A review on sonoelectrochemical technology as an upcoming alternative for pollutant degradation

A review of electro-Fenton and ultrasound processes: towards a novel integrated technology for wastewater treatment

Nowadays, the presence of persistent dissolved pollutants in water has received increasing attention due to their toxic effects on living organisms. Considering the limitations of conventional wastewater treatment processes for the degradation of these compounds, advanced oxidation processes such as electro-Fenton and sono-chemical process, as well as their combination, appear as potentially effective options for the treatment of wastewater contaminated with bio-recalcitrant pollutants. In view of the importance of the development of processes using real effluents, this review aims to provide a comprehensive perspective of sono-electro-Fenton-related processes applied for real wastewater treatment. In the first section, the fundamentals and effectiveness of both homogeneous and heterogeneous electro-Fenton approaches for the treatment of real wastewater are presented. While the second part of this work describes the fundamentals of ultrasound-based processes, the last section focuses on the coupling of the two methods for real wastewater treatment and on the effect of the main operational parameters of the process. On the basis of the information presented, it is suggested that sono-electro-Fenton processes substantially increase the efficiency of the treatment as well as the biodegradability of the treated wastewater. The combined effect results from mass transfer improvement, electrode cleaning and activation, water electrolysis, and the electro-Fenton-induced production of hydroxyl radicals. The information presented in this work is expected to be useful for closing the gap between laboratory-scale assays and the development of novel wastewater technologies.

Effect of plasma-activated water and buffer solution combined with ultrasound on fungicide degradation and quality of cherry tomato during storage

The purpose of this study was to examine plasma-activated buffer solution (PABS) and plasma-activated water (PAW) combined with ultrasonication (U) treatment on the reduction of chlorothalonil fungicide and the quality of tomato fruits during storage. To obtain PAW and PABS, an atmospheric air plasma jet was used to treat buffer solution and deionized water at different treatment times (5 and 10 min). For combined treatments, fruits were submerged in PAW and PABS, then sonicated for 15 min, and individual treatment without sonication. As per the results, the maximum chlorothalonil reduction of 89.29% was detected in PAW-U₁₀, followed by 85.43% in PABS. At the end of the storage period, the maximum reduction of 97.25% was recorded in PAW-U₁₀, followed by 93.14% in PABS-U₁₀. PAW, PABS, and both combined with ultrasound did not significantly affect the overall tomato fruit quality in the storage period. Our results revealed that PAW combined with sonication had a significant impact on post-harvest agrochemical degradation and retention of tomato quality than PABS. Conclusively, the integrated hurdle technologies effectively reduce agrochemical residues, which helps to lower health hazards and foodborne illnesses.

Ultrasound applications in drying of fruits from a sustainable development goals perspective

This review focuses on the many contributions of ultrasound technologies for fruit drying toward the United Nations Sustainable Development Goals (SDG). Along this review, several aspects attained from the application of ultrasound technologies are correlated with the SDGs. The main ultrasonic technologies applied for fruit drying, such as ultrasonic bath, probe ultrasound, air-borne ultrasound air-drying, and ultrasound-assisted contact air-drying, are presented. An in-depth discussion on ultrasound contributions, its advantages, disadvantages, and limitations are made. The effects of ultrasound on water diffusivity in several fruits are presented by correlating this effect with drying time and cost of energy. Ultrasound-assisted fruit drying, like other food processing technologies, directly impacts Zero Hunger, but ultrasound technologies contribute to much more than delivering long shelf-life food. This technology can be used to produce healthy foods and provide well-being, which will be discussed by correlating the effects of ultrasound-assisted air-drying with the concentration of nutritional compounds. Ultrasound-assisted fruit drying reduces wastewater toxicity and energy consumption and improves productivity, potentially improving workplaces and salaries. A walk through the technology is presented from Zero Hunger to No Poverty.

Efficient sonocatalytic degradation of orange II dye and real textile wastewater using peroxymonosulfate activated with a novel heterogeneous TiO2–FeZn bimetallic nanocatalyst

