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Fernandes, A.; Cruz-Lopes, L.; Esteves, B.; Evtuguin, D.V. Basic Principles of Microwave and Ultrasound Treatments. Encyclopedia. Available online: https://encyclopedia.pub/entry/52157 (accessed on 19 May 2024).
Fernandes A, Cruz-Lopes L, Esteves B, Evtuguin DV. Basic Principles of Microwave and Ultrasound Treatments. Encyclopedia. Available at: https://encyclopedia.pub/entry/52157. Accessed May 19, 2024.
Fernandes, Ana, Luísa Cruz-Lopes, Bruno Esteves, Dmitry V. Evtuguin. "Basic Principles of Microwave and Ultrasound Treatments" Encyclopedia, https://encyclopedia.pub/entry/52157 (accessed May 19, 2024).
Fernandes, A., Cruz-Lopes, L., Esteves, B., & Evtuguin, D.V. (2023, November 28). Basic Principles of Microwave and Ultrasound Treatments. In Encyclopedia. https://encyclopedia.pub/entry/52157
Fernandes, Ana, et al. "Basic Principles of Microwave and Ultrasound Treatments." Encyclopedia. Web. 28 November, 2023.
Basic Principles of Microwave and Ultrasound Treatments
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This entry is adapted from the peer-reviewed paper 10.3390/ma16237351

In the context of biorefinery and bioeconomy, lignocellulosic biomass is increasingly used to produce biofuels, biochemicals and other value-added products. Microwaves and ultrasound are emerging techniques that enable efficient and environmentally sustainable routes in the transformation of lignocellulosic biomass. 

microwaves ultrasound lignocellulosic biomass pretreatments

1. Introduction

Lignocellulosic biomass is the most abundant renewable resource in the world. The availability of non-food biomass is estimated to be (170–200) × 109 tons per day [1]. In the current context of biorefinery and bioeconomy, lignocellulosic biomass is increasingly used to obtain various alternative products to those of petroleum origin, namely biofuels, biochemicals, and other value-added products. Lignocellulosic biomass has a compact and robust structure that has been developed to acquire natural resistance in the cell wall and to protect itself from external physical or climatic actions and microbiologic attacks. This resistance is also called biomass recalcitrance [2][3][4][5]. Due to recalcitrance, lignocellulosic biomass must be pretreated before being chemically or biologically processed in order to obtain new bio-based products or biofuels. The goal of these pretreatments is to deconstruct the compact and recalcitrant structure of lignocellulosic biomass [3][5][6][7].
Pretreatments can be subdivided into four categories, depending on the approach: (i) physical: mechanical extrusion, grinding, microwave, ultrasound, pyrolysis, and pulsed electric field; (ii) chemical: acid, alkaline, ozonolysis, organosolv, ionic liquids, and deep eutectic solvents; (iii) physico-chemical: steam explosion, liquid hot water, wet oxidation, pretreatment with sulfite to recover lignocellulosic recalcitrance (SPORL), carbon dioxide explosion, and ammonia fiber explosion (AFEX); or (iv) biological: enzymatic, microbial, and fungal [3][6][8][9][10][11][12][13]. Depending on the type of biomass, the most appropriate pretreatment is selected. Often, it is necessary to perform a hybrid pretreatment—that is, a pretreatment that results from the combination between different types of pretreatments [8]. Each pretreatment has its advantages and drawbacks. Numerous studies have been published that describe, in detail, the specific drawbacks of each of the referred pretreatments (physical, chemical, physico-chemical, or biological) [2][13][14][15][16]. It should be noted that despite the large number of publications on the effect of different type of pre-treatments, when applied to lignocellulosic biomass, no single pretreatment was found to be superior in all respects [15][16]. This would also not be possible because, as will be discussed, the results of pretreatment vary from biomass to biomass.
Currently, within the scope of the concepts of biorefinery and circular bioeconomy, new pretreatment routes for lignocellulosic biomass have been developed in accordance with the principles of green Chemistry [14][15]. In this context, processes and routes are sought that simultaneously comply with four requirements: (i) do not consume too much energy; (ii) do not use toxic or dangerous solvents; (iii) minimize the amount of waste and be economically profitable [14][15]. In the search for approaches that meet these requirements, many studies on lignocellulosic biomass pretreatments have been published. Recent publications mention the following emerging techniques for the pretreatment of lignocellulosic biomass: ultrasound, microwaves, electron beams, gamma rays, high pressure homogenization, high-hydrostatic-pressure treatment, and pulsed electric field [15][17][18].

