Water Engineering Modeling and Mathematic Tools

By Pijush Samui and Hossein Bonakdari

Water Engineering Modeling and Mathematic Tools provides an informative resource for practitioners who want to learn more about different techniques and models in water engineering and their practical applications and case studies. The book provides modelling theories in an easy-to-read format verified with on-site models for specific regions and scenarios. Users will find this to be a significant contribution to the development of mathematical tools, experimental techniques, and data-driven models that support modern-day water engineering applications. Civil engineers, industrialists, and water management experts should be familiar with advanced techniques that can be used to improve existing systems in water engineering.

This book provides key ideas on recently developed machine learning methods and AI modelling. It will serve as a common platform for practitioners who need to become familiar with the latest developments of computational techniques in water engineering.

• Includes firsthand experience about artificial intelligence models, utilizing case studies
• Describes biological, physical and chemical techniques for the treatment of surface water, groundwater, sea water and rain/snow
• Presents the application of new instruments in water engineering

• Language: English
• Publisher: Elsevier Science
• Release date: Feb 5, 2021
• ISBN: 9780128208779

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globally.

1

Numerical insight into the sonolytic ozonation applied for water treatment

Nassim Kerabchi¹, Slimane Merouani² and Oualid Hamdaoui³,    ¹1Department of Process Engineering, Faculty of Engineering, Badji Mokhtar – Annaba University, Annaba, Algeria,    ²2Laboratory of Environmental Process Engineering, Department of Chemical Engineering, Faculty of Process Engineering, University Salah Boubnider Constantine 3, Constantine, Algeria,    ³3Chemical Engineering Department, College of Engineering, King Saud University, Riyadh, Saudi Arabia

Abstract

Sonolysis and ozonation are two advanced oxidation processes (AOPs) that are actually well documented and their mechanisms are well established. Recently, the combination of these two processes, that is sonolytic ozonation [ultrasound (US)/O3], has been successfully applied to improve the degradation of several water contaminants, as compared to each process separately. The mechanism by which US enhances the ozonation process and vice versa is utile now under discussion. Ozone injection can create more nucleation sites and improve the cavitation events by increasing the number of bubbles. Furthermore, US may enhance the mass transfer of O3 in the liquid. Besides, ozone can be thermally decomposed inside pulsing acoustic bubbles, which provokes reaction chain leading to higher rate of •OH generation. However, the use of high O3 concentration can decrease the sonochemical activity through favoring bubbles coalescence, which is considered as the main suppressor of inertial cavitation responsible for all chemical effects of US. The present study aimed at (1) giving an overview of the US/O3 sono-hybrid process and (2) illustrating the synergetic mechanisms of the process through confronting simulation results with experimental data. The simulations results provide the effect of ozone presence in the aqueous solution on the chemical yield, that is •OH production rate, of single acoustic bubble at various operating conditions of frequency, acoustic intensity, and ozone concentration. Based on results, a conceptual diagram for the chemical activity of the US/O3 process has been made.

Keywords

Advanced oxidation processes (AOPs); sonolytic ozonation (US/O3); •OH generation; computer simulation; reaction mechanism

Chapter Outline

Outline

1.1 Introduction 1

1.2 An overview of US/O3 process 2

1.3 Computational section 7

1.3.1 Mathematical model 7

1.3.2 Thermodynamic of bubble collapse in O2–O3-saturated water 10

1.3.3 Gas composition effect on the chemical activity of the acoustic bubble 12

1.3.4 O3–O2 gas composition effect dependence of operating conditions 12

1.4 Plausible mechanisms 16

1.5 Conceptual diagram for US/O3 system 17

1.6 Conclusion 19

Acknowledgment 19

References 19

1.1 Introduction

Recently, various chemical treatment processes were developed for effective treatment of industrial effluent and wastewater. Among them, advanced oxidation processes (AOPs) seem to be promising. The main advantage of AOPs is the production of powerful oxidizing agents, such as hydroxyl radicals (•OH, E⁰=2.8 V vs standard hydrogen electrode (SHE)), which can degrade a wide range of organic and inorganic pollutants quickly and nonselectively [1]. The effectiveness of various AOPs such as Fenton, photo–Fenton, ultraviolet (UV)/H2O2, ozonation, UV/ozone, sonication, and photocatalysis has been extensively studied. More recently, combination of these AOPs, that is hybrid processes, has also been employed to further enhance the efficiency of degradation [2]. Hybrid processes are in general more effective than the single-mode as well as the linear sum of the two singly processes.

