Cavitation based treatment of industrial wastewater: A critical review focusing on mechanisms, design aspects, operating conditions and application to real effluents.

Acoustic cavitation (AC) and hydrodynamic cavitation (HC) coupled with advanced oxidation processes (AOPs) are prominent techniques used for industrial wastewater treatment though most studies have focused on simulated effluents. The present review mainly focuses on the analysis of studies related to real industrial effluent treatment using acoustic and hydrodynamic cavitation operated individually and coupled with H2O2, ozone, ultraviolet, Fenton, persulfate and peroxymonosulfate, and other emerging AOPs. The necessity of using optimum loadings of oxidants in the various AOPs for obtaining maximum COD reduction of industrial effluent have been demonstrated. The review also presents critical analysis of designs of various HCRs that have been or can be used for the treatment of industrial effluents. The impact of operating conditions such as dilution, inlet pressure, ultrasonic power, pH, and operating temperature have been also discussed. The economic aspects of the industrial effluent treatment have been analyzed. HC can be considered as cost-efficient approach compared to AC on the basis of the lower operating costs and better transfer efficiencies. Overall, HC combined with AOPs appears to be an effective treatment strategy that can be successfully implemented at industrial-scale of operation.

Sonocatalytic recovery of ceria from graphite and inhibition of graphite erosion by ionic liquid based platinum nanocatalyst.

Use of ultrasound as an intensified non-destructive decontamination technique for processing graphite limits its reusability beyond a few number of decontamination cycles due to the exfoliation of graphite due to cavitation effects. The current work establishes that the use of platinum nanoparticles in the leachant reduces the erosion of graphite substrate due to cavitation. It presents an improved way of sonochemical recovery of ceria using a mixture of nitric acid, formic acid and hydrazinium nitrate in the presence of platinum nanoparticles and ionic liquid. The platinum nanoparticles catalyst in ionic liquid prevented the generation of the carbon residue due to the combined effect of denitration and reduced sonication. The presence of the catalyst showed a fivefold increase in dissolution kinetics of ceria as well as absence of graphite erosion, facilitating better chances of graphite recycling than the decontamination without the catalyst. The catalytic approach offers a better recycle strategy for graphite with reduced exfoliation and NOx generation due to denitration, making it a more sustainable decontamination process. Since ceria is used as a surrogate for plutonium oxide, the results can be extended to decontaminate such deposits clearly establishing the utility of the presented results in the nuclear industry.

Intensified sonochemical degradation of 2-Picoline in combination with advanced oxidizing agents.

2-picoline is a very important pyridine derivative with significant applications though it is also poisonous and harmful having considerable adverse influence on aquatic life, environment and organisms. The need for developing effective treatment methodologies for 2-Picoline directed the current work focusing on degradation of 2-Picoline using the combination of ultrasound and advanced oxidants such as hydrogen peroxide (H2O2), potassium persulphate (KPS), Fenton's reagent, and Peroxymonosulphate (PMS) along with the use of Titanium oxide (TiO2) as catalyst. Ultrasonic bath having 8 L capacity and operating frequency of 40 ± 2 kHz has been used. The effect of parameters like power, initial pH, temperature, time and initial concentration of 2-Picoline were studied to establish best operating conditions which were further used in the combination treatment approaches of ultrasound with oxidising agents. The chemical oxygen demand (COD) reduction for the optimized approaches of ultrasound in combination with oxidizing agents was also determined. Degradation experiments were performed using oxidising agents also in absence of ultrasound to investigate the individual treatment capacity of the oxidants and also the synergetic index for the combination. Kinetic study demonstrated that second order model suited for all the treatment approaches except US/Fenton where first order model fitted better. Ultrasound in combination with Fenton reagent demonstrated a substantial synergy for the degradation of 2-Picoline compared to other treatment approaches showing highest degradation of 97.6 %, synergetic index as 5.71, cavitational yield of 1.82 × 10-5 mg/J and COD removal of 82.4 %.