2. Basic Principles of Microwave and Ultrasound Treatments

2.1. Microwaves Radiation

Microwave radiation is non-ionizing radiation that lie between radio waves and infrared on the electromagnetic spectrum. Microwaves, like all electromagnetic waves, are composed of two fields perpendicular to each other, the electric and magnetic fields, which oscillate in the frequency range from 300 GHz to 300 MHz, corresponding to wavelengths from 1 mm to 1m, respectively [19][20]. This radiation is classified as non-ionizing because it does not have enough energy to remove electrons from the molecules or atoms on which it acts; it can only increase their kinetic energy, which translates into an increase in temperature [21].
Microwave radiation used at the industrial level has a frequency of 915 MHz, which allows more uniform heating and a transformation efficiency into heat of 85% [19][21]. Conventional microwave kitchen ovens typically use a frequency of 2.45 GHz, and their efficiency is 50% [19][21]. Most microwave reactors used for chemical synthesis also operate at 2.45 GHz [19]. Microwave photons do not ionize because they have low energy, unlike ionizing γ-ray or x-ray photons. Microwave photon frequencies of 915 MHz and 2.45 Hz correspond to energy values of 0.09 cal/mol and 0.23 cal/mol, respectively [21]. Typical photon energy from microwave radiation is ca. 0.03 kcal/mol, and the energy of chemical bonds ranges from 20 to 50 kcal/mol. [8]. Accordingly, these energy values of different orders show why microwave radiation is non-ionizing.
From a historical point of view, it is important to note that microwaves began to be used in industrial applications around the year 1980 [8][22]. Engineer Percy L. Spencer discovered in 1949 that microwave radiation can heat materials [8][14], but the first theory about the interaction of microwaves with matter was elaborated around the year 1954 by Von Hippel [8][14].

2.1.1. Conventional Heating and Microwave Heating

There are several differences between conventional and microwave heating. In conventional heating, the wall of the container is heated first, and only then is the material inside it heated (according to profile A, from Figure 1). In microwave heating, the process is reversed, first heating the inside and then the outside of the material (according to profile B).
The main differences between conventional and microwave heating in auto-hydrolysis treatment have been reported before [23][24]. The work conducted by Dávila et al. [23] highlighted the environmental sustainability of the microwave-assisted auto-hydrolysis method in the valorization of vine residues. In this method, a lower production of oligosaccharides was observed, as was an energy consumption only 28.8% of that required for conventional thermal treatment. Additionally, conventional hydrothermal treatment consumed 2.1 to 2.8 times more energy than microwave hydrothermal treatment, highlighting the energy efficiency of microwave-assisted auto-hydrolysis technology in the transformation of Paulownia elongata [24]. These results emphasize that the use of microwaves in the hydrolysis process is a sustainable and efficient alternative for this plant species.
These results are promising and encourage further research in this area, aiming to make the most of natural resources in a responsible manner. There are several studies that mention the advantages of heating lignocellulosic biomass with microwaves compared to conventional heating [8][15][20][23][24][25]. 
However, the heating of lignocellulosic biomass via microwave incidence also has disadvantages. The existence of materials that have low absorption, the presence of materials whose dielectric properties change with temperature, and the occurrence of heterogeneous materials (in composition, shape and/or size range) are some of these disadvantages. The existence of heterogeneous materials has as a consequence the differentiated absorption of heat, and a local overheating and the formation of the so-called “hot spots” can occur [8][14][20][21]. This phenomenon of the formation of “hot spots” can be minimized by increasing the size of the cavity, working at a higher frequency, or by coupling an agitator/turntable [21].

