Kinetics and Mechanism of Ultrasonic Defluorination of Fluorotelomer Sulfonates.

Ultrasound degrades "legacy" per- and polyfluoroalkyl substances (PFAS) via thermolysis at the interface of cavitation bubbles. However, compared to "legacy" PFAS, polyfluoroalkyl substances have a lesser affinity to the interface and may react with •OH. To understand the effect of size on degradation kinetics and mechanism of polyfluoroalkyl substances, this work compared ultrasonic treatment (f = 354 kHz) of n:2 fluorotelomer sulfonates (FTSAs) of varying chain lengths (n = 4, 6, 8). Of the congeners tested, 4:2 fluorotelomer sulfonate (FtS) degraded the fastest in individual solutions and in mixtures. Sonolytic rate constants correlated to diffusion coefficients of FTSAs, indicating that diffuse short-chain FTSAs outcompete long-chain FTSAs to adsorb and react at the bubble interface. Interestingly, 4:2 and 8:2 FtS had different evolutions of fluoride-to-sulfate ratios, [F-]/[SO42-], over time. Initially, [F-]/[SO42-]4:2 FtS and [F-]/[SO42-]8:2 FtS were respectively higher and lower than theoretical ratios. This difference was attributed to the lower maximum surface excess of 8:2 FtS, hindering its ability to pack and, consequently, defluorinate at the interface. In the presence of an •OH scavenger, FTSAs had similar %F- release compared to no scavenger, whereas %SO42- release was drastically diminished. Therefore, thermolysis is the primary degradation pathway of FTSAs; •OH supplements SO42- formation. These results indicate that ultrasound directly cleaves C-F bonds within the fluoroalkyl chain. This work shows that ultrasound efficiently degrades FTSAs of various sizes and may potentially treat other classes of polyfluoroalkyl substances.

Ohio Coronavirus Wastewater Monitoring Network: Implementation of Statewide Monitoring for Protecting Public Health.

CONTEXT: Prior to the COVID-19 pandemic, wastewater influent monitoring for tracking disease burden in sewered communities was not performed in Ohio, and this field was only on the periphery of the state academic research community. PROGRAM: Because of the urgency of the pandemic and extensive state-level support for this new technology to detect levels of community infection to aid in public health response, the Ohio Water Resources Center established relationships and support of various stakeholders. This enabled Ohio to develop a statewide wastewater SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) monitoring network in 2 months starting in July 2020. IMPLEMENTATION: The current Ohio Coronavirus Wastewater Monitoring Network (OCWMN) monitors more than 70 unique locations twice per week, and publicly available data are updated weekly on the public dashboard. EVALUATION: This article describes the process and decisions that were made during network initiation, the network progression, and data applications, which can inform ongoing and future pandemic response and wastewater monitoring. DISCUSSION: Overall, the OCWMN established wastewater monitoring infrastructure and provided a useful tool for public health professionals responding to the pandemic.

In Situ EPR Spin Trapping and Competition Kinetics Demonstrate Temperature-Dependent Mechanisms of Synergistic Radical Production by Ultrasonically Activated Persulfate.

Ultrasound coupled with activated persulfate can synergistically degrade aqueous organic contaminants. Here, in situ electron paramagnetic resonance spin trapping was used to compare radicals produced by ultrasonically activated persulfate (US-PS) and its individual technologies, ultrasound alone (US) and heat-activated persulfate (PS), with respect to temperature. Radicals were trapped using 5,5-dimethyl-1-pyrroline-N-oxide, DMPO, to form detectable nitroxide adducts. Using initial rates of radical adduct formation, and compared to US and PS, US-PS at 40 and 50 °C resulted in the largest synergistic production of radicals. Radicals generated from US were reasonably consistent from 40 to 70 °C, indicating that temperature had little effect on cavitational bubble collapse over this range. However, synergy indexes calculated from initial rates showed that ultrasonic activation of persulfate at the bubble interface changes with temperature. From these results, we speculate that higher temperatures enhance persulfate uptake into cavitation bubbles via nanodroplet injection. DMPO-OH was the predominant adduct detected for all conditions. However, competition modeling and spin trapping in the presence of nitrobenzene and atrazine probes showed that SO4•- predominated. Therefore, the DMPO-OH signal is derived from SO4•- trapping with subsequent DMPO-SO4- hydrolysis to DMPO-OH. Spin trapping is effective in quantifying total radical adduct formation but limited in measuring primary radical speciation in this case.

