Sonochemical degradation of naphthol blue black in water: Effect of operating parameters

Modified sono-Fenton process for oxidative degradation of chloramphenicol

Oxidative degradation of chloramphenicol (CAP) using a hybrid approach (US/HA+/n-Fe2O3/SPC) involving sodium percarbonate (SPC; "solid H2O2" carrier), Fe2O3 nanoparticles (n-Fe2O3; H2O2 decomposition catalyst), hydroxylamine in its protonated form (HA+; Fe (III) to Fe (II) reducer), and ultrasonic cavitation (to increase the generation of hydroxyl radicals) has been studied for the first time. The average size of n-Fe2O3 synthesized by the sonochemical method, as calculated according to the Debye-Scherrer equation, was ~ 18 nm. The maximum degradation degree of CAP (83.1%) and first-order oxidative degradation rate constant of CAP as 1.253 × 10-3 s-1 were achieved using the modified sono-Fenton process under the optimized conditions as the initial concentration of CAP - 50 mg/L, the molar ratio of CAP:HA+:n-Fe2O3:SPC of 1:100:100:100, pH as 3, the temperature as 318 K, the specific ultrasonic power as 53.3 W/L, and the treatment duration of 7200 s. In general, the efficiency and intensity of CAP degradation increased with a decrease in the pH value, an increase in the molar ratio of CAP:HA+:n-Fe2O3:SPC, a decrease in the initial concentration of CAP, an increase in temperature, and showed a minor change with the specific power of US. The synergistic coefficient for the combination of the US and the heterogeneous Fenton process was 17.9. The active participation of hydroxyl radicals in the oxidative degradation of CAP using the modified sono-Fenton process was confirmed by scavenging experiments performed using tert-butyl alcohol. The proposed process can be a promising direction in the remediation of pharmaceutical effluents with significant potential for commercial exploitation.

Effective treatment of 2,4,6-trinitrotoluene from aqueous media using a sono-photo-Fenton-like process with a zero-valent iron nanoparticle (nZVI) catalyst

In this study, we examine the effectiveness of using a combination of a sono-photo-Fenton-like procedure and nano zero-valent iron catalyst (nZVI) to treat 2,4,6-trinitrotoluene (TNT) in an aquatic environment. Zero-valent iron particles were generated by a chemical reduction technique. nZVI nanoparticles were analyzed using scanning electron microscopy (SEM) and X-ray diffraction (XRD) to characterize the nanocatalyst. The resulting nZVI nanoparticles were used as an addition in a sono-photo-Fenton method to remediate an aqueous solution contaminated with TNT. Furthermore, influences of operational factors such as temperature, catalyst dosage, wavelength, ultraviolet power, ultrasonic frequency and power, pH level, H2O2/nZVI ratio, initial TNT concentration, and reaction duration on the treatment of TNT were investigated. Under the conditions of an ideal pH of 3, temperature range of 40-45 °C, concentration of 50 mg per L TNT, dose of 2 mM of nZVI, and ratio of H2O2/Fe0 of 20, a treatment efficiency of 95.2% was achieved after a duration of 30 minutes. The sono-photo-Fenton process combined with nZVI showed a higher TNT removal efficiency compared to the Fenton, sono-Fenton, and photo-Fenton processes under the same conditions. Moreover, it promises a potential solution to treat TNT at the pilot scale while allowing reuse of the nZVI catalyst and the limitation of sludge.

Application of the general rate law model to the sonolytic degradation of nonvolatile organic pollutants in aqueous media

The pseudo-first and pseudo-second order equations have been the most commonly used models to characterize the sonolytic disappearance kinetics of nonvolatile pollutants in aqueous media. In this work, the general rate law model, i.e., pseudo-nth order kinetics equation, was applied for the first time to the sono-decomposition of different nonvolatile organic pollutants, naphthol blue black (NBB), furosemide (FSM), 4-isopropylphenol (4-IPP), and rhodamine B (RhB), in water. It was shown that the general rate law for a chemical reaction would apply to the kinetics of sonochemical decomposition. It is not feasible to set the order of ultrasonic pollutant degradation kinetics to pseudo-first or pseudo-second, as is typically used in numerous studies. The sonochemical oxidation reaction has a fractional order, the order is often non-integer, which frequently indicates a complex sonolytic decomposition reaction mechanism. The degradation mechanism of NBB and RhB does not change with the initial substrate concentration. They are ultrasonically degraded by hydroxyl radicals both in the bulk liquid solution and at the liquid/bubble interfacial layer. The destruction mechanism of FSM and 4-IPP changes as the initial contaminant concentration changes. At low initial substrate concentrations, these pollutants are oxidized mainly by reaction with hydroxyl radicals in the bulk liquid solution and at the interfacial shell of the cavitation bubbles. At high initial substrate concentrations, FSM and 4-IPP are degraded by thermal destruction in the liquid/bubble interfacial layer and by •OH radicals both in the bulk liquid solution and at the liquid/bubble interfacial layer. Additionally, the pseudo-nth order model predicts very well the sonolytic degradation at various sonication frequencies and intensities. The general rate law expression should be used to assess the real kinetics order of the sonolytic destruction process without any predetermined assumptions or constraints.

