Sonochemical degradation of azo dyes in aqueous solution: a new heterogeneous kinetics model taking into account the local concentration of OH radicals and azo dyes

The sonochemical decolorization and decomposition of azo dyes, such as C. I. Reactive Red 22 and methyl orange, were performed from the viewpoints of wastewater treatment and to determine the reaction kinetics. A low concentration of the azo dye solution was irradiated with a 200 kHz and 1.25 W/cm2 ultrasound in a homogeneous aqueous solution. The azo dye solutions were readily decolorized by the irradiation. The sonochemical decolorization was also depressed by the addition of the t-butyl alcohol radical scavenger. These results indicated that azo dye molecules were mainly decomposed by OH radicals formed from the water sonolysis. In this paper, we propose a new kinetics model taking into account the heterogeneous reaction kinetics similar to a Langmuir-Hinshelwood mechanism or an Eley-Rideal mechanism. The proposed kinetics model is based on the local reaction site at the interface region of the cavitation bubbles, where azo dye molecules are quickly decomposed because an extremely high concentration of OH radicals exists in this region. To confirm the proposed kinetics model, the effects of the initial concentration of azo dyes, irradiated atmosphere and pH on the decomposition rates were investigated. The obtained results were in good agreement with the proposed kinetics model.     

Sonochemical degradation of ethylbenzene in aqueous solution: a product study

The degradation of ethylbenzene in aqueous solution by 520 kHz ultrasound was investigated. The products formed were analysed using solid phase microextraction (SPME), a sampling technique that allows convenient GC-MS and GC-FID analysis in the micromolar range. A broad range of monosubstituted monocyclic and dicyclic aromatic hydrocarbons was found as well as some oxygenated products. The results clearly indicate that pyrolysis is an important pathway of ethylbenzene degradation. The side chain is dehydrogenated, forming styrene, or cleaved. The radicals formed upon cleavage are subsequently added to the double bond of the styrene side chain or recombined. This mechanism explains the formation of most of the products. Formation and breakdown of the reaction products follow first-order kinetics in spite of the fact that the selectivity of the reactions depends on the initial ethylbenzene concentration considerably. Changes in the temperature and the pressure of cavitation are expected to cause this dependence.     

Degradation of Rhodamine B in aqueous solution by laser cavitation

A novel method of laser cavitation (LC) was proposed for degrading organic dye wastewater. Rhodamine B (RhB) aqueous solution was employed as the simulated organic dye wastewater, and a LC system was designed to conduct the experiments of degrading RhB. The effects of laser energy, initial concentration and cavitation time on the degradation were investigated. Moreover, the degradation kinetics, degradation mechanism and energy efficiency were analyzed. The experimental results indicate that RhB aqueous solution can be degraded effectively by LC and the degradation follows the pseudo-first-order kinetics. The extent of degradation increases by 27.6% with the rise of laser energy (50–100 mJ) while it decreases by 7.8% with increasing the initial concentration from (20–40 mg/L), but RhB can not be degraded when exceeding 100 mg/L. The degradation extent of RhB at 100 mJ and 20 mg/L for 3 h is 81.11%, and the RhB solution is almost completely degraded at 150 mJ (98.4%). The degradation velocity of RhB rises firstly and then decreases as the cavitation time increases. The degradation of RhB by LC can be attributed to the N-de-ethylation and chromophore cleavage caused by oxidation of hydroxyl (OH) radical and thermal decomposition. LC has a higher energy efficiency compared with other methods and is more energy efficient at lower laser energy.     

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.     

Degradation of reactive brilliant red in aqueous solution by ultrasonic cavitation

Degradation of reactive brilliant red K-BP in aqueous solution by means of ultrasonic cavitation was investigated for a variety of operating conditions. It is found that the degradation of reactive brilliant red K-BP in aqueous solution follows pseudo-first-order reaction kinetics and the degradation rate is dependent on the initial concentration of reactive brilliant red K-BP, the temperature and acidity of the aqueous medium. The effects of Fe2+, Fenton reagent and NaCl addition on the sonochemical degradation of reactive brilliant red K-BP were also investigated. The results obtained here indicate that the degradation rate of brilliant red K-BP in aqueous solution was substantially accelerated by Fe2+, NaCl or Fenton reagent addition.     