TiO₂–FeZn nanocatalyst combined with sonolysis were used to activate peroxymonosulfate (PMS) as a highly efficient advanced oxidation process (US/TiO₂–FeZn/PMS) for the decoloration of orange II dye (OII) and real textile wastewater. The characterization of the as-synthesized NPs was performed by SEM, FTIR, EDX and XRD analyses. Optimal experimental conditions of operational parameters were obtained: pH = 3, 15 mg/L initial OII concentration, 0.2 g/L PMS, 0.7 g/L nanocatalyst dosing, and 300 W ultrasonic power. The decolorization was observed to increase with increasing the dose of nanocatalyst and the ultrasonic power, and with decreasing pH (under acidic conditions). Under optimal experimental conditions, decolorization and COD removal of textile wastewater were 99.9% and 74.6%, respectively, at 40 min. The TiO₂–FeZn/PMS/US as a novel process exhibited a higher removal of OII (95%) than TiO₂ NPs/PMS/US process (54%). The OII removal efficiency by the different processes decreased in the following order: TiO₂–FeZn/US/PMS > TiO₂–FeZn/PMS > TiO₂–FeZn/US > TiO₂ /US/PMS > US/PMS > TiO₂–FeZn > PMS > US. The recyclability study revealed that the process could be reused up to three consecutive cycles. The current US/nanocatalyst/PMS system was concluded to be an efficient, reusable and stable nanocatalyst for the oxidation of textile dyes.

Ultrasound-enhanced Magnéli phase Ti4O7 anodic oxidation of per- and polyfluoroalkyl substances (PFAS) towards remediation of aqueous film forming foams (AFFF)

Per-and polyfluoroalkyl substances (PFAS) remediation is still a challenge. In this study, we propose a hybrid system that combines electrochemical treatment with ultrasound irradiation, aiming for an enhanced degradation of PFAS. Equipped with a titanium suboxide (Ti₄O₇) anode, the electrochemical cell is able to remove perfluorooctanoic acid (PFOA) effectively. Under the optimal conditions (50 mA/cm² current density, 0.15 M Na₂SO₄ supporting electrolyte, and stainless steel/Ti₄O₇/stainless steel electrode configuration with a gap of ∼10 mm), the electrochemical process achieves ∼100 % PFOA removal and 43 % defluorination after 6 h. Applying ultrasound irradiation (130 kHz) alone offers a limited PFOA removal, with 33 % PFOA removal and 5.5 % defluorination. When the electrochemical process is combined with ultrasound irradiation, we observe a significant improvement in the remediation performance, with ∼100 % PFOA removal and 63.5 % defluorination, higher than the sum of 48.5 % (43 % achieved by the electrochemical process, plus 5.5 % by the ultrasound irradiation), implying synergistic removal/oxidation effects. The hybrid system also consistently shows the synergistic defluorination during degradation of other PFAS and the PFAS constituents in aqueous film forming foam (AFFF). We attribute the synergistic effect to an activated/cleaned electrode surface, improved mass transfer, and enhanced production of radicals.

Evaluation of anode materials in sonoelectrochemistry processes: Kinetic, mechanism, and cost estimation

Eco-friendly pollutant treatment technology has a developing tendency in future. The combination of ultrasound (US) and electrochemical (EC) is a promising technology, because they are efficient, clean and environmentally friendly. In this study, the impacts of anode material have been investigated in US (300 kHz) and EC (10V) system. The results of all systems revealed that the kinetic constant decreased with increasing pH. The results are also shown that ΔG^# > 0 and ΔH^# > 0 during PCP degradation in EC or US-EC systems are non-spontaneous and endothermic reactions. Meanwhile, in the US-EC system, TiO₂, Ti₄O₇, PbO₂, SnSb, RuIr, and BDD, except for TiO₂, all the anode materials showed a synergistic index (SI) of 106-197%, and the activation energies were 19.32, 33.4, 33.74, 32.84, 10.41, 36.44 kJ mol^-1, respectively. In EC and US-EC systems, PCP can be completely mineralized by BDD anode within 30 min. TBA scavenger experiments verified that hydroxyl radicals were the main oxidant in each system using BDD and PbO₂ anode. As a result of estimating the cost according to the anode material when removing PCP using the EC or US-EC system, BDD was the smallest in the two systems, 1.58 and 1.12 $ m^-3, respectively. Finally, this study may serve as a reference for implementation of US-EC system in wastewater treatment.

Sonoelectrochemical oxidation of sulfamethoxazole in simulated and actual wastewater on a piezo-polarizable FTO/BaZr x Ti(1-x)O3 electrode: reaction kinetics, mechanism and reaction pathway studies