2.1.2. Microwave Heating Mechanisms

Microwave heating is a non-contact energy transfer process that can be accomplished via two distinct mechanisms: dipole rotation and ion conduction [8][14][19].
In the case of polar molecules, these molecules have a tendency to orient according to the alternating electric field of the microwave radiation, and this generates heat due to rotation, as it causes friction and collision between the molecules [8][14][19]. This mechanism is called dipole rotation (Figure 2A). This mechanism occurs in polar molecules that have permanent dipoles but also in molecules with induced dipoles [19]. In the case of ions, the interaction with the alternating electric field causes these charged particles to move, constantly changing direction; that is, they move back and forth, which causes a local increase in temperature due to friction and collision between the ions [8][14][19]. This mechanism is called ion conduction heating (Figure 2B). This ion conduction mechanism has more influence on heat generation than the dipole rotation mechanism [19]. It should be pointed out that the electric field is called alternating because its direction is constantly changing. For a frequency of 2.45 GHz, the direction of the electric field oscillates about 4.9 billion times per second [26].

2.1.3. Behavior of Materials in Relation to Microwave Radiation

There are three parameters for evaluating the behavior of materials in the face of microwave radiation: the dielectric constant (𝜀′), the dielectric loss (𝜀″), and the dielectric loss tangent ( = ε ′ ε ″ ).
The dielectric constant (𝜀′), relates to the ability of molecules to be polarized by an electric field. In other words, it is the ability of molecules to store electromagnetic energy [2][19][27]. This quantity depends on the molecular mass and geometry of the molecule [8].
Dielectric loss (𝜀″), measures the ability of a material to convert energy into heat [2][19][27]. The lower the dielectric loss for a material, the lower its ability to absorb microwaves [20][22]. 𝜀″ decreases with increasing temperature [8][20].The fact that the electrical loss decreases with temperature makes this parameter possible to be modified by changing the temperature [8].
The dielectric loss tangent (tan 𝛿)  results from the mathematical relationship between these properties and so is a dimensionless parameter. This value reveals the ability of a material to be heated by microwave [8][19]. If this parameter is null it means that this material does not heat up with the incidence of microwave radiation [8][20][28].
According to the behavior of the materials in the face of microwave incidence, the materials can be classified into three categories: dielectric, conductive and non-conductive [8][21][28].
According to the literature, lignocellulosic biomass can be classified as a low-loss dielectric material, which—in other words—means that biomass absorbs microwave radiation but with some difficulties [19][27].
Another important parameter which reveals the behavior of a material affected by microwave radiation is the depth of penetration, Dp. This parameter estimates how deep microwave radiation reaches into a given material and can be predicted using the following expression [27].
D p = λ 0 2 π ( 2 ε ′ ) 0.5 { [ 1 + ( t a n   δ ) 2 ] 0.5 − 1   } − 0.5  
(λ0—is microwave wavelength in free space).
For water, the penetration depth is 1.4 cm at a temperature of 25 °C, but increasing the temperature to 90 °C increases it to 5.7 cm (maintaining a frequency of 2.45 GHz) [20].