Sonochemical synthesis of silver nanorods by reduction of sliver nitrate in aqueous solution

The sonochemical synthesis of sliver nanorods has been achieved by ultrasonic irradiation of the aqueous solution of sliver nitrate, methenamine (HMTA) and poly (vinyl pyrrolidone) (PVP) for 60 min. The silver nanorods obtained have lengths of 4–7 μm and mean diameters of about 100 nm. The structures of the samples were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), selected area electron diffraction (SAED) and X-ray powder diffraction (XRD), and the chemical composition of the sample was examined by energy-dispersive X-ray spectrum (EDS). The effects of the irradiation time, the concentration of PVP and the reaction temperature on the morphology of silver nanorods were discussed, and the mechanism of the silver nanorods formation was tentatively inferred.     

Effect of carbon tetrachloride on the luminol sonochemiluminescence reaction kinetics during multibubble cavitation

The sonochemiluminescence (SCL) of luminol reaction was studied in alkaline medium using a dissolution of luminol, sodium carbonate, hydrogen peroxide and carbon tetrachloride. The presence of carbon tetrachloride enhances the SCL reaction up to allow the study of the reaction in real time using a cell phone video camera. This experimental setup allows the study of the cavitation dynamics in real time and through all the reactor, including homogeneous and heterogeneous cavitation zones. Finally, it was tested the effect of ethanol, the ionic strength and pH on the SCL.     

Oxidation of alkylarenes to the corresponding acids using aqueous potassium permanganate by hydrodynamic cavitation

Oxidation of toluene using aqueous potassium permanganate was studied under heterogeneous condition in the presence of hydrodynamic cavitation and compared with the results of the reaction under acoustic cavitation. Various parameters, such as quantity of potassium permanganate, toluene to aqueous phase ratio, reaction time and cavitation parameters such as orifice plate, and pump discharge pressure were optimized. The reaction was found to be considerably accelerated at ambient temperature in the presence of cavitation. On comparison, it was found that when 1 kJ of energy was passed to the reaction mixture in the case of acoustic cavitation, the product obtained was 4.63 x 10(-6) mol, whereas when 1 kJ of energy was passed to the reaction mixture in the case of hydrodynamic cavitation the product obtained was 2.70 x 10(-5) mol. Hence, about six times more product would be obtained in the case of hydrodynamic cavitation than in the case of acoustic cavitation at same energy dissipation. It has been observed that further optimization is possible.     

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.     

Determination of the effective charge on muonium during its reaction in aqueous solution

The rate of reaction of muonium atoms with solutes of either charge is unaffected by the addition of a high concentration of an inert salt, therefore the effective charge on the muonium at the point of reaction is essentially zero.     

Sonochemical degradation of textile dyes in aqueous solution using sulphate radicals activated by immobilized cobalt ions

Decolorisation of dye solutions by cobalt activated persulphate and ultrasonication has been investigated. Rhodamine B, Methylene Blue dye (basic dyes) and Acid orange II, Acid scarlet red 3R (acid dyes) were used as model compounds in this study. Immobilized cobalt ions, activated the persulphate to form highly reactive sulphate radicals. The degradation studies were conducted with only persulphate (PS), cobalt activated persulphate (PS + Co), persulphate + ultrasonication (PS + US) and cobalt activated persulphate + ultrasonication (PS + US + Co). The decolorisation efficiency were in the order of PS < PS + Co < PS + US < PS + US + Co for all the four dye solutions. The effect of pH, dosage of persulphate as well as catalyst and contact time was investigated. Under the optimum condition, the decolorisation obeyed first-order kinetics. Nearly 90–97% of decolorisation was achieved with COD and TOC removal of about 65–73% and 53–62%, respectively, were achieved within an hour.     

Chemical effect of swirling jet-induced cavitation: degradation of rhodamine B in aqueous solution

The chemical effect of swirling jet-induced cavitation was investigated with the decomposing reaction of rhodamine B in aqueous solution. It was found that rhodamine B in aqueous solution can be degraded with swirling jet-induced cavitation and the degradation can be described by a pseudo-first-order kinetics. The effects of operating conditions such as pressure, temperature, initial concentration of rhodamine B, pH of water on the degradation rate of rhodamine B were discussed. It was found that the degradation rate of rhodamine B increased with increasing pressure and decreased with increasing initial concentration. It was also found that the degradation of rhodamine B was strongly dependent of temperature and pH of aqueous solution. The oxidation efficiency of swirling jet-induced cavitation for rhodamine B degradation was discussed and compared with ultrasonic cavitation. The result indicated that the swirling jet-induced cavitation is more energy efficient as compared to sonochemical cavitation.     