The sonoelectrochemical (SEC) oxidation of sulfamethoxazole (SMX) in simulated and actual wastewater on FTO/BaZr_(0.1)Ti_(0.9)O₃, FTO/BaZr_(0.05)Ti_(0.95)O₃ and FTO/BaTiO₃ electrodes is hereby presented. Electrodes from piezo-polarizable BaZr_(0.1)Ti_(0.9)O₃, BaZr_(0.05)Ti_(0.95)O₃, and BaTiO₃ materials were prepared by immobilizing these materials on fluorine-doped tin dioxide (FTO) glass. Electrochemical characterization performed on the electrodes using chronoamperometry and electrochemical impedance spectroscopy techniques revealed that the FTO/BaZr_(0.1)Ti_(0.9)O₃ anode displayed the highest sonocurrent density response of 2.33 mA cm^-2 and the lowest charge transfer resistance of 57 Ω. Compared to other electrodes, these responses signaled a superior mass transfer on the FTO/BaZr_(0.1)Ti_(0.9)O₃ anode occasioned by an acoustic streaming effect. Moreover, a degradation efficiency of 86.16% (in simulated wastewater), and total organic carbon (TOC) removal efficiency of 63.16% (in simulated wastewater) and 41.47% (in actual wastewater) were obtained upon applying the FTO/BaZr_(0.1)Ti_(0.9)O₃ electrode for SEC oxidation of SMX. The piezo-polarizable impact of the FTO/BaZr_(0.1)Ti_(0.9)O₃ electrode was further established by the higher rate constant obtained for the FTO/BaZr_(0.1)Ti_(0.9)O₃ electrode as compared to the other electrodes during SEC oxidation of SMX under optimum operational conditions. The piezo-potential effect displayed by the FTO/BaZr_(0.1)Ti_(0.9)O₃ electrode can be said to have impacted the generation of reactive species, with hydroxyl radicals playing a predominant role in the degradation of SMX in the SEC system. Additionally, a positive synergistic index obtained for the electrode revealed that the piezo-polarization effect of the FTO/BaZr_(0.1)Ti_(0.9)O₃ electrode activated during sonocatalysis combined with the electrochemical oxidation process during SEC oxidation can be advantageous for the decomposition of pharmaceuticals and other organic pollutants in water.

Pharmaceuticals and personal care products in water streams: Occurrence, detection, and removal by electrochemical advanced oxidation processes

Pharmaceutical and personal care products (PPCPs) are part of the emerging contaminants (ECs) in the environment due to their known or suspected adverse effects in aquatic and terrestrial organisms, as well as in human health. Presence of PPCPs in aquatic and terrestrial ecosystems has been mainly attributed to the effluents of wastewater treatment plants (WWTPs). Although several PPCPs have been detected in wastewater, their removal from wastewater via biological processes is limited. Removal of PPCPs depends on their chemical structure, concentration, solubility, and technology used to treat the wastewater. Electrochemical Advanced Oxidation Processes (EAOPs) are some of the most sought-after methods for dealing with organic pollutants in water including PPCPs, due to generation of strong oxidants such as •OH, H₂O₂ and O₃^- by using directly or indirectly electrochemical technology. This review is focused on the removal of main PPCPs via EAOPs such as, anodic oxidation, electro-Fenton, photoelectron-Fenton, solar photoelectron-Fenton, photoelectrocatalysis and sonoelectrochemical processes. Although more than 40 PPCPs have been identified through different analytical approaches, antibiotics, anti-inflammatory and antifungal are the main categories of PPCPs detected in different water matrices. Application of EAOPs has been centered in the removal of antibiotics and analgesics of high consumption by using model media, e.g. Na₂SO_4. Photoelectrocatalysis and Electro-Fenton processes have been the most versatile EAOPs applied for PPCPs removal under a wide range of operating conditions and a variety of electrodes. Although EAOPs have gained significant scientific interest due to their effectiveness, low environmental impact, and simplicity, further research about the removal of PPCPs and their by-products under realistic concentrations and media is needed. Moreover, mid-, and long-term experiments that evaluate EAOPs performance will provide knowledge about key parameters that allow these technologies to be scaled and reduce the potential risk of PPCPs in aquatic and terrestrial ecosystem.

Recent progress on ultrasound-assisted electrochemical processes: A review on mechanism, reactor strategies, and applications for wastewater treatment

The electrochemical advanced oxidation processes (EAOPs) have received significant attention among the many other water and wastewater treatment technologies. However, achieving a desirable removal effect with a single technique is frequently difficult. Therefore, the integration of ultrasound technique with other processes such as electrocoagulation, electro-Fenton, and electrooxidation is a critical way to achieve effective organic pollutants decomposition from wastewater. This review paper is focused on ultrasound-assisted electrochemical (US/electrochemical) processes, so-called sonoelectrochemical processes of various organic pollutants. Emphasis was given to recently published articles for discussing the results and trends in this research area. The use of ultrasound and integration with electrochemical processes has a synergistic impact owing to the physical and chemical consequences of cavitation, resulting in enhancing the mineralization of organic pollutants. Various types of sonoelectrochemical reactors (batch and continuous) employed in the US/electrochemical processes were reviewed. In addition, the strategies to avoid passivation, enhanced generation of reactive oxygen species, and mixing effect are reviewed. Finally, concluding remarks and future perspectives on this research topic are also explored and recommended.