Sonochemical reactor characterization in the presence of cylindrical and conical reflectors

Ultrasonic systems must be able to produce an acoustic field with the highest possible energy concentration in sonochemical reactors to accomplish maximum efficacy in the sonolytic degradation of water contaminants. In the present study, the impact of cylindrical and conical stainless-steel reflectors placed on the liquid surface on the sonochemical oxidation activity of ultrasonication reactors was investigated. The amount of effective acoustic power transferred to the ultrasonicated medium without and with reflectors was measured by calorimetric characterization of the sono-reactors at diverse ultrasonication frequencies in the interval of 300-800 kHz and different electrical powers in the range of 40-120 W. Iodide dosimetry without and with reflectors at diverse ultrasonication conditions (300-800 kHz and 40-120 W) and various aqueous solution volumes in the range of 300-500 mL was used to assess the sonochemical oxidation activity, i.e., the generation of oxidative species (mainly hydroxyl radicals). Sonochemiluminescence (SCL) imaging was used to study the active acoustic cavitation bubbles distribution in the sono-reactors without and with reflectors. Significant impacts of the position and shape of the reflectors on the active acoustic cavitation bubble distribution and the sonochemical oxidation activity were observed due to remarkable modifications of the ultrasonic field by directing and focusing of the ultrasonic waves. A significant augmentation in the triiodide formation rate was obtained in the presence of the conical reflector, especially at 630 kHz and 120 W (60.5% improvement), while iodide oxidation was quenched in the presence of the cylindrical reflector at all ultrasonication frequencies and powers. The SCL images show a noteworthy modification in the ultrasonic field and the acoustic cavitation bubble population when reflectors were used. The sonochemical oxidation activity was improved by the conical reflector when placed in the Fresnel zone (near field region).

General analytical solution expressions for analyzing Langmuir-type kinetics of sonochemical degradation of nonvolatile organic contaminants in water

Detailed kinetic studies of the ultrasonic decomposition of contaminants in water are scarce. Most of the work has used a pseudo-first order kinetics law, which is unrealistic. The model based on a Langmuir-type mechanism has been shown to fit the sonolytic decomposition data well, especially by using the non-linear technique. To avoid unrealistic assumptions, general analytical solutions to a time-dependent non-linear Langmuir-type equation may be the appropriate method. In this work, the sonolytic oxidation of organic contaminants, i.e., naphthol blue black and furosemide, in water was analyzed using two general analytical solution expressions of the Langmuir-type kinetics model, which describe the pollutant concentration in water. The validity of the two general analytical solution expressions was tested under a diversity of operating conditions, such as initial substrate concentration and varying ultrasonication frequency and intensity. As the initial substrate concentration increased, the sonolytic oxidation kinetics decreased, while the initial ultrasonic decomposition rate increased and then plateaued. Consequently, a heterogeneous kinetics equation based on a Langmuir-type mechanism can be used to simulate the sono-decomposition process. The decomposition yield increased with increasing sonication intensity and decreasing frequency. The two analytical solution expressions seem to be in excellent agreement with the experimental results of the sonochemical decomposition of the nonvolatile organic contaminants tested for the different operating conditions examined. These expressions provide a valuable tool for the analysis and simulation of advanced sonochemical oxidation processes under various experimental conditions.

Sonochemical advanced oxidation process for the degradation of furosemide in water: Effects of sonication's conditions and scavengers

