Enhanced sonochemical degradation of bisphenol-A by bicarbonate ions

Sonochemical elimination of organic pollutants can take place through two degradation pathways. Molecules with relatively large Henry’s law constants will be incinerated inside the cavitation bubble, while nonvolatile molecules with low Henry’s law constants will be oxidised by the OH ejected from the bubble of cavitation. Taking bisphenol-A as a model pollutant, this study points out an alternate degradation route, mediated by bicarbonate ions, which is significant for the elimination of micro-pollutants at concentrations present in natural waters. In this process, OH radicals react with bicarbonate ions to produce the carbonate radical, which, unlike the OH radical, can migrate towards the bulk of the solution and therefore induce the degradation of the micro-pollutants present in the bulk solution. As a consequence, initial degradation rate is increased by a factor 3.2 at low concentration of bisphenol-A (0.022 μmol l⁻¹) in presence of bicarbonate in water.     

Critical factors in sonochemical degradation of fumaric acid

The effects of critical factors such as Henry’s Law constant, atmospheric OH rate constant, initial concentration, H₂O₂, FeSO₄ and tert-butanol on the sonochemical degradation of fumaric acid have been investigated. The pseudo first-order rate constant for the sonochemical degradation of 1 mM fumaric acid is much lower than those for chloroform and phenol degradation, and is related to solute concentration at the bubble/water interface and reactivity towards hydroxyl radicals. Furthermore, fumaric acid is preferentially oxidized at the lower initial concentration. It is unreactive to H₂O₂ under agitation at room temperature. However, the degradation rate of fumaric acid increases with the addition of H₂O₂ under sonication. 0.1 mM of fumaric acid suppresses H₂O₂ formation thanks to water sonolysis, while degradation behavior is also dramatically affected by the addition of an oxidative catalyst (FeSO₄) or radical scavenger (tert-butanol), indicating that the degradation of fumaric acid is caused by hydroxyl radicals generated during the collapse of high-energy cavities.     

Sonoelectrochemical degradation of triclosan in water

The sonoelectrochemical degradation of triclosan in aqueous solutions with high-frequency ultrasound (850 kHz) and various electrodes was investigated. Diamond coated niobium electrode showed the best results and was used as standard electrode, leading to effective degradation and positive synergistic effect. The influence of different parameters on the degradation degree and energy efficiency were evaluated and favorable reaction conditions were found. It could be shown that 92% of triclosan (1 mg L⁻¹ aqueous solution) was degraded within 15 min, following pseudo-first order kinetics.     

Reaction kinetics of sonochemical oxidation of potassium hexacyanoferrate(II) in aqueous solutions

We studied sonochemical reactions resulting from ultrasonic treatment of potassium hexacyanoferrate(II) in aqueous solutions using a custom-built apparatus working at 536 kHz. We concluded that primary reactions are completely dominated by oxidation of Fe(II) to Fe(III) and did not find any evidences for degradation of cyanide. At the highest concentration used in the present study (0.1 M) we detected formation of pentacyanoaquaferrate(II) complex, which is most probably formed in reactions between hexacyanoferrate(III) anions and hydrogen atoms or hydrated electrons formed in sonochemical processes. We also determined that hydroxyl radicals formation rate in our system, (8.7 ± 1.5)∙10⁻⁸ M∙s⁻¹, is relatively high compared to other reported experiments. We attribute this to focusing of the ultrasonic wave in the sample vessel. Finally, we suggest that oxidation rate of hexacyanoferrate(II) anions can be a convenient benchmark of efficiency of sonochemical reactors.     

Effect of potassium monopersulfate (oxone) and operating parameters on sonochemical degradation of cationic dye in an aqueous solution

In this study, removal of Cresol Red (CR), a cationic triphenylmethane dye, by 300 kHz ultrasound was investigated. The effect of additive such as potassium monopersulfate (oxone) was studied. Additionally, sonolytic degradation of CR was investigated at varying power and initial pH. RC can be readily eliminated by the ultrasound process. The obtained results showed that. Sonochemical degradation of CR was strongly affected by ultrasonic power and pH. The degradation rate of the dye increased substantially with increasing ultrasonic power in the range of 20–80 W. This improvement could be explained by the increase in the number of active cavitation bubbles. The significant degradation was achieved in acidic conditions (pH = 2) where the color removal was 99% higher than those observed in higher pH aqueous solutions. The ultrasonic degradation of dye was enhanced by potassium monopersulfate (oxone) addition. It was found that the degradation of the dye was accelerated with increased concentrations of oxone for a reaction time of 75 min.     