Effects of combined treatment of plasma activated liquid and ultrasound for degradation of chlorothalonil fungicide residues in tomato

The effects of combined treatment (PAL-U) of plasma-activated liquid (PAL) including plasma-activated water (PAW) and plasma-activated buffer solution (PABS) and ultrasound (U) for the degradation of chlorothalonil fungicide on tomato fruit was investigated. Distilled water and buffer solution were activated by radiofrequency plasma jet for durations of 1, 3, 5, and 10 min to obtain PAL₁ to PAL₁₀. Fruits were immersed in PAL for 15 min and also in distilled water with sonication for 15 min for individual treatments, and in PAL with sonication for 15 min for combined treatments. The maximum chlorothalonil fungicide residues were reduced by 89.28 and 80.23% for PAW₁₀-U and PABS₁₀-U, respectively. HPLC-MS characterization revealed chlorothalonil degradation pathway and formation of 2,4,5-trichloroisophthalonitrile, 2,4-dichloroisophthalonitrile, 4-chloroisophthalonitrile, isophthalonitrile and phenylacetonitrile as degradation products. Treatments also showed no negative effects on tomato quality. Therefore, PAL and PAL-U treatments could serve as effective methods for degrading pesticides on tomatoes.

Sonoprocessing: From Concepts to Large-Scale Reactors

Intensification of ultrasonic processes for diversified applications, including environmental remediation, extractions, food processes, and synthesis of materials, has received attention from the scientific community and industry. The mechanistic pathways involved in intensification of ultrasonic processes that include the ultrasonic generation of cavitation bubbles, radical formation upon their collapse, and the possibility of fine-tuning operating parameters for specific applications are all well documented in the literature. However, the scale-up of ultrasonic processes with large-scale sonochemical reactors for industrial applications remains a challenge. In this context, this review provides a complete overview of the current understanding of the role of operating parameters and reactor configuration on the sonochemical processes. Experimental and theoretical techniques to characterize the intensity and distribution of cavitation activity within sonoreactors are compared. Classes of laboratory and large-scale sonoreactors are reviewed, highlighting recent advances in batch and flow-through reactors. Finally, examples of large-scale sonoprocessing applications have been reviewed, discussing the major scale-up and sustainability challenges.

A novel technique for the reduction of pesticide residues by a combination of low-intensity electrical current and ultrasound applications: A study on lettuce samples

In this study, effects of low-intensity electrical currents (200, 800 and 1400 mA), ultrasound frequencies (24 and 40 kHz) and their combinations were applied at the duration period of 2, 4, 6, 8, and 10 min for the degradation of captan, thiamethoxam and metalaxyl residues in lettuce samples. Residues of the pesticides were determined by gas chromatography with tandem mass spectrometry and electron capture detector. The results indicated that the combination of low-intensity electrical current and ultrasound was found to be effective for the reduction of the pesticides. The most effective combination was obtained to be current of 1400 mA and ultrasound frequency of 24 kHz at 10 min. Under this circumstance, 92.57, 81.99 and 93.09% of captan, thiamethoxam and metalaxyl residues were decreased, respectively. The findings suggest that the combination of low-intensity electrical current and ultrasound applications has an important potential for the degradation of pesticide residues.

Various wastewaters treatment by sono-electrocoagulation process: A comprehensive review of operational parameters and future outlook

Electrochemical processes are a promising alternative to traditional water treatment systems because they have advantages than conventional techniques such as chemical storage, small treatment systems, no alkalinity depletion, remote adjustment, and cost-effectiveness. The most crucial electrochemical method is Electrocoagulation (EC). Through creating cationic species, the EC causes the neutralization of pollutant surface charges and destabilizes suspended, emulsified or dissolved contaminants led to attracting particles of opposite charge and form flocculants. The main drawback of the EC process is a passive film forming on the electrode surface over time. Ultrasonic (US) waves breaking down sediments formed at the electrode surface and generate high amounts of radical species to remove pollutants by creating high-pressure points inside the solution during the cavitation phenomenon. Although EC systems are considered as an exemplary renaissance in water and wastewater treatment, various parameters related to these types of systems in pollutant degradation have not been fully addressed. To present a comprehensive vision of the current state of the art, and progress the treatment efficiency and agitate new studies in these fields, this review aimed to provide an overview of electrocoagulation's application in pollutant degradation, besides the advantages, associated disadvantages and further strategies for improving the performance of this technique. Moreover, this review discussed various parameters affecting the EC/US process, including nanoparticles addition, electrolyte concentration, current intensity, electrode distance, temperature, oxidant addition, pH, pollutant concentration, reaction time, and electrode combination, chloride addition, and ultrasonic frequency. Also, the efficiency of the EC/US process for disinfection, as well as treatment of car-washing, textile, pulp, and paper industry, oily, brewery wastewater, surfactant, humic acid, and heavy metals, are addressed.

