Effects of gas saturation and sparging on sonochemical oxidation activity in open and closed systems,Part I: H2O2 generation

Cavitational/sonochemical activity can be significantly enhanced or reduced depending on the gases dissolved in the liquid. Although many researchers have suggested the order of importance of dissolved gas conditions that affect the degree of sonoluminescence (SL), sonochemiluminescence (SCL), and compound degradation, the most suitable gas condition for sonochemical oxidation reactions is currently unknown. In this study (Part I), the effects of gas saturation and sparging on the generation of H₂O₂ were investigated in a 28-kHz sonoreactor system. Four gas modes, saturation/closed, saturation/open, sparging/closed, and sparging/open, were applied to Ar, O₂, N₂, and binary gas mixtures. The change in dissolved oxygen (DO) concentration during ultrasonic irradiation was measured and was used as an indicator of whether the gaseous exchange between liquid and air altered the gas content of the liquid. Considerable difference in the DO concentration was observed for the gas saturation/open mode, ranging from –11.5 mg/L (O₂ 100 %) to +4.3 mg/L (N₂ 100 %), while no significant difference was observed in the other gas modes. The change in the gas content significantly reduced the linearity for H₂O₂ generation, which followed pseudo-zero-order kinetics, and either positively or negatively affected H₂O₂ generation. Ar:O₂ (75:25) and Ar:O₂ (50:50) resulted in the highest and second-highest H₂O₂ generation for both gas saturation and sparging, respectively. In addition, gas sparging resulted in much higher H₂O₂ generation for all gas conditions compared to gas saturation; this was because of the significant change in the cavitational active zone and concentrated ultrasonic energy, which formed a bulb-shaped active zone, especially for the Ar/O₂ mixtures adjacent to the transducer at the bottom. The sparging flow rate and position also significantly affected H₂O₂ generation; the highest H₂O₂ generation was obtained when the sparger was placed at the bottom adjacent to the transducer, with a flow rate of 3 L/min.In Part II, the generation of nitrogen oxides, including nitrite (NO₂^–) and nitrate (NO₃^–), was investigated using the same ultrasonic system with three gas modes: saturation/open, saturation/closed, and sparging/closed.     

Effect of dissolved gases on sonochemical oxidation in a 20 kHz probe system: Continuous monitoring of dissolved oxygen concentration and sonochemical oxidation activity

Dissolved gases have a substantial influence on acoustic cavitation and sonochemical oxidation reactions. Little research on the changes in dissolved gases and the resultant changes in sonochemical oxidation has been reported, and most studies have focused only on the initial dissolved gas conditions. In this study, the dissolved oxygen (DO) concentration was measured continuously during ultrasonic irradiation using an optical sensor in different gas modes (saturation/open, saturation/closed, and sparging/closed modes). Simultaneously, the resulting changes in sonochemical oxidation were quantified using KI dosimetry. In the saturation/open mode using five gas conditions of Ar and O₂, the DO concentration decreased rapidly when O₂ was present because of active gas exchange with the atmosphere, and the DO concentration increased when 100% Ar was used. As a result, the order of the zero-order reaction constant for the first 10 min (k_0-10) decreased in the order Ar:O₂ (75:25) > 100% Ar ≈ Ar:O₂ (50:50) > Ar:O₂ (25:75) > 100% O₂, whereas that during the last 10 min (k_20-30) when the DO concentration was relatively stable, decreased in the order 100% Ar > Ar:O₂ (75:25) > Ar:O₂ (50:50) ≈ Ar:O₂ (20:75) > 100% O₂. In the saturation/closed mode, the DO concentration decreased to approximately 70–80% of the initial level because of ultrasonic degassing, and there was no influence of gases other than Ar and O₂. Consequently, k_0-10 and k_20-30 decreased in the order Ar:O₂ (75:25) > Ar:O₂ (50:50) > Ar:O₂ (25:75) > 100% Ar > 100% O₂. In the sparging/closed mode, the DO concentration was maintained at approximately 90% of the initial level because of the more active gas adsorption induced by gas sparging, and the values of k_0-10 and k_20-30 were almost the same as those in the saturation/closed mode. In the saturation/open and sparging/closed modes, the Ar:O₂ (75:25) condition was most favorable for enhancing sonochemical oxidation. However, a comparison of k_0-10 and k_20-30 indicated that there would be an optimal dissolved gas condition that was different from the initial gas condition. In addition, the mass-transfer and ultrasonic-degassing coefficients were calculated using changes in the DO concentration in the three modes.     

