Dr Judy Lee

The presence of nanoplastics in water has become a major environmental concern in the last decade however the knowledge on the origin and formation of these emerging contaminants is lacking due to analytical challenges in detection and quantification techniques. The release of nanoplastics due to the fragmentation of microplastics extracted from a facial scrub and the resulting toxicity on aquatic species are reported here for the first time. The daily use of 4 g of facial scrub could release up to 1011 microplastics of 400 nm in size per litre of wastewater from household drains. Turbulences created by mixing or pumping induced the fragmentation of microplastics into nanoplastics smaller than 10 nm via a crack propagation and failure mechanism, increasing the number of particles in water by one order of magnitude. Compared to microplastics at a fixed concentration number of 6.8×108 part./mL, the generated nanoplastics initiated the death of 54% more cells in zebra fish by passive ingestion via skin diffusion which therefore pose a real threat for aquatic living organisms. These results stress the need to reduce the release of nano/microplastics in the aquatic environment to prevent the contamination of all trophic levels

This editorial provides a perspective on current knowledge and issues associated with plastic pollution with an emphasis on (i) plastics in water and wastewater processing plants, (ii) the emerging concerns for the existence and presence of nanoplastics in the environment and (iii) the need for a standardised protocol for measuring and reporting nano/microplastics.

Mid-high-frequency ultrasound (200-1000 kHz) eliminates organic pollutants and also generates H O . To take advantage of H O , iron species can be added, generating a hybrid sono-Fenton process (sF). This paper presents the possibilities and limitations of sF. Heterogeneous (a natural mineral) and homogeneous (Fe and Fe ions) iron sources were considered. Acetaminophen, ciprofloxacin, and methyl orange were the target organic pollutants. Ultrasound alone induced the pollutants degradation, and the dual competing role of the natural mineral (0.02-0.20 g L ) meant that it had no significant effects on the elimination of pollutants. In contrast, both Fe and Fe ions enhanced the pollutants' degradation, and the elimination using Fe was better because of its higher reactivity toward H O . However, the enhancement decreased at high Fe concentrations (e.g., 5 mg L ) because of scavenger effects. The Fe addition significantly accelerated the elimination of acetaminophen and methyl orange. For ciprofloxacin, at short treatment times, the degradation was enhanced, but the pollutant complexation with Fe that came from the Fenton reaction caused degradation to stop. Additionally, sF did not decrease the antimicrobial activity associated with ciprofloxacin, whereas ultrasound alone did. Therefore, the chemical structure of the pollutant plays a crucial role in the feasibility of the sF process.

This study investigates the recovery of phosphorus from the process water obtained through hydrothermal carbonisation (HTC) of a ‘wet’ biomass waste, namely spent coffee grounds. HTC was shown to liberate more than 82% of the total phosphorus in the grounds in the form of dissolved ortho-phosphate. Nanofiltration was used to concentrate the inorganic nutrients of the HTC process water, achieving a mass concentration factor of 3.9 times. The natural stoichiometry of phosphorus, magnesium and ammoniacal nitrogen in the nanofiltration retentate was favourable for struvite precipitation. 92.8% of aqueous phosphorus was recovered as struvite through simple pH adjustment, yielding a total phosphorus recovery of 75% from the feedstock spent coffee grounds.

Per- and poly-fluoroalkyl substances (PFAS) are xenobiotics, present at variable concentrations in soils and groundwater worldwide. Some of the current remediation techniques being researched or applied for PFAS-impacted soils involve solidification-stabilisation, soil washing, excavation and disposal to landfill, on site or in situ smouldering, thermal desorption, ball milling and incineration. Given the large volumes of soil requiring treatment, there is a need for a more environmentally friendly technique to remove and treat PFASs from soils. Sorbents such as granular/powdered activated carbon, ion exchange resins and silicas are used in water treatment to remove PFAS. In this work, PFAS adsorption mechanisms and the effect of pore size, pH and organic matter on adsorption efficacy are discussed. Then, adsorption of PFAS to soils and sorbents is considered when assessing the viability of remediation techniques. Sonication-aided treatment was predicted to be an effective removal technique for PFASs from a solid phase, and the effect of varying frequency, power and particle size on the effectiveness of the desorption process is discussed. Causes and mitigation strategies for possible cavitation-induced particle erosion during ultrasound washing are also identified. Following soil remediation, degrading the extracted PFAS using sonolysis in a water-organic solvent mixture is discussed. The implications for future soil remediation and sorbent regeneration based on the findings in this study are given.

Pharmaceuticals in wastewater effluents represent a current environmental concern, which demands application of effective processes. In such regard, sonochemical advanced oxidation processes emerge as an attractive alternative, as illustrated herein. This chapter begins presenting some fundamental aspects of sonochemical treatments (e.g. effects of frequency, power and nature of pharmaceutical pollutants). In second place, the transformations of pharmaceuticals are described, considering topics such as the used analytical techniques and structural modifications of pollutants by chemical effects of ultrasound. Then, treatment of diverse wastewater containing pharmaceuticals are shown, paying special attention to degradations in complex matrices, reactors configurations and combination of ultrasound with membrane filtration processes. Final part is dedicated to highlight the key points presented along the chapter.

Abstract The enhancement of the ultrasound system by adding diverse oxidants to remove a model contaminant (acetaminophen, ACE) in water was investigated. Different parameters were evaluated to study their effect on both the degradation kinetics and the synergy of the combination. The variables studied were the ultrasonic frequency (575, 858, and 1135 kHz), type of oxidant (hydrogen peroxide, sodium peroxydisulfate (or persulfate, PDS), and potassium peroxymonosulfate (PMS)), ACE concentration (4, 8, and 40 µM), and oxidant concentration (0.01, 0.1, 1, and 5 mM). Particular interest was placed on synergistic effects, implying that one process (or both) is activated by the other to lead to greater efficiency. Interestingly, the parameters that led to the higher synergistic effects did not always lead to the most favorable degradation kinetics. An increase in ACE removal of 20% was obtained using the highest frequency studied (1135 kHz), PMS 0.1 mM, and the highest concentration of ACE (40 µM). The intensification of degradation was mainly due to the ability of ultrasound to activate oxidants and produce extra hydroxyl radicals (HO • ) or sulfate radicals (SO 4 •– ). Under these conditions, treatment of ACE spiked into seawater, hospital wastewater, and urine was performed. The hospital wastewater matrix inhibited ACE degradation slightly, while the urine components inhibited the pollutant degradation completely. The inhibition was mainly attributed to the competing organic matter in the effluents for the sono-generated radical species. On the contrary, the removal of ACE in seawater was significantly intensified due to “salting out” effects and the production of the strong oxidant HOCl from the reaction of chloride ions with PMS.

Sonolysis has been proposed as a promising treatment technology to remove per- and polyfluoroalkyl substances (PFASs) from contaminated water. The mechanism of degradation is generally accepted to be high temperature pyrolysis at the bubble surface with dependency upon surface reaction site availability. However, the parametric effects of the ultrasonic system on PFAS degradation are poorly understood, making upscale challenging and leading to less than optimal use of ultrasonic energy. Hence, a thorough understanding of these parametric effects could lead to improved efficiency and commercial viability. Here, reactor characterisation was performed at 44, 400, 500 and 1000 kHz using potassium iodide (KI) dosimetry, sonochemiluminescence (SCL), and sonoluminescence (SL) in water and PFOS solution. Then the degradation of PFOS (10 mg / L in 200 mL solution) was investigated at these four frequencies. At 44 kHz, no PFOS degradation was observed. At 400, 500 and 1000 kHz the amount of degradation was 96.9, 93.8 and 91.2%, respectively, over four hours and was accompanied by stoichiometric fluoride release, indicating mineralisation of the PFOS molecule. Close correlation of PFOS degradation trends with KI dosimetry and SCL intensity was observed, which suggested degradation occurred under similar conditions to these sonochemical processes. At 1000 kHz, where the overall intensity of collapse was significantly reduced (measured by SL), PFOS degradation was not similarly decreased. Discussion is presented that suggests a solvated electron degradation mechanism for PFOS may occur in ultrasonic conditions.

This paper presents an intensification study of an ozonation process through an ultrasonic pre-treatment for the elimination of humic substances in water and thus, improve the quality of water treatment systems for human consumption. Humic acids were used as representative of natural organic matter in real waters which present low biodegradability and a high potential for trihalomethane formation. Ultrasonic frequency (98 kHz, 300 kHz and 1 MHz), power (10-40 W) and sonicated volume (150-400 mL) was varied to assess the efficiency of the ultrasonic pre-treatment in the subsequent ozonation process. A direct link between OH radical (●OH) formation and fluorescence reduction was observed during sonication pre-treatment, peaking at 300 kHz and maximum power density. Ultrasound, however, did not reduce total organic carbon (TOC). Injected O3 dose and reaction time were also evaluated during the ozonation treatment. With 300 kHz and 40 W ultrasonic pre-treatment and the subsequent ozonation step (7.4 mg O3/Lgas), TOC was reduced from 21 mg/L to 13.5 mg/L (36% reduction). ●OH attack seems to be the main degradation mechanism during ozonation. A strong reduction in colour (85%) and SUVA254 (70%) was also measured. Moreover, changes in the chemical structure of the macromolecule were observed that led to the formation of oxidation by-products of lower molecular weight

Electrochemical oxidation (EO) and EO related processes, either alone or in combination with pre-ozonation, were investigated as a polishing step for slaughterhouse wastewater treatment. The wastewater had previously been subjected to grit removal, degreasing, biological treatment and settling, but failed to comply with European emission limits for treated urban wastewaters in regards to organic compounds, suspended solids and colour. Besides EO alone, the following processes were applied: EO with hydrogen peroxide (EO/H₂O₂), EO with ultraviolet C light (EO/UVC) and EO with ultraviolet C light and hydrogen peroxide (EO/UVC/H₂O₂). Without pre-ozonation, electrochemical processes could be arranged in the following order according to their ability to mineralisation and colour removal: EO ˂ EO/H₂O₂ ˂ EO/UVC ˂ EO/UVC/H₂O₂. To reach a colour of 25 mg Pt-Co/L, it took more than 480 min for EO, ῀400 min for EO/H₂O₂, ῀260 min for EO/UVC and ῀120 min for EO/UVC/H₂O₂. At this treatment time, chemical oxygen demand and suspended solids were below the European emission limit values. The pre-ozonation step improved organics removal by EO and all related processes by converting the original organic compounds into easily oxidisable compounds. Beyond that, ozonation itself led to suspended solids and colour abatement to values in agreement with the legislated/permissible discharge limits.

The main goal of this work is to evaluate the usage of ozone (O3) as a pre-treatment or simultaneously combined with UVC/H2O2 process for the polishing stage treatment of real bio-treated slaughterhouse wastewater. Two different treatment strategies were tested: i) pre-ozonation of the wastewater followed by an UVC/H2O2 process (two-step treatment); ii) simultaneous application of O3/UVC/H2O2 combined process (one-step treatment). For the two-step strategy, the pre-treatment with 30 mg O3/min for 10 min reduces significantly total suspended solids (TSS), turbidity and colour, reducing light filtering effects and increasing the efficiency of the following UVC/H2O2 process. In turn, the one-step treatment strategy (O3/UVC/H2O2) allows a more efficient use of injected O3 by reducing the amount of O3 required (from 273 to 189 mg O3/Leffluent) to achieve similar mineralization levels. The real bio-treated slaughterhouse wastewater treated by O3/UVC/H2O2 process achieved final colour values of 20 Pt/Co, TSS of 35 mg/L and COD of 61 mg O2/L, allowing its direct discharge into water compartments according to European Council Directive 91/271/EEC.