Evidence of Air Dispersion: HFPO-DA and PFOA in Ohio and West Virginia Surface Water and Soil near a Fluoropolymer Production Facility.

Perfluorooctanoic acid (PFOA) was used as a fluoropolymer manufacturing aid at a fluoropolymer production facility in Parkersburg, WV from 1951 to 2013. The manufacturer introduced a replacement surfactant hexafluoropropylene oxide dimer acid (HFPO-DA) that has been in use at this site since 2013. Historical releases of PFOA and related epidemiological work in this area has been primarily focused on communities downstream. To provide an update on the ongoing impacts from this plant, 94 surface water samples and 13 soil samples were collected mainly upstream and downwind of this facility. PFOA was detected in every surface water sample with concentrations exceeding 1000 ng/L at 13 sample sites within an 8 km radius of the plant. HFPO-DA was also found to be widespread with the highest levels (>100 ng/L) found in surface water up to 6.4 km north of the plant. One sample site, 28 km north of the plant, had PFOA at 143 ng/L and HFPO-DA at 42 ng/L. Sites adjacent to landfills containing fluorochemical waste had PFOA concentrations ranging up to >1000 ng/L. These data indicate that downwind atmospheric transport of both compounds has occurred and that the boundaries of the impact zone have yet to be fully delineated.

Fast Photomineralization of Dissolved Organic Matter in Acid Mine Drainage Impacted Waters.

Acid mine drainage (AMD) formed from pyrite (iron disulfide) weathering contributes to ecosystem degradation in impacted waters. Solar irradiation has been shown to be an important factor in the biogeochemical cycling of iron in AMD-impacted waters, but its impact on dissolved organic matter (DOM) is unknown. With a typical AMD-impacted water (pH 2.7-3) collected from the Perry State Forest watershed in Ohio, we observed highly efficient (>80%) photochemical mineralization of DOM within hours in a solar simulator resembling twice summer sunlight at 40°N. We confirmed that the mineralization was initially induced by •OH formed from FeOH2+ photodissociation and was inhibited 2-fold by dissolved oxygen removal, suggesting the importance of both the photochemical reaction and oxygen involvement. Size exclusion chromatography and Fourier transform ion cyclotron resonance mass spectrometry elucidated that any remaining organic matter was comprised of smaller and highly aliphatic compounds. The quantitative and qualitative changes in DOM are likely to constitute an important component in regional carbon cycling and nutrient release and to influence downstream aquatic ecosystems in AMD-affected watersheds.

Kinetics and Mechanism of Ultrasonic Activation of Persulfate: An in Situ EPR Spin Trapping Study.

Ultrasound (US) was shown to activate persulfate (PS) providing an alternative activation method to base or heat as an in situ chemical oxidation (ISCO) method. The kinetics and mechanism of ultrasonic activation of PS were examined in aqueous solution using an in situ electron paramagnetic resonance (EPR) spin trapping technique and radical trapping with probe compounds. Using the spin trap, 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), hydroxyl radical (•OH) and sulfate radical anion (SO4•-) were measured from ultrasonic activation of persulfate (US-PS). The yield of •OH was up to 1 order of magnitude greater than that of SO4•-. The comparatively high •OH yield was attributed to the hydrolysis of SO4•- in the warm interfacial region of cavitation bubbles formed from US. Using steady-state approximations, the dissociation rate of PS in cavitating bubble systems was determined to be 3 orders of magnitude greater than control experiments without sonication at ambient temperature. From calculations of the interfacial volume surrounding cavitation bubbles and using the Arrhenius equation, an effective mean temperature of 340 K at the bubble-water interface was estimated. Comparative studies using the probe compounds tert-butyl alcohol and nitrobenzene verified the bubble-water interface as the location for PS activation by high temperature with •OH contributing a minor role in activating PS to SO4•-. The mechanisms unveiled in this study provide a basis for optimizing US-PS as an ISCO technology.