Effects of gas saturation and sparging on sonochemical oxidation activity in open and closed systems,part II: NO2−/NO3− generation and a brief critical review

The sonochemical generation of NO₂⁻ and NO₃⁻ is considered to be one of the reasons for the low sonochemical oxidation activity in the presence of N₂ in the liquid phase. In this study, the generation characteristics of NO₂⁻ and NO₃⁻ were investigated using the same 28 kHz sonoreactor and the 12 gas conditions used in Part I of this study. Three gas modes, saturation/closed, saturation/open, and sparging/closed, were applied. N₂:Ar (25:75), N₂:Ar (50:50), and O₂:N₂ (25:75) in the saturation/closed mode generated the three highest values of NO₂⁻ and NO₃⁻. Ar and O₂ were vital for generating relatively large concentrations of NO₂⁻ and NO₃⁻. The absorption of N₂ from the air resulted in high generation of NO₂⁻ and NO₃⁻ for Ar 100 % and Ar/O₂ mixtures under the saturation/open mode. In addition, gas sparging enhanced the generation of NO₂⁻ and NO₃⁻ for N₂:Ar (25:75), O₂:N₂ (25:75), and N₂ significantly because of the change in the sonochemically active zone and the increase in the mixing intensity in the liquid phase, as discussed in Part I. The ratio of NO₃⁻ to NO₂⁻ was calculated using their final concentrations, and a ratio higher than 1 was obtained for the condition of Ar 100 %, Ar/O₂ mixtures, and O₂ 100 %, wherein a relatively high oxidation activity was detected. From a summary of the results and findings of previous studies, it was revealed that the observations of NO₂⁻ + NO₃⁻ could be more appropriate for investigating the NO₂⁻ and NO₃⁻ generation characteristics. In addition, H₂O₂/NO₂⁻/NO₃⁻ related activity rather than H₂O₂ activity was suggested to quantify the OH radical activity more appropriately in the presence of N₂.     

Superior selectivity of high-frequency ultrasound toward chorine containing-pharmaceuticals elimination in urine: A comparative study with other oxidation processes through the elucidation of the degradation pathways

This work considered the sonochemical degradation (using a bath-type reactor, at 375 kHz and 106.3 W L^-1, 250 mL of sample) of three representative halogenated pharmaceuticals (cloxacillin, diclofenac, and losartan) in urine matrices. The action route of the process was initially established. Then, the selectivity of the sonochemical system, to degrade the target pharmaceuticals in simulated fresh urine was compared with electrochemical oxidation (using a BDD anode, at 1.88 mA cm⁻²), and UVC/H₂O₂ (at 60 W of light and 500 mol L^-1 of H₂O₂). Also, the treatment of cloxacillin in an actual urine sample by ultrasound and UVC/H₂O₂ was evaluated. More than 90% of the target compounds concentration, in the simulated matrix, was removed after 60 min of sonication. However, the sono-treatment of cloxacillin in the real sample was less efficient than in the synthetic urine. The ultrasonic process achieved 43% of degradation after 90 min of treatment in the actual matrix. In the sonochemical system, hydroxyl radicals in the interfacial zone were the main degrading agents. Meanwhile, in the electrochemical process, electrogenerated HOCl was responsible for the elimination of pharmaceuticals. In turn, in UVC/H₂O₂ both direct photolysis and hydroxyl radicals degraded the target pollutants. Interestingly, the degradation by ultrasound of the pharmaceuticals in synthetic fresh urine was very close to the observed in distilled water. Indeed, the sonodegradation had a higher selectivity than the other two processes. Despite the sono-treatment of cloxacillin was affected by the actual matrix components, this contrasts with the UVC/H₂O₂, which was completely inhibited in the real urine. The sonochemical process led to 100% of antimicrobial activity (AA) elimination after 75 min sonication in the synthetic urine, and ∼ 20% of AA was diminished after 90 min of treatment in the real matrix. The AA decreasing was linked to the transformations of the penicillin nucleus on cloxacillin, the region most prone to electrophilic attacks by radicals according to a density theory functional analysis. Finally, predictions of biological activity confirmed that the sono-treatment decreased the activity associated with cloxacillin, diclofenac, and losartan, highlighting the positive environmental impact of degradation of chlorinated pharmaceuticals in urine.     