Ozonation combined with ultrasonic irradiation (US/O3) is a promising hybrid method for dealing with wastewater; it has been actively researched over the past several years [3–8]. Aqueous molecular ozone can react with organic substrates in acidic and neutral pH [9]. Under basic conditions, ozone tends to quickly decompose forming reactive free radicals, such as •OH, which is much more reactive than ozone itself (E⁰US=2.0 V vs SHE) [10,11]. On the other hand, sonication of an aqueous solution results in the formation of acoustic cavitation that grows and quickly collapses upon the rarefaction/compression cycles of the sonic wave [12]. The nearly adiabatic collapse of these microbubbles generates hot spots of extreme local temperature and pressure, that is as high as 5000K and 500 atm within the bubble and 1900K at the bubble/solution interface (Fig. 1.1, phase 1) [13]. Pollutants degradation can be produced inside the bubble itself, for volatile substrates, or at the interfacial sheath due to direct pyrolysis or reaction with radicals resulted from the gas-phase pyrolysis of H2O (Fig. 1.1, phase 2), for nonvolatile substrates [12]. Radicals escaping the acoustic bubble can also diffuse and react in the bulk [14].

Figure 1.1 Acoustic cavitation (phase 1), water sonolysis (phase 2), and ozone sonolysis (phase 3) processes occurred during the sonication of aqueous solution in the absence and presence of dissolved ozone.

In US/O3 hybrid process, ozone may diffuse in the bubble and take part in the pyrolytic reaction occurring therein. In this case, the thermal decomposition of ozone may improve the generation of hydroxyl radicals (Fig. 1.1, phase 3), while sonication can enhance the mass transfer of ozone in the liquid phase [2]. More interestingly, continuous injection of ozone can improve or suppress the inertial cavitation bubbles, depending on the concentration of ozone in the liquid phase. Therefore, the interaction between the mechanisms of individual AOPs that is manifested in terms of enhancement in degradation of the pollutant in the hybrid AOP is an interesting issue.

In this work, we have attempted to give a critical analysis of the source of the synergism produced by sonolytic ozonation when it is applied to pollutant degradation. Our approach is to confront simulation results for ozone decomposition in single acoustic bubble with experimental results. These simulations essentially give quantitative prediction of the physical and chemical effects, i.e. •OH production, induced by cavitation in the presence of ozone. Before proceeding to the analysis, we give a brief overview of the US/O3 hybrid process.

1.2 An overview of US/O3 process

Hart and Henglein [15] have first shown a rapid decomposition of ozone, associated with a fast formation rate of oxygen peroxide, when water containing dissolved ozone and oxygen was exposed to US waves. They attributed this event to the pyrolysis of ozone within bubbles, which can induce reaction chain conducting to higher formation rate of •OH. In 1994 Olson and Barbier [16] have reported that coupling low-frequency sonolysis, 20 kHz, with ozone increased substantially the removal of total organic carbon during the treatment of aqueous fulvic acid samples to 91% against 60% for the sole ozonation. Two years after, Barbier and Petrier [17] reported that the advantages of US/O3 process are frequency-dependent. After these works, numerous papers have been published aiming to explore the effectiveness of US/O3 process for the destruction of a wide range of organic contaminants. Table 1.1 lists the most significant studies reported in the field. Practically, consequences of coupling US with O3 are the same: (1) reduction of the treatment time, (2) improvement of the degradation rate, and (3) production of synergisms. These outcomes were attributed to the physical and chemical effects of sonication: (1) acoustic cavitation enhances volumetric mass transfer through ameliorating dispersion and solubility of O3 in water, and (2) sonolysis of ozone inside cavities improve the generation rates of hydroxyl radical and other oxidizing species that make the degradation faster.

