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Article

Ozone Nanobubble-Assisted Pretreatment of Lignocellulose: Enhancing Wood Liquefaction and Bio-Polyurea Development

by
Go Masuda
1,
Christian Ebere Enyoh
1,
Weiqian Wang
1,
Satoshi Anzai
2 and
Qingyue Wang
1,*
1
Graduate School of Science and Engineering, Saitama University, 255 Shimo-Okubo, Sakura-ku, Saitama 338-8570, Japan
2
Anzai Kantetsu Corporation, Komaoka 3-1-16, Tsurumi-ku, Yokohama City 230-0071, Kanagawa, Japan
*
Author to whom correspondence should be addressed.
Appl. Sci. 2025, 15(9), 4618; https://doi.org/10.3390/app15094618
Submission received: 28 February 2025 / Revised: 4 April 2025 / Accepted: 10 April 2025 / Published: 22 April 2025
(This article belongs to the Special Issue Industrial Chemical Engineering and Organic Chemical Technology)
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Abstract

:
Nanobubbles have emerged as a novel technology, yet their applications remain largely limited to cleaning and oxidation. This study explores the potential of ozone nanobubbles as a pretreatment method for liquefied wood. Wood meal was treated with ozone nanobubbles in tap water under three different conditions: room temperature, 50 °C, and room temperature followed by ultrasonic treatment. The treated samples were then compared with untreated wood meal through component analysis, FT-IR functional group evaluation, and X-Ray diffraction (XRD) analysis of cellulose crystallinity. In the liquefaction process, residue rates, FT-IR analysis, hydroxyl numbers, and viscosity were examined. Additionally, the mechanical properties of synthesized polyurea films were evaluated via tensile testing. The results showed a reduction in amorphous cellulose from 62.3% to 56.6% and hemicellulose from 42.8% to 35.9%, leading to liquefied wood with a high hydroxyl value from 341 KOH/mg to 387 KOH/mg and significantly lower viscosity from 684 cP to 264 cP. Furthermore, the polyurea films synthesized from the treated liquefied wood exhibited no deterioration in physical properties. These findings highlight ozone nanobubble pretreatment as a promising and industrially valuable process for producing low-residue, low-viscosity liquefied wood without compromising material performance.