Kinetic analysis of the pathways to naphthalene formation from phenyl + 1,3-Butadiyne reaction

Having a better understanding of polycyclic aromatic hydrocarbon (PAH) formation under flame conditions contributes to optimizing the fuel reforming process, where soot poisons the downstream catalyst. In this work, the phenyl + 1,3-Butadiyne reaction is systematically investigated to examine its contribution to naphthalene formation. The reaction potential energy surfaces were calculated using DFT/M06–2X/cc-pvtz and G4 methods. The temperature- and pressure-dependent reaction rate constants were calculated using RRKM theory with solving master equation. The results revealed that 2-naphthyl could be directly formed by phenyl + 1,3-Butadiyne reaction. With H assistance, naphthalene could be formed by the pathway of phenyl + 1,3-Butadiyne → C₆H₅CHCCCH (+H) → C₆H₅CHCHCCH (+H) →naphthalene +H. The proposed pathway is kinetically favorable, and featured by relatively low energy barrier. The importance of the proposed pathway reaction was confirmed in a premixed and a diffusion C₂H₄/O₂/Ar flame simulations, where the enhancement of naphthalene by the investigated reactions is notable. The mole fraction of A₂ is promoted by a factor of 10% in premix C₂H₄/O₂/Ar flame and 30% in C₂H₄/O₂/Ar counterflow flame, bringing the prediction results closer to the experimental results. The relative contribution of different reaction route to A₂ formation is evaluated for HACA, cyclopentadienyl radical-cyclopentadienyl radical, phenyl-vinylacetylene[1], benzyl radical-propargyl radical, indene-CH₂ and phenyl-1,3-Butadiyne routes in premixed and diffusion C₂H₄/O₂/Ar flames. This work suggests that the PAH growth by 1,3-Butadiyne addition reaction is an effective pathway for A₂ formation, which should be considered in future PAH mechanism.     

Sonocatalytic degradation of EDTA in the presence of Ti and Ti@TiO2 nanoparticles

The sonocatalytic degradation of EDTA (C₀ = 5 10⁻³ M) in aqueous solutions was studied under 345 kHz (P_ac = 0.25 W mL⁻¹) ultrasound at 22–51 °C, Ar/20%O₂, Ar or air, and in the presence of metallic titanium (Ti⁰) or core-shell Ti@TiO₂ nanoparticles (NPs). Ti@TiO₂ NPs have been obtained using simultaneous action of hydrothermal conditions (100–214 °C, autogenic pressure P = 1.0–19.0 bar) and 20 kHz ultrasound, called sonohydrothermal (SHT) treatment, on Ti⁰ NPs in pure water. Ti⁰ is composed of quasi-spherical particles (30–150 nm) of metallic titanium coated with a metastable titanium suboxide Ti₃O. SHT treatment at 150–214 °C leads to the oxidation of Ti₃O and partial oxidation of Ti⁰ and formation of nanocrystalline shell (10–20 nm) composed of TiO₂ anatase. It was found that Ti⁰ NPs do not exhibit catalytic activity in the absence of ultrasound. Moreover, Ti⁰ NPs remain inactive under ultrasound in the absence of oxygen. However, significant acceleration of EDTA degradation was achieved during sonication in the presence of Ti⁰ NPs and Ar/20%O₂ gas mixture. Coating of Ti⁰ with TiO₂ nanocrystalline shell reduces sonocatalytic activity. Pristine TiO₂ anatase nanoparticles do not show a sonocatalytic activity in studied system. Suggested mechanism of EDTA sonocatalytic degradation involves two reaction pathways: (i) sonochemical oxidation of EDTA by OH/HO₂ radicals in solution and (ii) EDTA oxidation at the surface of Ti⁰ NPs in the presence of oxygen activated by cavitation event. Ultrasonic activation most probably occurs due to the local heating of Ti⁰/O₂ species at cavitation bubble/solution interface.     