•Model PET nanofibres were produced by electrospinning and cryosectionning methods.•The adsorption of nanofibres and organic contaminants significantly reduced the membrane permeation.•Periodic gas scouring was efficient at removing nanofibers from the membranes. The fouling of water filtration membranes by nano/microplastic fragments can result in the decline of the water flux across membranes, however, the impact of other types of nano/microplastics on filtration processes, such as nano/micro fibres, has not been well characterised. The flexibility and high aspect ratio of nano/micro fibres could favor their adsorption onto filtration membranes, hence the presence of such fibres in water streams constitutes a potential threat to membrane units. In this work, the fouling of ultrafiltration membranes by a mixed system of poly(ethylene terephthalate) nanofibres and organic contaminants commonly found in textile industry effluents was investigated before mitigation strategies based on periodic gas scouring were developed. The nanofibres were prepared by electrospinning and cryo-sectioning using a freezing agent containing the organic contaminants of interest, poly(ethylene glycol) and poly(vinyl alcohol). Results showed that the organic contaminants induced a water flux decline across the membranes of 50% in less than an hour due to internal pore blocking whereas the adsorption of pure fibres as loose entanglements contributed to less than 10% of the permeation loss observed due to low pore coverage. Although applying periodic gas scouring was not efficient to dislodge the organic contaminants from the pores, this cleaning procedure removed up to 75% of the nanofibres from the surface of the membranes due to the shear forces generated by the gas bubbles. Hence, this study suggests that ultrafiltration membranes and gas scouring procedures could be successfully applied to control and limit the presence of fibres present in complex water streams.

Degradation of seven relevant pharmaceuticals with different chemical structures and properties: acetaminophen (ACE), cloxacillin (CXL), diclofenac (DCF), naproxen (NPX), piroxicam (PXC), sulfacetamide (SAM) and cefadroxil (CDX), in distilled water and mineral water by ultrasound was studied herein. Firstly, proper conditions of frequency and acoustic power were determined based on the degradation ability of the system and the accumulation of sonogenerated hydrogen peroxide (24.4 W and 375 kHz were found as the suitable conditions for the sonochemical treatment of the pharmaceuticals). Under such conditions, the pharmaceuticals degradation order in distilled water was: PXC>DCF~NPX>CXL>ACE>SAM>CDX. In fact, the initial degradation rate showed a good correlation with the Log P parameter, most hydrophobic compounds were eliminated faster than the hydrophilic ones. Interestingly, in mineral water, the degradation of those hydrophilic compounds (i.e., ACE, SAM and CDX) was accelerated, which was attributed to the presence of bicarbonate ions. Afterwards, mineral water containing six different initial concentrations (i.e., 0.331, 0.662, 3.31, 16.55, 33.1, and 331 µM) of selected pharmaceuticals was sonicated, the lowest concentration (0.331 µM) always gave the highest degradation of the pollutants. This result highlights the great ability of the sonochemical process to treat bicarbonate-rich waters containing pollutants at trace levels, as pharmaceuticals. Finally, the addition of ferrous ions to the sonochemical system to generate a sono-Fenton process resulted in an acceleration of degradation in distilled water but not in mineral water. This was attributed to the scavenging of sonogenerated HO• by bicarbonate anion, which decreases H2O2 accumulation, thus limiting the Fenton reaction.

This work reports the influence of ultrasound alone and combined with ozone for the treatment of real abattoir wastewater. Three different frequencies were studied (44, 300 and 1000 kHz) at an applied power of 40 W. The injected ozone dose was fixed at 71 mg/L and the treatment time varied from 1 to 60 min. Using ultrasound alone, 300 kHz was the only frequency showing a reduction in chemical oxygen demand (COD, 18% reduction) and biological oxygen demand (BOD, 50% reduction), while no diminution in microbial content was measured for any of the frequencies studied. Combining ultrasound with ozone, on the contrary, led to a significant decrease in COD (44%) and BOD (78%) removal for the three frequencies under study. A complete inactivation of total coliforms (TC) was obtained, as well as a final value of 99 CFU/mL in total viable counts (TVC, 5 log reduction). That is, the ozonation-sonication combined system was the only treatment method (compared to sonication and ozonation alone) reaching direct discharge limits, as well as meeting drinking water standards for microbial disinfection (TC and TVC).

•Pharmaceutical removal using biological digesters, sonication and membrane filtration.•Biotransformation of pharmaceutical in biological and ultrasound treatment.•Hybrid ultrasound coupled treatment processes with biological reactor and membrane filtration. Contaminants of emerging concern (CEC) such as pharmaceuticals commonly found in urban and industrial wastewater are a potential threat to human health and have negative environmental impact. Most wastewater treatment plants cannot efficiently remove these compounds and therefore, many pharmaceuticals end up in aquatic ecosystems, inducing problems such as toxicity and antibiotic-resistance. This review reports the extent of pharmaceutical removal by individual processes such as bioreactors, advanced oxidation processes and membrane filtration systems, all of which are not 100% efficient and can lead to the direct discharge of pharmaceuticals into water bodies. Also, the importance of understanding biotransformation of pharmaceutical compounds during biological and ultrasound treatment, and its impact on treatment efficacy will be reviewed. Different combinations of the processes above, either as an integrated configuration or in series, will be discussed in terms of their degradation efficiency and scale-up capabilities. The trace quantities of pharmaceutical compounds in wastewater and scale-up issues of ultrasound highlight the importance of membrane filtration as a concentration and volume reduction treatment step for wastewater, which could subsequently be treated by ultrasound.

The mechanical and chemical breakdown of plastic litter increases the release of nano- and microplastics, which have the potential to impact the performance of membranes processes used in water treatment plants. In this work, the extent of fouling of a commercial ultrafiltration poly(sulfone) membrane induced by nano- and microplastics ranging from 13 to 690 nm in size was investigated. The cross-flow filtration of the plastic particles over 48 h at a 1 bar pressure reduced the permeate water flux by 38 % compared to pure water filtration. Over 25 % of the nano- and microplastics initially present in the feed were absorbed onto the membrane surface within the 48 h of filtration. Particulate fouling mechanism was sequentially modelled into intermediate and complete pore blockage, followed by cake layer formation. Hydrophobic interactions and surface repulsion forces were found to dictate the adsorption rate of the nano- and microplastics onto the membrane surface. This work opens the understanding of NPs/MPs interactions with water filtration processes and demonstrates the need to develop solutions limiting the impact of NPs/MPs on current treating units.

Hospital wastewater (HWW) from three different cities in Colombia was characterized. Wastewater quality indicators and 38 relevant pharmaceuticals were examined. The HWW had pH from 6.82 to 8.06, chemical oxygen demand was between 235.5 and 1203 mg L-1, and conductivity ranged from 276.5 to 717.5 mu S cm(-1). Additionally, most of the target pharmaceuticals (20 of 38) had 100% occurrence frequency in the samples due to their high and continuous consumption in the hospitals. Indeed, acetaminophen, diclofenac, azithromycin, ciprofloxacin, sulfamethoxazole, losartan, metoprolol, and omeprazole were present in all samples at concentrations from one up to some hundreds of mu g L-1. Once pharmaceuticals are discharged into local sewage systems or rivers, because of the high dilution of HWW, the individual environmental hazards are low (i.e., risk quotients, RQ < 0.1 were determined). The action of conventional treatments on HWW also decreased the individual environmental risks of pharmaceuticals (RQ values < 0.1). However, the mixture of pharmaceuticals in the HWW had potential environmental risks (as RQ > 0.1 were found), remarking the need for efficient processes to eliminate pharmaceuticals from HWW. This work provides an initial view on the characterization of diverse Colombian HWW, which could be useful for the understanding of the current situation of pollution by pharmaceuticals in Latin America.

Membrane fouling by nano/microplastics (NP/MPs) is an emerging concern threatening the performance of water and wastewater treatment facilities. The NP/MPs can lead to surface adsorption, fouling and potential mechanical abrasion of the membranes. In this work, periodic gas scouring was applied during the filtration of nano/microplastics across ultrafiltration membranes to investigate the impact of shear forces on the adsorption of nano/microplastics. A series of surface energy and chemistry-modified membranes were also used including acrylic acid, cyclopropylamine and hexamethyldisiloxane plasma-modified membranes, allowing for a set of materials with controlled hydrophilicity, roughness and surface charge. Bubbling gas within the system at a gas flow rate of 0.5 to 1 L·min−1 and a water flow rate of 2 L·min−1 was found to limit the water flux decline across the pristine and hydrophobic membranes compared to the filtration experiments performed without cleaning from 38 to 22 and 23%, respectively. The adsorption of nano/microplastics onto the surface of the membranes was also simultaneously decreased from 40 to 25 and 19%, respectively. Interestingly, for the hydrophilised membranes no enhancement in permeance was observed when performing gas scouring due to the already low tendency for selective adsorption of the nano/microplastics onto their surface. The correlation of a dimensionless fouling number to the shear stress number suggested that the shear forces induced by gas scouring reduced nano/microplastics adsorption up to a gas injection ratio (volume fraction of gas) of 0.3, where the wall shear stress at the surface of the membrane was limited. This work offers an advanced physical strategy to reduce and control membrane fouling by nano/microplastics, with potential for this strategy to be adapted for more complex water matrices and plastic particles.

•PFAS sonolysis parameter meta-analysis.•Optimum pH, frequency and temperature derived for PFOX sonolysis.•Degradation occurs via adsorption bubble and headgroup removal.•PFOA/S sonolysis produces only CO2, H2, H+, and F-•Oxidative agents alter degradation mechanism. Human ingestion of Per- and Polyfluoroalkyl Substances (PFAS) from contaminated food and water is linked to the development of several cancers, birth defects and other illnesses. The complete mineralisation of aqueous PFAS by ultrasound (sonolysis) into harmless inorganics has been demonstrated in many studies. However, the range and interconnected nature of reaction parameters (frequency, power, temperature etc.), and variety of reaction metrics used, limits understanding of degradation mechanisms and parametric trends. This work summarises the state-of-the-art for PFAS sonolysis, considering reaction mechanisms, kinetics, intermediates, products, rate limiting steps, reactant and product measurement techniques, and effects of co-contaminants. A meta-analysis showed that mid-high frequency (100-1,000 kHz) sonolysis mechanisms are similar, regardless of reaction conditions, while the low frequency (20-100 kHz) mechanisms are specific to oxidative species added, less well understood, and generally slower than mid-high frequency mechanisms. Arguments suggest that PFAS degradation occurs via adsorption (not absorption) at the bubble interface, followed by headgroup cleavage. Further mechanistic steps toward mineralisation remain to be proven. For the first time, complete stoichiometric reaction equations are derived for PFOA and PFOS sonolysis, which add H2 as a reaction product and consider CO an intermediate. Fluorinated intermediate products are derived for common, and more novel PFAS, and a naming system proposed for novel perfluoroether carboxylates. The meta-analysis also revealed the transition between pseudo first and zero order PFOA/S kinetics commonly occurs at 15-40 µM. Optimum values of; ultrasonic frequency (300 – 500 kHz), concentration (>15-40 μM), temperature (≈20°C), and pH range (3.2 – 4) for rapid PFOX degradation are derived by evaluation of prior works, while optimum values for dilution of PFAS containing firefighting foams and applied power require further work. Rate limiting steps are debated and F- is shown to be rate enhancing, while SO42- and CO2 by products are theorised to be rate limiting. Sonolysis was compared to other PFAS destructive technologies and shown to be the only treatment which fully mineralises PFAS, degrades different PFAS in order of decreasing hydrophobicity, is parametrically well studied and has low-moderate energy requirements (several kWh g-1 PFAS). It is concluded that sonolysis of PFAS in environmental samples would be well incorporated within a treatment train for improved efficiency.

Wastewater treatment plants (WWTPs) are key components for the capture of microplastics (MPs) before they are released into natural waterways. Removal efficiencies as high as 99% may be achieved but sub-micron MPs as well as nanoplastics have been overlooked because of analytical limitations. Furthermore, short MP fibres are of concern because of their low capture rate as well as the lack of understanding of their influence on purification system efficiency. This study has investigated the impact of poly(ethylene terephthalate) (PET) short nanofibres on the performance of polyvinylidene fluoride (PVDF) ultrafiltration membranes during cross-flow operation. Model MP fibres with an average length of 10 +/- 7 mu m and a diameter of 142 +/- 40 nm were prepared via a combination of electrospinning and fine cutting using a cryomicrotome. The manufactured MPs were added to both pure and synthetic domestic wastewater at a concentration of 1 mg.L-1 to determine their impact on the performance of PVDF ultrafiltration membranes. The results show that PET fibres attach to the membrane in a disorganised manner with low pore coverage. The water flux was decreased by 8% for MPs in pure water and no noticeable effect in wastewater after 3 days of filtration. Additionally, the nutrient removal efficiency of the membrane was not altered by the presence of PET MPs. These findings show that MP fibres do not significantly influence the early stages of filtration for a standard concentration of MPs in wastewater treatment plant studies.