Table 1.1

C0, Initial pollutant concentration; Q0(O2/O3)mix, flow rate of O2–O3 gas mixture; Q0,O3, ozone flow rate in the gas mixture; NI, not indicated; T, solution temperature; US, ultrasound; V, solution volume; [O3]ss, ozone steady-state concentration in pure water (without US); [O3]g, ozone concentration in the inlet gas mixture; TOC, total organic carbon; COD, chemical oxygen demand; (+), positive synergy; (−), negative synergy.

However, the synergism coming from applying US/O3 process is only observed for nonvolatile contaminants and is greatly sensitive to operating conditions, that is concentration of dissolved O3, frequency, and power [8,27–33]. In general, low-frequency US (~20 kHz) provides best synergies than high frequency (Table 1.1). In fact, cavitation at low-frequency sonication generates intense mechanical effect, high acoustic streaming and turbulence, thereby accelerating O3 mass transfer coefficient (KLa) in the sonicated liquid [17,34]. In many cases, the synergy was recorded for TOC disappearance but not for the pollutant removal [25,35]. This may be attributed to the implication of new oxidation pathways arising from the formation of secondary active radicals (e.g., O2•− and HO2•) other than the primary •OH radical [4]. While some degradation byproducts are persistent to ozone, they can be reactive to the secondary generated radicals [35]. Finally, the negative synergism reported in works 2, 4, and 11 of Table 1.1 may be due to the use of high dissolved O3 concentration, which can provoke deactivation of inertial cavitation responsible for the chemical effect of US [36–38].

1.3 Computational section

1.3.1 Mathematical model

The present mathematical model has been fully described in our previous works [39–42]. In short, the physical situation is that an isolated single bubble in water saturated with specific gases oscillated under the action of an ultrasonic wave of known frequency and acoustic intensity continuously traveling the mediums. The radial fluctuation of the bubble is described by the Keller–Miksis equation [43]:

(1.1)

Dots in this equation means time derivatives (d/dt), R is the bubble radius, c is the sound speed in water, σ is the surface tension, ρL and μ are the liquid density and viscosity, respectively, p is internal bubble pressure, p∞ is the external static pressure, PA is the amplitude of the acoustic pressure, and f is the frequency of US. PA is related to the acoustic intensity Ia, by PA=(2IaρLc)¹/².

Isothermal expansion and adiabatic compression of the bubble have been widely assumed due to the very rapid happening of bubble oscillation and collapse [44,45]. Also, mass and heat transfers were ignored throughout the bubble oscillation. Reasons for these assumptions are available in detail in Refs. [39,46–48]. Temperature and pressure inside the bubble during adiabatic collapse phase were predicted by:

(1.2)

(1.3)

Pv is the water-vapor pressure, Pg0=p∞+(2σ/R0)−Pv is the initial gas pressure inside the bubble, R0 is equilibrium bubble radius, T∞ is the temperature of the bulk liquid, and γ is the polytropic index (cp/cv) of the gas/vapor mixture, given as

(1.4)

yk is the mole fraction of the species k and γk is the ratio of specific heat capacities of the species k.

The bubble is initially composed of water vapor and the dissolved gas. In our case, the combustion reaction occurring within the O2–O3 bubble at the strong collapse was simulated by a series of 25 reversible chemical reactions (Table 1.2) including O2, H2O, O3,•OH, H•, O, HO2•, H2, and H2O2 species. The chemical kinetics model consists of the reaction scheme and calculates the concentration profiles of all species during the bubble period. Rate expressions, which consider elementary reversible reactions including k species, are presented as

(1.5)

υki in the stoichiometric coefficients of the ith reaction and Xk is the symbol for the kth species. The superscript ′ denotes forward stoichiometric coefficients, whereas ″ of the kth species is given as

(1.6)

Table 1.2

M is the third body. Subscript "f denotes the forward reaction and r" denotes the reverse reaction. A is in (cm³ mol/s) for two-body reaction [(cm⁶ mol²/s) for a three-body reaction], and Ea is in (cal/mol).