The intensive consumption of pharmaceuticals and drugs in the last decades has led to their increased concentrations in wastewaters from industrial sources. The present paper deals, for the first time, with the sonochemical degradation and mineralization of furosemide (FSM) in water. FSM is a potent loop diuretic used to treat fluid build-up due to heart failure, liver scarring, or kidney disease. The influence of several operating parameters such as acoustic intensity, ultrasonic frequency, initial FSM concentration, solution's pH, nature of the dissolved gas (Ar, air and N2) and radical scavengers (2-propanol and tert-butanol) on the oxidation of FSM was assessed. The obtained results showed that the degradation rate of the drug increased significantly with the increase of the acoustic intensity in the range of 0.83 to 4.3 W cm-2 and decreased with the augmentation of the frequency in the range of 585-1140 kHz. It was also found that the initial rate of the sonolytic degradation of FSM increased with the increase of its initial concentration (2, 5, 10, 15 and 20 mg/L). The most significant degradation was achieved in acidic conditions at pH 2, while in terms of saturating gas, the rate of FSM degradation decreased in the order of Ar > air > N2. The FSM degradation experiments with radical scavengers showed that the diuretic molecule degraded mainly at the interfacial region of the bubble by hydroxyl radical attack. Additionally, in terms of acoustic conditions, the sono-degradation of 30.24 µmol L-1 of FSM solution demonstrate an optimal performance at 585 kHz and 4.3 W/cm2, the results indicated that even if the ultrasonic action eliminated the total concentration of FSM within 60 min, a low degree of mineralization was obtained due to the by-products formed during the sono-oxidation process. The ultrasonic process transforms FSM into biodegradable and environmentally friendly organic by-products that could be treated in a subsequent biological treatment. Besides, the efficiency of the sonolytic degradation of FSM in real environmental matrices such as natural mineral water and seawater was demonstrated. Consequently, the sonochemical advanced oxidation process represent a very interesting technique for the treatment of water contaminated with FSM.

Hydrodynamic cavitation coupled with zero-valent iron produces radical sulfate radicals by sulfite activation to degrade direct red 83

In the present research, hydrodynamic cavitation (HC) and zero-valent iron (ZVI) were used to generate sulfate radicals through sulfite activation as a new source of sulfate for the efficient degradation of Direct Red 83 (DR83). A systematic analysis was carried out to examine the effects of operational parameters, including the pH of the solution, the doses of ZVI and sulfite salts, and the composition of the mixed media. Based on the results, the degradation efficiency of HC/ZVI/sulfite is highly dependent upon the pH of the solution and the dosage of both ZVI and sulfite. Degradation efficiency decreased significantly with increasing solution pH due to a lower corrosion rate for ZVI at high pH. The corrosion rate of ZVI can be accelerated by releasing Fe2+ ions in an acid medium, reducing the concentration of radicals generated even though ZVI is solid/originally non-soluble in water. The degradation efficiency of the HC/ZVI/sulfite process (95.54 % + 2.87%) was found to be significantly higher under optimal conditions than either of the individual processes (<6% for ZVI and sulfite and 68.21±3.41% for HC). Based on the first-order kinetic model, the HC/ZVI/sulfite process has the highest degradation constant of 0.035±0.002 min-1. The contribution of radicals to the degradation of DR83 by the HC/ZVI/sulfite process was 78.92%, while the contribution of SO4•- and •OH radicals was 51.57% and 48.43%, respectively. In the presence of HCO3- and CO32- ions, DR83 degradation is retarded, whereas SO42- and Cl- ions promote degradation. To summarise, the HC/ZVI/sulfite treatment can be viewed as an innovative and promising method of treating recalcitrant textile wastewater.

Synthesis of magnetic graphene-like carbon nitride-cobalt ferrite (g-C3N4/CoFe2O4) nanocomposite for sonocatalytic remediation of toxic organic dyes

A novel magnetic g-C₃N₄/CoFe₂O₄ nanocomposite was successfully synthesized by a simple hydrothermal method and applied as a new graphene-like carbon nitride-based sonocatalyst for sonodegradation of pollutant dyes. The as-prepared samples were characterized by using X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, field-emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), vibrating sample magnetometry (VSM), X-ray photoelectron spectroscopy (XPS), UV-visible diffuse reflectance spectroscopy (DRS), BET surface area measurements and photoluminescence (PL) spectroscopy. The results indicate that the nanocomposite sample is composed of spherical CoFe₂O₄ nanoparticles adhered to g-C₃N₄ naosheets. The g-C₃N₄/CoFe₂O₄ nanocomposites were used as a new magnetically separable sonocatalyst in H₂O₂-assisted sonodegradation of methylene blue (MB), rhodamine B (RhB) and methyl orange (MO) dyes in aqueous media. The results showed complete degradation (ca. 100%) of dyes within short times (30-35 min). The sonocatalytic activity of graphitic carbon nitride (g-C₃N₄) was greatly enhanced with CoFe₂O₄ modification. Trapping experiments indicated that the g-C₃N₄/CoFe₂O₄ nanocomposites serves as a generator of hydroxyl radical (˙OH) via activation of H₂O₂ for degradation of dyes under ultrasound irradiation. Furthermore, the magnetic sonocatalyst can be separated from solution by an external magnet and reused several times without observable loss of activity. The possible mechanism of sonocatalytic activity was also proposed according to experimental results.