Sonochemical synthesis of supersaturated Ga–Al liquid-alloy fine particles and Al3+-doped γ-Ga2O3 nanoparticles by direct oxidation at near room temperature

In this study, we investigated the fabrication of supersaturated gallium (Ga)–aluminum (Al) liquid alloy and Al³⁺-doped γ-Ga₂O₃ nanoparticles (NPs) at near room temperature (60 °C) using sonochemical and sonophysical effects. Supersaturated Ga–Al liquid alloy microparticles (D_av = 1.72 µm) were formed and stabilized at 60 °C by the thermal nonequilibrium field provided by sonochemical hot spots. Compared with liquid Ga, supersaturated Ga–Al liquid alloy was rapidly oxidized to a uniform oxide without Al₂O₃ or Al deposition. Thus, ultrafine Al³⁺-doped γ-Ga₂O₃ NPs were obtained after only 1 h of ultrasonic irradiation at 60 °C. The oxidation of liquid Ga was remarkably accelerated by alloying with metallic Al and ultrasonic irradiation, and the time was shortened. The average diameter and surface area of the γ-Ga₂O₃-based NPs were 59 nm and 181 m²/g, respectively. Compared with γ-Ga₂O₃, the optical bandgap of the Al³⁺-doped γ-Ga₂O₃ NPs was broadened, and the thermal stability improved, indicating Al³⁺-doping into the γ-Ga₂O₃ lattice. However, the lattice constant of γ-Ga₂O₃ was almost unchanged with or without Al³⁺-doping. Al³⁺ was introduced into the defect sites of Ga³⁺, which were massively induced in the defective spinel structure during ultrasonic processing. Therefore, sonochemical processing, which provides nonequilibrium reaction fields, is suitable for the synthesis of supersaturated and metastable materials in metals and ceramics fields.     

Sono-enhanced degradation of dye pollutants with the use of H2O2 activated by Fe3O4 magnetic nanoparticles as peroxidase mimetic

Sono-enhanced degradation of a dye pollutant Rhodamine B (RhB) was investigated by using H₂O₂ as a green oxidant and Fe₃O₄ magnetic nanoparticles (MNPs) as a peroxidase mimetic. It was found that Fe₃O₄ MNPs could catalyze the break of H₂O₂ to remove RhB in a wide pH range from 3.0 to 9.0 and its peroxidase-like activity was significantly enhanced by the ultrasound irradiation. At pH 5.0 and temperature 55 °C, the ultrasound-assisted H₂O₂–Fe₃O₄ catalysis removed about 95% of RhB (0.02 mmol L⁻¹) in 15 min with a apparent rate constant of 0.15 min⁻¹ for the degradation of RhB, being 6.5 and 37.6 folds of that in the simple catalytic H₂O₂–Fe₃O₄ system, and the simple ultrasonic US-H₂O₂ systems, respectively. The beneficial synergistic behavior between Fe₃O₄ catalysis and ultrasonic was demonstrated to be dependent on Fe₃O₄ dosage, H₂O₂ concentration, pH value and temperature. As a tentative explanation, the observed significant synergistic effects was attributed to the positive interaction between cavitation effect accelerating the catalytic breakdown of H₂O₂ over Fe₃O₄ nanoparticles, and the function of Fe₃O₄ MNPs providing more nucleation sites for the cavitation inception.     

Degradation of imidacloprid containing wastewaters using ultrasound based treatment strategies

The present work deals with application of sonochemical reactors for the treatment of imidacloprid containing wastewaters either individually or in combination with other advanced oxidation processes. Experiments have been performed using two different configurations of sonochemical reactors viz. ultrasonic horn (20 kHz frequency and rated power of 240 W) and ultrasonic bath equipped with radially vibrating horn (25 kHz frequency and 1 kW rated power). The work also investigates the effect of addition of process intensifying agents such as H₂O₂ and CuO, which can enhance the production of free radicals in the system. The combination studies with advanced oxidation process involve the advanced Fenton process and combination of ultrasound with UV based oxidation. The extent of degradation obtained using combination of US and H₂O₂ at optimum loading of H₂O₂ was found to be 92.7% whereas 96.5% degradation of imidacloprid was achieved using the combination of US and advanced Fenton process. The process involving the combination of US, UV and H₂O₂ was found to be the best treatment approach where complete degradation of imidacloprid was obtained with 79% TOC removal. It has been established that the use of cavitation in combination with different oxidation processes can be effectively used for the treatment of imidacloprid containing wastewater.     