2.1.4. Behavior of lignocellulosic Biomass in Relation to Microwave Radiation

The behavior of lignocellulosic biomass against microwave radiation depends, as previously mentioned, on the effective parameters: 𝜀′, 𝜀″, tan 𝛿, and Dp. It is therefore essential to know the values of these parameters for lignocellulosic biomass when applying a microwave treatment. In the literature, there are examples of evaluation of these parameters for the various biomass, for examples: palm bark and fibers [26]; empty fruit bunches [29]; tropical wood [30]; banana fibers with polyurethane [31]; pinewood blocks [28]; pinewood and arabica coffee [32]; hay [33]; and karanja seeds [34]. Table 1 shows the dielectric constants, dielectric losses, dielectric loss tangents, and depths of microwave penetration for various biomasses.
Table 1. Dielectric constants, dielectric losses, dielectric loss tangents, and depths of microwave penetration for various biomasses.
Biomass 𝜺′ 𝜺″ 𝐭𝐚𝐧 𝜹 Dp
(cm)		 Frequencies and
Temperature		 Reference
Tropical wood 2.08 0.1849 0.0954 ---- 8.2 to 12.4 GHz [30]
Banana fibers with polyurethane 30% 137 26 ---- ---- 1 kHz [31]
Empty fruit bunch
(18 wt% moisture)		 6.4 1.9 0.3 3.5 2.45 GHz, 27 °C [29]
Empty fruit bunch char 3.5 0.47 0.13 ---- 2.45 GHz, 500 °C [29]
Pinewood 2.7 0.53 ---- 59 2.45 GHZ, 17 °C [28]
Oil palm fiber 1.99 0.16 0.08 24.8 2.45 GHZ, 500 °C [26]
Oil palm shell 2.76 0.35 0.12 13.4 2.45 GHz, 500 °C [26]
Oil palm char 2.83 0.23 0.08 20.6 2.45 GHz, 500 °C [26]
Hay ---- ---- ---- 0.02 2.45 GHz, 700 °C [33]
Pinewood 13.4 0.08 0.006 0.2 2.45 GHZ, 25 °C [32]
Arabica coffee 26.8 3.14 0.117 0.5 2.45 GHZ, 25 °C [32]
Wood ---- ---- 0.11 ---- ---- [34]
Fir plywood ---- ---- 0.01–0.05 ---- ---- [34]
Karanja seeds ---- ---- 1.3 1.26 0.1 to 3.0 GHz at room temperature [34]
As shown by the examples in Table 1, the dielectric properties vary substantially according to the type of biomass. These dielectric parameters are not constant and depend on temperature [22]. Regarding the loss tangents of the various biomasses presented in Table 1, it is necessary to highlight that of karanja seeds [34]. These seeds exhibit a value of 1.3 for dielectric loss tangent (tanδ) (at 2.45 GHz). This value is the highest known loss tangent value for lignocellulosic biomass. This makes this biomass ideal to be used in microwave pyrolysis for biodiesel production, moreover because these seeds are not edible.
The values of the dielectric constants of a biomass also depend on its humidity. The values of the dielectric parameters relative to the empty fruit bunch, in Table 1, are for 18% humidity [29]. However, if the moisture of the empty fruit cluster is 64% instead of 18%, the dielectric constant and the dielectric loss tangent increase from 6.4 to 57.4 and from 1.9 to 18.6, respectively.
There are several studies on the dielectric parameters of biomass. In one study, Salema et al. measured the dielectric properties of five different agricultural and forest residues (palm bark, empty fruit cluster, coconut husk, rice husk, and wood sawdust) from room temperature to approximately 700 °C [27]. This study mentions that the dielectric constants decrease slightly during the drying phase (from 24 to 200 °C); during pyrolysis, the dielectric constants continue to decrease (from 200 to 450 °C), but after 450 °C, the dielectric constants increase significantly. These researchers concluded that the dielectric constants depend on the type of biomass and vary during drying and pyrolysis but that they vary nonlinearly with temperature.
In another study, the parameters 𝜀′  and tan δ were measured for different lignocellulosic fibers (residual lemon, medlar, palm, and olive leaves) for a frequency range from 10 Hz to 8 MHz, and the authors concluded that with an increase in the frequency of the microwaves, the dielectric constant decreases [30]. However, the loss tangent decreases until it reaches a minimum and then remains constant [35]. It was verified that all fibers exhibited the same behavior. These dielectric parameters were measured for lignocellulosic fibers under the same conditions: at room temperature with a frequency of 10 Hz to 8 MHz for peak voltage 1 V and from 10 Hz to 100 KHz for peak voltage 5 V. It was concluded that these dielectric parameters depend in a notable way on the frequency. The dielectric materials were subdivided into four categories as follows: homogeneous (when the electrical properties are independent of position); dispersive (when the electrical properties depend on the frequency variations of the electric field); isotropic (when they are not affected by the direction of the applied electric field), and linear (when they are independent of the strength of the applied electric field [35]. This allowed classification of lignocellulosic biomass as a dispersive material, with a good degree of homogeneity and linearity.
In the treatment of lignocellulosic biomass, solvents are normally used. A group of researchers studied the incidence of microwaves in water, aqueous acidic, and alkaline media and in an ethanol–water mixture and concluded that the best solvent for the absorption of microwave radiation is water [36]. In fact, it is known that the presence of water inside the materials facilitates their heating and that humidity influences the dielectric properties, as it influences the Dp [8].
It is also important to know the dielectric parameters of the solvents used in the treatments assisted by microwaves: 𝜀′, 𝜀″, and tan δ.
The higher the tan δ, the more polar the solvent and the more easily it heats up due to the action of microwaves. In microwave-assisted pretreatments usually applied to lignocellulosic biomass the most commonly used solvents are water, aqueous acidic or basic solutions, deep eutectic solvents, and ionic liquids (as will be seen later) [37]. For these solvents, before using them, it is advisable to know their dielectric parameters.