1. Introduction

In recent years, the efficient utilization of woody biomass, a renewable resource, has been strongly emphasized as a key step toward achieving a sustainable society. Among various types of biomasses, waste wood remains largely underutilized, with its primary use still limited to fuel consumption [1,2,3]. This highlights an urgent need to explore alternative applications. Liquefaction, a chemical conversion technology for woody biomass, has garnered attention as a promising method for transforming wood into valuable chemical feedstocks. Since the 1980s, intensive research has been conducted on liquefaction techniques using solvents such as phenols and polyethylene glycol, leading to advancements in understanding reaction mechanisms and optimizing process conditions [4,5,6,7]. Notably, progress has been made in the development of functional polymers and adhesives derived from liquefied wood products [4,7,8,9]. However, these technologies remain confined to the laboratory scale.
At the same time, conventional liquefaction processes face several challenges, particularly in terms of reaction efficiency and environmental impact [10,11,12,13,14,15,16]. The rigid crystalline structures of lignin and cellulose, the primary components of wood, present significant barriers to efficient liquefaction. In particular, the rigid crystalline structure of cellulose, in addition to the strong bonds between each component, inevitably slows down the liquefaction process. Therefore, if these bonds and crystalline structures can be modified into states more conducive to liquefaction, the efficiency of the liquefaction process would naturally be enhanced. To solve this matter, traditional pretreatment methods, including strong acid or alkali treatments and high-temperature, high-pressure processing, have been investigated. However, these approaches are associated with high capital and operational costs as well as considerable environmental burdens. Against this backdrop, the development of more efficient and environmentally friendly pretreatment technologies has become a critical research focus for advancing the practical implementation of wood liquefaction processes [17].
Nanobubble technology, which enables the stable generation and maintenance of ultrafine bubbles with diameters less than 100 nm, has recently garnered attention across various industries as an innovative solution. Nanobubbles have unique physicochemical properties such as high specific surface area, long-term stability, high gas dissolution efficiency and strong oxidative potential [18,19,20,21,22].
The mechanism of organic matter decomposition is driven by free radicals generated during the disappearance of nanobubbles, involving various gases. Among these, ozone nanobubbles have been shown to possess several advantages over conventional ozone treatments. First, the utilization efficiency of ozone is improved by more than tenfold, which is attributed to the prolonged retention time enabled by nanobubble stability. Second, enhanced bubble stability allows for sustained oxidative effects, ensuring that ozone’s reactive capabilities are maintained over an extended period. Furthermore, the ultrafine size of nanobubbles facilitates uniform treatment even in deeper regions, achieving high treatment efficiency even at low concentrations [21,23,24,25].
The generation of nanobubbles can be achieved through various methods, each utilizing different physical and chemical principles. The major methods include ultrasonic cavitation [26], hydrodynamic cavitation [27,28,29,30], porous material diffusion [31,32], and solvent exchange techniques [28]. Each method has its advantages and disadvantages. The ultrasonic cavitation method generates nanobubbles by exposing a liquid to high-frequency ultrasonic waves, typically in the range of 20 kHz to several MHz. The ultrasonic waves induce alternating high-pressure and low-pressure cycles in the liquid, leading to the formation, growth, and subsequent collapse of microbubbles. When these bubbles collapse, they generate extreme localized conditions, such as high temperature and pressure, facilitating the production of nanobubbles. This method is widely used due to its ability to generate stable nanobubbles with high efficiency.
Hydrodynamic cavitation involves the formation of nanobubbles through rapid pressure changes within a liquid. This is typically achieved by forcing the liquid through a constricted or specially designed orifice, nozzle, or Venturi tube. As the liquid velocity increases at the constriction, the pressure decreases, causing the dissolved gas to nucleate and form microbubbles. These bubbles subsequently collapse due to downstream pressure recovery, generating nanobubbles. This method is advantageous for large-scale applications due to its simplicity and energy efficiency.
In the porous-material diffusion method, gas is introduced into a liquid through a porous membrane or material with nanometer-scale pores. The gas diffuses through the pores and forms bubbles as it enters the liquid phase. By controlling the pore size and flow rate, the size distribution and concentration of the nanobubbles can be precisely controlled. This method is particularly effective for generating monodisperse nanobubbles and is commonly used in applications requiring controlled gas dissolution. In this research, as shown in Figure 1, we used a carbon porous material to generate nanobubbles.
The solvent exchange method generates nanobubbles by exploiting differences in solubility between two miscible liquids. In this process, a liquid containing dissolved gas is mixed with a second solvent in which the gas has lower solubility. As the solubility equilibrium shifts, gas molecules are forced out of the solution, leading to the nucleation and stabilization of nanobubbles. This method is often used for producing nanobubbles in specific solvents or for applications requiring precise control over bubble stability and composition. Each of these methods has distinct advantages and is selected based on the desired application, efficiency, and scalability.
Although numerous studies and reports have demonstrated the effectiveness of nanobubbles in applications such as cleaning and purification [29,34,35,36,37], research on other potential applications remains limited [38]. To date, researchers have reported the promotion of seed germination in agriculture using ozone nanobubbles, the preservation of seafood freshness using nitrogen and oxygen nanobubbles, and the cooling effects of nanobubbles in cutting processes. Given this context, this research investigates whether the free radicals generated by nanobubbles can actively degrade or modify organic matter, thereby serving as a potential pretreatment step in liquefaction technology. In addition, polyurea films were prepared from liquefaction products of wood meal pretreated with ozone nanobubbles, and a comparative analysis of the physical properties was conducted between these films and those derived from liquefaction products of untreated wood meal.

2. Materials and Methods

2.1. Materials

The wood specimens utilized in this investigation were provided by Fujigen Co., Ltd. (Matsumoto, Nagano, Japan), a manufacturer specializing in the production of guitars along with various stringed musical instruments in the Nagano region. For experimental purposes, three distinct wood varieties—maple, mahogany, and rosewood—were selected as research materials. These wood varieties are well known for their hard and heavy wood. The bio-polyurea synthesis procedure incorporated cyanate resin (STABiO, manufactured by Mitsui Chemicals, Inc., Tokyo, Japan) and polyamine (Development product by Unitika Ltd., Osaka, Japan) in addition to the liquefied products. Both resin components were supplied for free by their respective manufacturing entities. The additional chemical reagents employed in this study, including polyethylene glycol 400 (PEG400) and glycerol, were acquired from FUJIFILM Wako Pure Chemical Corporation (Osaka, Japan) at chemical-grade quality and utilized without subsequent purification procedures.