Sonochemical dissolution of nanoscale ThO2 and partial conversion into a thorium peroxo sulfate

The influence of the sample morphology and experimental conditions towards the sonochemical dissolution of nanoscale ThO₂ samples in sulfuric acid media is described. Significant sonochemical dissolution rates and yields are observed at 20 kHz under Ar/O₂ atmosphere in dilute 0.5 M H₂SO₄ at room temperature, contrasting with the generally-reported high refractory behavior for ThO₂. The dissolution of ThO₂ combines the physical effects driven by acoustic cavitation phenomenon, the complexing affinity of Th(IV) in sulfuric medium and the sonochemical generation of H₂O₂. These sonochemical conditions further allow the observation of the partial conversion of ThO₂ into a scarce Th(IV) peroxo sulfate with 1D morphology resulting from one or both following processes: dissolution/reprecipitation or formation of an intermediate Th(IV) surface complex.     

Sonochemical water splitting in the presence of powdered metal oxides

Kinetics of hydrogen formation was explored as a new chemical dosimeter allowing probing the sonochemical activity of argon-saturated water in the presence of micro- and nano-sized metal oxide particles exhibiting catalytic properties (ThO₂, ZrO₂, and TiO₂). It was shown that the conventional sonochemical dosimeter based on H₂O₂ formation is hardly applicable in such systems due to catalytic degradation of H₂O₂ at oxide surface. The study of H₂ generation revealed that at low-frequency ultrasound (20 kHz) the sonochemical water splitting is greatly improved for all studied metal oxides. The highest efficiency is observed for relatively large micrometric particles of ThO₂ which is assigned to ultrasonically-driven particle fragmentation accompanied by mechanochemical water molecule splitting. The nanosized metal oxides do not exhibit particle size reduction under ultrasonic treatment but nevertheless yield higher quantities of H₂. The enhancement of sonochemical water splitting in this case is most probably resulting from better bubble nucleation in heterogeneous systems. At high-frequency ultrasound (362 kHz), the effect of metal oxide particles results in a combination of nucleation and ultrasound attenuation. In contrast to 20 kHz, micrometric particles slowdown the sonolysis of water at 362 kHz due to stronger attenuation of ultrasonic waves while smaller particles show a relatively weak and various directional effects.     

Sono-assisted preparation of highly-efficient peroxidase-like Fe3O4 magnetic nanoparticles for catalytic removal of organic pollutants with H2O2

Fe₃O₄ magnetic nanoparticles (Fe₃O₄ MNPs) with much improved peroxidase-like activity were successfully prepared through an advanced reverse co-precipitation method under the assistance of ultrasound irradiation. The characterizations with XRD, BET and SEM indicated that the ultrasound irradiation in the preparation induced the production of Fe₃O₄ MNPs possessing smaller particle sizes (16.5 nm), greater BET surface area (82.5 m² g^?1) and much higher dispersibility in water. The particle sizes, BET surface area, chemical composition and then catalytic property of the Fe₃O₄ MNPs could be tailored by adjusting the initial concentration of ammonia water and the molar ratio of Fe²⁺/Fe³⁺ during the preparation process. The H₂O₂-activating ability of Fe₃O₄ MNPs was evaluated by using Rhodamine B (RhB) as a model compound of organic pollutants to be degraded. At pH 5.4 and temperature 40 °C, the sonochemically synthesized Fe₃O₄ MNPs were observed to be able to activate H₂O₂ and remove ca. 90% of RhB (0.02 mmol L^?1) in 60 min with a apparent rate constant of 0.034 min^?1 for the RhB degradation, being 12.6 folds of that (0.0027 min^?1) over the Fe₃O₄ MNPs prepared via a conventional reverse co-precipitation method. The mechanisms of the peroxidase-like catalysis with Fe₃O₄ MNPs were discussed to develop more efficient novel catalysts.     

