OH-radical formation by ultrasound in aqueous solution--Part II: Terephthalate and Fricke dosimetry and the influence of various conditions on the sonolytic yield

Terephthalate and Fricke dosimetry have been carried out to determine the sonolytic energy yields of the OH free radical and of its recombination product H2O2 in aqueous solutions under various operating conditions (nature of operating gas, power, frequency, temperature). For example, in the sonolysis of Ar-saturated terephthalate solutions at room temperature, a frequency of 321 kHz, and a power of 170 W kg-1, the total yield [G(.OH) + 2 G(H2O2)], equals 16 x 10(-10) mol J-1. This represents the total of .OH that reach the liquid phase from gas phase of the cavitating bubble. The higher the solute concentration, the lower the H2O2 production as more of the OH free radicals are scavenged, in competition with their recombination. Fricke dosimetry, in the absence and presence of Cu2+ ions, shows that the yield of H atom reaching the liquid phase is much lower, with G(H.) of the order of 3 x 10(-10) mol J-1. These sonolytic yields are smaller in solutions that are at the point of gas saturation, and increase to an optimum as the initial sonication-induced degassing and effervescence subsides. The probing of the sonic field has shown that the rate of sonolytic free-radical formation may vary across the sonicated volume depending on frequency and power input.     

Effect of pH on the ultrasonic degradation of ionic aromatic compounds in aqueous solution

The sonolysis of 4-nitrophenol (4-NP) and aniline in O2-saturated aqueous solutions was performed at 610 kHz with ultrasonic power of 25 W and aqueous temperature of 15 +/- 1 degrees C. The initial rate of degradation of both 4-NP and aniline in sonolysis of aqueous media follows pseudo-first-order reaction kinetics. Investigation of the H2O2 generation rate in phosphate buffer media (0.01 M) over the range of pH 2-9 revealed a maximum yield at pH approximately 3.2. The pH, which results in modification of the physical properties (including charge) of molecules with ionisable functional groups, plays an important role in the sonochemical degradation of chemical contaminants. For hydrophilic substrates, the neutral species more easily diffuse to and accumulate at the hydrophobic interface of liquid-gas bubbles in comparison with their corresponding ionic forms. As a consequence, the degradation rate of 4-NP under ultrasonic irradiation decreases with increasing pH. In contrast, the disappearance rate of aniline exhibits a maximum under alkaline conditions due to the high solubility of the ionic anilinium ion and the (potentially) preferential movement of the uncharged form to the interface. Additionally, the rate of reaction of the uncharged aniline molecule (which dominates at pH > 4.6) with hydroxyl radicals is reported to be about three times as fast as the rate of reaction of the cationic anilinium species.     

Sonochemical degradation of martius yellow dye in aqueous solution

The sonolytic degradation of the textile dye martius yellow, also known as either naphthol yellow or acid orange 24, was studied at various initial concentrations in water. The degradation of the dye followed first-order kinetics under the conditions examined. Based on gas chromatographic results and sonoluminescence measurements of sonicated aqueous solutions of the dye, it is concluded that pyrolysis does not play a significant role in its degradation. The chromatographic identification of hydroxy added species indicates that an OH radical induced reaction is the main degradation pathway of the dye. Considering the non-volatility and surface activity of the dye, the degradation of the dye most probably takes place at the bubble/solution interface. The quantitative and qualitative formation of the degradation intermediates and final products were monitored using HPLC and ESMS. The analytical results suggest that the sonolytic degradation of the dye proceeds via hydroxylation of the aryl ring and also by C-N bond cleavage of the chromophoric ring, either through OH radical attack or through another unidentified process. The identification of various intermediates and end products also imply that the degradation of martius yellow proceeds through multiple reaction pathways. Total organic carbon (TOC) analyses of the dye solutions at various times following sonication revealed that sonolysis was effective in the initial degradation of the parent dye but very slow in achieving mineralization. The slow rate of mineralization is likely to be due to the inability of many of the intermediate products such as, the carboxylic acids, to accumulate at the bubble (air/water) interface and undergo decomposition due to their high water solubility (low surface activity).     

