlorophenols in water

Sonochemical degradation of dilute aqueous solutions of 2-, 3- and 4-chlorophenol and pentachlorophenol has been investigated under air or argon atmosphere. The degradation follows first-order kinetics in the initial state with rates in the range 4.5-6.6 microM min-1 under air and 6.0-7.2 microM min-1 under argon at a concentration of 100 microM of chlorophenols. The rate of OH radical formation from water is 19.8 microM min-1 under argon and 14.7 microM min-1 under air in the same sonolysis conditions. The sonolysis of chlorophenols is effectively inhibited, but not completely, by the addition of t-BuOH, which is known to be an efficient OH radical scavenger in aqueous sonolysis. This suggests that the main degradation of chlorophenols proceeds via reaction with OH radicals; a thermal reaction also occurs, although its contribution is small. The addition of appropriate amounts of Fe(II) ions accelerates the degradation. This is probably due to the regeneration of OH radicals from hydrogen peroxide, which would be formed from recombination of OH radicals and which may contribute a little to the degradation. The ability to inhibit bacterial multiplication of pentachlorophenol decreases with ultrasonic irradiation.

Formation of noble metal particles by ultrasonic irradiation

Abstract It was found that sonochemically prepared metal particles such as Ag, Pd, Au, Pt and Rh are of nanometer size with a fairly narrow distribution (e.g., about 5 nm for Pd particles obtained from a 1.0 mM Pd(II) in polyethylene glycol monostearate solution). We have suggested three different reduction pathways under sonication: (i) reduction by H atoms, (ii) reduction by secondary reducing radicals formed by hydrogen abstraction from organic additives with OH radicals and H atoms, (iii) reduction by radicals formed from pyrolysis of the additives at the interfacial region between cavitation bubbles and the bulk solution. The reduction of Ag(I) and Pt(II) mainly proceeds through reaction pathway (ii). In the cases of Pd(II) and Au(III), the reductions mainly proceed through reaction pathway (iii). The reduction of Rh(III) was not achieved under the same conditions; however, by the addition of sodium formate, reduction occurred and the preparation of Rh particles succeeded.

Sonochemical Formation of Gold Particles in Aqueous Solution

Gold(III) ions in aqueous solutions of NaAuCl4 were reduced to form gold particles by ultrasonic irradiation. The rate of formation of gold particles was accelerated in the presence of certain organic additives such as surfactants, water-soluble polymers and aliphatic alcohols and ketones. The rates of formation of gold particles from 1 mM Au(III) ions in pure water were 3 microM min-1 under argon atmosphere and approximately zero under air, and in solutions containing additive the rates were 9-133 microM min-1 under argon and 8-40 microM min-1 under air. Surfactants stabilized the particles as colloidal state for more than several months. The rates of formation of both hydrogen atoms and hydroxyl radicals were estimated to be equal to 25 microM min-1 in the sonolysis of pure water under argon. Three reaction pathways leading to the reduction of metal ions were proposed: (1) reduction by hydrogen atoms; (2) reduction by reducing radicals formed via reactions of hydroxyl radicals or hydrogen atoms with the additives; (3) reduction by radicals formed from thermal reaction of the additives at the interfacial region between cavitation bubbles and bulk solution and/or in the cavities. The order of the contribution of these three pathways to the reduction of gold ions was (3) > (2) > (1) in most cases. The number averages of the size of gold particles formed in surfactant solutions under argon atmosphere were about 10 nm with a fairly narrow size distribution.

Preparation of platinum nanoparticles by sonochemical reduction of the Pt(IV) ions: role of surfactants

Sonochemical reduction processes of Pt(IV) ions in water have been investigated in the presence of various kinds of surfactants such as sodium dodecylsulfate (SDS) and sodium dodecylbenzenesulfonate (DBS) as anionic surfactants, and polyethylene glycol monostearate (PEG-MS) as non-ionic, dodecyltrimethylammonium chloride (DTAC) and bromide (DTAB) as cationic surfactants. An improved colorimetric determination reveals that Pt(IV) ion is reduced to zero valent metal in two steps: step (1)--Pt(IV) ion to Pt(II) ion, and step (2)--Pt(II) ion to Pt(0), and after the completion of step (1), step (2) sets in. It appears that rapid scrambling reactions among platinum ions and/or atoms, that is, Pt(I) + Pt(IV)-->Pt(II) + Pt(III), etc. take place. In the sonolysis of aqueous solutions of SDS, DBS or PEG-MS, two kinds of organic reducing radicals, R(ab) and R(py), are proposed to contribute to the reduction. Radical R(ab) is formed from the reaction of the surfactants with primary radicals such as hydroxyl radicals and hydrogen atoms originated from the sonolysis of water, and radical R(py) is formed from the direct thermal decomposition of surfactants in the interfacial region between the collapsing cavities and the bulk water. R(ab) is effective for both the reduction steps, whereas R(py) is involved only in the reduction step (1). This fact coincides with the previous reported sonochemical reduction of Pt(II) ions. Hydrogen atoms themselves scarcely participate in the reduction. The average diameter (1.0 nm) of platinum particles prepared from the system of PEG-MS is smaller than those from the aqueous solution of anionic surfactant SDS (3.0 nm) and DBS (3.0 nm).