Isoproturon Reappearance after Photosensitized Degradation in the Presence of Triplet Ketones or Fulvic Acids.

Isoproturon (IPU) is a phenylurea herbicide used to control broad-leaf grasses on grain fields. Photosensitized transformation induced by excited triplet states of dissolved organic matter (3DOM*) has been identified as an important degradation pathway for IPU in sunlit waters, but the reappearance of IPU in the absence of light is observed after the initial photolysis. In this study, we elucidate the kinetics of this photodegradation and dark-reappearance cycling of IPU in the presence of DOM proxies (aromatic ketones and reference fulvic acids). Using mass spectrometry and nuclear magnetic resonance spectroscopic techniques, a semi-stable intermediate (IPUint) was found to be responsible for IPU reversion and was identified as a hydroperoxyl derivative of IPU. IPUint is photogenerated from incorporation of diatomic oxygen to IPU and is subjected to thermolysis whose rate depends on temperature, pH, the presence of DOM, and inorganic ions. These results are important to understand the overall aquatic fate of IPU and structurally similar compounds under diurnal conditions.

Kinetics and mechanism of sonochemical degradation of pharmaceuticals in municipal wastewater.

A series of six pharmaceuticals were degraded by continuous wave (CW) and pulsed wave (PW) ultrasound at 205 kHz using deionized water, wastewater effluent, and its isolated organic matter matrices. In deionized water, we observed that hydrophobicity is superior to diffusivity (D(W)) for predicting degradation kinetics. Enhancements in degradation kinetics by the PW mode were greatest for the highest DW (i.e., fluorouracil (5-FU)) and K(OW) (i.e., lovastatin (LOVS)) compounds, indicating that a pharmaceutical with either high diffusivity and low hydrophobicity or low diffusivity and high hydrophobicity benefits from additional time to populate the bubble-water interface during the silent cycle of PW ultrasound. Degradation of 5-FU and LOVS were inhibited by wastewater effluent to a greater extent than the other pharmaceuticals. In addition, a pulse enhancement (PE) for 5-FU and LOVS was not present in wastewater effluent. Irradiating 5-FU and LOVS in hydrophobic (HPO), transphilic (TPI), and hydrophilic (HPI) fractions of effluent organic matter (EfOM) showed that the TPI fraction reduced the PE the most, followed by the HPI and HPO fractions. The smaller size of the TPI over the HPO fraction and higher hydrophobicity of TPI over HPI implicate both size and hydrophobicity of EfOM in hindering degradation of pharmaceuticals.

The impact of inorganic salts on the ultrasonic degradation of contaminants: A review.

This comprehensive review explores the interplay between inorganic salts and ultrasound-assisted degradation of various contaminants. The addition of salt to aqueous matrices has been attributed to increasing contaminant degradation via the salting-out effect. However, research investigating the impact of salt on degradation has yielded inconsistent results. This review incorporated degradation information from 44 studies organizing data according to compound class and ionic strength to analyze the impact of inorganic salts on cavitation bubble dynamics, contaminant behavior, radical species generation, and contaminant degradation. Frequency and salt type were assessed for potential roles in contaminant degradation. The analysis showed that high intensity ultrasound was most beneficial to degradation in salt solutions. Unexpectedly, hydrophilic compounds showed marked enhancement with increasing ionic strength while many hydrophobic compounds did not benefit as greatly. Based on the collected data and analysis, enhanced degradation in the presence of salt appears to be primarily radical-mediated rather than due to the salting-out effect. Finally, the analysis provides guidance for designing sonolytic reactors for contaminant degradation.