2.2. Preparation of Ozone Nanobubbled Wood Meal

The wood meal used in this research was the same as that in our previous research, [39,40,41] consisting of 250 μm ground waste materials from wooden musical instruments. To achieve more uniform nanobubble treatment and a more effective liquefaction process, 250 μm wood meal was prepared. Ozone nanobubble treatment of the wood meal was performed by Anzai Kantetsu Co., Ltd. (Yokohama, Japan). This nanobubble treatment was carried out only once for each condition. The treatment was conducted under three different conditions using 20 g of wood meal in circulated tap water without any purification, as follows: 1. Ozone nanobubbles (O3 at 10 g/h) were introduced into 5 L of circulating water and treated for 180 min at 23 °C. 2. To promote the ozone nanobubble treatment, the water temperature was increased to 50 °C while maintaining the same conditions as in condition 1. 3. After the ozone nanobubble treatment under the same conditions as in condition 1, an additional ultrasonic treatment (38 kHz) was performed for 20 min. The ultrasonic treatment following the ozone nanobubble process was conducted using a Sono Cleaner 100a (Kaijo Corporation, Tokyo Japan).
After the ozone nanobubble treatment, the treated wood meal was dried in an oven at 105 °C, and the effects of the treatment were evaluated. In general, weight loss of the wood meal was not seen after the dry process. Subsequently, the wood meal was subjected to liquefaction and used for the preparation of polyurea films. The appearance of the wood meal before and after ozone nanobubble treatment is shown in Figure 2.

2.3. Analysis of Ozone Nanobubbled Wood Meal

To analyze the composition of wood meal treated with ozone nanobubbles, elemental analysis was performed for three samples using a CHN analyzer (MT-5, Yanaco Co., Ltd. Kyoto, Japan), while lignin and holocellulose quantification was conducted using the Klason lignin method and the Wise method. Additionally, Fourier-transform infrared spectroscopy (FT-IR) (JASCO Co., Ltd., Tokyo, Japan) was employed to obtain information on functional groups. These analyses were carried out using the same methods as in our previous research.
In the wood meal treated with ozone nanobubbles, a reduction in amorphous cellulose, similar to that observed in ozone-treated samples as reported by Kobayashi et al. [42], is expected. Therefore, the crystallinity before and after ozone nanobubble treatment was evaluated via X-Ray diffraction (XRD) using Ultima III (Rigaku Holdings Corporation, Tokyo, Japan). XRD spectra was obtained just once for each nanobubble condition. As shown in Figure 3, the crystallinity index was calculated according to Equations (1) and (2), and the XRD measurement conditions were set as shown in Table 1.
The rate of amorphous (%) = Applsci 15 04618 i001 × 100
The rate of crystalline (%) = 100 − The rate of amorphous (%)