Acoustic frequency and optimum sonochemical production at single and multi-bubble scales: A modeling answer to the scaling dilemma

The present work consists of an innovative approach aiming to address the scalability dilemma of the sonochemical activity dependency of acoustic frequency. The study originates from the discordance of observations between the theoretical investigations of the sonochemical activity of the single acoustic cavitation bubble in function of the acoustic frequency, in one hand, and the experimental findings regarding the optimal frequency condition, mainly in terms of pollutant degradation, in the other hand. A single bubble and an up-scaled model of the sonochemical activity are suggested and simulations were conducted based on both of them over the frequencies 20, 200, 300, 360, 443, 500, 600 and 800 kHz under an oxygen atmosphere. The results reveal that the sonochemical production at single bubble scale is monotonously decreasing with the increase of frequency, while all the products demonstrate an absolute optimum of sonochemical production at 200 kHz, except HO^• that attains its maximum molar yield under 300 kHz. Besides, the production of the predominant species, namely HO₂^•, HO^• and O₃, manifests a clear rebound at 500 kHz. All the present results were compared to and confirmed by experimental findings, while the scalability of the concentrations of sonochemically produced species was discussed using a parameter we introduced as “the mass focusing factor”.     

Effects of Na2SO4 or NaCl on sonochemical degradation of phenolic compounds in an aqueous solution under Ar: Positive and negative effects induced by the presence of salts

Sonochemical degradation of 4-chlorophenol, phenol, catechol and resorcinol was studied under Ar at 200 kHz in the absence and presence of Na₂SO₄ or NaCl. The rates of sonochemical degradation in the absence of salts decreased in the order 4-chlorophenol > phenol > catechol > resorcinol and this order was in good agreement with the order of log P (partition coefficient) value of each phenolic compound. The effects of salts on the rates of sonochemical degradation consisted of no effect or slight negative or positive effects. We discussed these unclear results based on two viewpoints: one was based on the changes in pseudo hydrophobicity and/or diffusion behavior of phenolic compounds and the other was based on the changes in solubility of Ar gas. The measured log P value of each phenolic compound slightly increased with increasing salt concentration. In addition, the dynamic surface tension for 4-chlorophenol aqueous solution in the absence and presence of Na₂SO₄ or NaCl suggested that phenolic compounds more easily accumulated at the interface region of bubbles at higher salt concentration. These results indicated that the rates of sonochemical degradation should be enhanced by the addition of salts. On the other hand, the calculated Ar gas solubility was confirmed to decrease with increasing salt concentration. The yield of H₂O₂ formed in the presence of Na₂SO₄ or NaCl decreased with increasing salt concentration. These results suggested that sonochemical efficiency decreased with decreasing gas amount in aqueous solution: a negative effect of salts was observed. Because negative and positive effects were induced simultaneously, we concluded that the effects of salts on the rates of sonochemical degradation of phenolic compounds became unclear. The products formed from sonochemical degradation of 4-chlorophenol were also characterized by HPLC analysis. The formation of phenol and 4-chloro-1,3-dihydroxy benzene was confirmed and these concentrations were affected by the presence of salts.     

Synthesis of MnO2 nanoparticles from sonochemical reduction of MnO4− in water under different pH conditions

MnO₂ was synthesized by sonochemical reduction of MnO₄⁻ in water under Ar atmosphere at 20 °C, where the effects of solution pH on the reduction of MnO₄⁻ were investigated. The obtained XRD results showed that poor crystallinity δ-MnO₂ was formed at pH 2.2, 6.0 and 9.3. When solution pH was increased from 2.2 to 9.3, the morphologies of δ-MnO₂ changed from aggregated sheet-like or needle-like structures to spherical nanoparticles and finally to cubic or polyhedron nanoparticles. After further irradiation, MnO₂ was readily reduced to Mn²⁺. It was confirmed that H₂O₂ and H atoms formed in the sonolysis of water acted as reductants for both reduction for MnO₄⁻ to MnO₂ and MnO₂ to Mn²⁺. The optimum irradiation time for the effective synthesis of MnO₂ was 13 min at pH 2.2, 9 min at pH 6.0, 8 min at pH 9.3, respectively.     