Radiation-grafted anion-exchange membranes (RG-AEM) are being developed to evaluate a range of chemistries that have relevance to a variety of electrochemical applications including reverse electrodialysis (RED) salinity gradient power. RG-AEMs are typically fabricated using an electron-beam activated (peroxidated) polymer substrate film. These activated films are first grafted with a monomer, such as vinylbenzyl chloride (VBC) and then reacted with a variety of tertiary amines to yield the desired RG-AEMs. The amination process forms covalently bound quaternary ammonium (QA) head-groups that allow the RG-AEMs to conduct anions such as Cl−. RG-AEMs are of interest as they exhibit high conductivities (100 mS cm−1 at elevated temperatures when containing Cl− anions). However, the current generation of RG-AEMs have two main Achilles' heels: (1) they exhibit low permselectivities; and (2) they exhibit a high degree of swelling in water. Introducing covalent crosslinking into ion-exchange membranes is a well-known strategy to overcome these issues but it often comes with a price – a significantly lowered conductivity (raised in situ resistance). Therefore, the level of crosslinking must be carefully optimised. RG-AEMs can be primarily crosslinked using two methods: (1) introduction of a divinyl monomer into the monomer mixture used during grafting; or (2) introduction of a diamine agent into the amination process. This study looks into both methods where either divinylbenzene (DVB) is added into the grafting mixture or N,N,N′,N′-tetramethylhexane-1,6-diamine (TMHDA) is added into the amination mixture. We show that on the balance of two application-relevant properties (resistances in aqueous NaCl (0.5 mol dm−3) solution and permselectivity), the diamine crosslinking method is the most effective for RG-AEMs being used in RED cells.

The degradation of paracetamol, a widely found emerging pharmaceutical contaminant, was investigated under a wide range of single-frequency and dual-frequency ultrasonic irradiations. For single-frequency ultrasonic irradiation, plate transducers of 22, 98, 200, 300, 400, 500, 760, 850, 1000, and 2000 kHz were employed and for dual-frequency ultrasonic irradiation, the plate transducers were coupled with a 20 kHz ultrasonic horn in opposing configuration. The sonochemical activity was quantified using two dosimetry methods to measure the yield of HO• and H2O2 separately, as well as sonochemiluminescence measurement. Moreover, the severity of the bubble collapses as well as the spatial and size distribution of the cavitation bubbles were evaluated via sonoluminescence measurement. The paracetamol degradation rate was maximised at 850 kHz, in both single and dual frequency ultrasonic irradiation. A synergistic index higher than 1 was observed for all degrading frequencies (200–1000 kHz) under dual frequency ultrasound irradiation, showing the capability of dual frequency system for enhancing pollutant degradation. A comparison of the results of degradation, dosimetry, and sonoluminescence intensity measurement revealed the stronger dependency of the degradation on the yield of HO• for both single and dual frequency systems, which confirms degradation by HO• as the main removal mechanism. However, an enhanced degradation for frequencies higher than 500 kHz was observed despite a lower HO• yield, which could be attributed to the improved mass transfer of hydrophilic compounds at higher frequencies. The sonoluminescence intensity measurements showed that applying dual frequency ultrasonic irradiation for 200 and 400 kHz made the bubbles larger and less uniform in size, with a portion of which not contributing to the yield of reactive oxidant species, whereas for the rest of the frequencies, dual frequency ultrasound irradiation made the cavitation bubbles smaller and more uniform, resulting in a linear correlation between the overall SL intensity and the yield of ROS.

Membrane processing and crystallization are common separation technologies that are widely used to recover, extract and purify food materials. A literature review is provided in this chapter, to highlight the role of ultrasound in mitigating fouling in membrane processing, as well as improving product quality, separation efficiency and repeatability in crystallization processes. Typical examples of ultrasound-assisted membrane processes include microfiltration and ultrafiltration of fruit juices, whey and milk. Sonocrystallization has been mainly studied for the formation of ice, salts, sugars, and lipids in food processing. To minimize energy consumption, ultrasound at low frequency with optimal intensity should be selected to avoid membrane damage and changes to physicochemical properties of food materials.

Influence of the bubble-bubble interaction on the pulsation of encapsulated microbubbles has been studied by numerical simulations under the condition of the experiment reported by Chang et al. [IEEE Trans. Ultrason Ferroelectr. Freq. Control48, 161 (2001)]. It has been shown that the natural (resonance) frequency of a microbubble decreases considerably as the microbubble concentration increases to relatively high concentrations. At some concentration, the natural frequency may coincide with the driving frequency. Microbubble pulsation becomes milder as the microbubble concentration increases except at around the resonance condition due to the stronger bubble-bubble interaction. This may be one of the reasons why the threshold of acoustic pressure for destruction of an encapsulated microbubble increases as the microbubble concentration increases. A theoretical model for destruction has been proposed.

This work proposes a disruptive tube-in-tube membrane microreactor for the intensification of photochemical UVC/H2O2 processes, towards contaminants of emerging concern (CECs) removal from urban wastewaters. The main novelty of this system relies on the radial addition of H2O2 through the porous membrane into the annular reaction zone, providing a more homogeneous distribution of the injected chemical across the whole reactor length. The proposed novel reactor consists of a ceramic ultrafiltration membrane inner tubing and a concentric quartz outer tubing that compose the annulus of the reactor (path length of 3.85 mm). The ultrafiltration membrane is used as a dosing system to deliver small amounts of H2O2 into the annulus of the reactor. In the annulus, where a 2 mg/L of oxytetracycline (OTC) solution flows, UVC light is provided via four mercury lamps located externally to the outer tube. The helical motion of OTC solution around the membrane shell-side enhances H2O2 radial mixing. The efficiency of the photochemical UVC/H2O2 process was evaluated as a function of the OTC flowrate, H2O2 dose, H2O2 dosage method and water matrix. OTC removal efficiencies of ~36% and ~7% were obtained for a synthetic OTC solution and an urban wastewater fortified with the same OTC concentration, using a H2O2 dose of 15.8 mg/L. Besides providing a good performance using low UVC fluence (34 mJ/cm2) and reactor residence time (4.6 s), the reactor has the advantage of an easy upscaling into a real plant by integrating multiple parallel membranes into a single shell.

To avoid structural changes within nanofiltration membranes during operation, pre-compaction of filtration membranes is usually performed. However, even after pre-compaction, the NF270 membrane has previously been shown to display a level of pressure susceptibility in pure water systems, particularly evident at low pressures. For the first time, this study provides experimental evidence for the effect of salinity on the pressure susceptibility of the NF270 membrane. Permeability was shown to decrease with increasing salinity up to 189 mM MgSO4, with the largest reduction (22 %) observed at the lowest MgSO4 concentration (31.49 mM MgSO4). A significant reduction (35 %) in the membrane susceptibility was also observed following the introduction of MgSO4 to a concentration of 31.49 mM. A mathematical expression, developed for pure water systems, was modified to account for salinity effects and fitted the experimental data well for concentrations up to 0.2 M. These results are explained by compaction of the membrane polymer, due to either charge neutralisation at the membrane surface, or electric double layer compression, or both. However, further increases in salinity had no significant effect on membrane susceptibility, suggesting that salt induced membrane compaction occurs at very low concentrations.

Ultrasound is known to promote crystal nucleation, but despite significant research there remains uncertainty about how the mechanisms are affected. Despite the proposal of various primary nucleation theories, most studies provide no way to quantify or observe the extent to which primary nucleation is taking place, leaving open the possibility that sonocrystallisation is occurring by a secondary nucleation-driven mechanism. By utilising the widely reported enantiomeric properties of sodium chlorate, the extent to which ultrasound can induce primary nucleation can clearly be observed. It was demonstrated during seeded cooling crystallisation that when stirring the seed similarity was 99.3% on average, indicating secondary nucleation had almost exclusively taken place. The application of ultrasound however, decreased the seed similarity to 85.8% and 92.4% when applying 98 kHz and 200 kHz ultrasound respectively, clearly showing that primary nucleation had been induced and indicating the frequency dependency of the induced primary nucleation. This frequency dependency suggests a link between crystal nucleation and high intensity cavitation collisions and collapses, and the potential existence of a collapse/collision intensity threshold required to induce primary nucleation. In addition, secondary nucleation rate was investigated using anti-solvent crystallisation and was observed to increase with the application of ultrasound, though it appeared frequency independent (between 98 kHz & 200 kHz), suggesting that higher energy cavitational events are less important in inducing secondary nucleation or that a lower cavitation intensity threshold exists compared to primary nucleation.

At high ultrasound frequencies, active bubble structures are difficult to capture due to the decrease in timescale per acoustic cycle and size of bubbles with increasing frequencies. However the current study demonstrates an association between the spatial distribution of visible bubbles and that of the active bubble structure established in the path of the propagating acoustic wave. By monitoring the occurrence of these visible bubbles, the development of active bubbles can be inferred for high frequencies. A series of still images depicting the formation of visible bubble structures suggest that a strong standing wave field exists at early stages of wave propagation and weakens by the increase in the attenuation of the acoustic wave, caused by the formation of large coalesced bubbles. This attenuation is clearly demonstrated by the occurrence of a force which causes bubbles to be driven toward the liquid surface and limit standing wave fields to near the surface. This force is explained in terms of the acoustic streaming and traveling wave force. It is found that a strong standing wave field is established at 168 kHz. At 448 kHz, large coalesced bubbles can significantly attenuate the acoustic pressure amplitude and weaken the standing wave field. When the frequency is increased to 726 kHz, acoustic streaming becomes significant and is the dominant force behind the disruption of the standing wave structure. The disruption of the standing wave structure can be minimized under certain pulse ON and OFF ratios.

Thin film composite membranes consist of an ultra thin active layer of polymer that governs the membrane's salt rejection and water permeation properties. However, the fragility of the ultrathin layer makes it challenging to select a technique capable of differentiating between the properties of this layer from the supporting layer. In this study, we isolated enough active layer material to characterise the water vapour sorption and free volume cavity size as functions of water activity. The sorption data were modelled using the Guggenheim–Anderson–De Boer (G.A.B.) isotherm and from this the number of sorption sites for water was calculated to be 189×1019 per gram for the active layer from a commercial Dow Filmtec SW30 membrane and 188×1019 per gram for an aromatic polyamide material prepared in house. The activation energy for diffusion of water through the active layer of SW30 was also evaluated and found to be 6.95 kcal/mol, lower than the heat of liquefaction of water. This suggests that water does not permeate as individual molecules in the vapour state, but rather as clusters of water molecules. The free volume cavity size in the active layer as a function of moisture uptake was also investigated and the results were explained using a pore filling and pore swelling mechanism. This study sheds light on the mechanisms of water entry into the active layer, water transport through the active layer, and the corresponding response of the polymer chains, thereby giving critical insight for the development of more novel systems.

Metal nano-porous architectures are a novel class of nanomaterials which has been applied in the fields of catalysis, sensing and gas storage because of their high surface-to-volume ratio, high mechanical strength and long-range ordered architectures. A commonly-used synthetic strategies to achieve architectures with high precision and diverse porosity design is the seed-and-growth method. In this work, using a dual-frequency sequential sonication approach, we have demonstrated a sonochemical-assisted one-pot seeding with a successive shell growth synthetic strategy for mesoporous metal deposition via a gold (Au) nanoparticle and poly(styrene) beads system. A uniform coating of gold nanoparticle seeds with dense surface coverage was formed by first employing 300 kHz ultrasound irradiation while the nano-porous shell growth was then performed under 1 MHz ultrasonic frequency. The precise control over the process conditions and parameters allowed for the design of well-defined shell thicknesses and surface roughness and area. The catalytic property of the MNMs was evaluated for the degradation of 4-nitrophenol and a high catalytic activity was achieved for the most porous gold structures, suggesting synergistic effects between the architecture of the nanomaterials and their surface reactivity.

Thin film composite membranes (TFCs) consist of a thin film of polymer that is responsible for high salt rejection. This layer is made via interfacial polymerisation of two monomers 1,3 phenylene diamine and trimesoyl chloride, with the membrane reported to reach a self limiting thickness of less than 200 nm. This paper reports for the first time the formation of thick free-standing aromatic polyamide membranes of greater than 50 μm in thickness via the well-known interfacial polymerisation technique. The membrane thickness as a function of polymerisation time and monomer concentration was investigated. The polyamide layer formed through interfacial polymerisation is not necessarily homogeneous, but can indeed feature areas of porosity. A mechanism for such a porous structure is proposed and discussed. The ability to form thick free-standing polyamide membranes allows bulk polymer properties to be evaluated for the first time. In particular, in this work we are able to measure the zeta potential of the membrane surface that usually faces the membrane support. We show that this surface is still negatively charged for all pH values above 4.0.