Cavitation-assisted decontamination of yttria from graphite of different densities.

Yttria coated graphite crucibles are widely used to handle molten refractory and radioactive metals like uranium and plutonium. However, the coated layer suffers damages like cracking and peeling off owing to thermal cycles. As a result, removal of the yttria layer from the graphite surface is essential to ensure reuse of the crucible and minimization of radioactive waste. The present work investigates intensified dissolution of yttria from the coated graphite samples using ultrasound as a non-destructive decontamination technique to recycle the graphite substrate. The optimum conditions established for maximum dissolution were 8 M as acid strength, frequency of 30 kHz, temperature of 45 °C and power density of 8 W cm-2 that resulted in maximum dissolution of 52% in 30 min. Use of an oxidant H2O2 to the acid, did not yield any improvement in the dissolution kinetics, instead, increased oxidation of the graphite substrate was observed, leading to the anomalous weight gain of the graphite substrate despite surface erosion. Effect of ultrasound on the dissolution was pronounced, with almost a threefold increase compared to dissolution performed under silent conditions. Rates of dissolution of yttria from the substrate of different densities and pore size distribution were also studied. The dissolution was slowest from graphite of density 1.82 g cm-3 as the pore size distribution was conducive to accommodate the yttria particles. The dissolution in nitric acid followed ash layer diffusion controlled kinetics. The study has demonstrated the efficacy of application of ultrasound for accelerated decontamination of graphite substrates.

Sonochemical recovery of uranium from nanosilica-based sorbent and its biohybrid.

Use of nanomaterials to remove uranium by adsorption from nuclear wastewater is widely applied, though not much work is focused on the recovery of uranium from the sorbents. The present work reports the recovery of adsorbed uranium from the microstructures of silica nanoparticles (SiO2M) and its functionalized biohybrid (fBHM), synthesized with Streptococcus lactis cells and SiO2M, intensified using ultrasound. Effects of temperature, concentration of leachant (nitric acid), sonic intensity, and operating frequency on the recovery as well as kinetics of recovery were thoroughly studied. A comparison with the silent operation demonstrated five and two fold increase due to the use of ultrasound under optimum conditions in the dissolution from SiO2M and fBHM respectively. Results of the subsequent adsorption studies using both the sorbents after sonochemical desorption have also been presented with an aim of checking the efficacy of reusing the adsorbent back in wastewater treatment. The SiO2M and fBHM adsorbed 69% and 67% of uranium respectively in the second cycle. The adsorption capacity of fBHM was found to reduce from 92% in the first cycle to 67% due to loss of adsorption sites in the acid treatment. Recovery and reuse of both the nuclear material and the sorbent (with some make up or activation) would ensure an effective nuclear remediation technique, catering to UN's Sustainable Development Goals.

Strategies to improve biological oxidation of real wastewater using cavitation based pre-treatment approaches.

The present work demonstrates the effective application of pretreatment based on cavitation to improve biological oxidation of real municipal and industrial wastewater. The optimum pretreatment conditions based on ultrasonic cavitation for treatment of municipal wastewater were observed as power dissipation of 90 W, a duty cycle of 70% and H2O2 dosage of 0.2 g/L resulting in about 24.9% COD reduction. The use of modified sludge and ultrasonic pretreatment for biological oxidation resulted in significant reduction in treatment time (36 h) than the treatment time (60 h) required for biological oxidation using untreated sludge as inoculum. Also, significantly enhanced biodegradability index (BI) from 0.33 to 0.6 was achieved using pretreatment for biological oxidation process. For the treatment of real industrial wastewater, different pretreatment approaches based on hydrodynamic cavitation (HC) in combination with H2O2, ozone or Fenton were investigated. The pretreatment using best approach of HC + Fenton resulted in 44.2% of COD reduction in total whereas only 28.1% of COD reduction was achieved for the untreated effluent being applied in the biological oxidation. Overall, the present work demonstrated the effectiveness of the pretreatment based on cavitation for the improved treatment of municipal and industrial wastewaters.

