Increasing efficiency and treatment volumes for sonolysis of per- and poly-fluorinated substances, applied to aqueous film-forming foam.

Sonolysis of per- and polyfluoroalkyl substances (PFAS) has recently matured to field studies, treating real world contamination. However, efficient sonolysis reactor designs are poorly researched. Moreover, the variety and complexity of PFAS pollution slows reactor optimisation and scale-up. In this work, the defluorination of 10.0 mg/L aqueous perfluorooctane sulfonic acid (PFOS) was used as a model metric for the optimisation of; reactor volume (0.6 or 1.4 L), power density (100 - 350 W L-1), number of modular reactors (1-3), and liquid height (56.7 - 340 mm). Note, the ultrasonic frequency (410 kHz) and flow rate (214.2 ml min-1) was optimised in this reactor previously. Peak PFOS defluorination rate (3.40 µmolL-1 min-1) occurred at 141.8 mm, in a 0.6 L reactor, under 200 WL-1 ultrasound. Increasing the number of transducers connected in parallel to one amplifier was able to increase treatment efficiency from 78.6 to 191.8 µmol kWh-1. The model was validated using legacy aqueous film forming foam (AFFF, 3 M FC-602 Lightwater) at different dilutions (×5, ×10, ×20 and ×100). Dilution played a role in AFFF sonolysis efficiency with optimal PFAS sonolysis rate (4.28 µmol L-1 min-1) at 20 × dilution. Overall AFFF was effectively modelled with a synthetic PFOS solution, attributed to limited matrix effects in AFFF sonolysis and high PFAS concentration (0.18-1.83 g L-1) dominated by PFOS (0.15 - 1.53 g L-1).

Flow and temporal effects on the sonolytic defluorination of perfluorooctane sulfonic acid.

The removal of per- and polyfluoroalkyl substance (PFAS) pollution from the environment is a globally pressing issue, due to some PFAS' recalcitrant, bioaccumulative, and carcinogenic nature. Destruction via ultrasonic waves (sonolysis) is a promising contender for industrialisation due to; moderate power consumption, applicability to several PFAS and sample types, and limited by-products. Liquid flow rate through an ultrasonic reactor can affect the size, shape, and spatial distribution of ultrasonic cavities and hence their chemical activity. Such effects have not been studied during PFAS sonolysis, and temporal effects have not been studied much beyond the reactant concentration. Here, the effects of varying recirculating flow rate on the ultrasonic defluorination of perfluorooctane sulfonic acid (PFOS) and implications for industrial scale up are presented. Under the ultrasonic power (200 W L-1, 2.27 W cm-2) and frequency (410 kHz) used, flow rates of 79 and 214 ml min-1 enhanced defluorination up to 14 % during 30 min of treatment. However, these effects were temporal and most significant in the initial minutes of treatment. This indicated a dynamic bubble size distribution which stabilised after around 15 min. Defluorination rates of PFOS were compared with measured potassium iodide dosimetry, calorimetry, sonoluminescence (SL), and sonochemiluminescence (SCL). Flow rates which enhanced defluorination correlated moderately with enhanced SCL and negatively impacted SL, calorimetry, and dosimetry. Effects were attributed to perturbed cavity surfaces, leading to asymmetric cavity collapse, and the possibility of enhanced solvated electron production/interaction. SL, SCL, dosimetry, and calorimetric measurements were also temporal, and each showed different times to equilibrate. Flow rates of 439 and 889 ml min-1 returned all sonochemical measurements to the levels without flow, likely due to continued collapse temperature quenching by furthered bubble asymmetry. Flow also enhanced reactor cooling, which is significant for industrial temperature control. The pump energy consumed was small (≈1.9 %) compared to that of the amplifier and chiller, hence, PFOS defluorination was more cost-effective using flow. However, the effect may be limited for the longer treatment times needed for environmental remediation.

Impact of sonication power on the degradation of paracetamol under single- and dual-frequency ultrasound.