Sonochemical and sonophotocatalytic degradation of malachite green: the effect of carbon tetrachloride on reaction rates

A comparative study between the sonolytic, photocatalytic and sonophotocatalytic oxidation processes of aqueous solutions of malachite green was carried out in the presence of carbon tetrachloride, under a low power ultrasonic field (<15 W) and using titanium dioxide as a photocatalyst. The effect of a number of parameters such as ultrasonic intensity, TiO2 crystalline structure and the presence of CCl4 were studied using an inexpensive reactor. Enhanced rates of sonolytic degradation of malachite green in the presence of CCl4 were demonstrated. On the other hand, the simultaneous use of sonolysis and photocatalysis in the presence of CCl4 does not improve the degradation rate of malachite green in comparison with the one obtained using only sonolysis, but it makes possible a faster oxidative degradation of some reaction intermediaries. Finally, in air saturated solutions both processes, the sonolytic and the photocatalytic one, follow a first-order rate law.     

Degradation of orange-G by advanced oxidation processes

The degradation and mineralization of orange-G (OG) in aqueous solutions by means of ultrasound irradiation at a frequency of 213 kHz and its combination with a heterogeneous photocatalyst (TiO₂) were investigated. The effects of various operational parameters such as, the concentration of the dye and solution pH on the degradation efficiency were studied. The degradation of the dye followed first-order like kinetics under the conditions examined. The sonolytic degradation of OG was relatively higher at pH 5.8 than that at pH 12. However, an alkaline pH was favoured for the photocatalytic degradation of OG using TiO₂. Total organic carbon (TOC) measurements were also carried out in order to evaluate the mineralization efficiency of OG using sonolysis, photocatalysis and sonophotocatalysis. The hybrid technique of sonophotocatalytic degradation was compared with the individual techniques of photocatalysis and sonolysis. A simple additive effect was observed during the sonophotocatalytic oxidation of OG using TiO₂ indicating that the combined treatment offers no synergistic enhancement. TOC results also support the additive effect in the dual treatment process.     

Modeling of ultrasonic degradation of non-volatile organic compounds by Langmuir-type kinetics

Sonochemical degradation of phenol (Ph), 4-isopropylphenol (4-IPP) and Rhodamine B (RhB) in aqueous solutions was investigated for a large range of initial concentrations in order to analyze the reaction kinetics. The initial rates of substrate degradation and H₂O₂ formation as a function of initial concentrations were determined. The obtained results show that the degradation rate increases with increasing initial substrate concentration up to a plateau and that the sonolytic destruction occurs mainly through reactions with hydroxyl radicals in the interfacial region of cavitation bubbles. The rate of H₂O₂ formation decreases with increasing substrate concentration and reaches a minimum, followed by almost constant production rate for higher substrate concentrations. Sonolytic degradation data were analyzed by the models of Okitsu et al. [K. Okitsu, K. Iwasaki, Y. Yobiko, H. Bandow, R. Nishimura, Y. Maeda, Sonochemical degradation of azo dyes in aqueous solution: a new heterogeneous kinetics model taking into account the local concentration OH radicals and azo dyes, Ultrason. Sonochem. 12 (2005) 255–262.] and Seprone et al. [N. Serpone, R. Terzian, H. Hidaka, E. Pelizzetti, Ultrasonic induced dehalogenation and oxidation of 2-, 3-, and 4-chlorophenol in air-equilibrated aqueous media. Similarities with irradiated semiconductor particulates, J. Phys. Chem. 98 (1994) 2634–2640.] developed on the basis of a Langmuir-type mechanism. The five linearized forms of the Okitsu et al.’s equation as well as the non-linear curve fitting analysis method were discussed. Results show that it is not appropriate to use the coefficient of determination of the linear regression method for comparing the best-fitting. Among the five linear expressions of the Okitsu et al.’s kinetic model, form-2 expression very well represent the degradation data for Ph and 4-IPP. Non-linear curve fitting analysis method was found to be the more appropriate method to determine the model parameters. An excellent representation of the experimental results of sonolytic destruction of RhB was obtained using the Serpone et al.’s model. The Serpone et al.’s model gives a worse fit for the sonolytic degradation data of Ph and 4-IPP. These results indicate that Ph and 4-IPP undergo degradation predominantly at the bubble/solution interface, whereas RhB undergoes degradation at both bubble/solution interface and in the bulk solution.     