Intensified dissolution of uranium from graphite substrate using ultrasound.

Decontamination of graphite structural elements and recovery of uranium is crucial for waste minimization and recycle of nuclear fuel elements. Feasibility of intensified dissolution of uranium-impregnated graphite substrate using ultrasound has been studied with objective of establishing the effect of operating parameters and the kinetics of sonocatalytic dissolution of uranium in nitric acid. The effect of operating frequency and acoustic intensity as well as the acid concentration and temperature on the dissolution of metal has been elucidated. It was observed that at lower acid concentrations (6 M-8 M), the dissolution ratio increases by 15% on increasing the bath temperature from 45 to 70 °C. At higher acid concentration (>10 M), the increase was only around 5-7% for a similar change in temperature. With 12 M HNO3, pitting was also observed on the graphite surface along with erosion due to high local reaction rates in the presence of ultrasound. For higher frequency of applied ultrasound, lower dissolution rate of uranium was observed though it also leads to high rates of erosion of the substrate. It was thus established that suitable optimization of frequency is required based on the nature of the substrate and the choice of recycling it. The dissolution rate was also demonstrated to increase with acoustic intensity till it reaches to the maximum at the observed optimum (1.2 W/cm2 at 33 kHz). Comparison with silent conditions revealed that enhanced rate was obtained due to the use of ultrasound under optimum conditions. The work has demonstrated the effective application of ultrasound for intensifying the extent of dissolution of metal.

A novel approach for continuous synthesis of calcium carbonate using sequential operation of two sonochemical reactors.

A novel continuous process for the synthesis of calcium carbonate based on precipitation reaction has been developed involving the sequential operation of two sonochemical reactors for the first time. The reactors were also operated as control (conventional approach without ultrasound) to clearly establish the process intensification benefits due to the use of ultrasound. The effect of different operating parameters such as Ca(OH)2 concentration, CO2 flow rate and Ca(OH)2 slurry flow rate on the particle size has been investigated. The obtained calcite particles were characterized using Fourier transform infrared (FTIR), wide angle X-ray diffraction (XRD) and particle size distribution (PSD) analysis. The morphology of the obtained particles was also analyzed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was established that the average particle size obtained in the presence of ultrasound was smaller with much narrow size distribution as compared to the conventional approach. Further, the average particle size was established to decrease with an increase in the Ca(OH)2 slurry concentration and CO2 flow rate with the optimum conditions giving a particle size of 164nm. The particle size was also influenced by the Ca(OH)2 slurry flow rate and under optimum condition of Ca(OH)2 slurry flow rate as 24mL/min, particle size of 135nm was obtained. Only calcite phase of CaCO3 was observed to be formed as established based on the XRD analysis during both the synthesis approaches confirming the stability of the obtained particles. It was also observed that the shape of the crystals varied with the method of synthesis. Rhombohedral calcite particles were formed in the presence of ultrasound whereas the conventional stirring method resulted in spindle shaped particles. Overall, the utility of the ultrasound assisted approach has been clearly established with novel results based on the use of sonochemical reactors in series.

Intensification of extraction of curcumin from Curcuma amada using ultrasound assisted approach: Effect of different operating parameters.

Curcumin, a dietary phytochemical, has been extracted from rhizomes of Curcuma amada using ultrasound assisted extraction (UAE) and the results compared with the conventional extraction approach to establish the process intensification benefits. The effect of operating parameters such as type of solvent, extraction time, extraction temperature, solid to solvent ratio, particle size and ultrasonic power on the extraction yield have been investigated in details for the approach UAE. The maximum extraction yield as 72% was obtained in 1h under optimized conditions of 35°C temperature, solid to solvent ratio of 1:25, particle size of 0.09mm, ultrasonic power of 250W and ultrasound frequency of 22kHz with ethanol as the solvent. The obtained yield was significantly higher as compared to the batch extraction where only about 62% yield was achieved in 8h of treatment. Peleg's model was used to describe the kinetics of UAE and the model showed a good agreement with the experimental results. Overall, ultrasound has been established to be a green process for extraction of curcumin with benefits of reduction in time as compared to batch extraction and the operating temperature as compared to Soxhlet extraction.