Effects of gas sparging and mechanical mixing on sonochemical oxidation activity

The effects of air sparging (0-16 L min^-1) and mechanical mixing (0-400 rpm) on enhancing the sonochemical degradation of rhodamine B (RhB) was investigated using a 28 kHz sonoreactor. The degradation of RhB followed pseudo first-order kinetics, where sparging or mixing induced a large sonochemical enhancement. The kinetic constant varied in three stages (gradually increased → increased exponentially → decreased slightly) as the rate of sparging or mixing increased, where the stages were similar for both processes. The highest sonochemical activity was obtained with sparging at 8 L min^-1 or mixing at 200 rpm, where the standing wave field was significantly deformed by sparging and mixing, respectively. The cavitational oxidation activity was concentrated at the bottom of the sonicator when higher sparging or mixing rates were employed. Therefore, the large enhancement in the sonochemical oxidation was attributed mainly to the direct disturbance of the ultrasound transmission and the resulting change in the cavitation-active zone in this study. The effect of the position of air sparging and mixing was investigated. The indirect inhibition of the ultrasound transmission resulted in less enhancement of the sonochemical activity. Moreover, the effect of various sparging gases including air, N₂, O₂, Ar, CO₂, and an Ar/O₂ (8:2) mixture was compared, where all gases except CO₂ induced an enhancement in the sonochemical activity, irrespective of the concentration of dissolved oxygen. The highest activity was obtained with the Ar/O₂ (8:2) mixture. Therefore, it was revealed that the sonochemical oxidation activity could be further enhanced by applying gas sparging using the optimal gas.

Enhanced electrochemical decontamination and water permeation of titanium suboxide reactive electrochemical membrane based on sonoelectrochemistry

Reactive electrochemical membrane (REM) allows electrochemical oxidation (EO) water purification under flow-through operation, which improves mass transfer on the anode surface significantly. However, O₂ evolution reaction (OER) may cause oxygen bubbles to be trapped in small-sized confined flow channels, and thus degrade long-term filterability and treatability of REM. In this study, ultrasound (ultrasonic vibrator, 28 kHz, 180 W) was applied to EO system (i. e. sonoelectrochemistry) containing titanium suboxide-REM (TiSO-REM) anode for enhanced oxidation of 4-chlorophenol (4-CP) target pollutant. Both experimental and modeling results demonstrated that ultrasound could mitigate the retention of O₂ bubbles in the porous structures by destructing large-size bubbles, thus not only increasing permeate flux but also promoting local mass transfer. Meanwhile, oxidation rate of 4-CP for EO with ultrasound (EO-US, 0.0932 min^-1) was 216% higher than that for EO without ultrasound (0.0258 min^-1), due to enhanced mass transfer and OH production under the cavitation effect of ultrasound. Density functional theory (DFT) calculations confirmed the most efficient pathway of 4-CP removal to be direct electron transfer of 4-CP to form [4-CP]⁺, followed by subsequent oxidation mediated by OH produced from anodic water oxidation on TiSO-REM anode. Last, the stability of TiSO-REM could be improved considerably by application of ultrasound, due to alleviation of electrode deactivation and fouling, indicated by cyclic test, scan electron microscopy (SEM) observation and Fourier transform infrared spectroscopy (FT-IR) characterization. This study provides a proof-of-concept demonstration of ultrasound for enhanced EO of recalcitrant organic pollutants by REM anode, making decentralized wastewater treatment more efficient and more reliable.

Sonoelectrochemistry for energy and environmental applications

Sonoelectrochemistry is the study of the effects and applications of ultrasonic waves on electrochemical processes. The integration of ultrasound and electrochemistry offers many advantages: fast reaction rates, enhanced surface activation, and increased mass transport at an electrode. Significant progress has been made in advancing basic and applied aspects of sonoelectrochemical techniques, which are herein reviewed by addressing the development and applications of sonoelectrochemical processes in energy and environmental areas. This review examines the experimental procedures that are used in various sonoelectrochemical techniques generally used for the synthesis of energy related materials (e.g., fuel cell electrocatalysts and materials for hydrogen production) and for the degradation of various organic compounds/pollutants. The challenges that remain for the sonoelectrochemical production of energy materials, the degradation of organic pollutants, and their associated reaction pathway mechanism(s) are also discussed. This review also highlights the significant improvements made to date. The provided information in this review may be helpful to scientists working in the research areas of environmental remediation, energy exploitation and exploration, as well as synthetic process-oriented research.