It is important to understand the fundamental behaviour of reverse osmosis membranes under a range of pH and salinity conditions. In this work, experiments and modelling are used in a complementary manner to better understand these fundamentals. We find experimentally that both pH and salinity can influence membrane charge, the fractional free volume (or pore size) and the membrane thickness. The thickness of the membrane is quantified using ellipsometry while the pore size is estimated through glucose filtration. The membrane swells marginally with increasing pH, but more so with increasing salt concentration up to 170mM. The flux of hydronium and hydroxide ions are investigated under similar conditions. At zero salinity, the rejection of H3O+ and OH- increases with increasing surface charge density and remains invariant with respect to applied pressure. Upon the addition of salt, negative rejection of both species is observed, corresponding to increasing flux of these highly mobile ions so as to maintain electroneutrality in the permeate solution. In particular, the solute flux of H3O+ increases significantly with increasing permeate flux, indicative of coupled flow. The Extended Nernst-Planck equation is employed to predict the rejection of these ions, with partitioning at the interface calculated based upon Donnan-equilibrium and steric hindrance. Good agreement with experimental results can be obtained without the adjustment of any parameters. © 2014 Elsevier B.V.

In this study, both the antisolvent sonocrystallization process of sodium chloride and cavitation activity were investigated as a function of frequency (22–1080 kHz) and acoustic calorimetric power (0–30 W). For frequencies between 20 and 139 kHz, the size of the sodium chloride crystals decreased sharply with increasing power. For frequencies 647 and 1080 kHz, a certain power threshold needs to be exceeded before a decrease in the crystal size was observed. This power threshold coincided with the power threshold for sonoluminescence emission from cavitation bubbles. It was found that the onset of cavitation bubble activity, irrespective of the magnitude (measured in terms of sonoluminescence), enhanced the crystal nucleation rate and decreased crystal size. The minimum crystal size obtained was found to decrease with increasing maximum total integrated sonoluminescence intensity. The results suggest sonoluminescence could be used as a measure to evaluate the sonocrystallization process and that a greater collapse intensity would yield the smallest crystals. In addition, photographs of the sonocrystallization process are reported, suggesting a link between nonsymmetrical transient cavitation activity and crystal nucleation.

The crystal nucleation rate of sodium chloride in ethanol was investigated by measuring the induction time at various supersaturation ratios under silent and ultrasound irradiation at frequencies between 22-500 kHz. Under silent conditions, the data follows the classical nucleation theory showing both the homogeneous and heterogeneous regions, giving an interfacial surface tension of 31.0 mJ/m2. Sonication led to a non-linearity in the data and was fitted by a modified classical nucleation theory to account for the additional free energy being supplemented by sonication. For 98 kHz, this free energy increased from 1.33 x 108 to 1.90 x 108 J/m3 for sonication powers of 2 to 15 W, respectively. It is speculated that the energy is being supplemented by the localized bubble collapse and collisions. Increasing frequency from 22 to 500 kHz revealed a minimum induction time obtained at frequencies between 44 and 98 kHz, which has been attributed to the overall collapse intensity being the strongest at these frequencies.

Bubbles in an acoustic field are affected by forces such as primary and secondary Bjerknes forces, which have been shown to be influenced by acoustic pressure, frequency, bubble size and separation distance between bubbles. However, such studies are predominantly theoretical, and are mostly focused on the sign reversal of the secondary Bjerknes force. This study provides experimental data on the effect of a range of bubble sizes (8-30 μm), distances (⩽0.2 mm), acoustic pressures (20-40 kPa) and frequencies (40-100 kHz) on the relative acceleration of two approaching bubbles. Under these conditions, only variations in the magnitude of the attractive force were observed. Using coupled equations of radial and translational motions, the acceleration and secondary Bjerknes force were calculated and compared to the experimental data. The variations in the magnitude of the secondary Bjerknes forces were explained by simulating bubble radius and coupled volume oscillation as a function of time.

When subjected to an ultrasonic standing-wave field, cavitation bubbles smaller than the resonance size migrate to the pressure antinodes. As bubbles approach the antinode, they also move toward each other and either form a cluster or coalesce. In this study, the translational trajectory of two bubbles moving toward each other in an ultrasonic standing wave at 22.4 kHz was observed using an imaging system with a high-speed video camera. This allowed the speed of the approaching bubbles to be measured for much closer distances than those reported in the prior literature. The trajectory of two approaching bubbles was modeled using coupled equations of radial and translational motions, showing similar trends with the experimental results. We also indirectly measured the secondary Bjerknes force by monitoring the acceleration when bubbles are close to each other under different acoustic pressure amplitudes. Bubbles begin to accelerate toward each other as the distance between them gets shorter, and this acceleration increases with increasing acoustic pressure. The current study provides experimental data that validates the theory on the movement of bubbles and forces acting between them in an acoustic field that will be useful in understanding bubble coalescence in an acoustic field.

In this study, the coalescence time between two contacting sub-resonance size bubbles was measured experimentally under an acoustic pressure ranging from 10kPa to 120kPa, driven at a frequency of 22.4kHz. The coalescence time obtained under sonication was much longer compared to that calculated by the film drainage theory for a free bubble surface without surfactants. It was found that under the influence of an acoustic field, the coalescence time could be probabilistic in nature, exhibiting upper and lower limits of coalescence times which are prolonged when both the maximum surface approach velocity and secondary Bjerknes force increases. The size of the two contacting bubbles is also important. For a given acoustic pressure, bubbles having a larger average size and size difference were observed to exhibit longer coalescence times. This could be caused by the phase difference between the volume oscillations of the two bubbles, which in turn affects the minimum film thickness reached between the bubbles and the film drainage time. These results will have important implications for developing film drainage theory to account for the effect of bubble translational and volumetric oscillations, bubble surface fluctuations and microstreaming.

Here we introduce a simple and effective experimental approach to measuring the interaction forces between two small bubbles (approximately 80-140 microm) in aqueous solution during controlled collisions on the scale of micrometers to nanometers. The colloidal probe technique using atomic force microscopy (AFM) was extended to measure interaction forces between a cantilever-attached bubble and surface-attached bubbles of various sizes. By using an ultrasonic source, we generated numerous small bubbles on a mildly hydrophobic surface of a glass slide. A single bubble picked up with a strongly hydrophobized V-shaped cantilever was used as the colloidal probe. Sample force measurements were used to evaluate the pure water bubble cleanliness and the general consistency of the measurements.

Numerical simulations of nonequilibrium chemical reactions inside an air bubble in liquid water irradiated by ultrasound have been performed for various ambient bubble radii. The intensity of sonoluminescence (SL) has also been calculated taking into account electron-atom bremsstrahlung, radiative attachment of electrons to neutral molecules, radiative recombination of electrons and ions, chemiluminescence of OH, molecular emission from nitrogen, etc. The lower bound of ambient radius for an active bubble in SL and sonochemical reactions nearly coincides with the Blake threshold for transient cavitation. The upper bound is in the same order of magnitude as that of the linear resonance radius. In actual experiments, however, the distribution of ambient radius for active bubbles may be narrow at around the threshold ambient radius for the shape instability. The threshold peak temperature inside an air bubble for nitrogen burning is higher than that for oxidant formation. The threshold peak temperatures depend on ultrasonic frequency and acoustic amplitude because chemical reactions inside a bubble are in nonequilibrium. The dominant emission mechanism in SL is electron-atom bremsstrahlung except at a lower bubble temperature than 2000 K, for which molecular emissions may be dominant.

Current industrial livestock production has one of the highest consumptions of water, producing up to ten times more polluted (biological oxygen demand, BOD) wastewaters compared to domestic sewage. Additionally, livestock production grows yearly leading to an increase in the generation of wastewater that varies considerably in terms of organic content and microbial population. Therefore, suitable wastewater treatment methods are required to ensure the wastewater quality meets EU regulations before discharge. In the present study, a combined lab scale activated sludge-filtration-ozonation system was used to treat a pre-treated abattoir wastewater. A 24-h hydraulic retention time and a 13-day solid retention time were used for the activated sludge process, followed by filtration (4–7 μm) and using ozone as tertiary treatment. Average reductions of 93% and 98% were achieved for chemical oxygen demand (COD) and BOD, respectively, obtaining final values of 128 mg/L COD and 12 mg/L BOD. The total suspended solids (TSS) average reduction reached 99% in the same system, reducing the final value down to 3 mg/L. Furthermore, 98% reduction in phosphorus (P) and a complete inactivation of total coliforms (TC) was obtained after 17 min of ozonation. For total viable counts (TVC), a drastic reduction was observed after 30 min of ozonation (6 log inactivation) at an injected ozone dose of 71 mg/L. The reduction percentages reported in this study are higher than those previously reported in the literature. Overall, the combined process was sufficient to meet discharge requirements without further treatment for the measured parameters (COD, BOD, TSS, P, TC and TVC).

In this work, cross-flow filtration experiments using a brackish water reverse osmosis polyamide membrane have been performed to gather boron rejection data as function of feed concentration, pressure, pH and salinity. Increasing transmembrane pressure increases the permeation of boron indicating that convective flow is important. This result is in contrast to the normal assumption that solution diffusion dominates in such systems. The extended Nernst–Planck equation with a Donnan-steric partition coefficient is used to analyse the transport mechanisms of both neutral boric acid and negatively charged borate ions through the RO membrane. The contribution of surface charge is experimentally determined by streaming potential measurements and the electrokinetic surface charge density is then calculated as a function of ionic strength and pH. It is found that a 0.380 nm pore radius and an effective membrane porosity of 0.05 shows good agreement with experimental data. Charge screening becomes more dominant with increasing ionic strength and this contribution is readily incorporated into the model. The study extends our understanding of the transport mechanism of boric acid and borate ions which can assist in predicting the performance of polyamide reverse osmosis membranes. It also raises questions as to the true mechanism of transport through such a membrane.

Sonoluminescence (SL) intensity can be increased with potassium iodide (KI) concentration, attributed to a reduction in the gas concentration of solution. However, bubble properties and active bubble distributions at different frequencies and powers also influence SL intensities. Hence, to elucidate how salt concentration affects SL intensity, a systematic study with parametric variation was undertaken. SL from KI solutions of 0.1, 1 and 2 M concentration, without flow and in the presence of flow at 24, 228 and 626 mL / min was investigated at 44, 300 and 1000 kHz. At all frequencies an increase in KI concentration caused a change in the active SL distributions. For 44 kHz, localised and standing wave field SL activity could be expanded. Flow at this frequency augmented SL and SL was maximised at the lowest power setting under stabilisation at the highest KI concentration. At 300 and 1000 kHz, attenuation of the sound field was reduced, allowing expansion of activity throughout solution. In this instance, augmentation of SL intensity was only observed under flow conditions at concentrations of 1 M (300 kHz) and 2 M (1000 kHz) under stabilisation. It was theorised that a combination of smaller bubbles at higher KI concentrations and flow effects could reduce bubble clustering and enhance field formations. This was most prevalent where the standing wave was reinforced under stabilised (44 and 300 kHz) or flow (1000 kHz) conditions, here the number of active bubbles in high pressure regions likely increases. Lastly, it was found that in KI solutions flow could localise SL activity beneath and at the flow inlet via reflection and aeration mechanisms.

An intensified charge-couped device (CCD) camera was used to capture raw images of multibubble sonoluminescence, generated by 168 and 448 kHz ultrasound. The effect of various air and surfactant concentrations, and pulse conditions on the acoustic pressure distribution, percentage of standing wave component, the structure of the sonoluminescence activity, and speed of streaming was investigated. It was observed that the enhancement in the sonoluminescence intensity by appropriate degassing, pulsing, and addition of sodium dodecylsulfate were closely related to an expansion in the spatial distribution of sonoluminescence activity. This broadening in the spatial distribution is correlated with a high percentage of standing wave component. This effect stems from the reduction in the attenuation of the acoustic field by inhibiting the formation of large coalesced bubbles.