Contact-Piezoelectric Bi-Catalysis of an Electrospun ZnO@PVDF Composite Membrane for Dye Decomposition

The treatment of organic pollutants in wastewater is becoming a great challenge for social development. Herein, a novel contact-piezoelectric bi-catalysis of a ZnO@ PVDF composite membrane was prepared by electrospinning technology. The obtained ZnO@PVDF composite membranes is superior to the pure PVDF membrane in decomposing methyl orange (MO) under ultrasonication at room temperature, which is mainly attributed to the synergy effect of the contact-electro-catalysis of dielectric PVDF, as well as the piezoelectric catalysis of tetrapodal ZnO and the β-phase of PVDF. The heterostructure of the piezoelectric-ZnO@dielectric-PVDF composite is beneficial in reducing the electron/hole pair recombination. As compared to the pure PVDF membrane, the catalytic degradation efficiency of the ZnO@PVDF composite membrane was improved by 444.23% under ultrasonication. Moreover, the reusability and stability of the composite membrane are comparable to those of the traditional powdered catalyst. This work offers a promising strategy for improving the pollutant degradation by combining contact-electro-catalysis with piezoelectric catalysis.

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.

Use of mesoporous BiOI microspheres for sonocatalytic degradation of tetracycline hydrochloride

Self-assembled mesoporous BiOI microsphere with nanosheets were prepared by a solvothermal method and used as sonocatalysts. The sonocatalytic performances of the BiOI microspheres were evaluated in terms of the degradation rate of tetracycline hydrochloride (TCH) as a model pollutant. We designed three comparative experiments to explore the degradation of TCH solution under natural light, namely with sonication, with BiOI alone, and with ultrasound (US)/BiOI synergy. The degradation rate of TCH with US/BiOI synergy was 227 times higher than that achieved with sonication and 83 times higher than that achieved with BiOI alone. The maximum TCH degradation rate was 93.0%. The synergistic effect was therefore significant, and the synergy factor was estimated to be 61. Many factors such as the ultrasonic duty cycle, applied power, catalyst concentration, and initial TCH dye concentration may affect the ultrasonic degradation efficiency. Box-Behnken design of the response surface method were used to optimize the parameters and to study the effects of the catalyst concentration, ultrasonic duty cycle, and applied power. Analysis of variance confirmed that the quadratic response surface model for predicting the sonocatalytic efficiency was good for the corresponding parameters (R² = 0.9936 and adjusted R² = 0.9854). The optimization results were verified by replicate experiments. The high TCH degradation rate may be related to the generation of reactive oxygen species at the end of cavitation bubble collapse, which can improve the chemical yields.

Galvano-Fenton Engineering Solution with Spontaneous Catalyst's Generation from Waste: Experimental Efficiency, Parametric Analysis and Modeling Interpretation Applied to a Clean Technology for Dyes Degradation in Water

In this paper, the degradation of the diazo dye naphthol blue black (NBB) using the Galvano-Fenton process is studied experimentally and numerically. The simulations are carried out based on the anodic, cathodic, and 34 elementary reactions evolving in the electrolyte, in addition to the oxidative attack of NBB by HO• at a constant rate of 3.35×107 mol-1·m3·s-1 during the initiation stage of the chain reactions. The selection of the operating conditions including the pH of the electrolyte, the stirring speed, and the electrodes disposition is performed by assessing the kinetics of NBB degradation; these parameters are set to 3, 350 rpm and a parallel disposition with a 3 cm inter-electrode distance, respectively. The kinetics of Fe(III) in the electrolyte were monitored using the principles of Fricke dosimetry and simulated numerically. The model showed more than a 96% correlation with the experimental results in both the blank test and the presence of the dye. The effects of H2O2 and NBB concentrations on the degradation of the dye were examined jointly with the evolution of the simulated H2O2, Fe2+, and HO• concentrations in the electrolyte. The model demonstrated a good correlation with the experimental results in terms of the initial degradation rates, with correlation coefficients exceeding 98%.

Sonochemical degradation of neonicotinoid pesticides in natural surface waters. Influence of operational and environmental conditions

Neonicotinoids sonochemical oxidation at high-frequency ultrasound (MHz range) has been carried out in ultrapure and natural surface-water matrices (river, reservoir and wastewater treatment plant effluent). To evaluate the influence of the operating variables, that is initial pollutant concentration, ultrasound frequency, ultrasound power, and pulse-stop time a Box-Behnken experimental design was planned. Optimal results were obtained using a frequency of 578 kHz, a power of 40 W L^-1, with a pollutant concentration of 1 μM (for each pesticide), and using a pulse-stop time of 100 ms. The experimental data adjustment using the Langmuir-Hinshelwood heterogeneous kinetic model showed that neonicotinoids oxidation was carried out in the bubble-liquid interface by the attack of hydroxyl radicals. Experiments performed in the presence of radical scavengers, that is, methanol, ethanol and tert-butyl alcohol corroborated this reaction mechanism. The influence of some environmental conditions such as pH, presence of soluble inorganic species (Cl^-, SO₄^2-, NO₃^-, HPO₄^2-, HCO₃^-) and soluble organic species (humic acids content) were established. Finally, the aqueous matrix's influence was investigated for three natural surface water cases, and the results were rationalized according to the main water physicochemical characteristics.