Using solid-phase microextraction during ultrasound reveals higher aqueous PAHs release from contaminated sediment.

Ultrasound (US) releases polycyclic aromatic hydrocarbons (PAHs) from contaminated creosote sediments and degrades PAHs in aqueous solution. However, it is unclear how much PAHs release occurs during active US compared to after US is stopped. In this study, solid-phase microextraction (SPME) was used to determine aqueous PAH concentrations desorbed from Little Scioto River creosote contaminated sediment during exposure to 20 kHz ultrasound (US) at a power of 430 W L-1. First, SPME fiber-water partition coefficients,KSPME, were experimentally determined and shown to be comparable with previous studies. Next, PAH concentrations released into aqueous solution were determined by sequentially exposing fresh, conditioned SPME fibers in a reactor containing the contaminated sediment and DI for 10 min periods. Three consecutive 10 min periods each were measured during US and after US. Compared to mixing only, PAHs desorbed during ultrasound was significantly higher. In addition, for phenanthrene, anthracene, and fluoranthene, US showed significantly higher aqueous concentrations during US compared with after US. For these less soluble PAHs, desorption into the aqueous phase reaches and surpasses expected equilibrium aqueous concentrations. However, when US is ceased, PAHs appear to resorb onto sediment resulting in the lower concentrations measured in these PAH compounds after US that are similar to expected equilibrium aqueous concentrations. Typical analytical extraction methods for determining the effects of US require stopping treatment and may underestimate the potential US has for release of contaminants from sediment during US. Using SPME during US treatment reveals that the concentration of PAHs during US may surpass the expected equilibrium aqueous concentration of some PAHs congeners. To our knowledge, this higher concentration observed during US compared to after US has not been shown previously.

Combined ultrasound-ozone treatment for reutilization of primary effluent-a preliminary study.

The present work is a preliminary study on the potential of low-frequency ultrasound irradiation coupled with O3 process for the disinfection of a primary effluent from a municipal wastewater treatment plant preserving nutrient levels (in particular nitrogen and phosphorous), for its possible reuse in civil, industrial, and agricultural sectors. The treated water could be reused, after appropriate dilution, contributing to the circular economy perspective and reducing the need for both chemical fertilizer addition and freshwater supply. The effect of different specific ultrasonic energies and ozone doses was assessed on a bench-top system, composed of an ultrasonic reactor and a semi-batch ozonation vessel. The results showed that the combined US-O3 process produces a good removal efficiency regarding soluble Chemical Oxygen Demand, sCOD (ca. 60%), anionic surfactants (ca. 50%), and formaldehyde (ca. 50%), and an optimal abatement for Methylene Blue Active Substances (MBAS, > 90%). The process also reached high disinfection performances, obtaining 4 logs for E. coli and 5 log abatement for Total Coliforms. The high removal efficiency is matched by an outstanding retention of nutrients (total nitrogen and orthophosphate) highlighting a high potential value for agricultural reuse of the treated primary effluent, with possible significant saving of chemical fertilizers. It was concluded that low-frequency ultrasound pre-treatment, combined with ozonation, could be a useful process for primary effluent recovery for several purposes. Further studies are expected to be planned and executed to evaluate system scale-up feasibility.

Synergistic, aqueous PAH degradation by ultrasonically-activated persulfate depends on bulk temperature and physicochemical parameters.