Cavitation intensifying bags improve ultrasonic advanced oxidation with Pd/Al2O3 catalyst

Advanced oxidation processes can potentially eliminate organic contaminants from industrial waste streams as well as persistent pharmaceutical components in drinking water. We explore for the first time the utilization of Cavitation Intensifying Bags (CIB) in combination with Pd/Al₂O₃ catalyst as possible advanced oxidation technology for wastewater streams, oxidizing terephthalic acid (TA) to 2-hydroxyterephthalic acid (HTA). The detailed characterization of this novel reaction system reveals that, during sonication, the presence of surface pits of the CIB improves the reproducibility and thus the control of the sonication process, when compared to oxidation in non-pitted bags. Detailed reaction kinetics shows that in the CIB reactor the reaction order to TA is zero, which is attributed to the large excess of TA in the system. The rate of HTA formation increased ten-fold from ~0.01 μM*min⁻¹ during sonication in the CIB, to ~0.10 μM*min⁻¹ for CIB in the presence of the Pd/Al₂O₃ catalyst. This enhancement was ascribed to a combination of improved mass transport, the creation of thermal gradients, and Pd/Al₂O₃ catalyst near the cavitating bubbles. Further analysis of the kinetics of HTA formation on Pd/Al₂O₃ indicated that initially the reaction underwent through an induction period of 20 min, where the HTA concentration was ~0.3 μM. After this, the reaction rate increased reaching HTA concentrations ~6 μM after 40 min. This behavior resembled that observed during oxidation of hydrocarbons on metal catalysts, where the slow rate formation of hydroperoxides on the metal surface is followed by rapid product formation upon reaching a critical concentration. Finally, a global analysis using the Intensification Factor (IF) reveals that CIB in combination with the Pd/Al₂O₃ catalyst is a desirable option for the oxidation of TA when considering increased oxidation rates and costs.     

Sonochemical activation-assisted biosynthesis of Au/Fe3O4 nanoparticles and sonocatalytic degradation of methyl orange

In this research, a sonochemical activation-assisted biosynthesis of Au/Fe₃O₄ nanoparticles is proposed. The proposed synthesis methodology incorporates the use of Piper auritum (an endemic plant) as reducing agent and in a complementary way, an ultrasonication process to promote the synthesis of the plasmonic/magnetic nanoparticles (Au/Fe₃O₄). The synergic effect of the green and sonochemical synthesis favors the well-dispersion of precursor salts and the subsequent growth of the Au/Fe₃O₄ nanoparticles.The hybrid green/sonochemical process generates an economical, ecological and simplified alternative to synthesizing Au/Fe₃O₄ nanoparticles whit enhanced catalytic activity, pronounced magnetic properties. The morphological, chemical and structural characterization was carried out by high- resolution Scanning electron microscopy (HR-SEM), Energy Dispersive X-Ray Spectroscopy (EDS) and X-Ray diffraction (XRD), respectively. Ultraviolet–visible (UV–vis) and X-ray photoelectron (XPS) spectroscopy confirm the Au/Fe₃O₄ nanoparticles obtention. The magnetic properties were evaluated by a vibrating sample magnetometer (VSM). Superparamagnetic behavior, of the Au/ Fe₃O₄ nanoparticles was observed (M_s = 51 emu/g and H_c = 30 Oe at 300 K). Finally, the catalytic activity was evaluated by sonocatalytic degradation of methyl orange (MO). In this stage, it was possible to achieve a removal percentage of 91.2% at 15 min of the sonocatalytic process (160 W/42 kHz). The initial concentration of the MO was 20 mg L⁻¹, and the Fe3O4-Au dosage was 0.075 gL⁻¹. The MO degradation process was described mathematically by four kinetic adsorption models: Pseudo-first order model, Pseudo-second order model, Elovich and intraparticle diffusion model.     