2.1.5. Microwave Absorbing Materials Addition to Lignocellulosic Biomass

Since lignocellulosic biomass does not absorb microwave radiation well [27], microwave-absorbing materials—that is, materials with high tan δ—should be added to the biomass; for example, before pyrolysis, the microwave absorbers are added [19]. Microwave-absorbing materials are carbon-based solid materials and metal oxides. In the category of metal oxides, the most used are: CuO, MgO, Fe2O3, Al2 O3, and SiO2, and in the category of carbon-based solid materials, the most common are coal, activated carbon, coke, graphite, and silica carbide (SiC) [19][33][35][38].
Although microwave-assisted pyrolysis is not the subject, for a better understanding of the behavior of lignocellulosic biomass when subjected to microwaves, the advantages and disadvantages of these two types of microwave-absorbing materials are presented below. Microwave-assisted pyrolysis (MAP) has proven to be an effective method of shortening pyrolysis reaction times and improving the quality of value-added products from several types of raw materials, eliminating the need for grinding [39]. Several reviews have been conducted in recent years on microwave-assisted pyrolysis [39][40][41][42][43][44][45][46]. There are several differences between conventional and microwave-assisted pyrolysis. Microwave-assisted pyrolysis has uniform heating—the whole material is heated simultaneously—while in conventional pyrolysis, there is superficial heating and then a transfer of energy via convection and/or conduction [45]. Furthermore, the heating in MAP is rapid and efficient and is more precise and controlled since it is possible to stop the heating immediately by turning off the power [45]. The most important operational parameters influencing product yield in MAP have been stated to be microwave power, temperature, addition and concentration of microwave absorbers, initial moisture content, and the flow rate/residence time of the initial sweep gas [40]. The addition of microwave absorbers can lead to enhancement of the pyrolysis reaction temperature at relatively low microwave power. There are several microwave absorbers used for lignocellulosic biomass—for example, silicon carbide used in MAP of pine wood sawdust [47] or MgCl2 and Na2HPO4 in corn stover pellets [48]. Sometimes, pyrolysis char is also used as a microwave-absorber [49][50][51]. With regard to carbon-based solid materials, the researchers point to three advantages with respect to microwave-assisted heating, namely: (1) the increased absorption capacity of microwaves of bulk materials; (2) increased heat transmission to surrounding materials; and (3) increased heating rate at low microwave powers [20]. Carbon-based solids are good microwave absorbers, as they have high Dp values when compared to metals. Activated carbon, for example, has a Dp of 0.7 to 3.43 cm, and silver has a Dp of 1.3 μm (values for frequency 2.45 GHz and room temperature [20]). As for the disadvantages of adding carbon-based solid materials, they can influence yields and alter the desired product yields [19].
Adding metal oxides to lignocellulosic biomass to make it more microwave-absorbent also has several advantages. At least three advantages are mentioned in the literature, namely: (1) the increase in the absorption capacity of microwaves; (2) increasing the rate of warming, and (3) “improving the devolatilization” of biomass [19].
Comparing carbon-based solids to metal oxides with materials, researchers report that metal oxides can affect the quality of the product obtained by pyrolysis [19], and carbon-based solids appear to be preferable because they mixed better and more evenly with the biomass [52].