Improved synthesis of aluminium nanoparticles using ultrasound assisted approach and subsequent dispersion studies in di-octyl adipate.

The present work reports on an efficient and simple one pot synthetic approach for aluminium nanoflakes and nanoparticles based on the intensification using ultrasound and provides a comparison with the conventional approach to establish the cutting edge process benefits. In situ passivation of aluminium particles with oleic acid was used as the method of synthesis in both the conventional and ultrasound assisted approaches. The aluminium nanoflakes prepared using the ultrasound assisted approach were subsequently dispersed in di-octyl adipate (DOA) and it was demonstrated that a stable dispersion of aluminium nanoflakes into di-octyl adipate (DOA) is achieved. The morphology of the synthesized material was established using the transmission electron microscopy (TEM) analysis and energy dispersive X-ray analysis (EDX) and the obtained results confirmed the metal state and nano size range of the obtained aluminium nanoflakes and particles. The stability of the aluminium nanoflakes obtained using ultrasound assisted approach and nanoparticles using conventional approach were characterized using the zeta potential analysis and the obtained values were in the range of -50 to +50mV and -100 to +30mV respectively. The obtained samples from both the approaches were also characterized using X-ray diffraction (XRD) and particle size analysis (PSA) to establish the crystallite size and particle distribution. It was observed that the particle size of the aluminium nanoflakes obtained using ultrasound assisted approach was in the range of 7-11nm whereas the size of aluminium nanoparticles obtained using conventional approach was much higher in the range of 1000-3000nm. Overall it was demonstrated that the aluminium nanoflakes obtained using the ultrasound assisted approach showed excellent morphological characteristics and dispersion stability in DOA showing promise for the high energy applications.

Ultrasonic degradation of poly(vinyl alcohol) in aqueous solution.

Solution of poly(vinyl alcohol) in water with different concentrations (by weight 1%, 1.5%, 2%) and different volumes (50, 75 and 100 ml) were subjected to ultrasonic degradation. A method of viscometry was used to study the degradation behavior and kinetic model was developed to estimate the degradation rate constant. The degradation rate constant was correlated with the power input due to ultrasonic irradiation and reaction volume. It was found that rate constant decreases as the reaction volume and concentration increases. The proportionality index of the relation between rate constant, power input and reaction volume was found to be nearly equal for all concentrations studied. The proportionality constant was found to be approximately equal for 1% and 1.5% solution and for 2% solution it was approximately half the value for that of 1% and 1.5% solutions. The decrease in rate constant and proportionality constant is attributed to the fact that at higher concentration and at higher volume, the intensity of cavitation phenomenon is depressed and therefore the extent of polymer chain breaking decreases. The difference in the values of limiting viscosities (constant solution viscosity which does not decrease by further ultrasonic irradiation) for 50, 75 and 100 ml solutions for each of 1% and 1.5% concentration was negligible. But 2% solution at 100 ml volume showed slightly higher value of limiting viscosity than that for 50 and 75 ml.

Ultrasound enhances lipase-catalyzed synthesis of poly (ethylene glutarate).

The present work explores the best conditions for the enzymatic synthesis of poly (ethylene glutarate) for the first time. The start-up materials are the liquids; diethyl glutarate and ethylene glycol diacetate, without the need of addition of extra solvent. The reactions are catalyzed by lipase B from Candida antarctica immobilized on glycidyl methacrylate-ter-divinylbenzene-ter-ethylene glycol dimethacrylate at 40°C during 18h in water bath with mechanical stirring or 1h in ultrasonic bath followed by 6h in vacuum in both the cases for evaporation of ethyl acetate. The application of ultrasound significantly intensified the polyesterification reaction with reduction of the processing time from 24h to 7h. The same degree of polymerization was obtained for the same enzyme loading in less time of reaction when using the ultrasound treatment. The degree of polymerization for long-term polyesterification was improved approximately 8-fold due to the presence of sonication during the reaction. The highest degree of polymerization achieved was 31, with a monomer conversion of 96.77%. The ultrasound treatment demonstrated to be an effective green approach to intensify the polyesterification reaction with enhanced initial kinetics and high degree of polymerization.