2.4. Characterization of the Liquefied Wood

For residue quantification purposes, a protocol involving the addition of 10 mL methanol to the liquefied product followed by thorough homogenization was implemented. The resulting mixture underwent filtration using a vacuum pump apparatus and paper filtration media (ADVANTEC C5, manufactured by Toyo Roshi Kaisha, Ltd., Tokyo, Japan). Subsequently, the filtered material was subjected to thermal drying in a laboratory oven maintained at 105 °C for a duration extending through the night. Residue percentage calculations were performed according to Equation (3):
Ratio of residue (%) = W1/W0 × 100,
where W1 represents the weight ratio of residue in the analyzed liquefied sample, and W0 denotes the weight ratio of wood in the initial specimen.
Molecular weight determinations for the liquefied wood specimens were conducted via the gel-permeation chromatography (GPC) methodology. The analytical system incorporated the chromatographic separation column from Shodex KF-802 (Resonac Holdings Co., Tokyo, Japan), a high-performance liquid chromatography pump (PU-2080, JASCO Co., Ltd., Japan), a temperature-controlled column environment (CO-2060, JASCO Co., Ltd., Tokyo, Japan), and a refractive index detection unit (RI-2031, JASCO Co., Ltd., Tokyo, Japan). The analytical parameters, consistent with previously established experimental protocols, are comprehensively presented in Table 2. Prior to chromatographic analysis, we precisely measured 10 mg quantities of the liquefied wood samples that were solubilized in 2.5 mL of tetrahydrofuran (THF).
The hydroxyl number of liquefied wood constitutes a parameter of critical significance in determining appropriate synthesis formulations, as this characteristic directly influences the mechanical properties exhibited by the resultant resin. Quantification of both acid numbers and hydroxyl value parameters was conducted by utilizing a methodological approach adapted from the protocol established by Ertas et al. [43]. The determination of hydroxyl value was performed through calculations based on the experimentally measured acid number. Both analytical parameters were quantified through titration procedures, with appropriate blank titrations executed following identical methodological protocols to ensure the analytical validity and accuracy of the obtained values. This measurement was performed two times for each sample.
The viscosity of the liquefied wood products was measured using a Brookfield DV-1 viscometer (AMETEK Brookfield, Middleborough, MA, USA). The measurement temperature was maintained at 23 ± 1 °C using a water bath before measurement. To obtain a stable value, the viscosity value was recorded after 1 min of spindle rotation.

2.5. Tensile Strength Test of Polyurea Films

Fabrication of polyurea film specimens for tensile strength evaluation was executed following methodological protocols established in our previous investigative work [39]. Bio-polyurea film was obtained at sizes of approximately 200 × 150 mm by the casting method. Most of the film samples’ thicknesses were from 100 to 200 µm. After casting the films, the substrate was set at room temperature overnight, and after that put into the oven at 80 °C for 48 h to cure completely. Tensile strength assessment was conducted in strict accordance with JIS K7127 (ISO 527-3) [44] standardized testing procedures. Each specimen’s dimension was 150 mm long and 10 mm wide. (±0.5 mm) Specimen preparation involved precise excision from the fabricated film using a precision cutting implement. Post-excision processing included edge refinement through controlled abrasive application to ensure dimensional and structural uniformity across all test specimens. Mechanical property characterization was performed by utilizing a precision universal testing apparatus, specifically the AGX-100kNV system (manufactured by Shimadzu Corporation, Kyoto, Japan), with the displacement rate established at 100 mm/min. Each sample was tested with fie specimens. Strain measurements were acquired through dual-point monitoring at grip positions, while stress calculations were performed according to the mathematical relationship expressed in Equation (4):
σ = F/A,
where σ: tensile stress (MPa), F: maximum tensile force (N), and A: specimen’s cross-sectional area (mm2).

3. Results and Discussion

3.1. Effect of Nanobubbling for Wood Meal

Despite no significant differences in the CHN ratio before and after ozone nanobubble treatment, as shown in Table 3, a slight decrease in the holocellulose content was observed. In this point, ozone nanobubbles may have selectively affected cellulose or hemicellulose. Additionally, the proportion of Soxhlet-extractable components tended to decrease after the ozone nanobubble treatment, indicating that some low-molecular-weight extractives were removed during the process.
To further analyze the composition in detail, holocellulose was fractionated into α-cellulose and hemicellulose to examine whether ozone nanobubble treatment selectively affected specific components. The results are presented in Table 4.
This analysis revealed a clear reduction in hemicellulose content in the samples treated with ozone nanobubbles, suggesting that the treatment may promote hemicellulose degradation. The lignin content seemed stable. Additionally, because ozone nanobubbles have been reported to affect the amorphous regions of cellulose, X-Ray diffraction (XRD) analysis was conducted. The results of this evaluation and the calculation results of the crystallinity are shown in Figure 4 and Table 5, respectively.
The crystallinity and amorphous content calculations indicated a reduction in the amorphous fractions in all ozone nanobubble-treated samples. This finding is consistent with the report by Kobayashi et al. [42], confirming that ozone nanobubbles exhibit effects similar to those of ozone gas. Although the number of tested samples was limited to one, a pronounced decrease in the amorphous fraction was observed in the sample subjected to ultrasonic treatment after ozone nanobubble processing (with a relative increase in the crystalline fraction). The condition to elevate temperature seemed less effective at this point. On the other hand, this result suggests that post-treatment with ultrasound may enhance the effects of nanobubble processing. Alternatively, the simultaneous application of nanobubble treatment and ultrasonic processing may further amplify the observed effects. From this point, nanobubble treatment could affect the specific components of the wood meal, such as its hemicellulose and amorphous cellulose.