Sonolysis of 4-chlorophenol in aqueous solution: effects of substrate concentration, aqueous temperature and ultrasonic frequency

The sonolysis of 4-chlorophenol (4-CP) in O2-saturated aqueous solutions is investigated for a variety of operating conditions with the loss of 4-CP from solution following pseudo-first-order reaction kinetics. Hydroquinone (HQ) and 4-chlorocatechol (4-CC) are the predominant intermediates which are degraded on extended ultrasonic irradiation. The final products are identified as Cl-, CO2, CO, and HCO2H. The rate of 4-CP degradation is dependent on the initial 4-CP concentration with an essentially linear increase in degradation rate at low initial 4-CP concentrations but with a plateauing in the rate increase observed at high reactant concentrations. The results obtained indicate that degradation takes place in the solution bulk at low reactant concentrations while at higher concentrations degradation occurs predominantly at the gas bubble-liquid interface. The aqueous temperature has a significant effect on the reaction rate. At low frequency (20 kHz) a lower liquid temperature favours the sonochemical degradation of 4-CP while at high frequency (500 kHz) the rate of 4-CP degradation is minimally perturbed with a slight optimum at around 40 degrees C. The rate of 4-CP degradation is frequency dependent with maximum rate of degradation occurring (of the frequencies studied) at 200 kHz.     

A statistical thermodynamic approach to sonochemical reactions

The calculation of the equilibrium constants K of the sonolysis reactions of CO2 into CO and O atom, the recombination of O atoms into O2 and the formation of H2O starting with H and O atoms, has been studied by means of statistical thermodynamic. The constants have been calculated at 300 kHz versus the pressure and the temperature according to the extreme conditions expected in a cavitation bubble, e.g. in the range from ambient temperature to 15200 K and from ambient pressure to 300 bar. The decomposition of CO2 appears to be thermodynamically favored at 15200 K and 1 bar with a constant K1=1.52 x 10(6), whereas the formation of O2 is not expected to occur (K2=1.8 x10(-8) maximum value at 15200 K and 300 bar) in comparison to the formation of water (K3=3.4 x 10(47) at 298 K and 300 bar). The most thermodynamic favorable location of each reactions is then proposed, the surrounding shell region for the thermic decomposition of CO2 and the wall of the cavitation bubble for the formation of water. Starting from a work of Henglein on the sonolysis of CO2 in water at 300 kHz, the experimental amount of CO formed (7.2 x 10(20)molecules L(-1)) is compared to the theoretical CO amount (1.4 x 10(27)molecules L(-1)) which can be produced by the sonolysis of the same starting amount CO2. With the help of the literature data, the number of cavitation bubble has been evaluated to 6.2 x 10(15) bubbles L(-1) at 300 kHz, in 15 min. This means that about 1 bubble on 1900000 is efficient for undergoing the sonolysis of CO2.     