The effect of heterogeneous solvent systems on sonochemical reactions: accelerated degradation of alkyl thiols in emulsion

Sonochemical reactions of alkyl and aryl thiols in water–organic mixed solvent systems were kinetically investigated. The reaction in the liquid–liquid interface apparently depends on the polarities, surface activities, vapor pressures and hydrophobicities of organic solvents and thiols. Interestingly, the rate jump in sonochemical disappearance of alkyl thiols was observed under emulsified conditions.     

A critical review on the sonochemical degradation of organic pollutants in urine,seawater, and mineral water

Substances such as pharmaceuticals, pesticides, dyes, synthetic and natural hormones, plasticizers, and industrial chemicals enter the environment daily. Many of them are a matter of growing concern worldwide. The use of ultrasound to eliminate these compounds arises as an interesting alternative for treating mineral water, seawater, and urine. Thereby, this work presents a systematic and critical review of the literature on the elimination of organic contaminants in these particular matrices, using ultrasound-based processes. The degradation efficiency of the sonochemical systems, the influence of the nature of the pollutant (volatile, hydrophobic, or hydrophilic character), matrix effects (enhancement or detrimental ability compared to pure water), and the role of the contaminant concentration were considered. The combinations of ultrasound with other degradation processes, to overcome the intrinsic limitations of the sonochemical process, were considered. Also, energy consumptions and energy costs associated with pollutants degradation in the target matrices were estimated. Moreover, the gaps that should be developed in future works, on the sonodegradation of organic contaminants in mineral water, seawater, and urine, were discussed.     

Ultrasonic degradation, mineralization and detoxification of diclofenac in water: Optimization of operating parameters

The 20 kHz ultrasound-induced degradation of non-steroidal, anti-inflammatory drug diclofenac (DCF) was investigated. Several operating conditions, such as power density (25–100 W/L), substrate concentration (2.5–80 mg/L), initial solution pH (3.5–11), liquid bulk temperature and the type of sparging gas (air, oxygen, argon), were tested concerning their effect on DCF degradation (as assessed measuring absorbance at 276 nm) and hydroxyl radicals generation (as assessed measuring H₂O₂ concentration). Sample mineralization (in terms of TOC and COD removal), aerobic biodegradability (as assessed by the BOD₅/COD ratio) and ecotoxicity to Daphnia magna and Artemia salina were followed too.DCF conversion is enhanced at increased applied power densities and liquid bulk temperatures, acidic conditions and in the presence of dissolved air or oxygen. The reaction rate increases with increasing DCF concentration in the range 2.5–5 mg/L but it remains constant in the range 40–80 mg/L, indicating different kinetic regimes (i.e. first and zero order, respectively). H₂O₂ production rates in pure water are higher than those in DCF solutions, implying that decomposition basically proceeds through hydroxyl radical reactions. Mineralization is a slow process as reaction by-products are more stable than DCF to total oxidation; nonetheless, they are also more readily biodegradable. Toxicity to D. magna increases during the early stages of the reaction and then decreases progressively upon degradation of reaction by-products; nevertheless, complete toxicity elimination cannot be achieved at the conditions in question. Neither the original nor the treated DCF samples are toxic to A. salina.     

Effects of Na2SO4 or NaCl on sonochemical degradation of phenolic compounds in an aqueous solution under Ar: Positive and negative effects induced by the presence of salts

Sonochemical degradation of 4-chlorophenol, phenol, catechol and resorcinol was studied under Ar at 200 kHz in the absence and presence of Na₂SO₄ or NaCl. The rates of sonochemical degradation in the absence of salts decreased in the order 4-chlorophenol > phenol > catechol > resorcinol and this order was in good agreement with the order of log P (partition coefficient) value of each phenolic compound. The effects of salts on the rates of sonochemical degradation consisted of no effect or slight negative or positive effects. We discussed these unclear results based on two viewpoints: one was based on the changes in pseudo hydrophobicity and/or diffusion behavior of phenolic compounds and the other was based on the changes in solubility of Ar gas. The measured log P value of each phenolic compound slightly increased with increasing salt concentration. In addition, the dynamic surface tension for 4-chlorophenol aqueous solution in the absence and presence of Na₂SO₄ or NaCl suggested that phenolic compounds more easily accumulated at the interface region of bubbles at higher salt concentration. These results indicated that the rates of sonochemical degradation should be enhanced by the addition of salts. On the other hand, the calculated Ar gas solubility was confirmed to decrease with increasing salt concentration. The yield of H₂O₂ formed in the presence of Na₂SO₄ or NaCl decreased with increasing salt concentration. These results suggested that sonochemical efficiency decreased with decreasing gas amount in aqueous solution: a negative effect of salts was observed. Because negative and positive effects were induced simultaneously, we concluded that the effects of salts on the rates of sonochemical degradation of phenolic compounds became unclear. The products formed from sonochemical degradation of 4-chlorophenol were also characterized by HPLC analysis. The formation of phenol and 4-chloro-1,3-dihydroxy benzene was confirmed and these concentrations were affected by the presence of salts.     