Sonoelectrochemical Degradation of Propyl Paraben: An Examination of the Synergy in Different Water Matrices

The synergistic action of anodic oxidation using boron-doped diamond and low-frequency ultrasound in different water matrices and operating conditions for the decomposition of the emerging contaminant propyl paraben was investigated. The degree of synergy was found to decrease with an increase in current in the range 1.25–6.25 mA/cm² or the ultrasound power until 36 W/L, where a further decrease was observed. Despite the fact that the increased propyl paraben concentration decreased the observed kinetic constant for both the separated and the hybrid process, the degree of synergy was increased from 37.3 to 43.4% for 0.5 and 2 mg/L propyl paraben, respectively. Bicarbonates (100–250 mg/L) or humic acid (10–20 mg/L) enhanced the synergy significantly by up to 55.8%, due to the higher demand for reactive oxygen species. The presence of chloride ions decreased the observed synergistic action in comparison with ultrapure water, possibly due to the electro-generation of active chlorine that diffuses to the bulk solution. The same behavior was observed with the secondary effluent that contained almost 68 mg/L of chlorides. The efficiency was favored in a neutral medium, while the hybrid process was delayed in alkaline conditions.

A review of the application of sonophotocatalytic process based on advanced oxidation process for degrading organic dye

Nowadays the use of conventional wastewater treatment methods is becoming increasingly challenging mainly due to the presence of organic matter in wastewater. Therefore, an emerging technology is needed to deal with these highly concentrated and toxic non-biodegradable organic matters. In the last few decades, advanced oxidation process (AOP) has emerged to treat wastewaters discharged from industries. Recently, researchers have shown interest to use the application of ultrasound (US) in photocatalysis, i.e. sonophotocatalysis, to improve the performance of the treatment process in the degradation of organic and inorganic contaminants in aqueous streams. Sonophotocatalysis is the combination of the use of ultraviolet (UV) and US.

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.

Acoustic cavitation assisted plasma for wastewater treatment: Degradation of Rhodamine B in aqueous solution

A novel wastewater treatment process, acoustic cavitation assisted plasma (ACAP) is proposed in this study aiming at expanding the treatable range of water pollutants due to a synergetic effect of ultrasound irradiation and high voltage plasma discharge. In this process, the role of acoustic cavitation is not only to provide generation of chemically active OH radicals, as for example in conventional ultrasonic wastewater treatment techniques, but also to ensure conditions for stable plasma generation in wastewater and, thus, to extend the treatable range of water pollutants. Rhodamine B (RhB) was used as a model pollutant in experiments examining effects of ultrasound amplitude, RhB initial concentration, output voltage, solution pH and electrical conductivity on the RhB degradation efficiency. The results revealed that the ultrasound-assisted plasma generation requires lower output voltages and allows to increase the acceptable range of electrical conductivity of treatable solutions up to 1000 μS/cm, that is about 24 times higher than in the case of conventional plasma discharge treatment. The alkaline and acid medium were found to be favorable for higher degradation efficiency. Additional measurements and results of recent investigations concerning underwater plasma showed that microbubbles presented in cavitation zone could serve as "bridges" making the pulse discharge propagation between the electrodes easier than in the conventional case. Besides, acoustic cavitation assists a faster transition of plasma discharge from ineffective streamer type to more effective spark type that further contributes to the improvement of the treatment performance.

Removal of phenol from steel wastewater by combined electrocoagulation with photo-Fenton

Phenol and its derivatives are available in various industries such as refineries, coking plants, steel mills, drugs, pesticides, paints, plastics, explosives and herbicides industries. This substance is carcinogenic and highly toxic to humans. The purpose of the study was to investigate the removal of phenol from wastewater of the steel industry using the electrocoagulation-photo-Fenton (EC-PF) process. Phenol and chemical oxygen demand (COD) removal efficiency were investigated using the parameters pH, Fe²⁺/H₂O₂, reaction time and current density. The highest removal efficiency rates of phenol and COD were 100 and 98%, respectively, for real wastewater under optimal conditions of pH = 4, current density = 1.5 mA/cm², Fe²⁺/H₂O₂ = 1.5 and reaction time of 25 min. Combination of the two effective methods for the removal of phenol and COD, photocatalytic electrocoagulation photo-Fenton process is a suitable alternative for the removal of organic pollutants in industry wastewater because of the low consumption of chemicals, absence of sludge and other side products, and its high efficiency.