2.1.6. Factors to Consider in a Microwave Pretreatment for Lignocellulosic Biomass

There are several factors that affect the heating of lignocellulosic biomass when subjected to microwaves and these factors should be studied in detail beforehand. Prior knowledge of dielectric properties (𝜀′, 𝜀″, tan δ, and Dp) of the lignocellulosic biomass is important not only for a better understanding of the microwave heating process but also for a proper sizing of the necessary equipment [27]. A recent review [3] summarizes seven factors that affect the heating of lignocellulosic biomass by microwaves: (1) the dynamics of dipole biomass molecules; (2) the composition and size of the biomass; (3) the induction current of magnetic materials present in the biomass and the ionic conduction of electrolytes; (4) reaction time (residence time) and heating rate; (5) the moisture content of the biomass; (6) the power of the microwave; and (7) the depth of penetration. Therefore, in the selection of a microwave pretreatment, or in the sizing of a microwave equipment, these variables must be considered.
Since several factors influence the behavior of lignocellulosic biomass, when subjected to microwave radiation, and these factors being interconnected with each other, it becomes difficult to find the best conditions for a given pretreatment employing microwaves. To overcome this barrier, a computational simulation was recently developed on the Comsol Multiphysics platform, using Maxwell’s mathematical equations and the heat transfer equation to simulate microwave heating for three types of lignocellulosic biomass: sugarcane bagasse, palm oil, and green algae [20]. The goal was to find the best conditions for microwave pretreatment for these three types of biomass, and they concluded that these conditions depend on temperature, humidity (from 20 to 80%), volume (from 10−5 to 100 × 10−5 m3), and the shape of these samples (cylindrical or spherical). This work allowed us to reach the following conclusions: (i) the selection of the microwave power is fundamental to finding the best conditions (temperature, humidity, volume, and shape of the sample); (ii) since there is a homogeneous temperature distribution profile inside the sample, the sample size and the penetration depth (Dp) must have dimensions of the same order of magnitude; (iii) materials with high values of dielectric constants (ε′) and dielectric losses (ε″) will have low penetration depth values (Dp); (iv) the distribution of the electric field depends on the geometry of the sample, the humidity, and also the type of biomass; and (v) the power absorbed by the sample increases with its volume but decreases with the quotient of its surface area/volume [20].