Sonolytical preparation of various types of metal nanoparticles in aqueous solution

Composite nanoparticles of Au, Pd and Pt, and nanoparticles of transition metal oxides, MnO2, were prepared by the sonochemical reduction of corresponding ions (Au(III), Pd(II), Pt(II), Pt(IV) and Mn(VII)) in an aqueous system in the presence of surfactants. The prepared noble metal particles were stable and the sizes were of the order of nanometer with narrow distribution. Surfactants were found to be stabilizers for the noble metal nanoparticles as well as important sources of reducing species for the metal ions. In binary metallic particles, which were prepared from aqueous mixed solutions of noble metal ions, three possible constitutional forms were obtained. MnO2 particles were reduce to water soluble Mn(II) ions by further sonication.

Sonolysis of surfactants in aqueous solutions: an accumulation of solute in the interfacial region of the cavitation bubbles.

The sonolysis of surfactants (such as sodium dodecylbenzenesulfonate (DBS), sodium dodecylsulfate (SDS), and polyethylene glycol monostearate), sodium 4-toluenesulfonate (STS), and 1-hexanol in aqueous solutions was investigated under an argon atmosphere with ultrasound of 200 kHz in order to compare the scavenging efficiency of the hydroxyl radical and the accumulation in the gas-liquid interfacial region of the cavitation bubbles. The degradation rate of the solute follows the order 1-hexanol > DBS and SDS > STS. The scavenging efficiency of the hydroxyl radical by non-volatile surfactants was much greater than that of the non-volatile and hydrophilic solute (e.g., STS). The surfactant was accumulated in a relatively high ratio in the interfacial region. The degradation of surfactants occurred by reaction with the hydroxyl radical and also by pyrolysis at high temperature. On the other hand, STS, due to its non-volatile and hydrophilic properties, was principally present in the bulk solution and the degradation by pyrolysis was not observed at the investigated concentration ranges.

Formation of colloidal silver in water by ultrasonic irradiation

Stable colloidal dispersions of silver were prepared by ultrasonic irradiations of aqueous AgClO4 or AgNO3 solutions in the presence of surfactants.

Hydrogen gas evolution from alumina nanoparticles dispersed in water irradiated with γ-ray

Abstract Hydrogen gas evolution induced by γ-ray irradiation of nanoparticles dispersed in aqueous solution was studied by measuring hydrogen yields from TiO 2 , α- and γ-Al 2 O 3 nanoparticles. Hydrogen gas was detected by a gas chromatograph after an irradiation with 60 Co γ-ray of the sample solution in a closed vial at room temperature. TiO 2 nanoparticles, a highly photo-catalysis mainly in anatase structure with average diameter of 21 nm, was found to have a relatively low evolution yield, about three times of the back ground hydrogen evolution due to the water radiolysis, while the α- and γ-Al 2 O 3 showed a much higher yield, 7 – 8 times of the background, depending on the total dose and dose rate of the γ-ray.

Decomposition of chlorofluorocarbons and hydrofluorocarbons in water by ultrasonic irradiation

Abstract The sonochemical degradation of CFC-113 (F 2 ClCCCl 2 F), HCFC-225ca (F 3 CCF 2 CCl 2 H), HCFC-225cb (F 2 ClCCF 2 CClFH) and HFC-134a (F 3 CCF 2 H) in water was investigated. The decomposition rates of CFC-113 increased with increasing the concentration of the CFC and at high concentration the rates far exceeded the rate of OH radical formation by water sonolysis, and OH radicals seemed to have little effect on the decomposition. The pyrolysis in the cavitation bubbles was suggested.

Sonochemical degradation of chlorinated hydrocarbons using a batch and continuous flow system

The sonochemical degradation of chlorinated hydrocarbons such as 1,1,1-trichloroethane, trichloroethylene and tetrachloroethylene in aqueous solution was carried out in the batch and continuous flow systems at an ultrasonic frequency of 100kHz under an air atmosphere. In the batch experiment, the rate of degradation follows the order 1,1,1-trichloroethane>tetrachloroethylene>trichloroethylene, and the chlorinated hydrocarbon were readily degraded by ultrasonic irradiation. The experiments in the continuous flow system were performed in the range of volumetric flow rate from 7 to 30 x 10(-3)lmin(-1). The conversion of the chlorinated hydrocarbons at a steady-state of reactor depended on the volumetric flow rate. The yield of Cl(-) (as a measurement of mineralization of chlorinated hydrocarbons) was 70-90% of the chlorine atoms in the parent chlorinated hydrocarbon molecules. From the viewpoint of the scale-up process, the sonochemical degradation of trichloroethylene was simulated in a three stage reactor, and the conversion (>99%) in a third stage reactor was showed the good results that can be satisfied a desired water quality standard.