3.2. Effect of Nanobubbling on Wood Liquefaction

To evaluate the liquefaction rate of ozone nanobubble-treated wood meal, liquefaction was conducted under conditions previously identified in our research as yielding a low residue rate. These liquefaction conditions included a 3% sulfuric acid catalyst, a PEG-to-glycerin ratio of 7:3, a solvent-to-wood meal ratio of 5:1 (by weight), and a liquefaction temperature of 150 °C. The extent to which the residue rate varied depended on the presence or absence of ozone nanobubble treatment was assessed, and the results of the residue rate measurements are presented in Figure 5.
In the first 30 min of liquefaction, no significant difference was observed between the ozone nanobubble-treated wood meal and untreated wood meal. However, from 30 to 180 min, the residue rate of the ozone nanobubble-treated wood meal was noticeably lower. In the liquefaction mechanism of wood, hemicellulose and lignin liquefy first, followed by cellulose. Considering this, ozone nanobubble treatment appears to enhance the liquefaction of cellulose specifically.
In addition, the time required to reach a residue rate of 10% was 180 min for untreated wood meal, whereas it was reduced to 90 min for the ozone nanobubble-treated wood meal, effectively halving the required time. This suggests that the energy consumption for liquefaction is also reduced by half while simultaneously facilitating the production of low-residue liquefied wood. Under this assumption, nanobubble treatment enables us to attain a more eco-friendly liquefaction condition.
These findings are consistent with the XRD crystallinity measurements and the observed decrease in hemicellulose content, further supporting the fact that ozone nanobubble treatment enhances the liquefaction efficiency of wood meal. To assess how this treatment affects the physical properties of the liquefied wood itself, further analyses were conducted using FT-IR and GPC for molecular weight determination, as well as hydroxyl numbers and viscosity measurements. The FT-IR spectra of these liquefied wood samples are shown in Figure 6.
The FT-IR spectra of liquefied wood derived from ozone nanobubble-treated and untreated wood meal revealed notable changes, particularly around 1000 cm−1. Specifically, the absorption peaks associated with the C–O bond in cellulose (1070 cm−1) and the C–O–C bond (1150 cm−1) exhibited variations. These changes are likely attributable to alterations in cellulose crystallinity and molecular interactions such as hydrogen bonding. Consistent with previous findings, this suggests a reduction in the amorphous fraction and a loosening of the rigid crystalline structure.
Notably, in Sample 3, which underwent both nanobubble treatment and ultrasonic processing, the peak intensity ratios changed significantly. This distinctive spectral shift further supports the characteristic effects of nanobubble treatment.
Figure 7 shows the comparison of GPC chromatography between non-nanobubble treatment and nanobubble treatment at room temperature.
The GPC analysis revealed that, excluding the solvent peaks corresponding to PEG400 and glycerin (from 7–9 min), the liquefied products derived from ozone nanobubble-treated wood meal contained a higher proportion of high-molecular-weight components (Mw ≒ 2190), as indicated by the peak observed around the 6-min mark. Despite this, the residue rate remained low, suggesting that ozone nanobubble treatment may alter the structural characteristics of the wood meal, making it more soluble in the solvent system. This phenomenon is similarly observed for the high-molecular-weight peak appearing around 5 min, and the maintained state of dissolution suggests potential differences in conditions such as the branched structure of the molecules and the hydrogen bonding of the hydroxyl groups.
Alternatively, the recondensation mechanism may be modified, resulting in a structure that is more readily soluble in polyol solvents such as PEG and glycerin. However, because GPC analysis alone provides limited insight into these structural changes, further evaluations were conducted, including hydroxyl (OH) number measurements and viscosity analysis of the liquefied wood. The results are shown in Table 6.
At room temperature, ozone nanobubble treatment resulted in only a slight increase in hydroxyl numbers. However, treatments at 50 °C and those combining ozone nanobubbles with ultrasonic processing exhibited significantly higher hydroxyl numbers. Previously, prolonged liquefaction was found to promote the depolymerization of wood components, leading to an increase in hydroxyl numbers. This suggests that ozone nanobubble treatment may facilitate the depolymerization of wood components, making them more reactive.
On the other hand, despite the GPC results indicating a higher proportion of high-molecular-weight components, the viscosity of the liquefied wood was lower than that of untreated samples. This suggests that the number of hydrogen bonds formed within the liquefied wood may have been reduced, indicating changes in its chemical structure. These effects were particularly pronounced under conditions that combined nanobubble treatment with ultrasonic processing, further correlating with the peak shifts observed around 1100 cm−1 in the FT-IR spectra.
Based on these findings, for wood meal pretreatment, the most effective approach for achieving a low-residue, low-viscosity liquefied product is the combination of the ozone nanobubble and ultrasonic treatments rather than ozone nanobubbles alone.
Thus, the ability to achieve a low residue rate within a shorter reaction time while obtaining a highly reactive liquefied wood with a high hydroxyl number and process-friendly low viscosity demonstrates the industrial potential of ozone nanobubble treatment. However, further process optimization, including more effective treatment conditions and the integration of nanobubble treatment with the liquefaction process, will be necessary for maximizing its effectiveness.