Recent trends in the applications of sonochemical reactors as an advanced oxidation process for the remediation of microbial hazards associated with water and wastewater: A critical review

Water is one of the major sources that spread human diseases through contamination with bacteria and other pathogenic microorganisms. This review focuses on microbial hazards as they are often present in water and wastewater and cause various human diseases. Among the currently used disinfection methods, sonochemical reactors (SCRs) that produce free radicals combined with advanced oxidation processes (AOPs) have received significant attention from the scientific community. Also, this review discussed various types of cavitation reactors, such as acoustic cavitation reactors (ACRs) utilizing ultrasonic energy (UE), which had been widely employed, involving AOPs for treating contaminated waters. Besides ACRs, hydrodynamic cavitation reactors (HCRs) also effectively destroy and deactivate microorganisms to varying degrees. Cavitation is the fundamental phenomenon responsible for initiating many sonochemical reactions in liquids. Bacterial degradation occurs mainly due to the thinning of microbial membranes, local warming, and the generation of free radicals due to cavitation. Over the years, although extensive investigations have focused on the antimicrobial effects of UE (ultrasonic energy), the primary mechanism underlying the cavitation effects in the disinfection process, inactivation of microbes, and chemical reactions involved are still poorly understood. Therefore, studies under different conditions often lead to inconsistent results. This review investigates and compares other mechanisms and performances from greener and environmentally friendly sonochemical techniques to the remediation of microbial hazards associated with water and wastewater. Finally, the energy aspects, challenges, and recommendations for future perspectives have been provided.     

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/L_gas), 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.     

Sonochemical degradation of trimethoprim in water matrices: Effect of operating conditions,identification of transformation products and toxicity assessment

The sonochemical degradation of trimethoprim (TMP), a widely used antibiotic, in various water matrices was investigated. The effect of several parameters, such as initial TMP concentration (0.5–3 mg/L), actual power density (20–60 W/L), initial solution pH (3–10), inorganic ions, humic acid and water matrix on degradation kinetics was examined. The pseudo-first order degradation rate of TMP was found to increase with increasing power density and decreasing pH, water complexity (ultrapure water > bottled water > secondary wastewater) and initial TMP concentration.TMP degradation is accompanied by the formation of several transformation products (TPs) as evidenced by LC-QToF-MS analysis. Nine such TPs were successfully identified and their time-trend profiles during degradation were followed. An in silico toxicity evaluation was performed showing that several TPs could potentially be more toxic than the parent compound towards Daphnia magna, Pimephales promelas and Pseudokirchneriella subcapitata.     

Experimental and numerical investigation of the effect of liquid temperature on the sonolytic degradation of some organic dyes in water

This paper presents a comprehensive experimental and numerical investigation of the effects of liquid temperature on the sonochemical degradation of three organic dyes, Rhodamine B (RhB), Acid orange 7 (AO7) and Malachite green (MG), largely used in the textile industry. The experiments have been carried out for an ultrasonic frequency of 300 kHz. The obtained experimental results were discussed using a new approach combining the results of single-bubble event and the number of active bubbles. The single-bubble event was predicted using a model that combines the bubble dynamics with chemical kinetics occurring inside a bubble during the strong collapse. The number of active bubbles was predicted using a method developed in our previous work. The experiments showed that the degradation rate of the three dyes increased significantly with increasing liquid temperature in the range 25–55 °C. It was predicted that the main pathway of pollutants degradation is the attack by OH radicals. The simulations showed that there exists an optimum liquid temperature of about 35 °C for the production of OH inside a bubble whereas the number of active bubbles increased sharply with the rise of the liquid temperature. It was predicted that the overall production rate of OH increased with increasing liquid temperature in the range 25–55 °C. Finally, it was concluded that the effect of liquid temperature on the sonochemical degradation of the three dyes in aqueous phase was controlled by the number of active bubbles in the range 35–55 °C and by both the number of bubbles and the single bubble yield in the range 25–35 °C.     