Sonolytic hydrolysis of peptides in aqueous solution upon addition of catechol

The sonolytic hydrolysis of peptides with addition of phenolic reagents to aqueous solutions is described. Sonolysis of an aqueous solution of peptides to which catechol (o-dihydroxybenzene) had been added resulted in hydrolytic products reflecting the amino acid sequence without any side reactions, while sonolysis without any additives resulted in oxidation analytes and degradation products caused by side reactions. Although the use of additives such as resorcinol (m-dihydroxybenzene), hydroquinone (p-dihydroxybenzene) and phenol was also effective in producing sequence related products, several degradation products were produced by side reactions. A characteristic of the sonolysis of peptides is that the N-terminal side of proline, Xxx-Pro, is more susceptible than other amino acid residues to the process. This characteristic of sonolysis is superior to that of acid hydrolysis in which cleavage at the C-terminal side of proline, Pro-Xxx is difficult, and where dehydration products result due to side reactions.     

Sonolysis of chlorobenzene in aqueous solution: organic intermediates

The ultrasonic degradation of 1.72 mM chlorobenzene was investigated. The sonolysis of chlorobenzene followed first-order kinetics. The influence of the pH of the aqueous solution and the effect of the saturating gás, air or argon, was measured. No pH effect was noticed, and saturation with the monoatomic argon accelerated the degradation. Furthermore, the addition of the radical scavenger benzoate demonstrated that no significant degradation took place in the bulk solution. For air-saturated solutions, the following organic degradation products were identified: methane, acetylene, butenyne, butadiyne, benzene, chlorophenols, phenylacetylene and other chlorinated and non-chlorinated monocyclic and dicyclic hydrocarbons. For argon-saturated solutions, the same products were found, except for the chlorophenols. The presence of the chlorophenols in the case of air-saturation only demonstrated the interaction between the radicals formed and oxygen, and no direct degradation by OH. radicals. The kinetics of several organic degradation products and chloride were determined for the sonolysis of air- and argon-saturated solutions.     

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.     

Scavenging of OH(*) radicals produced from H(2)O sonolysis with nitrate ions

The scavenging of OH(?) radicals formed during H(2)O sonolysis with nitrate-ions was studied in HNO(3)/NaNO(3) mixture at the constant NO(3)(-) ions concentration ([HNO(3)]+[NaNO(3)])=1 M in Ar atmosphere. Small amounts of N(2)H(5)NO(3) was added to solutions to avoid HNO(2) accumulation due to HNO(3) sonolysis. It was shown that the increase of [H(+)] causes the increase of H(2)O(2) formation rate (W(H(2)O(2)). (W(H(2)O(2)) values reach the plateau at [HNO(3)] approximately 1 M. The (W(H(2)O(2)) ratio in solution with [H(+)]=1 M and pure water was found to be equal to 2.4+/-0.4. It was assumed that (W(H(2)O(2)) increase in nitric acid medium is related to the changing of H(2)O(2) formation mechanism. In pure water H(2)O(2) is formed due to the OH(*) radicals recombination. In HNO(3)+NaNO(3) mixture the mechanism of H(2)O(2) formation consists in conversion of OH(*) radicals to NO(3)(*) radicals followed by NO(3)(*) radicals hydrolysis. Results obtained show that OH(*) radicals recombination mainly occurs in the liquid phase surrounding the cavitating bubble.     

Exploring the effects of pulsed ultrasound at 205 and 616 kHz on the sonochemical degradation of octylbenzene sulfonate

Compared to continuous wave (CW) ultrasound, pulsed wave (PW) ultrasound has been shown to result in enhanced sonochemical degradation of octylbenzene sulfonate (OBS). However, pulsed ultrasound was investigated under limited pulsing conditions. In this study, pulse-enhanced degradation of OBS was investigated over a broad range of pulsing conditions and at two ultrasonic frequencies (616 and 205 kHz). The rate of OBS degradation was compared to the rate of formation of 2-hydroxyterephthalic acid (HTA) following sonolysis of aqueous terephthalic acid (TA) solutions. This study shows that sonication mode and ultrasound frequency affect both OBS degradation and HTA formation rates, but not necessarily in the same way. Unlike TA, OBS, being a surface active solute, alters the cavitation bubble field by adsorbing to the gas/solution interface of cavitation bubbles. Enhanced OBS degradation rates during pulsing are attributed to this adsorption process. However, negative or smaller pulse enhancements compared to enhanced HTA formation rates are attributed to a decrease in the high-energy stable bubble population and a corresponding increase in the transient bubble population. Therefore, sonochemical activity as determined from TA sonolysis cannot be used as a measure of the effect of pulsing on the rate of degradation of surfactants in water. Over relatively long sonolysis times, a decrease in the rate of OBS degradation was observed under CW, but not under PW conditions. We propose that the generation and accumulation of surface active and volatile byproducts on the surface and inside of cavitation bubbles, respectively, during CW sonolysis is a contributing factor to this effect. This result suggests that there are practical applications to the use of pulsed ultrasound as a method to degrade surface active contaminants in water.     