The rate ri for the ith reaction is given by

(1.7)

where [Xk] is the molar concentration of the kth species, and kfi and kri are the forward and reverse rate constants of the ith reaction, respectively. The forward and reverse rate constants for the ith reactions are written by Arrhenius equations as

(1.8)

(1.9)

Efi (Eri) is the activation energy, Rg is the ideal gas constant, Afi (Ari) is the preexponential factor, and bfi (bri) is the temperature exponent. Arrhenius parameters are given in Table 1.2. Details of the simulation procedures are available in Refs. [39,46]. Briefly, the bubble dynamic equation [Eq. (1.1)] was resolved by the fourth-order Runge–Kutta method after converting it into two ordinary first-order differential equations, whereas the chemical reaction systems were resolved by the finite difference method.

1.3.2 Thermodynamic of bubble collapse in O2–O3-saturated water

Computer simulations of bubble oscillation in water saturated with different O2–O3 gas composition have been performed for different frequencies of US in the range of 20–1000 kHz when the acoustic intensity and the liquid temperature were fixed at 1 W/cm² and 20°C. Fig. 1.2A–D gives the predicted effect of O2–O3 gas composition, as function of frequency, on water vapor and O3 fractions inside the bubble at its maximum size (Rmax) as well as the collapse duration and the maximum bubble temperature attained at the end of the bubble collapse (at time corresponding to Rmin). The fraction of water vapor and the duration of the bubble collapse were not affected by the saturation gas composition. However, the maximum bubble temperature and the ozone content were sensitively dependent on the saturation gas compositions. Besides, the variations of the four parameters of Fig. 1.2A–D are frequency-dependent. While the collapse time and the water vapor fraction decreased with increasing the frequency, the ozone fraction inside the bubble increased with increasing both O3 content in the saturation gas matrix and frequency. The resulted maximum bubble temperature decreased linearly with increasing O3 content inside the bubble because of the lower polytropic index γ=cp/cv of O3 as compared to that of oxygen (1.29 against 1.4). In parallel, the maximum bubble temperature decreased with increase in frequency due to the significant impact of frequency on the bubble compression ratio (Rmax/Rmin), which was adversely affected by the rise of frequency in the range of 20–1000 kHz (detailed numerical analysis of the frequency effect is available in Ref. [50]). The bubble collapse is therefore less intense at higher frequency under predominated O3 saturation atmosphere.

Figure 1.2 Fraction of the trapped water vapor (A) and ozone (B) in the bubble (noted as y) as well as the collapse duration (C) and (D) the maximum bubble temperature achieved at the end of the bubble collapse, as function of ozone fraction in the O3–O2 saturating atmosphere (acoustic intensity: 1 W/cm², liquid temperature: 20°C).

1.3.3 Gas composition effect on the chemical activity of the acoustic bubble

In Fig. 1.3, the chemical bubble yield on all species formed at the end of the bubble collapse is shown as function of O2–O3 gas composition for a frequency of 355 kHz and an acoustic intensity of 1 W/cm². The maximum bubble temperature evolution under the same conditions was also inserted. The production rates of reactive species and hydrogen decreased with increasing O3 content in the gas matrix. Moreover, hydroxyl radical was the major product of the combustion reactions inside the bubble whatever the O3–O2 gas composition. As compared to pure O2 saturation, the •OH-yield decreased by 5%, 8%, 15%, 29%, 61%, and 85% for, respectively, 3%, 5%, 10%, 20%, 50%, and 100% O3 molar fraction in the inlet saturation gas. More significant reductions were observed for the other species, as shown in Fig. 1.3. Simultaneously, the maximum bubble temperature drops slightly at low O3 percentages (<7%) but it markedly descents by about 450K and 900K for 50% and 100% of O3 in the gas matrix, respectively.