2.1.7. The Reasons Justifying Microwave Absorption and Lignocellulosic Biomass Recalcitrance

As already discussed, lignocellulosic biomass is a dielectric material, which absorbs microwaves. The main causes of the absorption of microwaves by lignocellulosic biomass are the presence of water and the polarity of the macromolecules that constitute the biomass (cellulose, hemicellulose, and lignin). With regard to polarity, it is the polarity of the macromolecules from the biomass that provides its heating when microwave irradiation reaches these macromolecules (dipole rotation mechanism as discussed before).
It is important to emphasize that the polarity of the constituent macromolecules of biomass is due, in part, to hydroxyl groups (-OH) [8][14]. These OH groups establish hydrogen bonds within the polymeric molecules of the biomass and between these polymers (intra-polymer and inter-polymer bonds, respectively) [5]. A set of all these intra-polymer and inter-polymer hydrogen bonds and cross-links between macromolecules gives robustness to lignocellulosic biomass and makes it recalcitrant and very difficult to deconstruct.
Several factors related to the structural features and morphology of lignocellulosic biomass were referred as contributing to the recalcitrance of biomass: (i) the crystallinity of cellulose, the degree of polymerization, (ii) the size of the biomass particle, (iii) the size and volume of the pores in biomass tissues, (iv) the accessible surface area, and (v) the structural complexity of biomass components [5]. Before lignocellulosic biomass is used in the processing to produce bio-based products or biofuels, it has to be deconstructed, and this is the main function of any pretreatment (Part II). The deconstruction of lignocellulosic biomass by the incidence of microwave radiation is possible because the microwaves force the dipolar macromolecules of the biomass to align with the oscillating electric field, which results in the rupture of hydrogen bonds and the breakdown of cell walls [53].

2.2. Ultrasound and Two Categories of Ultrasound

Sounds in the sound spectrum can be classified, according to applied frequency (f), into three groups: infrasounds (f < 20 HZ), audible sounds (20 Hz < f < 20 kHz), and ultrasound (f > 20 kHz). In turn, ultrasound can be subdivided into two categories: (1) low- to medium-frequency waves (20–100 kHz), also called “power ultrasounds” and (2) high-frequency waves (3–10 MHz), also called “diagnostic ultrasounds”.
Regarding an ultrasound treatment applied to the pretreatment of lignocellulosic biomass, low- and medium-frequency ultrasound are commonly reported.

2.2.1. Basic Principles of Cavitation

An ultrasound is a cyclic pressure wave consisting of compression and rarefaction zones (areas of low pressure) alternating in space and time. When an ultrasound (of low or medium frequency) propagates inside a liquid, the phenomenon of acoustic cavitation occurs. This phenomenon originates in a zone of rarefaction (or zone of negative pressure) that, when propagating inside a liquid, forces its particles to separate, thus generating cavities or bubbles. As the wave travels through the liquid, the bubbles grow for successive cycles until they reach an unstable size and then suffer a violent collapse [54][55][56][57].
Briefly, the phenomenon of acoustic cavitation has three phases: (1) the formation of the bubble; (2) the rapid growth of the bubble during the successive alternating compression-rarefaction cycles until it reaches an unstable size; and (3) the violent collapse of the bubble inside the liquid.
In most situations, after the collapse of the bubble, new smaller bubbles result and the cavitation cycle repeats. During the collapse of the bubble, the temperature and pressure inside it can reach very high values. The literature suggests temperature values between 500 K to 15,000 K and pressure values from 100 atm to 5000 atm [3]. Researchers say the life cycle of a bubble can last only a few microseconds [2] and that the rate of warming can reach 1010 kelvins per second [56]. Bubble collapse is a violent phenomenon that causes points of high temperature and pressure called “hot spots”. It is these “hot spots” that are the theoretical foundation of any pretreatment using ultrasound [54].
From a historical point of view, the phenomenon of cavitation was discovered by Thomycroft and Bamby in 1895, but it was not in demand until the year 1917, when the first mathematical model of the phenomenon was realized and disseminated by Lord Rayleigh [56].