Acoustic cavitation plays a significant role in many applications that include therapeutic medicine, sonochemsitry and materials processing. The type of cavitation bubbles generated, namely transient or stable (repetitive transient), is important for optimizing the efficiency of these applications. We report on a multibubble sonoluminescence (MBSL)-based experimental technique for the detection and the control of type of cavitation at low and high ultrasound frequencies. It is observed that the use of a horn-type sonicator operating at 20 kHz primarily generates transient cavitation bubbles, as reported earlier. However, the use of plate type transducers at low frequencies (25 and 37 kHz) generates a significant amount of stable cavitation, as evidenced from the quenching of sonoluminescence by volatile solutes. Evidence for the existence of transient cavitation at a higher ultrasound frequency (440 kHz) is also provided.

The concentration of skim milk and whey was investigated at a pilot scale using forward osmosis membranes with an installed membrane area of 24 m2. The pilot plant was operated in batch mode using a draw solution (48–57 g/L of NaCl) that mimics the potential brine streams available in a dairy processing plant. This approach avoids or limits the need for the regeneration of a synthetic draw solution. A concentration factor of ∼2.5 was achieved for both the skim milk and fresh whey, resulting in a total solids concentration of ∼21 wt% and 15 wt%, respectively. Increasing the transmembrane pressure was found to be effective in improving the water flux, whereas a much greater increase in the draw solution osmotic pressure would be needed to achieve the same enhancement of flux. This study also showed that small organic molecules, such as lactose, were not fully rejected by the forward osmosis membranes. A cleaning protocol was established for recovering the membrane performance after milk and whey concentration. The specific energy required for milk and whey concentration using only the forward osmosis step (5–10 kWh/t water removed) is much lower than that required by reverse osmosis. Forward osmosis is an energy efficient and effective process for dairy applications if unlimited access to a brine stream can be made available within or in the proximity of dairy processing plants.

Numerical simulations of cavitation noise have been performed under the experimental conditions reported by Ashokkumar et al. (2007) [26]. The results of numerical simulations have indicated that the temporal fluctuation in the number of bubbles results in the broad-band noise. "Transient" cavitation bubbles, which disintegrate into daughter bubbles mostly in a few acoustic cycles, generate the broad-band noise as their short lifetimes cause the temporal fluctuation in the number of bubbles. Not only active bubbles in light emission (sonoluminescence) and chemical reactions but also inactive bubbles generate the broad-band noise. On the other hand, "stable" cavitation bubbles do not generate the broad-band noise. The weaker broad-band noise from a low-concentration surfactant solution compared to that from pure water observed experimentally by Ashokkumar et al. is caused by the fact that most bubbles are shape stable in a low-concentration surfactant solution due to the smaller ambient radii than those in pure water. For a relatively high number density of bubbles, the bubble-bubble interaction intensifies the broad-band noise. Harmonics in cavitation noise are generated by both "stable" and "transient" cavitation bubbles which pulsate nonlinearly with the period of ultrasound.

To understand the behaviour of systems containing clouds of bubbles (multibubble system) in real sonochemical reactors, a new diagnosis method, i.e., optical cavitation probe (OCP), has been proposed. When a laser beam is introduced into the cavitation bubble cloud, the scattered light intensity changes by the collective oscillation of cavitation bubbles. The frequency domain spectrum of the scattered light contains rich information on the cavitation bubble clouds, comparable with the acoustic emission spectra detected by a hydrophone. The significant merits of OCP, such as capability for spatially resolved, non-invasive measurement of the cavitation bubble clouds, robustness even in a violent cavitation field have been experimentally demonstrated. © 2008 Elsevier B.V. All rights reserved.

A simple bubble population model, with emphasis on the bubble–bubble coalescence, is proposed. In this model, the bubble size distribution is simulated through the iteration of fundamental bubble population process: generation, dissolution, and coalescence. With this simple modelling, the bubble size distribution experimentally observed by the pulsed laser diffraction method and the void rate obtained by the capillary system at 443 kHz were successfully simulated. The experimental results on the bubble population growth by the repetitive pulsed sonication and the effect of pulse width on the bubble population were recreated by the numerical simulation in a semi-quantitative manner. The importance of coalescence of bubbles especially for the effect of addition of surfactant is demonstrated. By decreasing the coalescence frequency by one order of magnitude in the simulation, both the drastic decrease in the total bubble volume as well as the depression of bubble size distribution centring from a few tens of microns in water to a few microns in a dilute surfactant solution can be simultaneously derived.

Using a standing-wave field, it is possible to trap small objects at nodes of a sound pressure distribution. In the present study, a sound wave was generated by a transducer outside of a microchannel, and propagated into a microchannel on a glass plate, where it generated a standing wave field. When water containing alumina particles was injected into the microchannel, several layers of particles were formed in the sound field. Moreover, when the ultrasound driving frequency was swept, it was possible to control the direction of the particle flow. The sound field was numerically calculated and the experimental results are discussed.

The influence of sodium dodecyl sulfate (SDS) on waveform distortion of 141 kHz ultrasonic standing waves in liquids containing air bubbles was investigated for various transducer powers. Fast Fourier transform (FFT) operations were performed on the pressure waveform to obtain the harmonic components. In addition, the intensity of sonoluminescence (SL) was measured as a function of the power. Waveform distortion was observed for water at high applied power, with the curve exhibiting a steeper gradient for positive pressures and a broadened minimum for negative pressures. This was in reasonable agreement with theoretical studies reported in the literature. Much less distortion was found for a 1 mM SDS solution as the applied power was increased than for water or a 10 mM SDS solution. This may be attributed to a lower population of large coalesced bubbles in the 1 mM solution due to electrostatic repulsion, leading to damping of the sound energy and little cavitation noise because of viscous resistance to bubble radial motion in addition to adsorption and desorption of surfactant molecules at the bubble-liquid interface. For 10 mM SDS, the power threshold for the harmonic components was lower than that for the SL. In this case, it appears that there is a range of applied powers where most bubbles are stable and cannot collapse. The influence of the addition of an electrolyte and a nonionic surfactant was also investigated.

Aqueous solutions of reconstituted whey protein- concentrate (WPC) & isolate (WPI) powders were sonicated at 20 kHz in a batch process for 1–60 min. Sonication at 20 kHz increased the clarity of WPC solutions largely due to the reduction in the size of the suspended insoluble aggregates. The gel strength of these solutions when heated at 80°C for 20 min also increased with sonication, while gelation time and gel syneresis were reduced. These improvements in gel strength were observed across a range of initial pH values, suggesting that the mechanism for gel promotion is different from the well known effects of pH. Examining the microstructure of the whey protein gels indicated a compact network of densely packed whey protein aggregates arising from ultrasound treatment. Comparable changes were not observed with whey protein isolate solutions, which may reflect the absence of larger aggregates in the initial solution or differences in composition.

Sonocrystallisation is the application of ultrasound to the crystallisation process. The benefits obtained by sonication have been widely studied since the beginning of the 20th century and so far it is clear that ultrasound can be a very useful tool for enhancing crystallisation and controlling the properties of the final product. Crystal size, polymorphs, purity, process repeatability and lower induction time are only some of the advantages of sonocrystallisation. Even though the effects of sonication on crystallisation are quite clear, the physical explanation of the phenomena involved is still lacking. Is the presence of cavitation necessary for the process? Or is only the bubbles surface responsible for enhancing crystallisation? Are the strong local increases in pressure and temperature induced by cavitation the main cause of all the observed effects? Or is it the strong turbulence induced in the system, instead? Many questions still remain and can only be appreciated with an understanding of the complexity behind the individual processes of crystallisation and acoustic cavitation. Therefore, this review will first summarise the theories behind crystallisation and acoustic cavitation, followed by a description of all the current proposed sonocrystallisation mechanisms, and conclude with an overview on future prospects of sonocrystallisation applications.

Ultrasonic processing is an emerging technology in food processing. When ultrasound passes through a liquid, bubble nuclei present in the liquid grow by bubble coalescence and rectified diffusion. When these bubbles reach a critical size range, they collapse under near-adiabatic conditions generating extreme conditions within the bubbles and in the surrounding liquid that include intense shear forces, turbulence and microstreaming effects. These ultrasound-induced physical effects are finding increasing use in food and dairy processing, in applications such as the enhancement of whey ultrafiltration, extraction of functional foods, reduction of product viscosity, homogenization of milk fat globules, crystallization of ice and lactose and the cutting of cheese blocks. After a brief introduction to the ultrasonic processing of food systems in general, this review presents a critical discussion of applications in dairy processing, together with the findings of some recent research on the use of ultrasound to modify the functionality of dairy protein ingredients.

Ultrasound is known to promote nucleation of crystals and produce a narrower size distribution in a controlled and reproducible manner for the crystallisation process. Although there are various theories that suggest cavitation bubbles are responsible for sonocrystallisation, most studies use power ultrasonic horns that generate both intense shear and cavitation and this can mask the role that cavitation bubbles play. High frequency ultrasound from a plate transducer can be used to examine the effect of cavitation bubbles without the intense shear effect. This study reports the crystal size and morphology with various mixing speeds and ultrasound frequencies. The results show high frequency ultrasound produced sodium chloride crystals of similar size distribution as an ultrasonic horn. In addition, ultrasound generated sodium chloride crystals having a more symmetrical cubic structure compared to crystals produced by a high shear mixer.

To characterize the bubble populations (size and its number distribution) in a sonochemical reactor, a simple but powerful technique based on the Fraunhofer laser diffraction (LD) has been proposed. In this method, the acoustic wave disturbance to the laser probe in the sonochemical reaction field was eliminated by the temporal separation using pulsed sonication (pulsed LD). With this relatively simple strategy, the temporal development of the bubble size distribution could be evaluated by pulsed LD. A number density of bubbles was estimated by using a calibration data obtained with monosized standard particles. In addition, the effect of pulse length and a surfactant on the bubble population phenomena in a multibubble system are discussed.

The search for lower energy consumption desalination systems has been driving research in the past decade towards the investigation of osmotically driven membrane processes, such as forward osmosis (FO) or osmotic distillation (OD). Despite similarities with reverse osmosis (RO) membranes, thin film composite (TFC) for FO membranes require careful design to reduce salt concentration polarization formation within the large pores composing the supporting layer. An investigation of a novel, highly stable, robust support made solely of carbon nanotubes (CNTs), which could find applications in both RO and FO was undertaken. TFC membranes were fabricated by interfacially polymerizing for the first time a dense poly(amide) (PA) layer on self-supporting bucky-papers (BPs) made of hydroxyl-functionalized entangled CNTs. These hydrophilic supports exhibited low contact angle with water (90%), making it a promising material when compared with poly(sulfone) (PSf), the traditional polymer used to fabricate TFC membrane supports in RO. In addition, the impact of the support hydrophilicity on the stability of the interfacially polymerized film and on water adsorption was investigated by oxygen-plasma treating various potential support materials, exhibiting similar geometrical properties. The morphology and salt diffusion of both CNT BP and PSf supports were investigated, and the novel BP–PA composite membranes were found to be superior to commercially available TFC membranes.

It has been established that the addition of sodium dodecylsulfate (SDS) to water to a concentration of 1 mM increased the integrated sonoluminescence (SL) intensity to a maximum. Moreover, further increase in the SDS concentration to 10 mM decreased the SL intensity to a level comparable to that obtained for water. Photographic images of water and 10 mM SDS have revealed a localized distribution of SL bubbles near the liquid surface. For 1 mM SDS, a homogeneous distribution of SL bubbles was observed throughout the liquid. In this study, a comprehensive investigation was performed to determine the variations in the spatial distribution of SL bubbles as a function of SDS concentration, with and without the addition of sodium chloride (NaCl). It was found that the integrated SL intensity passed through a local minimum as the distribution of SL bubbles transformed from an isolated to a homogeneous distribution at 0.25 and 2.4 mM SDS. Similar transformations in the spatial distribution of SL bubbles within these SDS solutions were also observed upon the addition of a few millimolar NaCl. These variations in the spatial distribution of SL bubbles in aqueous solutions containing an ionic surfactant and electrolyte were believed to be the result of changes in the coalescence stability of bubbles, the attenuation of the acoustic wave, and the standing wave ratio. © 2010 American Chemical Society.