Sonochemical degradation of PAH in aqueous solution. Part I: monocomponent PAH solution

The sonolysis of selected monocomponent PAH aqueous solution is studied at 20 and 506 kHz in the microg l(-1) range. The highest activity observed at 506 kHz, compared to 20 kHz, is tentatively explained by examination of the physical characteristics of bubbles (size and life-time) as well as by the calculation of the number of bubble at both frequency (5 x 10(3)bubbles l(-1) at 20 kHz and 4.5 x 10(9)bubbles l(-1) at 506 kHz). It is demonstrated that the main mechanism of sonodegradation is the pyrolysis of PAHs in the heart of the cavitation bubbles, and that a possible PAH oxidation by means of HO degrees appears as a minor way, since gaseous byproducts such as CO, CO2, C2H2 and CH4 have been detected. Correlations have been found by examination of kinetic variations in terms of the physical-chemical properties of PAHs. The rate constants of PAH degradation increase when the water solubility, the vapour pressure and the Henry's law constant increase.     

氯柱硼镁石在氯化镁水溶液中相转化平衡液相的红外光谱分析

采用差示FTIR光谱技术,研究了氯柱硼镁石在30℃下浓度为0.5,12和18％MgCl_2水溶液中溶解及相转化平衡饱和水溶液中硼氧配阴离子的FTIR光谱。给出了溶液中硼氧配阴离子的FTIR光谱振动频率的归属,515cm~(-1)为单和二硼氧配阴离子特征峰,630cm~(-1)为三硼氧配阴离子特征峰,550cm~(-1)为四硼氧配阴离子特征峰。对饱和水溶液中硼氧配阴离子的存在形式及其相互作用以及与相转化析出固相的关系进行了讨论。     

Comparative study of sonochemical reactors with different geometry using thermal and chemical probes

Laboratory scale 20 kHz sonochemical reactors with different geometries have been tested using thermal probes, the kinetics of H(2)O(2) formation, and the kinetics of diphenylmethane (DPhM) sonochemical darkening. Results revealed that the overall sonochemical reaction rates in H(2)O and DPhM are driven by the total absorbed acoustic energy and roughly independent the geometry of the studied reactors. However, the sonochemical efficiency, defined as eta=VG/S, where G is a sonochemical yield of H(2)O(2), V is a volume of sonicated liquid, and S is a surface of the sonotrode, was proved to increase with the decrease of S. This phenomenon was explained by growing of the maximum cavitating bubble size with ultrasonic intensity and its independence towards the specific absorbed acoustic power. For the cleaning bath reactor the kinetics of the sonochemical reactions in H(2)O and DPhM depends strongly on the reaction vessel materials: the reaction rates decreased with the increase of the materials elasticity. Kinetic study of H(2)SO(4) sonolysis using a sonoreactor without direct contact with titanium sonotrode showed that sulphate anion is an effective scavenger of OH() radicals formed during water sonolysis.     