3.3. Effect of Nanobubbling on Bio-Polyurea Film

A bio-polyurea film was prepared using liquefied wood obtained from ozone nanobubble-treated wood meal by the same method as in our previous research. The tensile test was conducted in accordance with JIS K7201, and the results of three different nanobubble samples are presented in Figure 8 and Table 7, respectively.
Since our previous research [39] showed that hydroxyl numbers above 300 exhibit comparable tensile strength, the combination of cyanate resin, liquefied wood, and polyamine used in this study (with a cyanate-to-hydroxyl-to-amino group ratio of 2:1:1) can be considered representative of typical performance characteristics.
On the other hand, the elongation at breaks tended to be higher in liquefied wood derived from ozone nanobubble-treated wood meal. While the tensile stress primarily depends on chemical bonding and is unaffected by ozone nanobubble treatment, elongation is influenced by factors such as molecular chain slippage, chain length, and branching structure. This phenomenon is known as the “entanglement effect” in polymers, which is schematically illustrated in Figure 9.
Polymers, as long-chain molecules, inherently exhibit chain entanglement in both their solution and solid states. This entanglement restricts molecular mobility and enhances resistance to external forces. Under tensile stress, the entangled chains gradually slide past one another, absorbing energy and improving both the strength and ductility of the material. In the case of ozone nanobubble treatment, it is likely that this entanglement effect was enhanced.
This hypothesis is further supported by the observed differences in the viscosity of liquefied wood where molecular branching structures and reduced hydrogen bonding were indicated. These findings are consistent with the enhanced entanglement effect in ozone nanobubble-treated samples.

4. Conclusions

This study examined whether the treatment of wood meal using ozone nanobubbles could serve as an effective pretreatment for liquefied wood. To enhance temperature conditions and promote nanobubble treatment, additional ultrasonic processing was applied under three different conditions. The results showed a decrease in hemicellulose content and a reduction in the amorphous regions of cellulose in the wood meal. Notably, the condition of nanobubble + ultrasonic was significantly changed. Furthermore, liquefaction of this treated wood meal significantly reduced the residue rate compared with the same conditions without nanobubble treatment. The resulting liquefied wood exhibited high hydroxyl functionality and low viscosity. These characteristics of the liquefied wood are definitely beneficial for a commercial product due to its high reaction ability and high-speed processability. This result suggests that nanobubble treatment on biomass can contribute to furthering new research and applications. Then, bio-polyurea was synthesized from the obtained liquefied wood and evaluated through tensile testing. The results indicated that the bio-polyurea exhibited comparable physical strength to the bio-polyurea synthesized from liquefied wood derived from untreated wood meal. However, the elongation tended to be 1–2% higher than that of the untreated sample, demonstrating a potential advantage of nanobubble treatment. This time, we examined the results based on simple condition changes such as temperature and ultrasonic treatment. More effective ozone nanobubble conditions, however, are likely to exist. Nevertheless, several considerations remain, including how outcomes might differ with various biomass species and the extent of effectiveness when gases other than ozone are employed. Thus, this study could unveil the possibility of applying nanobubble technology in this field, but more effort and deep investigation is needed to optimize this treatment.