Images of sonoluminescence, sonophotoluminescence and sonochemiluminescence are recorded in order to semi-quantitatively compare the spatial distribution of the cavitation activity at three different ultrasound frequencies (170 kHz, 440 kHz and 700 kHz) and at various acoustic amplitudes. At all ultrasound frequencies investigated, the sonochemically active cavitation zones are much larger than the cavitation zones where sonoluminescence is observed. Also, the sonochemically active bubbles are observed at relatively lower acoustic amplitudes than that required for sonoluminescence bubbles to appear. The acoustic power required for the observation of the initial cavitation bubbles increases with an increase in the ultrasound frequency. The cavitation bubbles are observed relatively uniformly throughout the reactor at 170 kHz whereas they are located away from the transducer at the higher frequencies used in this study. While these observations highlight the complexities involved in acoustic cavitation, possible reasons for the observed results are discussed.

This paper reports for the first time the development in the size and shape of sodium chloride crystals during the anti-solvent crystallization in ethanol under different sonication modes. Sonication using 98 kHz and calorimetric power of 6 W was applied either continuously for a range of crystallisation times (5 – 90 s) or intermittently (5 s pulse). Under silent conditions, crystallization time of 90 s generated crystals with an average size of 73.8 ± 6.9 μm, compared to 8.7 ± 2.8 μm under 90 s of continuous sonication. However, it was observed that within the first 5 s of sonication at the beginning of the crystallization, the average crystal size was already reduced to 7.0 ± 3.3 μm. If the system was left to crystallise further to 90 s without ultrasound, the crystal size grew only slightly to 8.2 ± 1.4 μm. When 5 s burst of ultrasound was applied during the crystallization process, a bimodal distribution of small (from sonication) and large crystals (from the silent period) was obtained. These results imply that the major influence of sonication is crystal nucleation rather than fragmentation, and equilibrium is reached with 5 s sonication by precipitating most of the crystals in solution.

Skim milk was concentrated at 10 °C using forward osmosis (FO), reverse osmosis (RO) and pressure-assisted forward osmosis (PAFO). A pressure of 40 bar, in the form of draw solution osmotic pressure (FO and PAFO modes) or transmembrane hydraulic pressure (RO mode) was applied; an additional hydraulic pressure of 2 bar was applied in the PAFO mode. More severe protein fouling was observed in RO, followed by PAFO and then FO. This was credited to the difference in the initial permeate flux, induced by the different effective driving pressures, with RO having a greater deviation of the initial flux from the critical flux value. The critical flux was determined for the FO and RO modes using a step-wise increase of draw solution osmotic pressure or hydraulic pressure, at a constant milk solids content. The critical flux was between 5.4 L/m2h (1.5 × 10−6 m3/m2s) and 7.2 L/m2h (2 × 10−6 m3/m2s) for both the FO and RO modes at a cross flow velocity of 0.2 m/s. The similarities in the critical flux for FO and RO suggests that the critical flux does not depend on the nature of pressure applied on the system (hydraulic or osmotic). Therefore, when operated at the same flux and crossflow velocity, FO would not fundamentally provide a lower fouling environment compared to RO. An increase of the solids content from 8.7% to 17.3% caused a reduction in the critical flux from 5.4 L/m2h to 3.1 L/m2h (8.5 × 10−7 m3/m2s).

The presence of nano and microplastics in water has increasingly become a major environmental challenge. A key challenge in their detection resides in the relatively inadequate analytical techniques available preventing deep understanding of the fate of nano/microplastics in water. The occurrence of nano/microplastics in water and wastewater treatment plants poses a concern for the quality of the treated water. Due to their broad but small size and diverse chemical natures, nano/microplastics may travel easily along water and wastewater treatment processes infiltrating remediation processes at various levels, representing operational and process stability challenges. This review aims at presenting the current understanding of the fate and impact of nano/microplastics through water and wastewater treatment plants. The formation and fragmentation mechanisms, physical-chemical properties and occurrence of nano/microplastics in water are correlated to the interactions of nano/microplastics with water and wastewater treatment plant processes and potential solutions to limit these interactions are comprehensively reviewed. This critical analysis offers new strategies to limit the number of nano/microplastics in water and wastewater to keep water quality up to the required standards and reduce threats on our ecosystems.

Bubbles generated by acoustic cavitation may be efficient in light production by direct emission (sonoluminescence) or indirect emission (sonochemiluminescence) depending on operating parameters such as acoustic pressure and surface tension. These conditions are quite difficult to reach at very high frequencies, even by concentrating the acoustic power at a given location via focusing the acoustic field thanks to the transducer shape (High Intensity Focused Ultrasound). The current work aims at probing the cavitation bubble behaviour under short frequency sweeps by monitoring sonochemiluminescence and sonoluminescence activities. When the frequency was swept in reverse (negative sweep), an enhancement in the SCL, relative to the SCL observed under a single frequency irradiation, was observed. Conversely, a positive frequency sweep resulted in the quenching of SCL intensity. The degree of SCL enhancement and quenching was also dependent on the rate at which the frequency was being swept and on the change in the size of cavitation bubbles. The size of cavitation bubbles varied with varying starting sweep frequency (3.4, 3.6 and 4.2 MHz), affecting both SCL and sonoluminescence (SL) emissions. The addition of a surfactant (sodium dodecyl sulphate) affected the observed results, possibly due to its influence on coalescence between cavitation bubbles. The results suggest that the enhancement and quenching are related to the response of bubbles generated by the starting frequency to the direction of the frequency sweep and the influence of the sweep rate on growth and coalescence of bubbles, which affected the population of the active bubbles.

The presence of ultrasound-induced cavitation in sonodynamic therapy (SDT) treatments has previously enhanced the activity and delivery of certain sonosensitisers in biological systems. The purpose of this work was to investigate the potential for two novel anti-cancer agents from natural derivatives, sanguinarine and ginger root extract (GRE), as sonosensitisers in an SDT treatment with in vitro PANC-1 cells. Both anti-cancer compounds had a dose-dependent cytotoxicity in the presence of PANC-1 cells. A range of six discreet ultrasound power-frequency configurations were tested and it was found that the cell death caused directly by ultrasound was likely due to the sonomechanical effects of cavitation. Combined treatment used dosages of 100 μM sanguinarine or 1mM of GRE with 15 s sonication at 500 kHz and 10 W. The sanguinarine-SDT and GRE-SDT treatments showed a 6% and 17% synergistic increase in observed cell death, respectively. Therefore both sangunarine and GRE were found to be effective sonosensitisers and warrant further development for SDT, with a view to maximising the magnitude of synergistic increase in toxicity.

The ability of direct contact membrane distillation to concentrate the waste effluent from salty whey, a by-product from the cheese making industry has been investigated. The effect of trace protein in the feed, cross-flow velocity and feed acidity were the factors examined. Flat Sheet PTFE membranes of nominal pore sizes 0.05, 0.22 and 0.45 μm were utilised. A decline in feed flux in the presence of trace protein in the feed was observed, but liquid penetration through the membrane could still be prevented by utilization of a membrane of smaller pore size, to achieve a final total solids concentration of ±30% w/w with water recovery from 37 to 83 %. The pressure-drop across the channel length was also predicted accounting for the feed spacer. To increase the channel length up to 1 m will require operation using the smallest pore size of 0.05 μm, unless very low cross-flow velocities are used. The fouling of the membrane is primarily governed by precipitation of a calcium phosphate salt. However, operation at low pH does not improve the flux or the final salt concentration significantly.

A technique based on pulsed ultrasound and sonoluminescence emission was used to measure the size and size distribution of bubbles generated by 355 kHz power ultrasound under continuous Ar or He flow in aqueous NaCl solutions. It was observed that the bubble size strongly decreased with increasing NaCl concentration and that this decrease was much stronger than in solutions presaturated with Ar or He. This size decrease is attributed to the combination of the salting-out effect of the salt and the introduction of bubble nuclei by the continuous gas flow. Besides, the comparison of Ar and He bubbles underlines the effect of the gas diffusion coefficient on the bubble size reached.

In this review the phenomenon of ultrasonic cavitation and associated sonochemistry is presented through system parameters. Primary parameters are defined and considered, namely; pressure amplitude, frequency and reactor design; including transducer type, signal type, vessel-transducer ratio, liquid flow, liquid height, liquid temperature and the presence of a reflective plate. Secondary parameters are similarly characterised and involve the use of gas and liquid additives to influence the chemical and physical environments. Each of the parameters are considered in terms of their effect on bubble characteristics and subsequent impact on sonochemical activity. Evidence suggests that via parametric variation, the reaction products and efficiency may be controlled. This is hypothesised to occur through manipulation of the structural stability of the bubble.

The effects of ultrasound (frequency and pressure amplitude) and external parameters (fluid flow rate and surface stabilisation) on active sonoluminescence (SL) and sonochemical (SC) bubbles were investigated using common characterisation techniques. The SL from water, sonochemiluminescence (SCL) from luminol solutions and iodide dosimetry were studied at flow rates of 0, 24, 228 and 626 mL / min at 44, 300 and 1000 kHz with and without surface stabilisation. An increase in flow, in general, decreased SL, SCL and dosimetry caused by a reduction in collapse intensity. However, all flow rates were also able to increase SL intensity and the highest flow rate (626 mL / min) could also increase SCL and dosimetry. For SL, augmentation with flow was attributed to a reduction in coalescence bubbles which cause growth to inactive size (44 kHz) and enhancement of the standing wave at the surface of solution (300 and 1000 kHz). Where agitation at the solution surface (44 kHz) caused aeration (without stabilisation), flow may have circulated additional cavitation nuclei, increasing SL. Increases in SCL intensity and dosimetry yields were attributed to increased bubble fragmentation which was more influential for the latter process. Disparities between SCL and dosimetry are discussed in terms of gas concentration and reaction energy requirements influenced by the transient nature of the bubbles. SL and SCL had complimentary behaviour when they were located in the same regions i.e. a reduction in SL resulted in an increase in SCL as bubbles moved from stable to transient in nature. The same was not observed when SL and SCL bubbles were located in different regions. The active region for SL / SCL could differ or overlap, depending on the standing to travelling wave proportions at each frequency effecting active regions. In some cases, increased standing wave proportions throughout the reactor (with surface stabilisation) did not facilitate an increase in SL intensity, as was expected. Here, the travelling wave without stabilisation enabled a stronger area of activity toward the surface with a localised standing wave.

The solubility of calcium phosphate in concentrated dairy brine streams is important in understanding mineral scaling on equipment, such as membrane modules, evaporators, and heat exchangers, and in brine pond operation. In this study, the solubility of calcium phosphate has been assessed in the presence of up to 300 g/L sodium chloride as well as lactose, organic acids, and anions at 10, 30, and 50 °C. As a neutral molecule, lactose has a marginal but still detectable effect upon calcium solubility. However, additions of sodium chloride up to 100 g/L result in a much greater increase in calcium solubility. Beyond this point, the concentrations of ions in the solution decrease significantly. These changes in calcium solubility can readily be explained through changes in the activity coefficients. There is little difference in calcium phosphate speciation between 10 and 30 °C. However, at 50 °C, the ratio of calcium to phosphate in the solution is lower than at the other temperatures and varies less with ionic strength. While the addition of sodium lactate has less effect upon calcium solubility than sodium citrate, it still has a greater effect than sodium chloride at an equivalent ionic strength. Conversely, when these organic anions are present in the solution in the acid form, the effect of pH dominates and results in much higher solubility and a calcium/phosphate ratio close to one, indicative of dicalcium phosphate dihydrate as the dominant solid phase.

The rectified diffusion growth of a single air bubble levitated in an acoustic field (frequency = 22.35 kHz) in water and in aqueous solutions containing surfactants (sodium dodecyl sulfate and sodium dodecylbenzene sulfonate) was investigated. As reported by Crum (J. Acoust. Soc. Am. 1980, 68, 203), the presence of surfactants at the bubble/liquid interface enhanced the growth rate of the bubble by rectified diffusion. It is suggested in this paper that in addition to the effect of surfactants on the surface tension and interfacial resistance to mass transfer, the effect of surface rheological properties may also contribute to the cause of the enhancement observed in the bubble growth rate.