Synthesis of titanium dioxide by ultrasound assisted sol-gel technique: effect of calcination and sonication time.

Nanostructured titanium dioxide has been synthesized using both conventional and ultrasound assisted sol-gel technique with an objective of understanding the role of cavitational effects in the synthesis process. The experiments were conducted at a constant calcination temperature of 750 °C and the calcination time was varied from 30 min to 3 h to study the effect of calcination time on the properties of the synthesized TiO2. The TiO2 specimens were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The influence of the sonication time on the phase transformation process from anatase to rutile and also on the crystallite size and percentage crystallinity of the synthesized TiO2 has also been investigated. It was observed that 100% phase transformation occurred after 3 h of calcination for the ultrasound assisted sol-gel synthesized TiO2. The study on the phase transformation via variation of sonication time yielded interesting results. It was observed that as the sonication time increased, an initial increase in the rutile content is obtained and beyond optimum sonication time, the rutile content decreased. In general, the ultrasound assisted process results in synthesis of TiO2 material with higher rutile content as compared to the conventional sol-gel process.

Ultrasonic degradation of 2:4:6 trichlorophenol in presence of TiO2 catalyst.

The degradation of trichlorophenol has been studied at different intensities of irradiation using ultrasonic horn by changing the power input to the system. Effect of presence of catalyst TiO2 and concentration of catalyst on the degradation rates has also been investigated. The rates of degradation are found to be higher at higher intensities in the absence of catalyst but reverse trend is observed in the presence of catalyst. Adsorption and desorption characteristics of trichlorophenol on TiO2 catalyst have been examined. The catalyst enhances the rates of degradation but it also adsorbs some amount of TCP during the degradation process protecting it from ultrasonic degradation. Thus it is essential to consider the adsorption and desorption kinetics to get proper estimates of the degradation rates when the rates of degradation of TCP are calculated by analyzing the residual concentration of the compound in the liquid. The degradation of the pollutant seems to take place in the liquid only and that too only after desorption of the same from the solid particles. Solid particles seem to play a physical role in the overall degradation scheme, providing additional surface cavitation.

Mixing time analysis of a sonochemical reactor.

Mixing time measurements have been carried out in a cylindrical reactor irradiated with an ultrasonic horn fitted with different size tips. Liquid phase bulk velocities induced by the vibrating horn surface have been estimated from the mixing time measurements. A relationship has been established between the mean horn surface velocities (frequency x amplitude) and the mean velocities estimated from the mixing time measurements. A correlation has been developed for the prediction of the mixing time using a method similar to that used for liquid jet mixing. This could be the first step in defining the overall flow field, the information about which can then be used to get realistic numerical solutions of the Rayleigh-Plesset equation for a travelling cavity to understand the cavity dynamics in the various parts of the ultrasonic horn reactor.

Hybrid reactor based on combined cavitation and ozonation: from concept to practical reality.

The present work gives an in depth discussion related to the development of a hybrid advanced oxidation reactor, which can be effectively used for the treatment of various types of water. The reactor is based on the principle of intensifying degradation/disinfection using a combination of hydrodynamic cavitation, acoustic cavitation, ozone injection and electrochemical oxidation/precipitation. Theoretical studies have been presented to highlight the uniform distribution of the cavitational activity and enhanced generation of hydroxyl radicals in the cavitation zone, as well as higher turbulence in the main reactor zone. The combination of these different oxidation technologies have been shown to result in enhanced water treatment ability, which can be attributed to the enhanced generation of hydroxyl radicals, enhanced contact of ozone and contaminants, and the elimination of mass transfer resistances during electrochemical oxidation/precipitation. Compared to the use of individual approaches, the hybrid reactor is expected to intensify the treatment process by 5-20 times, depending on the application in question, which can be confirmed based on the literature illustrations. Also, the use of Ozonix® has been successfully proven while processing recycled fluids at commercial sites on over 750 oil and natural gas wells during hydraulic operations around the United States. The superiority of the hybrid process over conventional chemical treatments in terms of bacteria and scale reduction as well as increased water flowability and better chemical compatibility, which is a key requirement for oil and gas applications, has been established.