Author Contributions

G.M.: Writing—original draft, data curation, conceptualization, methodology, visualization, investigation, and writing—review and editing; C.E.E.: supervision and writing—review and editing; W.W.: supervision and writing—review and editing; S.A.: data curation and methodology; Q.W.: supervision, writing—revision and editing, and resources. All authors have read and agreed to the published version of the manuscript.

Funding

This study was partially supported by the Special Funds for Basic Research (B) (No. 22H03747, FY2022-FY2024) of the Grant-in-Aid for Scientific Research of the Japanese Ministry of Education, Culture, Sports, Science, and Technology (MEXT).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

This study was partially supported by the Special Funds for Basic Research (B) (No. 22H03747, FY2022-FY2024) of the Grant-in-Aid for Scientific Research of the Japanese Ministry of Education, Culture, Sports, Science, and Technology (MEXT). Our heartfelt thanks go to Fujigen Co., Ltd., Unitika Ltd., and Mitsui Chemicals, Inc. for their kind donations of materials, which made this study possible. We are truly grateful for their generosity and the trust they placed in our research endeavors.

Conflicts of Interest

Author Satoshi Anzai was employed by Anzai Kantetsu Corporation. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: GO MASUDA reports was provided by Saitama University.

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Figure 1. Porous material for generating nanobubbles [33].
Figure 1. Porous material for generating nanobubbles [33].
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Figure 2. Appearances of the samples under three different conditions. The top images are of nanobubbling, the middle images are after the process, and the bottom images are of wood meal with water and supernatant liquid.
Figure 2. Appearances of the samples under three different conditions. The top images are of nanobubbling, the middle images are after the process, and the bottom images are of wood meal with water and supernatant liquid.
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Figure 3. How to calculate the crystallinity index of the samples.
Figure 3. How to calculate the crystallinity index of the samples.
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Figure 4. XRD spectra of untreated and nanobubble-treated wood meal samples.
Figure 4. XRD spectra of untreated and nanobubble-treated wood meal samples.
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Figure 5. Liquefaction rate of wood meal with different nanobubble treatments.
Figure 5. Liquefaction rate of wood meal with different nanobubble treatments.
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Figure 6. FT-IR spectra of the liquefied wood from untreated wood and nanobubble-treated wood (Nanobubble sample #1: nanobubbling at room temperature; nanobubble sample #2: nanobubbling at 50 °C; and nanobubble #3: nanobubbling and ultrasonicating at room temperature).
Figure 6. FT-IR spectra of the liquefied wood from untreated wood and nanobubble-treated wood (Nanobubble sample #1: nanobubbling at room temperature; nanobubble sample #2: nanobubbling at 50 °C; and nanobubble #3: nanobubbling and ultrasonicating at room temperature).
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Figure 7. GPC chromatography of untreated and nanobubble-treated liquefied wood samples.
Figure 7. GPC chromatography of untreated and nanobubble-treated liquefied wood samples.
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Figure 8. Stress–strain curve of three different nanobubble-treated and untreated samples (only the typical curve is shown for each).