Coupling ultrasound with other remediation technologies has potential to result in synergistic degradation of contaminants. In this work, we evaluated synergisms from adding high-power ultrasound (20 kHz; 250 W) to activated persulfate over a range of bulk temperatures (20-60 °C). We studied the aqueous degradation kinetics of three polycyclic aromatic hydrocarbons (PAHs: naphthalene, phenanthrene, and fluoranthene) treated by ultrasound-alone, heat-activated persulfate, and combined ultrasonically-activated persulfate (US-PS). At 20 °C, observed US-PS rate constants strongly correlated with Wilke-Chang diffusion coefficients. This correlation indicates PAH molecules diffuse to the bubble-water interface prior to reaction with sulfate radicals (SO4-) generated at the interface. At higher temperatures, observed US-PS rate constants appear to be a more complicated function of temperature and diffusion coefficients. Synergy indexes for PAHs with fast diffusion coefficients were greatest at 20 °C. Fluoranthene, the largest and most hydrophobic PAH, had a maximum synergy index at 30 °C; it benefited from additional thermal persulfate activation in bulk solution. Fluoranthene synergy indexes, however, decreased above 30 °C and became antagonistic at 60 °C. Electron paramagnetic resonance (EPR) spin trapping was used to quantify hydroxyl radical (OH) produced from acoustic cavitation in the absence of persulfate. These data showed consistent OH production from 20 to 60 °C, indicating PAH antagonisms at 60 °C were not due to lower bubble collapse temperatures. Instead, the results suggest that PAH antagonisms are caused by increased radical-radical recombination as bulk temperature increases. In effort to develop an efficient, combined remediation technology, this work suggests bulk temperatures between 20 and 40 °C maximize US-PS synergisms.

Effect of sediment particle size on polycyclic aromatic hydrocarbon bioaccessibility and degradation by ultrasound.

The effect of particle size on sonochemical desorption, degradation and change in bioaccessibility of polycyclic aromatic hydrocarbons (PAHs) on contaminated sediments was investigated. Batch experiments were performed with the whole sediment (WS < 850 µm), a large size range fraction (150 µm < LSR < 850 µm), and a small size range fraction (SSR < 150 µm) of the whole sediment. PAH degradation followed pseudo first-order kinetics; PAHs on LSR sediments underwent more rapid degradation than on SSR and WS sediments (νPAH,LSR > νPAH,WS > νPAH,SSR). In addition, a higher sediment slurry concentration resulted in slower degradation of PAHs. Results are consistent with the more rapid particle size reduction of the LSR. More rapid particle size reduction and faster PAH degradation for the LSR fraction combined with analysis of particle velocities in both size ranges indicates that microjets as opposed to particle-particle collisions due to shockwaves are effective in rapid particle size reduction and PAH degradation. Moreover, the bioaccessible fraction (FPAH,fast,t) of sorbed PAHs in both particle size fractions was found to increase with sonication time but was more rapid with the LSR. Likewise, the more tightly bound PAHs, those in the slow desorbing fraction (FPAH,slow,t) of PAHs, decreased faster with sonication of LSR particles compared to SSR particles, consistent with the trend of particle size reduction. Results of this study suggest that ultrasonic treatment is more effective for larger size particle sediments, although sonication is also viable for small sediment sizes.

Effect of ultrasound frequency on pulsed sonolytic degradation of octylbenzene sulfonic acid.

It has been shown that pulsed ultrasound can influence the amount of surfactant that can adsorb to and decompose at the surface of cavitation bubbles. However, the effect of ultrasound frequency on this process has not been considered. The current study investigates the effect of ultrasound frequency on the pulsed sonolytic degradation of octyl benzenesulfonate (OBS). Furthermore, the effect of pulsing and ultrasound frequency on the rate of *OH radical formation was determined. OBS degradation rates were compared to the rates of *OH radical formation. In this way, conclusions were made regarding the relative importance of accumulation of OBS at cavitation bubble surfaces versus sonochemical activity to the sonochemical decomposition of OBS under different conditions of sonolysis. Comparisons of the data in this way indicate that sonolytic degradation of OBS depends on both the sonochemical activity (i.e., *OH yield) and the accumulation of OBS on cavitation bubble surfaces. However, under a certain set of pulsing and ultrasound frequency exposure conditions, enhanced accumulation of OBS at the gas/solution interface of cavitation bubbles is the sole mechanism of enhanced degradation due to pulsing. On the basis of this finding, conclusions on how pulsing at various ultrasound frequencies affects cavitation bubbles were made.