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".

Impact of seawater salinity on the sonochemical removal of emerging organic pollutants

The results presented in this study illustrate the multiple roles of seawater salinity toward the sonochemical degradation, at variable frequencies (300-1700 kHz), of several hazardous substances, i.e. propylparaben (PPR) endocrine disruptor and several synthetic dyes: naphthol blue black (NBB), malachite green (MG), basic red 29 (BR29), acid orange 7 (AO7), Rhodamine B (RhB) and basic fuchsin (BF). Sonochemical treatment degraded all pollutants in seawater at faster rates than in deionized water. The seawater-salts through increasing the ionic strength of the solution act as a potential pusher of hydrophilic pollutants toward the reactive interfacial area of cavitation bubbles. Additionally, the salts reduce the bubble coalescence, which yields higher number of active bubbles in the irradiating media. Analysing the degradation rate of PPR and NBB with two heterogeneous models based on Langmuir kinetics mechanism indicated that the bubble interfacial area was the preferred reaction zone for the ultrasonic degradation of PPR and NBB in seawater.

The Effects of Ultrasound on the Electro-Oxidation of Sulfate Solutions at Low pH

The electro‐oxidation of sulfate solutions is a well‐established route for the generation of powerful oxidants such as persulfate. Despite this, the effects of simultaneous ultrasound irradiation during this process has attracted little attention. Herein, we investigate the effects of a low‐intensity ultrasonic field on the generation of solution‐phase oxidants during the electro‐oxidation of sulfate solutions. Our results show that at high current densities and high sulfate concentrations, ultrasound has little effect on the Faradaic and absolute yields of solution‐phase oxidants. However, at lower current densities and sulfate concentrations, the amount of these oxidants in solution appears to decrease under ultrasonic irradiation. A mechanism explaining these results is proposed (and validated), whereby anodically‐generated sulfate and hydroxyl radicals are more effectively transported into bulk solution (where they are quenched) during sonication, whereas in the absence of an ultrasonic field these radicals combine with one another to form more persistent species (such as persulfate) that can be detected by iodometry.

Sonochemical synthesis of WS2 nanosheets and its application in sonocatalytic removal of organic dyes from water solution

In this research, tungsten disulfide (WS₂) nanosheets as sonocatalyst were synthesized through a sonochemical route. Characterization of as-synthesized sonocatalyst was carried out by X-ray diffraction (XRD), scanning electron microscopy (SEM), electron dispersive X-ray spectroscopy (EDX), Dot-mapping, high resolution transmission electron microscopy (HRTEM), Brunauer-Emmett-Teller (BET), Fourier transform infrared spectroscopy (FTIR), and ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS) analysis. WS₂ nanosheets were evaluated for their sonocatalytic performance in order to remove basic violet 10 (BV10) under ultrasonic irradiation. The removal efficiency was maximized (94.01%) via the use of 1 g L^-1 catalyst and 10 mg L^-1 BV10 at pH = 4.5 and an ultrasonic power of 400 W within a reaction time of 150 min. In addition to BV10, the sonocatalytic elimination for a number of organic dyes viz. direct blue 71, acid blue 92, methylene blue, basic orange 2 and basic red 46 was examined to demonstrate the performance of WS₂ nanosheets under the ultrasonic irradiation. The experimentation of trapping was conducted using edetate disodium (EDTA-2Na), tert-butyl alcohol (t-BuOH), and benzoquinone (BQ). According to the results, all radicals participated in the sonocatalytic activity. OH played a more prominent role than h⁺ and O₂^-∙ in the process of BV10 separation. Following five repetitive runs, the nanocomposites revealed a reusability of circa 18% drop in the elimination efficiency. The main removal intermediates were recognized by GC-MS technique.