Figure 1.3 Maximum bubble temperature and production rates of the different sepcies formed in the bubble at the end of the bubble collapse, that is time corresponding to Tmax, as function of ozone composition in O2–O3 gas matrix.

O3. Therefore the loss in the chemical bubble yield at 355 kHz and 1 W/cm² with augmenting O3 dosage in the saturation gas (2•OH.

1.3.4 O3–O2 gas composition effect dependence of operating conditions

Since hydroxyl radical is the main oxidant involved in sonochemistry, we will follow only the production rate of this reactive species under different conditions of sonolytic ozonation such as the frequency, the acoustic intensity delivered to the reaction system, and O3 fraction in the saturating gas matrix (i.e., oxygen). The ambient bubble radius (R0) employed for the numerical simulation of the bubble oscillation is selected as function of frequency (Table 1.3) according to the literature experimental reports. In reality, the initial size of active bubbles (R0) in cavitation field is not a single value, whereas it has a range, but there is a certain size of ambient bubble radius at which a dominant number of bubbles were observed in the cavitation region. This size represents the mean ambient radius. Additionally, experiments [55] showed that the range of ambient radius for active bubbles is rather narrow and it closes around the mean ambient radius. The present study has tried to use the same frequencies at which the mean ambient bubble radii were determined experimentally. Also we assume that the acoustic intensity did not affect the initial bubble size (R0).

Table 1.3

In Fig. 1.4, the acoustic generation of hydroxyl radical as function of O3 content in the gas matrix is shown for various frequencies (140, 231, 355, 515, 647, and 1000 kHz) and acoustic intensities (for up to 5 W/cm²). Note that numerical simulations were also conducted for 20 kHz, but no effect of O3 was observed on the production rate of •OH (data not shown), which was mainly due to the vaporous nature of the bubble at very low frequency, as illustrated in Refs. [50,57]. The bubble at 20 kHz is at ~99% vapor, and the presence of O3 in the liquid phase could not yield any effect on the chemical bubble yield (see Refs. [50,57]). The major statement that can be made from Fig. 1.4 is that the ozone effect on the sonochemical activity of a single acoustic bubble is frequency- and intensity-dependent. The most important remarks are:

1. The presence of ozone in the gas matrix can improve the acoustic production of •OH, particularly for lower operating frequencies and higher delivered acoustic intensities. This is in fact due to the higher bubble temperature generated upon collapse (>5000K) under these conditions, which allow the generation of important amount of O atoms from the pyrolysis pathway of ozone. However, the use of low acoustic intensity showed, in general, a negative impact on the effect of ozone, specifically when the frequency is much higher than 140 kHz.

2. An optimum ozone dosage for the production of hydroxyl radical was observed. The optimum value of O3 concentration depends on both the frequency and the acoustic intensity. In general, the optimal dose decreased with increasing frequency or decreasing applied acoustic intensity, i.e. 70% for 140 kHz, 50% for 213 kHz, and 30% for 647 kHz. The existence of this optimum reflects a strong concurrence between the bubble temperature reduction by O3 addition and the pyrolytic decomposition of ozone, which can compensate the former via the generation of excessive amount of O atoms. This competition is of course operating conditions–dependent.

3. For higher-frequency sonolysis, a higher acoustic intensity should be applied for obtaining a positive effect of ozone on the single bubble yield, i.e., 2 W/cm² is required at 674 kHz and more than this value is necessary at 1000 kHz. In fact, the lower bubble temperature generated at higher acoustic intensities can be increased through increasing the acoustic amplitude of the bubble oscillation via augmenting the acoustic intensity. A detailed numerical simulation of this event is available in many previous simulation papers [50,58].

4. In some cases, like those of much higher frequencies such as 647 and 100 kHz, the ozone can significantly enhance the production of hydroxyl radical. However, for an application point of view, lower acoustic intensity is not suitable for sonochemistry because it can produce insufficient number of bubbles in the sonicating medium. Therefore the overall sonochemical effect under low acoustic power is in general very low, particularly for higher-frequency sonolysis (>100 kHz).