The effects of sonication power on the ultrasonic cavitation and sonochemistry as well as the degradation of paracetamol were studied and compared for single- and dual-frequency sonoreactors. For the single-frequency sonication, a 500 kHz plate transducer was employed, with three different calorimetric powers of 8.4, 16.7 and 27.9±3.9 W. For the dual-frequency sonication, the plate transducer was perpendicularly coupled with a low-frequency 20 kHz ultrasonic horn, and three calorimetric powers of 27.9, 33.4, 44.6±3.9 W were studied. At all the studied powers, dual-frequency sonication led to a synergistic effect in the degradation of paracetamol, though varying the power of the horn did not affect the degradation rate. A comparison of the degradation data versus the yield of oxidants as well as the overall intensities of sonoluminescence and sonochemiluminescence suggested the degradation is by the action of oxidants near the surface of the bubbles as the major reaction mechanism. Despite the enhancement observed for the degradation, dual-frequency sonication had no significant effect on the yield of either of the oxidants, regardless of the applied power to the horn. In contrast, dual-frequency sonication decreased the overall sonoluminescence and sonochemiluminescence intensities at all powers studied, suggesting that the application of dual-frequency sonication reduces the size of cavitation bubbles. Normal distribution function analysis confirmed dual-frequency sonication resulted in smaller sonoluminescing bubbles, hence the reduction in the sonoluminescence intensity. The increase in degradation rate under DFUS is attributed to the increase in the transfer of paracetamol from the bulk towards the bubbles. As a result, the availability of the pollutant molecules in the vicinity of the bubbles to react with HO• would increase and consequently, the degradation rate would enhance under DFUS.

A fundamental study on the degradation of paracetamol under single- and dual-frequency ultrasound.

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 sonoluminescence intensity and the yield of reactive oxidant species.

Sonolysis of per- and poly fluoroalkyl substances (PFAS): A meta-analysis.

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. The 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 perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (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 the dilution factor applied to 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.

The effect of ultrasound treatment in combination with nisin on the inactivation of Listeria innocua and Escherichia coli.

Ultrasound, alone or in combination with natural antimicrobials, is a novel food processing technology of interest to replace traditional food decontamination methods, as it is milder than classical sterilisation (heat treatment) and maintains desirable sensory characteristics. However, ultrasound efficacy can be affected by food structure/composition, as well as the order in which combined treatments are applied. More specifically, treatments which target different cell components could result in enhanced inactivation if applied in the appropriate order. The microbial properties i.e. Gram positive/Gram negative can also impact the treatment efficacy. This work presents a systematic study of the combined effect of ultrasound and nisin on the inactivation of the bacteria Listeria innocua (Gram positive) and Escherichia coli (Gram negative), at a range of cavitation conditions (44, 500, 1000 kHz). The order of treatment application was varied, and the impact of system structure was also investigated by varying the concentration of Xanthan gum used to create the food model systems (0 - 0.5% w/v). Microbial inactivation kinetics were monitored, and advanced microscopy and flow cytometry techniques were utilised to quantify the impact of treatment on a cellular level. Ultrasound was shown to be effective against E. coli at 500 kHz only, with L. innocua demonstrating resistance to all frequencies studied. Enhanced inactivation of E. coli was observed for the combination of nisin and ultrasound at 500 kHz, but only when nisin was applied before ultrasound treatment. The system structure negatively impacted the inactivation efficacy. The combined effect of ultrasound and nisin on E. coli was attributed to short-lived destabilisation of the outer membrane as a result of sonication, allowing nisin to penetrate the cytoplasmic membrane and facilitate cell inactivation.

Use of enantiomeric properties of sodium chlorate to assess primary and secondary nucleation under sonication.

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.

A review on pharmaceuticals removal from waters by single and combined biological, membrane filtration and ultrasound systems.

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.

Sonocrystallisation of ZIF-8 in water with high excess of ligand: Effects of frequency, power and sonication time.

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.

Per- and poly-fluoroalkyl substance remediation from soil and sorbents: A review of adsorption behaviour and ultrasonic treatment.

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 PFAS 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.

Analysis of ultrasonic pre-treatment for the ozonation of humic acids.

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 hydroxyl radical (HO) 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 ozone (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). HO 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.

The impact of food model system structure on the inactivation of Listeria innocua by cold atmospheric plasma and nisin combined treatments.