Direct and indirect photolysis of polycyclic aromatic hydrocarbons in nitrate-rich surface waters.

The photolysis of three polycyclic aromatic hydrocarbons (PAHs)-pyrene, phenanthrene, and naphthalene-were studied in waters taken from creosote-contaminated sites in Gary (IN, USA) and Wilmington (NC, USA). Direct photolysis of all PAHs was observed under simulated solar radiation, with pyrene degrading at a faster rate than either phenanthrene or naphthalene. Phenanthrene degradation, when compared to its direct photolysis rate, increased in Gary water but decreased in Wilmington water. Analysis of the waters for dissolved organic carbon (DOC) and nitrate revealed higher levels of DOC in the Wilmington sample (9.29 mg/L) compared with the Gary sample (6.73 mg/L), as well as significantly less nitrate (0.046 mM vs 0.205 mM for the Gary sample). The slightly lower rate of phenanthrene degradation observed for the Wilmington sample, corrected for light attenuation effects, is statistically the same as that in the direct photolysis experiments. Therefore, we attribute the lower rate of degradation in the presence of Wilmington water to light screening by DOC, but we believe the faster reaction rate observed for the Gary water results from hydroxyl radical (OH*) chemistry generated by nitrate photolysis. Indeed, degradation of the target compound increased when nitrate (at 0.2 and 0.4 mM) was added to the Wilmington sample, further corroborating this conclusion. Overall photoreaction rates decreased for the lower-molecular-weight PAHs, because the fastest naphthalene photolytic rate was roughly two orders of magnitude slower than that of pyrene.

Photochemical acetochlor degradation induced by hydroxyl radical in Fe-amended wetland waters: Impact of pH and dissolved organic matter.

Iron (Fe) plays a critical role in the formation of hydroxyl radical (OH) which may participate in the indirect photodegradation of aquatic contaminants. While Fe photochemistry has been extensively studied, the efficacy of iron amendments for contaminant attenuation in sunlit natural waters has not been well researched. We studied the efficacy of this approach by monitoring OH induced acetochlor (AC) degradation and determining OH production rates with terephthalate (TPA) as a probe. Surface wetland waters as well as model fulvic acid (FA) solutions were amended with Fe(III) salt at different concentrations at pH values of 2.7, 5, and 7.6. We observed no significant enhancement in the AC degradation rate at circumneutral pH. At pH 5, AC degradation increased by more than 50% with an Fe addition up to an [Fe]T ≈ 6 µM and plateaued at high [Fe]T. At the highly acidic pH of acid mine drainage (AMD) waters, AC degradation was enhanced by two-orders-of magnitude with increasing [Fe]T and no plateau was observed under the conditions tested ([Fe]T ≤ 500 µM). While the Fe induced relative difference in OH production rates determined using TPA was useful in elucidating the reaction mechanism for different dissolved organic matter types at different pH values, the absolute value of OH production rates over-predicted the transformation of AC suggesting the existence of unknown side reactions and/or alternative reactive intermediates.

Effects of pulsed ultrasound on the adsorption of n-alkyl anionic surfactants at the gas/solution interface of cavitation bubbles.

Sonolysis of argon-saturated aqueous solutions of the nonvolatile surfactants sodium dodecyl sulfate (SDS) and sodium 1-pentanesulfonate (SPSo) was investigated at three ultrasonic frequencies under both continuous wave (CW) and pulsed ultrasound. Secondary carbon-centered radicals were detected by spin trapping using 3,5-dibromo-4-nitrosobenzenesulfonic acid (DBNBS) and electron paramagnetic resonance (EPR) spectroscopy. Following sonolysis, -*CH- radicals were observed for both surfactants under both sonication modes. Under CW at 354 kHz, the maximum plateau -*CH- radical yield was higher for SPSo than for SDS, indicating that SDS, which is more surface active under equilibrium conditions, accumulates at the gas/solution interface of cavitation bubbles to a lesser degree, compared with the less surface active surfactant, SPSo. However, after sonolysis (354 kHz) under pulsed ultrasound with a pulse length of 100 ms and an interval of 500 ms, the -*CH- radical yield at the plateau concentrations was higher for SDS than for SPSo due to increased amounts of SDS accumulation on the bubble surfaces. In contrast to the findings following sonolysis at 354 kHz, sonolysis of aqueous surfactant solutions at 620 kHz and 803 kHz showed a higher -*CH- radical yield for SDS compared with SPSo under CW but lower -*CH- radical yield with increasing pulsing interval, indicating a frequency dependence on accumulation. Results indicate that pulsing the ultrasonic wave has a significant effect on the relative adsorption ability of n-alkyl surfactants at the gas/solution surface of cavitation bubbles.