Power density modulated ultrasonic degradation of perfluoroalkyl substances with and without sparging Argon

The power density modulates the dynamics of the chemical reactions during the ultrasonic breakdown of organic compounds. We evaluated the ultrasonic degradation of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) at various power densities (30 W/L–262 W/L) with and without sparging Argon. We observed pseudo-first-order degradation kinetics at an initial PFASs concentration of 100 nM over a range of power density. The rate kinetics of degradation shows a non-linear increase with an increase in power density. We proposed a four-parameter logistic regression (4PLR) equation that empirically fits the degradation rate kinetics with the power density. The 4PLR equation predicts that the maximum achievable half-life of PFOA and PFOS sonochemical degradation are 1 and 10 min under a given set of experimental conditions. The high bulk-water temperature (i.e., 30 °C) of the aqueous sample helps increase the degradation rate of PFOA and PFOS. The addition of oxidants such as iodate and chlorate help enhance PFOA degradation in an argon environment at an ultrasonic frequency of 575 kHz.     

Insights into ultrasound-promoted degradation of naphthenic acid compounds in oil sands process affected water. Part I: Accelerated H-abstraction and decarboxylation of aromatic and alicyclic compounds

Propelled by enormous increase in demand for fuel sources, Canadian oil sands are becoming increasingly important as a fuel source due to their abundance and upgrading capability. However, extraction of bitumen, a high acid crude (HAC) oil, requires 2–3 units of water per unit of oil resulting in naphthenic acid (NA)-rich oil sands process affected water (OSPW) collected in effluent ponds. This study illustrates the role of sonochemistry in the accelerated degradation through H-abstraction and subsequent decarboxylation of aromatic and alicyclic naphthenic acid model compounds. Benzoic acid and 3-methylcyclohexane carboxylic acid were selected as model NA compounds to investigate the mechanism of hydroxyl radical (OH^•) initiated carboxylic acid degradation in 378 KHz sonochemical reactor. Established FTIR methods with low resolution LCMS spectroscopy confirmation were applied to determine the extent of carboxylic acid degradation and identify the formation of products. FTIR monitoring showed a non-linear degradation of carboxylic acids with formation of many intermediates highlighting the shift from cyclic carboxylic acids to cyclic alcohols during BA degradation. Subsequent decrease in carboxylic acid groups signifies scission of cyclic structures before complete mineralization. This is confirmed with the LCMS identification of products such as: 3-hydroxybenzoic acid and phenol. This study postulated new breakdown pathways for degradation of benzoic acid with complete mineralization at a sonochemical reaction time (SRT) of 4 h. A radical quenching process was also inferred through the formation of conglomerates during sonochemical degradation of BA. Extension of the study to 3-methylcyclohexane carboxylic acid (3mCHA) shows similar non-linearity with an increase in carboxylic acid groups indicating H-abstraction followed by ring-opened compounds. However, due to the complex nature of 3mCHA’s ring-opened compounds, complete mineralization is not achieved. The putative role of sonochemistry is a promising and sustainable degradation method for mitigating NAs in OSPW, but sonication periods need to be considered carefully to ensure adequate mineralization of their constituents and combinatorial methods with other advanced oxidation methods may be needed to enhance industrial application.In Part II, an in silico screening approach using first principles is reported to identify the breakdown of the organic compounds and determine molecular rates of reaction to confirm the mechanistic origins of the compounds formed.     

Ultrasound for the remediation of contaminated waters with persistent organic pollutants: A short review

The rising amount of persistent organic contaminants released into water reservoirs in the last years became a cause of concern for the industry, academy, and public administration, due to their bioaccumulation, mutagenicity, and photosynthesis reduction. Therefore, the search for processes that efficiently remove such contaminants became of primary importance. In this context, ultrasound (US) is one of the most promising and economically viable alternatives to degrade organic pollutants in varied environments. Whereas the use of other advanced oxidation processes (AOPs), such as Fenton and photocatalysis, has been widely reported for this purpose, only a few papers deal with ultrasound application as a possible AOP. In this review, a general overview of ultrasound is provided, covering the last twenty years. It includes fundamental aspects of ultrasound and applications, individually or combined with other AOPs, to deplete organic pollutants from various classes in an aqueous environment. Finally, the review concludes by indicating that additional research should be conducted worldwide to explore the full potential of ultrasound as a useful AOP.     

Benzophenone-3 ultrasound degradation in a multifrequency reactor: Response surface methodology approach

Response Surface Methodology was used for optimizing operating variables for a multi-frequency ultrasound reactor using BP-3 as a model compound. The response variable was the Triclosan degradation percent after 10 sonication min. Frequency at levels from 574, 856 and 1134 kHz were used. Power density, pulse time (PT), silent time (ST) and PT/ST ratio effects were also analyzed. 2² and 2³ experimental designs were used for screening purposes and a central composite design was used for optimization. An optimum value of 79.2% was obtained for a frequency of 574 kHz, a power density of 200 W/L, and a PT/ST ratio of 10. Significant variables were frequency and power level, the first having an optimum value after which degradation decreases while power density level had a strong positive effect on the whole operational range. PT, ST, and PT/ST ratio were not significant variables although it was shown that pulsed mode ultrasound has better degradation rates than continuous mode ultrasound; the effect less significant at higher power levels.     