Ultrasound-enhanced electrokinetic remediation for removal of Zn, Pb, Cu and Cd in municipal solid waste incineration fly ashes

Low-frequency ultrasound generated by a transducer was investigated to activate the raw municipal solid waste incineration (MSWI) fly ashes in the electrokinetic process, aiming at enhancing heavy metal (HM) removal and achieving better remedial efficacy. The maximum removal efficiencies of 69.84%, 64.24%, 67.74% and 59.93% were obtained in the orthogonal tests of ultrasonication for Zn, Pb, Cu and Cd, respectively. The acoustic time of 30 min and controlling temperature of 45 °C in the operating parameters were quantitatively determined to optimize the ultrasonication of the MSWI fly ash matrices. The changes of acoustic time had a significant effect on the extraction efficiencies of all the four heavy metal elements in the sonication optimal experiments. The longer running time was preferred for the pretreatment of the fly ashes in according to the marginal mean removal results. The voltage gradient of 2 V/cm was most likely to improve the removals of four HMs during the electrokinetics in the range of 0.5-2 V/cm. The synergetic application of acidification and ultrasonication for the media treatment was demonstrated to be most effective in enhancing the remedial efficiencies in the further electrokinetic experiments compared with the other activation systems. Correspondingly, the leaching concentrations of Zn, Pb, Cu and Cd in the samples were reduced by 85.92%, 98.22%, 88.53% and 98.34%, respectively. The contaminants were continuously extracted from the solid grains of the fly ashes by the protonic attack and bubble implosion. The obtained risk-assessment-code values indicated the adoption of AUS-EKR system reduced the environmental toxicity for the fly ashes to the maximum extent.

Decomposition of ibuprofen in water via an electrochemical process with nano-sized carbon black-coated carbon cloth as oxygen-permeable cathode integrated with ultrasound

The main aim of the present investigation was the treatment of ibuprofen (IBP)-polluted aquatic phase using a novel oxygen-permeable cathode (OPC)-equipped electrochemical process (ECP) integrated with ultrasound (US). According to kinetic modeling, the decomposition rate of IBP by the integrated process was 3.2 × 10^-2 min^-1 which was significant in comparison with the OPC-equipped ECP (1.4 × 10^-2 min^-1) and US alone (2.4 × 10^-3 min^-1). Increasing the current resulted in the enhanced generation of H₂O₂ and consequently, improved the degradation of IBP in the solution. Excessive concentrations of Na₂SO₄ as supporting electrolyte led to no significant enhancement in the reactor efficiency. At initial IBP concentration of 1 mg L^-1, complete removal of IBP with reaction rate of 1.7 × 10^-1 min^-1 was happened within a short reaction time of 30 min. The pulse mode of US led to more than 10% increase in the removal efficiency compared with the normal mode. The presence of scavenging compound of methanol caused the highest drop in the efficiency of the integrated treatment process, indicating the substantial role of free hydroxyl radicals in the degradation of IBP. Intermediate byproducts generated in the solution during the decomposition were also identified and interpreted.

Non-Noble Metal Oxide Catalysts for Methane Catalytic Combustion: Sonochemical Synthesis and Characterisation

The aim of this study was to obtain nanocrystalline mixed metal-oxide–ZrO₂ catalysts via a sonochemically-induced preparation method. The effect of a stabiliser’s addition on the catalyst parameters was investigated by several characterisation methods including X-ray Diffraction (XRD), nitrogen adsorption, X-ray fluorescence (XRF), scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectrometer (EDS), transmission electron microscopy (TEM) and µRaman. The sonochemical preparation method allowed us to manufacture the catalysts with uniformly dispersed metal-oxide nanoparticles at the support surface. The catalytic activity was tested in a methane combustion reaction. The activity of the catalysts prepared by the sonochemical method was higher than that of the reference catalysts prepared by the incipient wetness method without ultrasonic irradiation. The cobalt and chromium mixed zirconia catalysts revealed their high activities, which are comparable with those presented in the literature.

Investigation of mass transfer intensification under power ultrasound irradiation using 3D computational simulation: A comparative analysis

This paper aims at investigating the influence of acoustic streaming induced by low-frequency (24kHz) ultrasound irradiation on mass transfer in a two-phase system. The main objective is to discuss the possible mass transfer improvements under ultrasound irradiation. Three analyses were conducted: i) experimental analysis of mass transfer under ultrasound irradiation; ii) comparative analysis between the results of the ultrasound assisted mass transfer with that obtained from mechanically stirring; and iii) computational analysis of the systems using 3D CFD simulation. In the experimental part, the interactive effects of liquid rheological properties, ultrasound power and superficial gas velocity on mass transfer were investigated in two different sonicators. The results were then compared with that of mechanical stirring. In the computational part, the results were illustrated as a function of acoustic streaming behaviour, fluid flow pattern, gas/liquid volume fraction and turbulence in the two-phase system and finally the mass transfer coefficient was specified. It was found that additional turbulence created by ultrasound played the most important role on intensifying the mass transfer phenomena compared to that in stirred vessel. Furthermore, long residence time which depends on geometrical parameters is another key for mass transfer. The results obtained in the present study would help researchers understand the role of ultrasound as an energy source and acoustic streaming as one of the most important of ultrasound waves on intensifying gas-liquid mass transfer in a two-phase system and can be a breakthrough in the design procedure as no similar studies were found in the existing literature.