Multibubble sonoluminescence as a tool to study the mechanism of formic acid sonolysis

Sonoluminescence spectra collected from 0.1 to 3.0 M aqueous solutions of formic acid sparged with argon show the OH(A²Σ⁺−X²Π_i) and C₂(d³Π_g → a³Π_u) emission bands and a broad continuum typical for multibubble sonoluminescence. The overall intensity of sonoluminescence and the sonochemical yield of HCOOH degradation vary in opposite directions: the sonoluminescence is quenched while the sonochemical yield increases with HCOOH concentration. By contrast, the concentration of formic acid has a relatively small effect on the intensity of C₂ Swan band. It is concluded that C₂ emission originates from CO produced by HCOOH degradation rather than from direct sonochemical degradation of HCOOH. The intensity of C₂ band is much stronger at high ultrasonic frequency compared to 20 kHz ultrasound which is in line with higher yields of CO at high frequency. Another product of HCOOH sonolysis, carbon dioxide, strongly quenches sonoluminescence, most probably via collisional non-radiative mechanism.     

Intensification of sonochemical decolorization of anthraquinonic dye Acid Blue 25 using carbon tetrachloride

In this work, the influence of CCl₄ on the sonochemical decolorization of anthraquinonic dye Acid Blue 25 (AB25) in aqueous medium was investigated using high frequency ultrasound (1700 kHz). This frequency, reputed ineffective, was tested in order to introduce the ultrasound waves with high frequency in the field of degradation or removal of dyes from wastewater, due to its limited use in this field, and to increase the application of high frequency ultrasound wave in the field of environmental protection. The effects of various parameters such as the concentration of CCl₄, frequency (22.5 and 1700 kHz), solution pH, temperature and tert-butyl alcohol adding on the decolorization rate of AB25 was studied. The obtained results clearly demonstrated the significant intensification of AB25 decolorization in the presence of CCl₄. The enhancement effect of CCl₄ increased by decreasing temperature and by increasing the CCl₄ concentration. The pH has a significant influence on the bleaching of dye both in the absence and presence of CCl₄. The three investigated dosimeter methods (KI oxidation, Fricke reaction and H₂O₂ production) well corroborate the improvement of the sonochemical effects in the presence of CCl₄. The best sonochemical decolorization rate of AB25 in aqueous solution both in the absence and presence of CCl₄ is observed to occur at 1700 kHz compared to 22.5 kHz. The sonochemical oxidation of CCl₄ generates oxidizing species in the liquid phase that are highly beneficial for oxidation of hydrophilic and non-volatile pollutant, such as dyes, because they are less susceptible to free radical attack due to lower stability of the generated free radicals.     

How do dissolved gases affect the sonochemical process of hydrogen production? An overview of thermodynamic and mechanistic effects – On the “hot spot theory”

Although most of researchers agree on the elementary reactions behind the sonolytic formation of molecular hydrogen (H₂) from water, namely the radical attack of H₂O and H₂O₂ and the free radicals recombination, several recent papers ignore the intervention of the dissolved gas molecules in the kinetic pathways of free radicals, and hence may wrongly assess the effect of dissolved gases on the sonochemical production of hydrogen. One may fairly ask to which extent is it acceptable to ignore the role of the dissolved gas and its eventual decomposition inside the acoustic cavitation bubble? The present opinion paper discusses numerically the ways in which the nature of dissolved gas, i.e., N₂, O₂, Ar and air, may influence the kinetics of sonochemical hydrogen formation. The model evaluates the extent of direct physical effects, i.e., dynamics of bubble oscillation and collapse events if any, against indirect chemical effects, i.e., the chemical reactions of free radicals formation and consequently hydrogen emergence, it demonstrates the improvement in the sonochemical hydrogen production under argon and sheds light on several misinterpretations reported in earlier works, due to wrong assumptions mainly related to initial conditions. The paper also highlights the role of dissolved gases in the nature of created cavitation and hence the eventual bubble population phenomena that may prevent the achievement of the sonochemical activity. This is particularly demonstrated experimentally using a 20 kHz Sinaptec transducer and a Photron SA 5 high speed camera, in the case of CO₂-saturated water where degassing bubbles are formed instead of transient cavitation.