Current literature shows a direct correlation between the sonochemical (SC) process of iodide oxidation and the degradation of phenol solution. This implies phenol degradation occurs primarily via oxidisation at the bubble surface. There is no work at present which considers the effect of fluid flow on the degradation process. In this work, parametric analysis of the degradation of 0.1 mM phenol solution and iodide dosimetry under flow conditions was undertaken to determine the effect of flow. Frequencies of 44, 300 and 1000 kHz and flow rates of 0, 24, 228 and 626 mL / min were applied with variation of power input, air concentration, and surface stabilisation. Phenol degradation was analysed using the 4-aminoantipyrine (4-AAP) method and sonoluminescence (SL) images were evaluated for 0.1, 20 and 60 mM phenol solutions. Flow, at all frequencies under certain conditions, could augment phenol degradation. At 300 kHz there was excellent correlation between phenol degradation and dosimetry indicating a SC process, here flow acted to increase in bubble transience, fragmentation and radical transfer to solution. At 300 kHz, although oxidation is the primary phenol degradation mechanism, it is limited, attributed to degradation intermediates which reduce •OH radical availability and bubble collapse intensity. For 44 and 1000 kHz there was poor correlation between the two SC processes. At 44 kHz (0.01 mM), there was little to suggest high levels of intermediate production, therefore it was theorised that under more transient bubble conditions additional pyrolytic degradation occurs inside the bubbles via diffusion / nanodroplet injection mechanisms. At 1000 kHz, phenol degradation was maximised above all other systems attributed to increased numbers of active bubbles combined with the nature of the ultrasonic field. SL quenching by phenol was reduced in flow systems for the 20 and 60 mM phenol solutions. Here, where the standing wave field was reinforced, and bubble localisation increased, flow and the intrinsic properties of phenol acted to reduce coalescence / clustering. Further, at these higher concentrations, the accumulation of volatile phenol degradation products inside the bubbles are likely reduced in flow conditions leading to an increase SL.

The effect of alcohols on the initial growth of the multibubble sonoluminescence (MBSL) intensity in aqueous solutions has been investigated. With increasing concentrations of the alcohols, the number of pulses required to grow the MBSL intensity to a steady state (Ncrit) increases (relative to that of water) initially to a maximum for all the alcohols used in this study, followed by a decrease for methanol and ethanol. The cause of the initial increase in Ncrit is attributed to the inhibition of bubble coalescence in the system. This inhibition in bubble coalescence results in a population of bubbles with a smaller size range and thus a larger number of pulses is required to grow the bubbles to their sonoluminescing size range. It is suggested that the decrease in the Ncrit at higher alcohol concentrations may be caused by an increase in the bubble growth by rectified diffusion.

The dissolution of a single stationary bubble held in place by a horizontal plate is commonly observed experimentally. For several decades the standard approach to the analysis of such dissolution data has been to apply a correction factor of ln(2)=0.69ln(2)=0.69 to the Epstein–Plesset equation for an isolated bubble. In this paper, the transport equations for a stationary bubble touching a plate are solved numerically for the common case where the flow field caused by the change in system volume as the bubble dissolves can be neglected. It is found that the total bubble lifetime is not well characterised by the use of the ln(2)ln(2) factor. However, in most experimental situations, the initial stages of bubble dissolution are not captured. For low gas solubilities the use of a correction factor of 0.69 to the Epstein–Plesset equation is appropriate once the initial transients have dissipated. The correction factor varies from 0.69 to 0.77 across the full range of situations described in this paper. The mathematical model is validated by comparison to experimental data.

A simple method is described for determining the size of sonoluminescence bubbles generated by acoustic cavitation. The change in the intensity of sonoluminescence, from 4 ms pulses of 515 kHz ultrasound, as a function of the “off” time between acoustic pulses, is the basis of the method. The bubble size determined in water was in the range of 2.8−3.7 μm.

It has previously been reported that the addition of low concentrations of ionic surfactants enhances the steady-state sonoluminescence (SL) intensity relative to water (Ashokkumar; et al. J. Phys. Chem. B 1997, 101, 10845). In the current study, both sonoluminescence and passive cavitation detection (PCD) were used to examine the acoustic cavitation field generated at different acoustic pulse lengths in the presence of an anionic surfactant, sodium dodecyl sulfate (SDS). A decrease in the SL intensity was observed in the presence of low concentrations of SDS and short acoustic pulse lengths. Under these conditions, the inhibition of bubble coalescence by SDS leads to a population of smaller bubbles, which dissolve during the pulse “off time”. As the concentration of surfactant was increased at this pulse length, an increase in the acoustic cavitation activity was observed. This increase is partly attributed to enhanced growth rate of the bubbles by rectified diffusion. Conversely, at long pulse lengths acoustic cavitation activity was enhanced at low SDS concentrations as a larger number of the smaller bubbles could survive the pulse “off time”. The effect of reduced acoustic shielding and an increase in the “active” bubble population due to electrostatic repulsion between bubbles are also significant in this case. Finally, as the surfactant concentration was increased further, the effect of electrostatic induced impedance shielding or reclustering dominates, resulting in a decrease in the SL intensity.

This research investigates surface coated ultrafiltration (UF) polyvinylidene fluoride (PVDF) hollow fiber membrane for the removal of organic micropollutants (OMPs) in water. Coating of PVDF membranes with Poly (1-phenylethene-1,2-diyl) - Polystyrene solution through physical adsorption was carried out under two modes, ‘dipped’ and ‘sprayed’. The performance of the coated membrane in the rejection of model organic micropollutants, caffeine and carbamazepine spiked in deionised water (at 300 g/L and 500 g/L), correlated with the coating methods used. Dipped coated membrane showed a better removal of recalcitrant hydrophobic carbamazepine compared to the ‘sprayed’ coated membrane; while for both methods of coating, removal of caffeine was relatively insignificant. Inferably, hydrophobic interaction and size exclusion may be the major removal mechanism involved in the rejection by the coated membranes. The coating layer potentially enhanced reduction of pore size with resulting effect on membrane permeability and providing more sites for possible hydrophobic interaction.

An understanding of crystallisation within saline effluents is important for the design of both brine crystallisers and brine disposal ponds. In this work, crystallisation of a saline effluent concentrate from the Australian dairy industry has been examined at 22 wt% and 30 wt% total solids and at temperatures between 10 and 70 °C. The precipitation occurs more rapidly at higher temperatures. This trend is dictated by precipitation of calcium phosphate salts, albeit the major constituents of the mixture are NaCl and lactose. The crystallisation induction time can be shortened by introducing cavitation induced by ultrasound. In particular, the use of two short acoustic pulses between 3.7 J/g and 16 J/g at 20 kHz spaced ten minutes apart has maximum impact upon both induction time and crystal size. It is believed that the first ultrasound pulse either generates new nuclei or enhances the mass transfer of solute toward the surface of sub-micron growing crystals. Conversely, the second pulse disrupts the growing crystals and forms secondary nuclei. The ultrasound cannot shift the solution equilibrium and so is not able to improve the low crystal yield. To increase this total yield, further evaporation is necessary. The work provides direction to personnel in the dairy industry of the feasibility of brine crystallisation with respect to energy demand and solid recovery.

The composite consisting of silver nanoparticles deposited on boehmite hybrid was synthesized by NaBH4 reduction technique. The morphology of the composite was studied by TEM, UV/Vis spectrophotometer and particle sizer. The size of the silver nanoparticles deposited on the surface of the boehmite ranged from 10 nm to 100 nm. The contact of silver nanoparticles increased by means of deposition of silver nanoparticles on the boehmite sol and the aggregation of the composites. This leads to the appearance of a shoulder at 450 nm in the UV–Vis absorption spectra with the addition of 0.15 mg and 1.5 mg boehmite. It was found that the intensity of the SERS in the case of the composite was higher than for silver colloids consisting of a concentration of silver greater than 3.2 mM.

High-speed imaging was used to capture the effect of surface-active solutes on the behavior of acoustic bubbles, generated in a microspace using low-frequency ultrasound (60 kHz). By confining cavitation within a microspace, the dynamic behavior of bubbles, such as bubble coalescence, clustering, and fragmentation, could be observed directly. It was observed that bubbles coalesced instantly in water; however, in the presence of surface-active solutes (n-propanol and sodium dodecyl sulfate, SDS) the coalescence was hindered. Low-density bubble clusters were observed in the presence of 1 mM SDS and 0.1 M n-propanol. When 0.1 M sodium chloride was added to 1 mM SDS solution, the extent of clustering and the density of the clusters enhanced significantly; a similar observation was made at a higher SDS concentration (10 mM). The importance of these results in understanding multibubble sonoluminescence data published previously has been addressed. The collective oscillation of a bubble cluster consisting of different sized bubbles and images of bubbles emitting a fountain of microbubbles have also been presented.

The thickening or gelling of protein-based dairy streams and ingredients upon exposure to heat has been an ongoing problem in dairy processing for many decades. This phenomenon can restrict the range of dairy product options and reduce manufacturing efficiencies by limiting the type and extent of heat treatment that can be used. In this report, we outline a novel approach to overcoming this problem. The use of preheating treatments to induce whey protein aggregate formation in whey products is well known in the field. However, we show that the application of ultrasound for a very short duration after such a heating step breaks down these aggregates and prevents their reformation on subsequent heating, thereby reducing the viscosity increase that is usually associated with this process. This novel technique has the potential to provide significant economic benefit to the dairy manufacturing industry

The sonication of an aqueous solution generates cavitation bubbles, which may coalesce and produce larger bubbles. This paper examines the effect of surface-active solutes on such bubble coalescence in an ultrasonic field. A novel capillary system has been designed to measure the change in the total volume resulting from the sonication of aqueous solutions with 515 kHz ultrasound pulses. This volume change reflects the total volume of larger gas bubbles generated by the coalescence of cavitation bubbles during the sonication process. The total volume of bubbles generated is reduced when surface-active solutes are present. We have proposed that this decrease in the total bubble volume results from the inhibition of bubble coalescence brought about by the surface-active solutes. The observed results revealed similarities with bubble coalescence data reported in the literature in the absence of ultrasound. It was found that for uncharged and zwitterionic surface-active solutes, the extent of bubble coalescence is affected by the surface activity of the solutes. The addition of 0.1 M NaCl to such solutes had no effect on the extent of bubble coalescence. Conversely, for charged surface-active solutes, the extent of bubble coalescence appears to be dominated by electrostatic effects. The addition of 0.1 M NaCl to charged surfactant solutions was observed to increase the total bubble volume close to that of the zwitterionic surfactant. This suggests the involvement of electrostatic interactions between cavitation bubbles in the presence of charged surfactants in the solution.

Acoustic cavitation is the fundamental process responsible for the initiation of most of the sonochemical reactions in liquids. Acoustic cavitation originates from the interaction between sound waves and bubbles. In an acoustic field, bubbles can undergo growth by rectified diffusion, bubble-bubble coalescence, bubble dissolution or bubble collapse leading to the generation of primary radicals and other secondary chemical reactions. Surface active solutes have been used in association with a number of experimental techniques in order to isolate and understand these activities. A strobe technique has been used for monitoring the growth of a single bubble by rectified diffusion. Multibubble sonoluminescence has been used for monitoring the growth of the bubbles as well as coalescence between bubbles. The extent of bubble coalescence has also been monitored using a newly developed capillary technique. An overview of the various experimental results has been presented in order to highlight the complexities involved in acoustic cavitation processes, which on the other hand arise from a simple, mechanical interaction between sound waves and bubbles.

The enhancement of sonochemical-reaction efficiency by pulsed ultrasound at 152 kHz has been studied experimentally through absorbance measurements of triiodide ions from sonochemical oxidation of potassium iodide at different liquid volumes to determine sonochemical efficiency defined by reacted molecules per input ultrasonic energy. The mechanism for enhancement of the reaction efficiency by pulsed ultrasound is discussed using captured images of sonochemiluminescence (SCL), and measured time-resolved signals of the SCL pulses and pressure amplitudes. The high sonochemical-reaction efficiency by pulsed ultrasound, compared with that by continuous-wave ultrasound, is attributed both to the residual pressure amplitude during the pulse-off time and to the spatial enlargement of active reaction sites.