Limitations of the Weissler reaction as a model reaction for measuring the efficiency of hydrodynamic cavitation

The Weissler reaction in which iodide is oxidised to a tri-iodide complex (I(3)(-)) has been widely used for measurement of the intensity of ultrasonic and hydrodynamic cavitation. It was used in this work to compare ultrasonic cavitation at 24kHz with hydrodynamic cavitation using two different devices, one a venturi and the other a sudden expansion, operated up to 8.7bar. Hydrodynamic cavitation had a maximum efficiency of about 5x10(-11) moles of I(3)(-) per joule of energy compared with the maximum of almost 8x10(-11)molJ(-1) for ultrasonic cavitation. Hydrodynamic cavitation was found to be most effective at 10 degrees C compared with 20 degrees C and 30 degrees C and at higher upstream pressures. However, it was found that in hydrodynamic conditions, even without cavitation, I(3)(-) was consumed at a rapid rate leading to an equilibrium concentration. It was concluded that the Weissler reaction was not a good model reaction for the assessment of the effectiveness of hydrodynamic cavitation.     

Sonochemical degradation of various monocyclic aromatic compounds: relation between hydrophobicities of organic compounds and the decomposition rates

Various aromatic compounds, i.e., nitrobenzene, aniline, phenol, benzoic acid, salicylic acid, 2-chlorophenol, 4-chlorophenol, styrene, chlorobenzene, toluene, ethylbenzene and n-propylbenzene were decomposed under identical ultrasonic irradiation conditions. The relationships between the initial rates of degradation of these aromatic compounds and their physicochemical parameters were systematically investigated. It was revealed that some correlations between the degradation rates and parameters of volatility, Henry's law constant and vapor pressure, were observed only in the limited high range of parameters. It was suggested that the Henry's law constant and vapor pressure had influenced on the rate of degradation for some of the tested aromatic compounds. In contrast, better correlations between the initial rates of degradation and hydrophobic parameters, water solubility and LogP (water-octanol partition coefficient), were observed over the wide range of chosen parameters. These results meant that the hydrophobicity of the compounds significantly affected their accumulation at the gas-liquid interface of the bubbles and it was the most important factor for the sonochemical degradation of aromatic compounds. In particular, for the sonolysis of water-insoluble organic compounds, LogP was found to be the representative parameter for understanding the hydrophobic properties of water-insoluble compounds.     

Kinetics and mechanism of the reaction of an arenediazonium ion with hydrophilic aminocarboxylic acids in aqueous buffered solution

The reaction of 4‐nitrobenzenediazonium ion, 4NBD, with the aminocarboxylic acids (AA) glycine and serine was studied under acidic conditions by using Linear Sweep Voltammetry (LSV), which allows simultaneous monitoring of 4NBD loss and product formation. Voltammograms of the reaction mixture are complex, showing up to five reduction peaks. The reduction peaks at E_p = ?0.5 and ?1.0 V, not detected in the absence of AA, are associated to products formed in the course of the reaction. The variation of their peak current, i_p, with time shows a complex behavior; that of i_p (E_p = ?1.0 V) follows a biphasic profile with i_p increasing with time up to a maximum after which a decrease is detected, suggestive of formation and subsequent decomposition of a transient intermediate, meanwhile i_p (E_p = ?0.5 V) increases with time after an induction period. The peaks at E_p = ?0.1 and ?0.8 V are associated to the reduction of the diazonium group of 4NBD and, in the presence of AA ([AA] >>> [4NBD]), their peak currents decrease exponentially with time following clean first‐order kinetics for more than 3t_1/2. The variation of k_obs with [AA] at a given pH is linear with an intercept equal to zero and that of log(k_obs) with pH at constant [AA] is also linear. Kinetic evidence is consistent with a reaction mechanism involving an irreversible, rate‐limiting bimolecular step which leads to the formation of an unstable triazene, which further decomposes yielding 4‐nitroaniline among other reaction products. Copyright © 2007 John Wiley & Sons, Ltd.     

Evolution of the heterogeneous structure of model sodium silicate glasses during binodal decomposition: X-ray small-angle scattering investigation

The kinetics of binodal decomposition in model glasses of the Na₂O-SiO₂ system from the initial stage to the stage of the Ostwald ripening inclusive has been investigated in situ. It has been found that the spatial-temporal evolution of the heterogeneous structure during binodal decomposition has a multistage character. The characteristic size of the phase regions at each stage of the binodal decomposition varies with time according to the power law L ～ t ^β. The values of exponents β for different stages of the binodal decomposition are as follows: 1/20 (fluctuation stage), 1/2 (growth stage), 0 (transient stage), and 1/3 (Ostwald ripening stage). The particle size distributions for all stages of the binodal decomposition have been presented.     