Investigation of applicability of Electro-Fenton method for the mineralization of naphthol blue black in water

In this study, mineralization and color removal performance of electro-Fenton method were examined in water containing naphthol blue black (NBB), a diazo dye. NBB was totally converted to intermediate species in a 15-min electrolysis at 60 mA, but complete de-colorization took 180 min. A very high oxidation rate constant ((3.35 ± 0.21) x 10¹⁰ M^-1s^-1) was obtained for NBB, showing its high reactivity towards hydroxyl radicals. A very high total organic carbon (TOC) removal value (45.23 mg L^-1) was obtained in the first 60 min of the electro-Fenton treatment of an aqueous solution of NBB (0.25 mM) at 300 mA, indicating the mineralization efficiency of the electro-Fenton method. Mineralization current efficiency values obtained at 300 mA gradually decreased from 24.18% to 4.47% with the electrolysis time, indicating the presence of highly parasitic reactions. Gas chromatography-mass spectrometry analyses revealed that the cleavage of azo bonds of NBB led to formation of different aromatic and aliphatic oxidation intermediates. Ion chromatography analysis showed that ammonium, nitrate and sulfate were the mineralization end-products. The concentration of sulfate ion reached to its quantitative value at the 4th h of electrolysis. On the other hand, the total concentration of ammonium and nitrate ions reached to only 61% of the stoichiometric amount of initial nitrogen after a 7 h electrolysis. Finally, it can be said that the electro-Fenton method is a suitable and efficient method for the removal of NBB and its intermediates from water.

A review on heterogeneous sonocatalyst for treatment of organic pollutants in aqueous phase based on catalytic mechanism

Heterogeneous sonocatalysis, as an emerging advanced oxidation process (AOP), has shown immense potential in water treatment and been widely demonstrated to remove persistent organic compounds in the past decade. The present article aims to provide a comprehensive review on the development of a heterogeneous catalyst for enhancing the ultrasonic degradation rate of organic pollutants from a viewpoint of sonocatalytic mechanism. The rational design and fundamentals for preparing sonocatalysts are presented in the context of facilitating the heterogeneous nucleation and photo-thermal-catalytic effects as well as considering the mechanical stability and separation capacity of the heterogeneous catalyst. In addition, some new trends, ongoing challenges and possible methods to overcome these challenges are also highlighted and proposed.

Insights into numerical simulation of controlled ultrasonic waveforms driving single cavitation bubble activity

A computational study treating cavitation phenomenon within a single bubble undergoing various controlled ultrasonic waveforms is presented in this paper. Numerical simulations using sinusoidal, square, triangular and sawtooth waves crossing an aqueous media, saturated with oxygen, are conducted upon various operational conditions of frequency and amplitude. Bubble radius, temperature and pressure were estimated over time for 64 combined cases. The obtained results show that at relatively low acoustic pressure, i.e. 1.5 and 2 atm, the square wave is proved to generate the highest temperature and pressure inside the bubble, while triangular and sawtooth ones remain the less interesting waveforms for sonochemical application within the same operational conditions. At higher amplitudes above 2.5 atm, this trend is changed, especially at low frequencies, i.e. 200 and 300 kHz, where square wave showed some limitations in attaining the optimal values of the strong collapse within one acoustic cycle.

Ultrasonic waveform upshot on mass variation within single cavitation bubble: Investigation of physical and chemical transformations

The mechanical disturbance created by an ultrasonic wave travelling through a liquid medium induces the formation of cavitation that oscillates due to rarefaction and compression of the wave. The duration and the magnitude of the pressure applied by the ultrasonic wave at each instant would generate a specific impact on the variation of the bubble radius, the temperature, the pressure and the mass inside it. In this paper, a numerical study is conducted to simulate four waveforms (sinusoidal, square, triangular and sawtooth) travelling an aqueous media saturated with oxygen with an amplitude of 1.5 and 2 atm and a frequency of 200, 300 and 500 kHz. The purpose is to highlight the mass evolution within acoustic cavitation bubble during one cycle due to physical transformations and sonochemical effect. The obtained results demonstrated that square signal enhances temperature and pressure growth inside the bubble, as well as mass transfer by evaporation and condensation. This leads to an improvement of produced quantities of free radicals but also to a selectivity of O as a major product in the detriment of HO₂ and OH. These trends are less and less observed when passing to sinusoidal, triangular and square signal.