Sonochemical and photosonochemical degradation of 4-chlorophenol in aqueous media

The degradation of 4-chlorophenol (4-CP) in aqueous media by 516 kHz ultrasonic irradiation was investigated in order to clarify the degradation mechanism. The degradation of concentrated 4-CP solution by means of ultrasound, UV irradiation and their combined application was also studied. The obtained results indicate that *OH radical are the primary reactive species responsible for 4-CP ultrasonic degradation. Very little 4-CP degradation occurs if the sonolysis is carried out in the presence of the *OH radical scavenger tert-butyl alcohol, also indicating that little or no pyrolysis of the compound occurs. The dominant degradation mechanism is the reaction of substrate with *OH radicals at the gas bubble-liquid interface rather than high temperature direct pyrolysis in ultrasonic cavities. This mechanism can explain the lower degradation rate of the ionic form of 4-CP that is partly due to the rapid dissociation of *OH radicals in alkaline solutions. The sonochemical destruction of concentrated 4-CP aqueous solution is obtained with low rate. Coupling photolysis with ultrasound irradiation results in increased efficiency compared to the individual processes operating at common conditions. Interestingly, the photosonochemical decomposition rate constant is greater than the additive rate constants of the two processes. This may be the result of three different oxidative processes direct photochemical action, high frequency sonochemistry and reaction with ozone produced by UV irradiation of air, dissolved in liquid phase because of the geyser effect of ultrasound streaming. Additionally, the photodecomposition, at 254 nm, of hydrogen peroxide produced by ultrasound generating *OH radical can partly explain the destruction enhancement.     

原子吸收光谱法间接研究组氨酸锌的配合反应

编程计算了不同pH条件下的组氨酸和锌离子的各种存在形式并分析了拟合分布图,研究了在硫化锌法原子吸收间接测定组氨酸时的pH值对原子吸收响应的影响及络合反应的机理。指出在pH 9.5左右最佳测定条件下,所形成的可溶性组氨酸锌配合物是由电中性的组氨酸基His+- 和带负一价电荷形态的组氨酸基His-与Zn(OH)₂共同形成的Zn(OH)₂·(C₆N₃O₂H₉)₂,Zn(OH)₂·[(C₆N₃O₂H₈)-]₂,Zn(OH)2·(C₆N₃O₂H₉)·[(C₆N₃O₂H₈)-]。结果表明,理论计算分析的结果与实验数据基本吻合,确定了硫化锌法原子吸收间接测定组氨酸时的配合物反应机理及配合物的组成结构。     

Sonolysis and radiolysis of glyceraldehyde in deaerated aqueous solution

The objective of this work was to contribute to the mechanism of the sonolytic and radiolytic reactions leading in deaerated aqueous solutions of sugars to products by radical-radical combination. For this purpose glyceraldehyde, the first homologue of the series of aldoses, was investigated. Primary glyceraldehyde radicals are produced by abstraction of carbon-bonded hydrogen atoms by sonolytic or radiolytic H and OH radicals. Secondary glyceraldehyde radicals are derived from primary radicals by elimination of water. Both kinds of radicals were found to participate in dimer production.     

Degradation of organic water pollutants through sonophotocatalysis in the presence of TiO(2)

The degradation of 2-chlorophenol and of the two azo dyes acid orange 8 and acid red 1 in aqueous solution was investigated kinetically under sonolysis at 20 kHz and under photocatalysis in the presence of titanium dioxide particles, as well as under simultaneous sonolysis and photocatalysis, i.e. sonophotocatalysis. The influence on the degradation and mineralisation rates of the initial substrate concentration and of the photocatalyst amount was systematically investigated to ascertain the origin of the synergistic effect observed between the two degradation techniques. The evolution of hydrogen peroxide during kinetic runs was also monitored. Small amounts of Fe(III) were found to affect both the adsorption equilibria on the semiconductor and the degradation paths. Ultrasound may modify the rate of photocatalytic degradation by promoting the deaggregation of the photocatalyst, by inducing the desorption of organic substrates and degradation intermediates from the photocatalyst surface and, mainly, by favouring the scission of the photocatalytically and sonolytically produced H(2)O(2), with a consequent increase of oxidising species in the aqueous phase.