Ultrasound assisted synthesis of doped TiO2 nano-particles: characterization and comparison of effectiveness for photocatalytic oxidation of dyestuff effluent.

The present work deals with the synthesis of titanium dioxide nanoparticles doped with Fe and Ce using sonochemical approach and its comparison with the conventional doping method. The prepared samples have been characterized using X-ray diffraction (XRD), FTIR, transmission electron microscopy (TEM) and UV-visible spectra (UV-vis). The effectiveness of the synthesized catalyst for the photocatalytic degradation of crystal violet dye has also been investigated considering crystal violet degradation as the model reaction. It has been observed that the catalysts prepared by sonochemical method exhibit higher photocatalytic activity as compared to the catalysts prepared by the conventional methods. Also the Ce-doped TiO(2) exhibits maximum photocatalytic activity followed by Fe-doped TiO(2) and the least activity was observed for only TiO(2). The presence of Fe and Ce in the TiO(2) structure results in a significant absorption shift towards the visible region. Detailed investigations on the degradation indicated that an optimal dosage with 0.8 mol% doping of Ce and 1.2 mol% doping of Fe in TiO(2) results in higher extents of degradation. Kinetic studies also established that the photocatalytic degradation followed the pseudo first-order reaction kinetics. Overall it has been established that ultrasound assisted synthesis of doped photocatalyst significantly enhances the photocatalytic activity.

Analysis of semibatch emulsion polymerization: role of ultrasound and initiator.

In this work semibatch miniemulsion was carried out wherein the effect of free radicals produced by ultrasound and an external addition of initiator was examined. Influence of different variables on polymerization rate and polymer particle size has also been investigated. Over a range of 0-4% (by wt) initiator, the polymerization rate was found to increase over a range of 0.56-1.33 g L(-1) min(-1). Similarly monomer concentration range (7.2-15 wt.%) changed the polymerization rate from 1.33 to 2.61 g L(-1) min(-1). Under optimum parametric conditions polymer particle size 50 nm were obtained with a narrow size distribution. Syndiotactic phase of PMMA was observed by controlling the formulation recipe. Although, number of reports could be found in the literature [9,13,17,18,20,22] related to batch emulsion polymerization, this experimental data could be useful for the production of large scale monodispersed PMMA latex as all of the scale-up and design parameters have been qualitatively addressed.

Effect of dissolved gas on efficacy of sonochemical reactors for microbial cell disruption: Experimental and numerical analysis.

In the present work the effect of dissolved gases on the extent of ultrasonically induced microbial cell disruption has been explored using a mathematical model and it has been validated by experimental data from literature. Degassing experiments are carried out and a degassing kinetics model for horn type ultrasonic device is presented. An overall model combining hydrodynamic and kinetics of cell disruption for horn type reactor is then proposed. The model includes several important operational parameters such as stress generated by the cavity, cell wall strength, dissolved gas concentration, degassing due to sonication, acoustic streaming generated due to sonication and attenuation of ultrasound in water. Model basically realizes in categorizing the volume of sonochemical reactor as active cavitation zone (ACZ) and inactive cavitation zone (ICZ). All the transformations are seen to occur only in ACZ. The two regions, i.e. ACZ and ICZ are assumed to behave as two mixed flow reactor arranged in closed loop. Suggestions have been also made for efficient design and scale up of ultrasonic devices for microbial cell disruption. The same model can be extended for other applications like particle size reduction, nano particle synthesis, leaching, emulsification with the knowledge of critical rate controlling parameter.