Intensification of Red-G dye degradation used in the dyeing of alpaca wool by advanced oxidation processes assisted by hydrodynamic cavitation

Red-G dye is one of the main dyes used in the textile industry to dye alpaca wool. Therefore, considering the large volume of processed wool in Perú, the development of efficient technologies for its removal is a present scientific issue. In this study, an integrated system based on hydrodynamic cavitation (HC) and photo-Fenton process was evaluated to remove the Red-G dye. Using a hybrid cavitation device (venturi + orifice plate), the effect of pH was evaluated, achieving 21 % of removal at pH 2 which was more than 80 % higher compared to pH 4 and 6. The effect of temperature was also evaluated in HC-system at pH 2, where percentage of dye degradation increased at lower temperatures (around 20 °C). Then, 50.7 % of dye was removed under optimized condition of HC-assisted Fenton process (FeSO₄:H₂O₂ of 1:30), that value was improved strongly by UV-light incorporation in the HC-system, increasing to 99 % removal efficiency with respect to HC-assisted Fenton process and reducing the time to 15 min. Finally, the developed cavitation device in combination with photo-Fenton process removed efficiently the dye and thus could be considered an interesting option for application to real wastewater.     

Simultaneous hydrodynamic cavitation and glow plasma discharge for the degradation of metronidazole in drinking water

In this study, a novel hydrodynamic cavitation unit combined with a glow plasma discharge system (HC-GPD) was proposed for the degradation of pharmaceutical compounds in drinking water. Metronidazole (MNZ), a commonly used broad-spectrum antibiotic, was selected to demonstrate the potential of the proposed system. Cavitation bubbles generated by hydrodynamic cavitation (HC) can provide a pathway for charge conduction during glow plasma discharge (GPD). The synergistic effect between HC and GPD promotes the production of hydroxyl radicals, emission of UV light, and shock waves for MNZ degradation. Sonochemical dosimetry provided information on the enhanced formation of hydroxyl radicals during glow plasma discharge compared to hydrodynamic cavitation alone. Experimental results showed a MNZ degradation of 14% in 15 min for the HC alone (solution initially containing 300 × 10⁻⁶ mol L⁻¹ MNZ). In experiments with the HC-GPD system, MNZ degradation of 90% in 15 min was detected. No significant differences were observed in MNZ degradation in acidic and alkaline solutions. MNZ degradation was also studied in the presence of inorganic anions. Experimental results showed that the system is suitable for the treatment of solutions with conductivity up to 1500 × 10⁻⁶ S cm⁻¹. The results of sonochemical dosimetry showed the formation of oxidant species of 0.15 × 10⁻³ mol H₂O₂ L⁻¹ in the HC system after 15 min. For the HC-GPD system, the concentration of oxidant species after 15 min reached 13 × 10⁻³ mol H₂O₂ L⁻¹. Based on these results, the potential of combining HC and GPD systems for water treatment was demonstrated. The present work provided useful information on the synergistic effect between hydrodynamic cavitation and glow plasma discharge and their application for the degradation of antibiotics in drinking water.     

Sonochemical degradation of ciprofloxacin and ibuprofen in the presence of matrix organic compounds

Ciprofloxacin (CIPRO) and ibuprofen (IBU), a hydrophilic and a hydrophobic compound, respectively, were degraded by ultrasound at the frequencies of 20 and 620 kHz in aqueous solution containing matrix organic compounds. Compared to in its absence, in the presence of terephthalate (TA), a commonly used OH scavenger, CIPRO degradation was inhibited by a factor of 40–1500 depending on the frequency and initial concentration. However, the degradation rates of IBU were only reduced between 30% and 80% with TA present compared to in its absence. Similar to TA, the presence of Suwannee River Fulvic Acid (SRFA) inhibited CIPRO degradation to a greater extent than that of IBU but overall inhibition by SRFA was dramatically less than by TA. Although both TA and SRFA inhibited the degradation of CIPRO and IBU, the mechanisms of inhibition are different. TA reacts with OH in bulk solution and our evidence also indicates that it accumulates on or interacts with cavitation bubbles. On the other hand, SRFA stays in bulk solution, quenching OH and/or associating with the target compounds.