Novel processing methods such as cold atmospheric plasma (CAP) and natural antimicrobials like nisin, are of interest to replace traditional food decontamination approaches as, due to their mild nature, they can maintain desirable food characteristics, i.e., taste, texture, and nutritional content. However, the microbial growth characteristics (planktonic growth/surface colonies) and/or the food structure itself (liquid/solid surface) can impact the inactivation efficacy of these novel processing methods. More specifically, cells grown as colonies on a solid(like) surface experience a completely different growth environment to cells grown planktonically in liquid, and thus could display a different response to novel processing treatments through stress adaptation and/or cross protection mechanisms. The order in which combined treatments are applied could also impact their efficacy, especially if the mechanisms of action are complementary. This work presents a fundamental study on the efficacy of CAP and nisin, alone and combined, as affected by food system structure. More specifically, Listeria innocua was grown planktonically (liquid broth) or on a viscoelastic Xanthan gum gel system (1.5% w/v) and treated with CAP, nisin, or a combination of the two. Both the inactivation system, i.e., liquid versus solid(like) surface and the growth characteristics, i.e., planktonic versus colony growth, were shown to impact the treatment efficacy. The combination of nisin and CAP was more effective than individual treatments, but only when nisin was applied before the CAP treatment. This study provides insight into the environmental stress response/adaptation of L. innocua grown on structured systems in response to natural antimicrobials and novel processing technologies, and is a step towards the faster delivery of these food decontamination methods from the bench to the food industry.

The application of flow to an ultrasonic horn system: Phenol degradation and sonoluminescence.

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.

Ultrasonic degradation of perfluorooctane sulfonic acid (PFOS) correlated with sonochemical and sonoluminescence characterisation.

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 a solution of potassium salt of PFOS (hereafter, K-PFOS). Then the degradation of K-PFOS (10 mg L-1 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 hydrated electron degradation mechanism for PFOS may occur in ultrasonic conditions. This mechanism is a novel hypothesis in the field of PFAS sonolysis.

Tertiary treatment of real abattoir wastewater using combined acoustic cavitation and ozonation.

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).

Flow effects on phenol degradation and sonoluminescence at different ultrasonic frequencies.

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 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, and in flow conditions, the accumulation of volatile phenol degradation products inside the bubbles are likely reduced leading to an increase SL.

Combined effects of flow, surface stabilisation and salt concentration in aqueous solution to control and enhance sonoluminescence.

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.

Disparities between sonoluminescence, sonochemiluminescence and dosimetry with frequency variation under flow.

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.

A review on possible mechanisms of sonocrystallisation in solution.

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.

Modelling the microbial dynamics and antimicrobial resistance development of Listeria in viscoelastic food model systems of various structural complexities.

Minimal processing for microbial decontamination, such as the use of natural antimicrobials, is gaining interest in the food industry as these methods are generally milder than conventional processing, therefore better maintaining the nutritional content and sensory characteristics of food products. The aim of this study was to quantify the impact of (i) structural composition and complexity, (ii) growth location and morphology, and (iii) the natural antimicrobial nisin, on the microbial dynamics of Listeria innocua. More specifically, viscoelastic food model systems of various compositions and internal structure were developed and characterised, i.e. monophasic Xanthan gum-based and biphasic Xanthan gum/Whey protein-based viscoelastic systems. The microbial dynamics of L. innocua at 10 °C, 30 °C and 37 °C were monitored and compared for planktonic growth in liquid, or in/on (immersed or surface colony growth) the developed viscoelastic systems, with or without a sublethal concentration of nisin. Microscopy imaging was used to determine the bacterial colony size and spatial organisation in/on the viscoelastic systems. Selective growth of L. innocua on the protein phase of the developed biphasic system was observed for the first time. Additionally, significant differences were observed in the colony size and distribution in the monophasic Xanthan gum-based systems depending on (i) the type of growth (surface/immersed) and (ii) the Xanthan gum concentration. Furthermore, the system viscosity in monophasic Xanthan gum-based systems had a protective role against the effects of nisin for immersed growth, and a further inhibitory effect for surface growth at a suboptimal temperature (10 °C). These findings give a systematic quantitative insight on the impact of nisin as an environmental challenge on the growth and spatial organisation of L. innocua, in viscoelastic food model systems of various structural compositions/complexities. This study highlights the importance of accounting for system structural composition/complexity when designing minimal food processing methods with natural antimicrobials.