Figure 1.4 Production rate of hydroxyl radical (•OH) from a single collapsing bubble as function of O3 fraction in O3–O2 saturation gas matrix for various frequencies ((A): 140 kHz, (B): 213 kHz, (C): 355 kHz, (D): 515 kHz, (E): 647 kHz, (F): 1000 kHz) and acoustic intensities.

O2+HO2•, which yields the lesser oxidant radical HO2•. Moreover, the bubble temperature could be another enhancer/reducer parameter of the chemical bubble yield as lower ones (i.e., obtained at lower intensities and too higher frequencies) could not promote efficient ozone sonolysis, and in this case, the most atomic oxygen would be consumed by the oxygen trapped initially in the bubble with ozone.

1.4 Plausible mechanisms

The single bubble yield is not the sole factor that controls the overall sono-ozonolytic activity in aqueous solutions. In fact, the synergistic effect resulted from the application of US/O3 process for water treatment may be controlled by several competitive parameters including: (1) cavitation-enhanced mass transfer of ozone in the bulk solution, (2) the single bubble yield, (3) the number of active bubbles, (4) coalescence of bubbles, and (5) ozone aqueous concentration.

Moreover, the aqueous concentration of ozone has, in parallel to its effect on single bubble yield, a paramount influence on cavitation nuclei. Using gas of higher solubility, such as CO2, N2O, or even ozone, can have different effects depending on the circumstances: It increases the dissolved gas concentration and the number of cavitation nuclei; the latter can either lead to a higher number of cavitation bubbles and consequently to higher sonochemical activity (scenario 1), or on the contrary to a higher extent of coalescence, that leads to a decrease in sonochemical activity (scenario 2). The occurrence of scenarios (1) and (2) depends on the ozone concentration. At lower ozone concentration, scenario 1 is highly probable, whereas scenario (2) could be the dominant event at high concentration of dissolved O3. A very similar circumstance has been recently reported by Merouani and Hamdaoui [57] for the case of CO2, which has also a too high solubility in water (i.e., 1688 mg/L at 20°C), as compared to conventional gases usually used for sonochemistry (argon, oxygen, helium, air, etc.). The presence of CO2 at trace amount (<5%) in argon can improve the sonochemical activity of the system, while the saturation of the liquid by CO2 alone quenches totally the chemical activity of the system [57]. Based on a systematic numerical study, Merouani and Hamdaoui [57] have reported that the quenching effect of CO2 could be due to its very high solubility in water that suppresses total inertial cavitation through favoring bubbles coalescence at nucleation. However, using trace CO2 in argon saturation atmosphere can improve the formation of hydroxyl radical through enhancing the number of inertial cavitation bubbles in the medium [57]. This scenario is probably the same for the case of ozone in oxygen since O3 has also higher water solubility than O2 (i.e., 482 mg/L against 43.4 mg/L at 20°C).

Based on above statements, we can highlight many plausible interaction mechanisms for the source of synergism coming from the combination of US and ozonation for the degradation of organic pollutants:

1. At low-frequency sonolysis (~20 kHz): The numerical results revealed that no effect of dissolved O3 on the acoustic production of hydroxyl radicals was produced. Therefore the experimental reported synergy at this frequency (Table 1.1) was attributed to (1) the US-enhanced mass transfer of O3 in the aqueous phase which increases the rate of the ozonation process and (2) the increase of the number of active bubbles due to the high O3 solubility in water. This last scenario may only be possible when O3 content in the saturation gas matrix is low because for high O3 dosage, the coalescence phenomenon (suppressor of inertial cavitation bubbles) could be dominant. However, given that an almost positive synergy was experimentally reported at 20 kHz (Table 1.1), we can conclude that the physical effects of US (acoustic streaming and micro-turbulence) were the dominant events responsible for the synergistic effect whatever the concentration of O3 in the bulk solution. This affirmation is supported by the fact that the low-frequency sonolysis alone (frequency<100 kHz) was in general not effective for the degradation of organic pollutants.