Probing the radical chemistry and the reaction zone during the sono-degradation of endocrine disruptor 2-phenoxyethanol in water

Sonochemical degradation at 600 kHz of 2-phenoxyethanol (PhE), an endocrine disrupting compound, was performed in the presence of several organic additives, namely: 2-propanol, Triton X-100 and sucrose, of different volatilities to obtain detailed information on the reaction zone and the oxidation pathway of this priority emerging water contaminate. It was found that sonication at 600 kHz and 120 W completely remove PhE (10 mg L^-1) from aerated solutions within 100 min of irradiation. Very little removal of PhE (∼7%) and low accumulation of H₂O₂ took place in the presence of adequate amount of 2-propanol, indicating that reaction with OH radical outside the bubble is the major degradation pathway of PhE. Addition of the hydrophobic surfactant Triton X-100, as an OH-probe for the interfacial region, at 10 and 100 mM reduced the degradation event by 57% and 72% and resulted in more than 50% decrease in the yield of H₂O₂, confirming that PhE degradation occurs mainly at the bubble/solution interface with hydroxyl radical attack. Addition of the hydrophilic substrate glucose at high doses decreased slightly (∼7%) the degradation of PhE and the formation rate of H₂O₂, meaning that the bulk of the solution participate marginally in the degradation of the pollutant. Finally, analyzing the degradation rates at various initial PhE concentrations (2-400 mg L^-1) with a heterogeneous Langmuir type mechanism underlined the predominance of interfacial radical reactions during the oxidation of PhE, particularly at high initial pollutant concentrations.

Sono-photo-Fenton oxidation of bisphenol-A over a LaFeO3 perovskite catalyst

In this study, oxidation of bisphenol-A (IUPAC name - 2,2-(4,4-dihydroxyphenyl, BPA), which is an endocrine disrupting phenolic compound used in the polycarbonate plastic and epoxy resin industry, was investigated using sono-photo-Fenton process under visible light irradiation in the presence of an iron containing perovskite catalyst, LaFeO₃. The catalyst prepared by sol-gel method, calcined at 500°C showed a catalytic activity in BPA oxidation using sono-photo-Fenton process with a degradation degree and a chemical oxygen demand (COD) reduction of 21.8% and 11.2%, respectively. Degradation of BPA was studied by using individual and combined advanced oxidation techniques including sonication, heterogeneous Fenton reaction and photo oxidation over this catalyst to understand the effect of each process on degradation of BPA. It was seen, the role of sonication was very important in hybrid sono-photo-Fenton process due to the pyrolysis and sonoluminescence effects caused by ultrasonic irradiation. The prepared LaFeO₃ perovskite catalyst was a good sonocatalyst rather than a photocatalyst. Sonication was not only the effective process to degrade BPA but also it was the cost effective process in terms of energy consumption. The studies show that the energy consumption is lower in the sono-Fenton process than those in the photo-Fenton and sono-photo- Fenton processes.

Kinetic, product, and computational studies of the ultrasonic induced degradation of 4-methylcyclohexanemethanol (MCHM)

A massive spill of 4-methylcyclohexanemethanol (MCHM), a semi-volatile organic compound, contaminated the Elk river and forced the recent closure of tap water for nearly 300,000 residents. Typical water treatment methods are not effective for MCHM remediation, however ultrasonic irradiation leads to its rapid pseudo-first order degradation. The degradation processes were effectively modeled employing heterogeneous kinetic models with the reaction surface corresponding to the gas-liquid interface of the cavitation bubble. The Freundlich model which takes into account non-uniform distribution within the reactive zone showed the strongest correlation to the observed degradation kinetic data with R² > 0.99. Solute-solute clustering behavior is proposed to explain non-uniform distribution of MCHM. The results indicate the degradation occurs predominantly at the gas-liquid interface as a result of hydroxyl radical reactions and pyrolysis with primary reaction products, (4-methylcyclohexenyl) methanol and 4-methylcyclohexanone. Computational methods using density functional B3YPL/6-311G** calculations with Gaussian 09 provided insight of the hydroxyl radical and pyrolytic degradation pathways for the isomeric and conformational forms of MCHM. Our studies demonstrate that heterogeneous kinetic models and computational methods are important tools for the fundamental understanding and effective application of ultrasonically mediated degradation of MCHM which may be extended to a number of semi-volatile compounds.

Valorization of Crataegus azarolus stones for the removal of textile anionic dye by central composite rotatable design using cubic model: optimization, isotherm, and kinetic studies