Treating soil-washing fluids polluted with oxyfluorfen by sono-electrolysis with diamond anodes

This works is focused on the treatment by sono-electrolysis of the liquid effluents produced during the Surfactant-Aided Soil-Washing (SASW) of soils spiked with herbicide oxyfluorfen. Results show that this combined technology is very efficient and attains the complete mineralization of the waste, regardless of the surfactant/soil radio applied in the SASW process (which is the main parameter of the soil remediation process and leads to very different wastes). Both the surfactant and the herbicide are completely degraded, even when single electrolysis is used; and only two intermediates are detected by HPLC in very low concentrations. Conversely, the efficiency of single sonolysis approach, for the oxidation of pollutant, is very low and just small changes in the herbicides and surfactant concentrations are observed during the tests carried out. Sono-electrolysis with diamond electrodes achieved higher degradation rates than those obtained by single sonolysis and/or single electrolysis with diamond anodes. A key role of sulfate is developed, when it is released after the electrochemical degradation of surfactant. The efficient catalytic effect observed which can be explained by the anodic formation of persulfate and the later, a sono-activation is attained to produce highly efficient sulfate radicals. The effect of irradiating US is more importantly observed in the pesticide than in the surfactant, in agreement with the well-known behavior of these radicals which are known to oxidize more efficiently aromatic compounds than aliphatic species.

Facile preparation of yolk–shell structured Si/SiC@C@TiO2 nanocomposites as highly efficient photocatalysts for degrading organic dye in wastewater

The prepared yolk–shell Si/SiC@C@TiO₂ nanospheres show a strong photon absorbance throughout the visible light region, and excellent photocatalytic performance for degrading methyl blue in water under UV and visible light irradiation.

Alternate pulses of ultrasound and electricity enhanced electrochemical process for p-nitrophenol degradation

A novel alternated ultrasonic and electric pulse enhanced electrochemical process was developed and used for investigating its effectiveness on the degradation of p-nitrophenol (PNP) in an aqueous solution. The impacts of pulse mode, pH, cell voltage, supporting electrolyte concentration, ultrasonic power and the initial concentration of PNP on the performance of PNP degradation were evaluated. Possible pathway of PNP degradation in this system was proposed based on the intermediates identified by GC-MS. Experimental results showed that 94.1% of PNP could be removed at 2h in the dual-pulse ultrasound enhanced electrochemical (dual-pulse US-EC) process at mild operating conditions (i.e., pulse mode of electrochemical pulse time (TEC)=50 ms and ultrasonic pulse time (T US)=100 ms, initial pH of 3.0, cell voltage of 10 V, Na2SO4 concentration of 0.05 M, ultrasonic powder of 48.8 W and initial concentration of PNP of 100mg/L), compared with 89.0%, 58.9%, 2.4% in simultaneous ultrasound enhanced electrochemical (US-EC) process, pulsed electrochemical (EC) process and pulsed ultrasound (US), respectively. Moreover, energy used in the dual-pulse US-EC process was reduced by 50.4% as compared to the US-EC process. The degradation of PNP in the pulsed EC process, US-EC process and dual-pulse process followed pseudo-first-order kinetics. Therefore, the dual-pulse US-EC process was found to be a more effective technique for the degradation of PNP and would have a promising application in wastewater treatment.

Insights into the scalability of magnetostrictive ultrasound technology for water treatment applications

To date, the successful application of large scale ultrasound in water treatment has been a challenge. Magnetostrictive ultrasound technologies for constructing a large-scale water treatment system are proposed in this study. Comprehensive energy evaluation of the proposed system was conducted. The effects of chosen waveform, scalability and reactor design on the performance of the system were explored using chemical dosimetry. Of the fundamental waveforms tested; sine, triangle and square, the highest chemical yield resulted from the square wave source. Scaling up from the 0.5L bench-scale system to the 15 L large-scale unit resulted in a gain of approximately 50% in sonochemical efficiency (SE) for the system. The use of a reactor tank with 45° inclined sides further increased SE of the system by 70%. The ability of the large scale system in removing contaminants from natural water samples was also investigated. The results revealed that the large-scale unit was capable of achieving a maximum removal of microbes and dissolved organic carbon (DOC) of 35% and 5.7% respectively at a power density approximately 3.9 W/L. The results of this study suggest that magnetostrictive ultrasound technology excited with square wave has the potential to be competitive in the water treatment industry.