2. For high-frequency sonolysis (superior than 100 kHz, coupled with higher acoustic intensity), ozonation with low-to-moderate O3 flow in oxygen gas can improve the sonochemical yield of hydroxyl radicals. The resulted synergy in this case can be attributed to both (1) US-enhanced mass transfer and (2) O3-enhanced the single bubble yield and the number of active bubbles. However, for higher O3 concentrations, the effect of bubbles coalescence may overlap the beneficial effect of ozone on the sonochemical activity, and in this case, the overall sono-ozonolytic activity of the reacting system may be decreased. Therefore, higher O3 concentration is to be avoided when applying US/O3 process for water treatment when operating with high-frequency US.

1.5 Conceptual diagram for US/O3 system

Weavers et al. [3,8] have initially established a conceptual diagram for pollutants destruction pathways in US/O3 system. Then, the Weavers’ diagram has been partially modified by Merouani and Hamdaoui [2] by including the effect of high ozone dosage on acoustic cavitation. Herein, we will present an updated diagram of US/O3 process (Fig. 1.5) to further include the multiple effects of the low ozone concentration on cavitation and sonochemical activity. Routes 1 and 2 of the diagram reflect O3 mass transfer in water and direct reactions with the pollutant upon the sole ozonation. Additionally, sonication enhances mass transfer of O3, which may result in additional O3 being transferred to solution via pathway 5. Routes 6 and 7 happen by the sole sonication through pollutant pyrolysis in the bubble or by reaction with the sonochemically produced •OH. Routes 2, 6, and 7 still happen in the US/O3-coupled system but not at the same rates as in the separated processes, for the following reasons: The sonolytic decomposition of O3 produces further •OH to destruct substrates (pathway 3) and O3 can react with O(³P) to yield O2 (pathway 4). Both these pathways decrease the concentration of O3 available to react via pathway 2. Furthermore, O3 may scavenge •OH formed from sonication via pathway 8 (•OH+O3→HO2•+O2), which lessens the aqueous amount of both •OH and O3. Note that the occurrence of routes 3 or 4 depends strongly on the sonochemical conditions as stated in Section 1.3.4. Higher sonochemical conditions (i.e., low frequencies or high acoustic intensities) favors pathway 3, whereas pathway 4 likely occurs at lower sonochemical conditions (higher frequencies associated with lower acoustic intensities). Additionally, both routes 3 and 4 are more favorable at low-to-moderate O3 concentration in the aqueous phase, where higher number of bubbles is also believed to be formed as a consequence of the high solubility of ozone (pathway 11). However, the use of higher O3 concentration may quench pathways 3, 4, and 11 via suppressing the inertial cavitation responsible of the chemical effect of US (pathway 9), as stated in Section 1.3.4. This may be the reason for which negative synergy between US and O3 was reported for many cases shown in Table 1.1. The aqueous reaction of O3 with the acoustically generated H2O2 is another possible route for increasing the efficiency of the sono-hydride process (pathway 11). Note that the occurrence of pathways 1–11 is to a great extent affected by the operating conditions like frequency, power, and O3-dissolved concentration, which control all physical and chemical effects of acoustic cavitation.

Figure 1.5 Diagram of possible pathways of substrate (S) degradation and interactions of sonolysis and ozonolysis. US, Ultrasound.

1.6 Conclusion

Comparing with ozonation or US alone, US/O3 sono-hybrid process appears to be more effective and uses less energy, and it has the advantages of being nonselective, leading to no secondary pollution, and being particularly effective in removing biorefractory pollutants from water. The simulation results of the present study support the fact that the application of this hybrid technique can be synergistic. However, some limits of this synergistic effect have been shown based on the effect of ozone solubility on inertial cavitation event. Based on gathering different experimental and numerical considerations, a conceptual diagram for the chemical activity of the US/O3 process has been