Kinetic investigation on carbamate formation from the reaction of carbon dioxide with amino acids in homogeneous aqueous solution

The kinetics of carbamate formation from the reaction of carbon dioxide with α‐amino acids in D₂O was first investigated by means of nuclear magnetic resonance spectroscopy. Potassium carbonate was used as the CO₂ source. For each amino acid, the maximum carbamate yield, the apparent rate constant for the carbamate formation k_app, and the rate constants for the formation k₁ and the breakdown k_?1 of the carbamate were estimated. Plots of log k₁ or log k_?1 versus pK_a of amino acids indicated that the formation rate k₁ increased with the basicity (pK_a) of amino acid, while the decomposition rate k_?1 decreased. A Br?nsted β value of 0.39 was obtained from the former plot, being in good agreement with the previously reported ones (0.26–0.43). The observed negative pK_a dependence of log k_?1 (Br?nsted α = 0.34) is reasonable, because the carbamate decomposition is acid‐catalyzed and the steady‐state concentration of H⁺ should be higher for weaker basic amines. The charge (σ) and the lone‐pair energy (E_N) at the nitrogen atom of the amino group were calculated. Although log k₁ correlated with σ and E_N, log k_?1 was unrelated with both of these parameters. Considering that the carbamate formation (k₁) is not only base‐catalyzed but should also be promoted by the nucleophilicity of the amino nitrogen, its correlation with σ and E_N in addition to pK_a is rational. The irrelevance of log k_?1 to σ and E_N is not surprising, because σ and E_N are not a direct measure of [H⁺] of the solution. Copyright © 2011 John Wiley & Sons, Ltd.     

Comparative study on the process behavior and reaction kinetics in sonocatalytic degradation of organic dyes by powder and nanotubes TiO2

Sonocatalytic degradation of various organic dyes (Congo Red, Reactive Blue 4, Methyl Orange, Rhodamine B and Methylene Blue) catalyzed by powder and nanotubes TiO₂ was studied. Both catalysts were characterized using transmission electron microscope (TEM), surface analyzer, Raman spectroscope and thermal gravimetric analyzer (TGA). Sonocatalytic activity of powder and nanotubes TiO₂ was elucidated based on the degradation of various organic dyes. The former catalyst was favorable for treatment of anionic dyes, while the latter was more beneficial for cationic dyes. Sonocatalytic activity of TiO₂ nanotubes could be up to four times as compared to TiO₂ powder under an ultrasonic power of 100 W and a frequency of 42 kHz. This was associated with the higher surface area and the electrostatic attraction between dye molecules and TiO₂ nanotubes. Fourier transform-infrared spectrometer (FT-IR) was used to identify changes that occurred on the functional group in Rhodamine B molecules and TiO₂ nanotubes after the reaction. Sonocatalytic degradation of Rhodamine B by TiO₂ nanotubes apparently followed the Langmuir-Hinshelwood adsorption kinetic model with surface reaction rate of 1.75 mg/L min. TiO₂ nanotubes were proven for their high potential to be applied in sonocatalytic degradation of organic dyes.     

The kinetics and mechanism of catalytic reactions by molecular beam relaxation spectroscopy: HCOOH decomposition

The reactive scattering of formic acid from Ni(110) was studied over the temperature range of 175–920 K with MBRS in the millisecond time region by employing a modulation frequency of 36.8 Hz. The steady-state carbon and oxygen composition of the surface varied over the range of temperatures studied. For beam fluxes of 10¹³ molecules/cm² sec the onset of decomposition on the steady-state surface occurred at 300 K. By 400 K decomposition was essentially complete, and the products CO₂, CO, H₂ and H₂O were detected. All reaction events were prceded by a common step, and the products were then produced by a series/parallel mechanism. The rate constants measured for H₂ and H₂O formation indicated stringent limitations on the efficiency of second-order collisions on the surface for producing gaseous products. This study illustrates the use of MBRS for surface reaction mechanistic studies in the millisecond time scale.     

CP dynamics of heterogeneous organic material: characterization of molecular domains in coals

Simplified equations are used in common approaches to describe cross polarization (CP) dynamics of solids. The CP behavior may be modulated by several nuclei interactions and physicochemical sample properties. At high magnetic fields and spinning speeds, these modulations can obscure the results. To elucidate their impact on the CP behavior of natural organic materials variable contact time (VCT) experiments were acquired with a high temporal resolution for two coal samples. The measurements were distinctly influenced by interfering fluctuations. Conventional approaches showed insufficient flexibility in terms of degrees of freedom to calculate the CP dynamics. The use of an original fundamental equation as model resulted in sufficient flexibility for such heterogeneous systems. The best results were obtained assuming a two component system. On these conditions a differentiation between amorphous and crystalline domains within the coal samples was enabled.