Figure 8. Stress–strain curve of three different nanobubble-treated and untreated samples (only the typical curve is shown for each).
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Figure 9. Image of the “entanglement effect” in polymers [45].
Figure 9. Image of the “entanglement effect” in polymers [45].
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Table 1. Conditions of the XRD measurement.
Table 1. Conditions of the XRD measurement.
EquipmentRIGAKU Ultima Ⅲ
CondituonsRange: 2θ = 5–40° (Reflection Method)
Method: 2θ/θ
Step: 0.02°
Scan speed: 2°/min (2θ)
CalculationDIFFRAC EVA 3.2 (Bruker AXS)
Table 2. Analytical conditions of GPC.
Table 2. Analytical conditions of GPC.
GPC MeasurementConditions
Mobile phaseTHF
Flow rate1 mL/min
Dilute sample concentration0.4 g/L
THF column temp.40 °C
Sample loop volume100 μL
Table 3. Componential analysis of three different offcut woods.
Table 3. Componential analysis of three different offcut woods.
Wood MealC (%)H (%)N (%)Extractive (%)Holocellulose (%)
Untreated46.6 (0.1)5.7 (0.1)0.1 (0.1)6.477.4
Nanobubble @RT46.1 (1.4)5.6 (0.2)0.1 (0.1)4.670.9
Nanobubble @50 °C45.1 (1.4)5.5 (0.3)0.2 (0.1)4.769.7
+Ultrasonic @RT44.8 (0.4)5.7 (0.1)0.1 (0.1)2.072.3
( ) shows the standard deviation.
Table 4. Results of the Klason lignin and Wise methods.
Table 4. Results of the Klason lignin and Wise methods.
Wood MealKlason Lignin (%)α-Cellulose (%)Hemicellulose (%)
Untreated23.857.142.8
Nanobubble @RT23.864.135.9
Nanobubble @50 °C23.865.134.9
+Ultrasonic @RT20.761.538.4
Table 5. Results of the calculations of the crystallinity for each sample.
Table 5. Results of the calculations of the crystallinity for each sample.
XRD SampleCrystallinity (%)Amorphous (%)
Untreated37.762.3
Nanobubble @RT43.456.6
Nanobubble @50 °C40.359.7
+Ultrasonic @RT49.750.3
Table 6. The comparison of the OH numbers, residues, and viscosities of untreated and nanobubble-treated samples.
Table 6. The comparison of the OH numbers, residues, and viscosities of untreated and nanobubble-treated samples.
Liquefied ProductsOH Number (KOH/mg)Residue (wt%)Viscosity (cP)
Untreated34114.7684
Nanobubble @RT3870.4246
Nanobubble @50 °C5572.9232
+Ultrasonic @RT5817.5198
Table 7. Comparison of the tensile tests of the nanobubble-treated and untreated samples.
Table 7. Comparison of the tensile tests of the nanobubble-treated and untreated samples.
Polyurea FilmOH Number (KOH/mg)Stress (MPa)Strain (%)
Non treatment34143.6 (3.2)4.3 (0.6)
Nanobubble @RT38740.9 (4.8)5.8 (1.8)
Nanobubble @50 °C55746.7 (10.3)5.9 (1.3)
+Ultrasonic @RT58142.8 (6.0)6.1 (0.6)
( ) shows the standard deviation.
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MDPI and ACS Style

Masuda, G.; Enyoh, C.E.; Wang, W.; Anzai, S.; Wang, Q. Ozone Nanobubble-Assisted Pretreatment of Lignocellulose: Enhancing Wood Liquefaction and Bio-Polyurea Development. Appl. Sci. 2025, 15, 4618. https://doi.org/10.3390/app15094618

AMA Style

Masuda G, Enyoh CE, Wang W, Anzai S, Wang Q. Ozone Nanobubble-Assisted Pretreatment of Lignocellulose: Enhancing Wood Liquefaction and Bio-Polyurea Development. Applied Sciences. 2025; 15(9):4618. https://doi.org/10.3390/app15094618

Chicago/Turabian Style

Masuda, Go, Christian Ebere Enyoh, Weiqian Wang, Satoshi Anzai, and Qingyue Wang. 2025. "Ozone Nanobubble-Assisted Pretreatment of Lignocellulose: Enhancing Wood Liquefaction and Bio-Polyurea Development" Applied Sciences 15, no. 9: 4618. https://doi.org/10.3390/app15094618

APA Style

Masuda, G., Enyoh, C. E., Wang, W., Anzai, S., & Wang, Q. (2025). Ozone Nanobubble-Assisted Pretreatment of Lignocellulose: Enhancing Wood Liquefaction and Bio-Polyurea Development. Applied Sciences, 15(9), 4618. https://doi.org/10.3390/app15094618

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