This letter reports on the use of frequency sweeps to probe acoustic cavitation activity generated by high-intensity focused ultrasound (HIFU). Unprecedented enhancement and quenching of HIFU cavitation activity were observed when short frequency sweep gaps were applied in negative and positive directions, respectively. It was revealed that irrespective of the frequency gap, it is the direction and frequency sweep rate that govern the cavitation activity. These effects are related to the response of bubbles generated by the starting frequency to the direction of the frequency sweep, and the influence of the sweep rate on growth and coalescence of bubbles, which in turn affects the active bubble population. These findings are relevant for the use of HIFU in chemical and therapeutic applications, where greater control of cavitation bubble population is critical.

Antisolvent crystallization of paracetamol was conducted using ultrasound. The effect of various ultrasonic frequencies and power on the mean crystal size, crystal size distribution, induction time, and type of polymorph obtained was studied. Multibubble sonoluminescence intensity was used to correlate the crystallization results with cavitation activity. Results showed that under optimum conditions ultrasound can significantly (i) reduce the mean crystal size from 170 to 13 μm, (ii) lower the induction time from 360 to 30 s, and (iii) narrow the size distribution. A close association between cavitation activity and rate of nucleation was observed. In addition, crystallization under sonication led to the formation of not only monoclinic polymorph (form I) but also orthorhombic polymorph (form II) of paracetamol, which is otherwise difficult to obtain in the absence of ultrasound

Cheese whey was concentrated to a concentration factor of 2.7 in a pilot scale forward osmosis filtration system, using a commercial cellulose triacetate membrane in a spiral-wound configuration. The whey was concentrated in a batch mode, using sodium chloride as the draw solution at initial osmotic pressures of 53–75 bar. During the process, flux was shown to reduce due to the simultaneous decrease in the bulk osmotic pressure of the draw solution, increase in the bulk osmotic pressure of the whey and the effect of concentration polarisation on both sides of the membrane. The flux is known to be driven by the effective osmotic pressures of whey and the draw solution on the surface of the membrane active layer. A short-cut approach that requires minimal information in advance about the osmotic pressure of whey and the geometry of the filtration system was implemented, enabling the determination of these effective osmotic pressures. The results obtained were shown to be in agreement with the fundamental forward osmosis flux model. The short-cut approach can be utilised for estimating effective osmotic pressures of other liquid food streams to be concentrated by forward osmosis, without the need of external measurements.

Advances made in recent years have allowed the application of colorants obtained from natural sources into textile dyeing. The use of ultrasound in the dyeing method is reported to increase dye uptake and decrease dyeing times. The aim of this work is to further extend the knowledge of natural hair dyes considering the use of ultrasound in the dyeing method with commercially available herbal dyes and using goat hair as a model for human hair. Optimal ultrasonic parameters were selected by considering the effects of sonication times (5, 10 and 15 min), frequencies (44, 400 and 1000 kHz) and total dyeing times (30, 60 and 120 min) in the morphology of the dyed hair and the colour intensity. Damage to the hair surface was evaluated by scanning electron microscopy (SEM) images, differences in colour of the dyed hair was obtained by ImageJ analysis and quantification of dye uptake was determined by UV-visible spectroscopy. The evidence from this study suggests an increase in goat hair coloration with the use of ultrasonic energy. Optimal dyeing conditions in consideration of colouration efficacy without hair damage were identified as sonication at 400 kHz for 10 min with a total dyeing time of 60 min.

A systematic study on the sonocrystallisation of ZIF-8 (zeolitic imidazolate framework-8) in a water-based system was investigated under different mixing speeds, ultrasound frequencies, calorimetric powers and sonication time. Regardless of the synthesis technique, pure crystals of ZIF-8 with high BET (Brunauer, Emmett and Teller) specific surface area (SSA) can be obtained in water after only 5 s. Furthermore, 5 s sonication produced even smaller crystals (~ 0.08 µm). The type of technique applied for producing the ZIF-8 crystals did not have any significant impact on crystallinity, purity and yield. Crystal morphology and size were affected by the use of ultrasound and mixing, obtaining nanoparticles with a more spherical shape than in silent condition (no ultrasound and mixing). However, no specific trends were observed with varying frequency, calorimetric power and mixing speed. Ultrasound and mixing may have an effect on the nucleation step, causing the fast production of nucleation centres. Furthermore, the BET SSA increased with increasing mixing speed. With ultrasound, the BET SSA is between the values obtained under silent condition and with mixing. A competition between micromixing and shockwaves has been proposed when sonication is used for ZIF-8 production. The former increases the BET SSA, while the latter could be responsible for porosity damage, causing a decrease of the surface area.

In a prior paper [Bance-Soualhi et al., J. Mater. Chem. A, 2021, 9, 22025], we showed that the crosslinking of radiation-grafted anion-exchange membranes (RG-AEM) was necessary to obtain high enough apparent permselectivities for use in applications such as (reverse)electrodialysis. However, a separate result in this prior study suggested that comparable AEMs (similar ion-exchange capacity, IEC, and backbone chemistry) with different cationic headgroups may yield different balances between permselectivity and conductivity. This short follow-up study compares the permselectivities and Cl À conductivities of a series of non-crosslinked RG-AEMs with either aliphatic quaternary ammonium headgroups (N-benzyl-N-methylpiperidinium, MPIP, and benzyltrimethylammonium, TMA) or aromatic cationic headgroups (N-benzylpyridinium, PYR, or 1-benzyl-2,3-dimethylimidazolium, DMI). We show that a change in the headgroup chemistry modified the permselectivity-conductivity balance of the RG-AEM, but this was primarily due to the different headgroups inducing varying intrinsic water contents: higher water content RG-AEMs yield lower permselectivites. As also expected from this water content observation, higher IEC variants yielded RG-AEMs with lower permselectivities. The addition of N,N,N 0 ,N 0-tetramethylhexane-1,6-diamine-(TMHDA)-based ionic crosslinking to a DMI-based RG-AEM also raised permselectivity, confirming the observation of the prior study also applies to aromatic headgroup RG-AEMs. In summary, high IEC AEMs containing imidazolium-type headgroups along with an optimal amount of ionic crosslinking could be promising and warrant more study (i.e. a comparison of RG-AEMs with cheaper, more scalable non-RG-analogues that contain these attributes).

Sonochemical (SC) processes can be increased with the application of fluid flow due to changes in bubble characteristics. In this work, a novel flow through set-up was applied to an ultrasonic horn system to investigate the effects of flow on the degradation of phenol. KI dosimetry and sonochemiluminescence (SCL) were also analysed, under the same conditions, to provide comparison of degradation to other SC processes. Further, sonoluminescence (SL) in water and different concentrations of potassium iodide (KI) and phenol solutions was studied to determine the effect of flow on processes inside the bubble that result in SL. The degradation of 0.1 mM phenol solutions, KI dosimetry and SL from phenol (0.1, 20 and 60 mM) and KI (0.1, 1 and 2 M) solutions were analysed under flow rates of 0, 24, 228 and 626 mL/min. For an ultrasonic horn system, all flow rates could augment phenol degradation beyond that of the systems without flow. At the lowest applied power, the amount of degradation was significantly increased with flow, becoming greater than degradation observed at the highest power. A strong correlation between phenol degradation and SC processes indicated that degradation followed an oxidative process. SL intensity from water, KI, and phenol solutions could also be increased with flow beyond the no flow system. For water this occurred most readily at higher powers, then for the solutes there was varied behaviour dependent upon the solute concentration. It was theorised that flow may increase the transfer of radical species to solution to enhance SC processes. An increase in SL, with flow, indicates that flow is acting to change the properties of the bubbles and/or the bubble field such that the active bubbles present collapse with greater total intensity.

This study investigates the long-term performance of the mesophilic (35 °C) anaerobic mono-digestion of process waters (PW) from the hydrothermal carbonisation (HTC) of spent coffee grounds. At an organic loading rate (OLR) of 0.4 gCOD L−1 d−1, initial instability was seen, but after 40 days and supplementary alkalinity, the digestion stabilised with the chemical oxygen demand (COD) in the untreated PW degraded with 37.8–64.6% efficiency and the yield of methane at 0.16 L gCOD−1. An increase in OLR to 0.8 gCOD L−1 d−1 caused a collapse in biogas production, and resulted in severe instability in the reactor, characterised by falling pH and an increasing volatile fatty acid concentration. Comparatively, the digestion of a treated PW (concentrated in nanofiltration and reverse osmosis after removal of the fouling fraction), at OLR between 0.4 and 0.8 gCOD L−1 d−1, was stable over the entire 117 days of treated PW addition, yielded methane at 0.21 L gCOD−1 and the COD was degraded with an average efficiency of 93.5% - the highest efficiency the authors have seen for HTC PW. Further anaerobic digestion of untreated PW at an average OLR of 0.95 gCOD L−1 d−1 was stable for 38 days, with an average COD degradation of 69.6%, and methane production between 0.15 and 0.19 L gCOD−1. The digestion of treated PW produced significantly higher COD degradation and methane yield than untreated PW, which is likely to be related to the removal of refractory and inhibitory organic material in the post-HTC treatment by adsorption of hydrophobic material.

Nano/microplastic materials fouling across filtration membranes can impact the performance of filtration systems, which constitutes a critical challenge for water facilities operation. In this study, plasma surface modifications aiming at reducing nano/microplastic materials adsorption on ultrafiltration membranes were investigated. Hydrophilic acrylic acid and cyclopropylamine plasma coatings caused a water flux decline of less than 8% after 6 h of crossflow filtration. Both hydrophilic coatings reduced the percentage of nano/microplastics adsorbed on the membranes by more than 60%. On the contrary, the hydrophobic hexamethyldisiloxane layer had no impact on the cumulative percentage of adsorbed nano/microplastics compared to that of the pristine poly(sulfone) membranes, which culminated at 40%, resulting in a water flux decline of 40% upon filtration for both membranes. The extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) theory was then applied to the system particle-membrane, which identified polar forces as the predominant intermolecular interactions contributing to membrane fouling. Tuning the hydrophilicity of the membranes was, therefore, a more efficient strategy to reduce nano/microplastic materials adsorption during filtration than tailoring the surface charge of the membranes, showing potential for complex water matrices remediation.

[Display omitted] •Nano and micro plastics are emerging are complex persistent pollutants.•Particulate matter characterisation is challenging and requires advances.•Routes to support capture and remediation are reviewed critically.•Guidelines need to be applied at global level to benchmark data.•Strategies to standardize NP/MP evaluation are discussed. Nano and micro plastics (NP/MPs) represent one of the most challenging classes of micro-pollutants, with occurrence across all ecosystems and size distributions ranging from the nanometre to the millimetre scale. These broad composition and size distribution ranges limit the efficiency of detection methods, often inherently focused on a single and narrow class of NP/MPs sizes. In addition to their demonstrated native toxicity, NP/MPs may act as efficient carriers of pollutants and pathogens onto their surface, facilitating the transfer and penetration of other classes of hazardous materials. This comprehensive review presents the key challenges related to soil, air and water NP/MPs: (i) sampling and extraction, (ii) using defined synthetic NP/MPs for spiking environmental samples and for model studies, and (iii) characterisation. A major challenge discussed in this paper relates to the lack of relevant characterisation strategies for the NP/MPs materials, enabling simultaneous identification, quantification and generation of statistical size distributions. This critical discussion ends with a series of well-informed propositions to support the systematic assessment of the impact of NP/MPs materials, spanning from materials science and characterisation, as well as environmental and chemical engineering.

There are numerous reasons why the exploration of dynamic free volume is particularly important for membrane materials; a few of these include the ability to gain knowledge on plasticization and competitive sorption effects on transport properties. Positron annihilation lifetime spectroscopy (PALS) is a versatile characterisation technique able to measure angstrom to meso sized pores in a variety of materials including polymeric, hybrid, and inorganic membrane materials. Within this work we present the PALS technique and its ability to determine the pore sizes and free volume distribution of a variety of membrane materials. We demonstrate how PALS can be used to observe structural changes in different environments so that we can get a better understanding of how materials behave in dynamic environments. This capability enables understanding of the material behaviour for the particular application environment being investigated and allows us to better tailor the material chemistry and processing to optimize performance.

The viscosity and gelation properties of dairy ingredients may be modified by the use of carefully selected ultrasonication treatments. These properties are retained after freeze or spray drying. The method includes the use of ultrasonication alone at a frequency below 213 kHz. It further includes the use of heating dairy ingredients to a temperature above 65°C followed by ultrasonication at a frequency below 213 kHz, or with simultaneous ultrasonication. This treatment is shown to increase the heat stability of the final product.