Sonochemical degradation of alkylbenzene sulfonate surfactants in aqueous mixtures.

The degradation of nonvolatile surfactants sodium 4-octylbenzene sulfonate (OBS) and dodecylbenzenesulfonate (DBS) and a nonvolatile nonsurfactant 4-ethylbenzene sulfonic acid (EBS), as single components and binary mixtures, were studied under 354 kHz ultrasound. In addition, the effects of pulsed ultrasound on degradation were also examined. Results show that in mixtures of the surfactant OBS and nonsurfactant EBS, the surfactant is selectively degraded. The reduced degradation of EBS was dependent on the mixed molar ratio of EBS/OBS. The degradation of OBS was unaffected by the presence of EBS at a molar ratio of OBS/EBS > or = 1. Furthermore, OBS degradation was significantly enhanced under pulsed ultrasound. In OBS and DBS surfactant mixtures sonicated under pulsed ultrasound, surfactants strongly affected each other's degradation rates due to competition for the reaction sites on the cavitation bubble surfaces. OBS exhibits a faster degradation rate than DBS at shorter pulse intervals due to its faster rate of transfer to the cavitation bubble interfaces. At longer pulse intervals, DBS, which is more surface active, degrades faster than OBS due to the increased amounts of DBS accumulation on the bubble surfaces.

Ultrasonic control of ceramic membrane fouling: Effect of particle characteristics.

In this study, the effect of particle characteristics on the ultrasonic control of membrane fouling was investigated. Ultrasound at 20 kHz was applied to a cross-flow filtration system with gamma-alumina membranes in the presence of colloidal silica particles. Experimental results indicated that particle concentration affected the ability of ultrasound to control membrane fouling, with less effective control of fouling at higher particle concentrations. Measurements of sound wave intensity and images of the cavitation region indicated that particles induced additional cavitation bubbles near the ultrasonic source, which resulted in less turbulence reaching the membrane surface and subsequently less effective control of fouling. When silica particles were modified to be hydrophobic, greater inducement of cavitation bubbles near the ultrasonic source occurred for a fixed concentration, also resulting in less effective control of fouling. Particle size influenced the cleaning ability of ultrasound, with better permeate recovery observed with larger particles. Particle size did not affect sound wave intensity, suggesting that the more effective control of fouling by large particles was due to greater lift and cross-flow drag forces on larger particles compared to smaller particles.

Ultrasonic control of ceramic membrane fouling by particles: effect of ultrasonic factors.

Ultrasound at 20 kHz was applied to a cross-flow ultrafiltration system with gamma-alumina membranes in the presence of colloidal silica particles to systematically investigate how ultrasonic factors affect membrane cleaning. Based on imaging of the ultrasonic cavitation region, optimal cleaning occurred when the membrane was outside but close to the cavitation region. Increasing the filtration pressure increased the compressive forces driving cavitation collapse and resulted in fewer cavitation bubbles absorbing and scattering sound waves and increasing sound wave penetration. However, an increased filtration pressure also resulted in greater permeation drag, and subsequently less improvement in permeate flux compared to low filtration pressure. Finally, pulsed ultrasound with short pulse intervals resulted in permeate flux improvement close to that of continuous sonication.