In this study, the central composite rotatable design (CCRD) was used in the optimization of the operating parameters for the removal of the direct blue 86 (DB86), an anionic dye, because of its hazardous impact on human health and aquatic environment. In addition, DB86 is a recalcitrant and non-biodegradable dye whose presence considerably inhibits photosynthesis. Its removal in aqueous medium was achieved by biosorption onto the novel biosorbent Crataegus azarolus stones (CAS). The parameters like the solution pH, biosorbent dose, initial DB86 concentration, and temperature were studied in the ranges 2-6, 0.8-4 g L^-1, 20-100 mg L^-1, and 10-50 °C, respectively. The significance of the experimental parameters and their interactions was investigated by the Student's t test and p values with 5% error limits using JMP 11.0.0 software. The regression analysis of the experimental data obtained from 31 batch runs provides a cubic model. The optimum conditions obtained for the maximum DB86 elimination from the synthetic solution were found to be pH 2, biosorbent dose of 4 g L^-1, initial DB86 concentration of 20 mg L^-1, and temperature of 10 °C, leading to a theoretical maximum removal of 123%. The experimental data were analyzed by the Langmuir, Freundlich, and Temkin equilibrium models. The Langmuir isotherm gave the best fit with a maximum biosorption capacity of 24.02 mg g^-1. The results of the kinetic study revealed that the biosorption kinetic of DB86 follows a pseudo-second-order model. All results confirmed that CAS are an efficient, economic, and ecological alternative for the treatment of industrial wastewaters loaded with anionic dyes.

Improvement of sonochemical degradation of Brilliant blue R in water using periodate ions: Implication of iodine radicals in the oxidation process

In this paper, the effect of periodate (IO₄^-) on the ultrasonic degradation at 300kHz of Brilliant Blue R (BBR), an organic dye pollutant, was investigated. The experiments were realized in the absence and presence of periodate for various operating conditions including initial solution pH (2-8) and delivered ultrasonic power (20-80W). It was found that periodate greatly enhanced the sonochemical degradation of BBR. The degradation rate increased significantly with increasing IO₄^- concentration up to 10mM and decreased afterward. With 10mM of periodate, the degradation rate was 2.4-fold higher than that with ultrasound alone. The chemical probes experiments showed that periodate activation into free radicals (IO₃, IO₄ and OH) takes place by sonolysis and iodine radicals contribute significantly in the oxidation process. It was found that the periodate-enhanced effect was strongly experimental parameters dependent. The advantageous effect of periodate increased significantly with decreasing power and the best enhancing effect was obtained for the lowest power. Correspondingly, the periodate-enhanced effect increased with pH increase in the range 2-8 and it was more remarkable at near alkaline condition (pH 8). A reaction scheme for periodate sonolysis was proposed, for the first time, discussed and then used for interpreting the obtained results.

Ultrasonic Destruction of Acid Orange 7: Effect of Humic Acid, Surfactants and Complex Matrices

  The ultrasonic degradation at 600 kHz of an azo dye, acid orange 7 (AO7), in the presence of various dissolved natural organic matters (humic acid and surfactants) and in environmentally relevant matrices (natural water and seawater) was investigated. Additionally, the dependence of AO7 degradation on several operating parameters was clarified. The obtained results showed that ultrasound completely destroyed AO7 in 90 min of treatment but only 10% of TOC was removed after a long irradiation time. Investigations using the radical scavengers tert-butyl alcohol and KI revealed that AO7 degradation proceeds through radical reactions occurring at the bubble-liquid interface. AO7 conversion was strongly affected by the operating conditions. While the degradation of the dye was not affected by the presence of humic acid, it was impacted negatively by the presence of surfactants. Replacing deionized water by natural water and seawater as real environmental matrices did not affect the degradation of the dye.

Persulfate-enhanced sonochemical degradation of naphthol blue black in water: Evidence of sulfate radical formation

This work explores the effect of persulfate (PS) on the sonochemical degradation of organic pollutants taking naphthol blue black (NBB), an anionic diazo dye, as a substrate model. The sonolytic experiments were conducted in the absence and presence of PS under various experimental conditions including acoustic power (10-80W), frequency (20 and 585kHz) and saturating gas (argon, air and nitrogen). Experimental results showed that PS decomposition into sulfate radical (SO₄^-) takes place by sonolysis and increasing PS concentration up to 1g/L would result in an increase in the NBB degradation rate. It was found that the PS-enhanced effect was strongly operating parameters dependent. The positive effect of PS decreased with increasing power and the best enhancing effect was obtained for the lowest acoustic power. Correspondingly, the PS-enhanced effect was more remarkable at low frequency (20kHz) than that observed at high frequency ultrasound (585kHz). Nitrogen saturating gas gave the best enhanced effect of PS than argon and air atmospheres. Theoretical (computer simulation of bubble collapse) and experimental measurements of the yields of free radical generation under the different experimental conditions have been made for interpreting the obtained effects of PS on the sonochemical degradation of the dye pollutant. The experimental findings were attributed to the fact that radical-radical recombination reactions occur at faster rate than the radical-organic reaction when the concentration of free radicals is too high (at higher sonochemical conditions).

Supramolecular hydrogels for environmental remediation: G4-quartet gels that selectively absorb anionic dyes from water

Binary mixtures of guanosine and 8-aminoguanosine form stable, transparent hydrogels with Ba²⁺ that selectively bind anionic dyes.