2.2.2. Factors That Influence the Cavitation of Lignocellulosic Biomass

The phenomenon of cavitation, when applied to lignocellulosic biomass, is a complex phenomenon that depends on numerous factors, such as the physical properties of the solvent used in the pretreatment, viscosity, surface tension, and volatility [4][58]. The literature states that the phenomenon of cavitation occurs preferably in liquids with low volatility, medium viscosity, and high surface tension [58]. In addition to these factors, in a pretreatment with ultrasound, it is necessary to consider not only the frequency of the ultrasound, the sonication time, and the acoustic power of the ultrasound but also the geometry of the reactor (ultrasonic baths and probes are different) [11][55][58][59]. Temperature is also a factor that has an influence when applying an ultrasonic treatment to lignocellulosic biomass [59]. Thus, in aqueous solvents, cavitation is at its maximum at low temperatures [55].
In addition to the aforementioned factors, the effectiveness of an ultrasonic pretreatment also depends on the type of lignocellulosic biomass. Studies show that when different biomasses are applied for the same treatment, this leads to different experimental results [55]. For this reason, when optimizing a treatment route for a given biomass, this route is commonly suitable for that biomass only [55]. The same route applied to other biomass may prove to be ineffective.
Thus, briefly, it can be stated that the effectiveness of ultrasonic pretreatment depends on the following factors: the properties of the medium (solvent viscosity, surface tension, and volatility); the characteristics of ultrasound (frequency, sonication time and power); the operating temperature; and the type of biomass.

2.2.3. Physical Effects and Chemical Effects of Ultrasound on Lignocellulosic Biomass

The ultrasound effects on lignocellulosic biomass are very diverse and complex but can be subdivided into two main groups: (i) the physical or mechanoacoustic effects and (ii) the chemical or sonochemical effects [2][55][56][58][60]. The physical effects during cavitation deal with the formation of strong shear forces and the creation of microjets [58][61]. Shear forces and microjets are the consequence of symmetrical cavitation (usually spherical) or asymmetric cavitation, respectively [58]. Microjets form when cavitation occurs on the surface of a solid particle larger than the bubble [55]. It is important to mention that microjets can reach projection speeds of hundreds of kilometers per hour [55].
As for the chemical effects of ultrasound, the formation of several radicals stands out [2][54][55][56]. The most important free radicals to consider are those that result from the ultrasonic decomposition of water, the hydroxyl radical (OH●) and the hydrogen radical (H●) being the most important [61]. Note that the decoupling of these OH● and H● radicals can, in turn, form water, or the two hydroxyl radicals can react with each other and form hydrogen peroxide [55][56]. However, the dominant species is the hydroxyl radical, as its reduction potential (+2.06 V) is higher than that of hydrogen peroxide (+1.78 V) [62]. In addition to these radicals, others can be formed depending on the solvent applied in the ultrasonic treatment. In the literature, eight techniques are listed for measuring the concentration of different radicals resulting from cavitation [62]. Although the formation of radicals is the most relevant chemical effect, within the bubbles generated in cavitation, high temperature and pressure can cause luminescence phenomena [55].
The shear forces and the formation of microjets at high speeds lead to the detachment and destruction of the chemical bonds between the macromolecules of the lignocellulosic biomass [4]. As for the oxidizing radicals resulting from cavitation, they trigger numerous chemical reactions, which promote the decomposition of the macromolecules that constitute the lignocellulosic biomass and which are catalyzed by ultrasound (Part IV exposes the main reactions triggered by ultrasound in the lignocellulosic biomass).
Still, regarding the effects of acoustic cavitation, it is noteworthy that the main general physicochemical effect is the promotion of mass and energy transfers and—as a consequence—an increase in the speed of the chemical reactions involved [4][63]. This is due to the fact that acoustic cavitation promotes localized increases in temperature and pressure in very short time intervals, which—in addition to the turbulence and intensity of the shear forces and microjets—can cause morphological changes in the lignocellulosic biomass as well as an increase in the speed of the chemical reactions involved [3]. As for changes in the lignocellulosic structure due to ultrasonic pretreatment, as examples, the breakdown of the α-O-4 and β-O-4 bonds of the lignin is reported [14][61] as is the rupture of ether bonds between hemicellulose and lignin [14].

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Subjects: Materials Science, Paper & Wood
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Ana Fernandes
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Luísa Cruz-Lopes
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Bruno Esteves
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Dmitry V. Evtuguin
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Update Date: 30 Nov 2023
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