1. Introduction
The impending depletion of fossil fuels, coupled with the escalating environmental degradation, has intensified the search for renewable and environmentally benign energy sources [1,2,3]. Bioethanol, derived from lignocellulosic biomass, has emerged as a key contender due to its abundance, widespread availability, and sustainability [4,5]. Lignocellulosic biomass, such as rice straw, constitutes a vast and renewable resource with considerable potential for the production of biofuels and biochemicals, thereby promoting sustainable agriculture [6,7,8,9]. However, the complex structure of lignocellulose, characterized by its interwoven cellulose, hemicellulose, and lignin components, presents a formidable challenge to the efficient enzymatic hydrolysis required for bioconversion processes. Consequently, the pretreatment of lignocellulosic biomass is crucial for altering its structural composition, which in turn enhances the accessibility of cellulose and hemicellulose to enzymatic hydrolysis [10,11,12].
A variety of pretreatment strategies have been investigated to overcome the recalcitrance of lignocellulosic biomass, including mechanical disintegration, chemical treatments with acids or bases, and ammonia-based processes [13,14,15]. However, traditional chemical pretreatments are frequently resource intensive, necessitating large volumes of water and chemicals, and substantial waste effluents are generated. These factors significantly contribute to the increased costs associated with the pretreatment phase in lignocellulosic biorefineries. Furthermore, the formation of inhibitors during these chemical processes can adversely affect the subsequent saccharification and fermentation stages [16,17,18]. To address these limitations, there is a growing interest in the development of more sustainable and efficient pretreatment methods [19]. These methods aim to effectively deconstruct the lignocellulosic matrix while concurrently minimizing the environmental footprint.
Mechanochemical pretreatment, employing mechanical forces to disrupt the lignocellulosic structure, has risen as a compelling alternative to traditional methods. Hick et al. [20] first introduced the concept of acid-assisted mechanocatalytic in the field of biomass refining, which involves the addition of trace amounts of acidic catalysts during the mechanical ball milling of cellulose. Compared to traditional mechanical ball milling for size reduction, the acid-assisted mechanocatalytic process not only disintegrates the crystalline cellulose structure but also substantially amplifies the interaction between the straw and the acidic catalyst [21,22]. This interaction efficiently cleaves the β-1,4 glycosidic bonds within the cellulose, resulting in a significant decrease in its degree of polymerization [23]. Importantly, this technology is advantageous in that it requires less acid and operates under milder conditions of ambient temperature and pressure, in contrast to the substantial acid usage and severe conditions demanded by traditional acid treatments. Research has shown that the application of acid-assisted mechanocatalytic treatments can significantly improve the solubility of cellulose in water, which may be attributed to the formation of α (1–6) glycosidic linkages during the pretreatment phase [24,25]. Nonetheless, current research predominantly concentrates on the acid-induced degradation of cellulose, with scant attention given to its enzymatic degradation [26,27]. Furthermore, it is essential to recognize that rice straw possesses a more intricate composition than pure cellulose. The synergistic effect of mechanical forces and chemical catalysts within the acid-assisted mechanocatalytic pretreatment is known to alter the physicochemical structure of rice straw, subsequently influencing the efficiency of enzymatic hydrolysis. However, the precise mechanisms governing these interactions remain unclear.
In this study, we investigated the efficacy of acid-assisted mechanocatalytic depolymerization (AAMD) as an innovative pretreatment strategy for rice straw. The AAMD methodology integrates mechanical disruption of the biomass with the catalytic effects of acids, thereby facilitating a more efficient disassembly of the lignocellulosic matrix. We elaborated on the effects of AAMD on the microstructure properties and enzymatic hydrolysis of rice straw. This study aims to contribute to the development of more sustainable and efficient bioconversion technologies, paving the way for the utilization of lignocellulosic biomass in the production of renewable energy.
2. Materials and Methods
2.1. Rice Straw and Chemicals
Rice straw (RS) was collected from a field in Zhenjiang City, Jiangsu Province, crushed, and passed through a 1.00 mm sieve. The moisture content was determined by subjecting the sample to oven drying until a constant mass was achieved at 105 °C. The chemical composition of the RS was analyzed in accordance with the standard procedure established by the National Renewable Energy Laboratory (NREL) [28]. The cellulose, hemicellulose, and lignin contents of RS were determined to be 30.74%, 15.50%, and 15.44%, respectively. H2SO4, CaCO3, and NaOH were purchased from Sinopharm (Beijing, China).
2.2. Acid-Assisted Mechanocatalytic Depolymerization Pretreatment
The acid catalyst (98 mg sulfuric acid per 1 g rice straw to achieve final concentrations of 1.0 mmol/g rice straw) was mixed with 5 mL of distilled water to form a solution. Subsequently, 1 g of rice straw was introduced into this acid solution, and the mixture was agitated for 30 min at ambient temperature. The acid-impregnated rice straw was then dried by an oven at 40 °C before mechanocatalytic treatment, denoted as A-RS.
The depolymerization process was conducted in a 4 mL centrifuge tube containing two 4 mm stainless steel balls, using a vibratory ball mill (Scientz-48L, Scientz, Ningbo, China). The rice straw suffered impact and compression stresses when collisions between balls and walls occur in this vibratory ball mill [29]. To avoid overheating in the depolymerization process, a cooling system was employed to maintain the internal temperature of the ball mill below 20 °C. A-RS (0.5 g) was milled at 50 Hz for 1 h. The acid-assisted mechanocatalytic depolymerization pretreated rice straw was denoted as AAMD-RS.
To ascertain the influence of the acid catalyst on the mechanical degradation of rice straw, a control experiment was performed. Untreated rice straw (RS) was milled under identical conditions (50 Hz for 1 h) in the absence of acid impregnation, and the resulting material was denoted as BM-RS.
2.3. Microstructure Characterization of Rice Straw
2.3.1. Scanning Electron Microscopy (SEM)
Rice straw samples were sputtered with Pt for 2 min by E-1010 Iron Sputter (Hitachi, Tokyo, Japan). SEM images of the coated samples were characterized with a Regulus-8100 microscope (Hitachi, Tokyo, Japan) at 5.0 kV.
2.3.2. Specific Surface Area (SSA)
The N2 isothermal adsorption and desorption curves at the temperature of liquid nitrogen were measured using a JW-BK200B surface area and pore size analyzer (JWGB Instruments, Beijing, China). The specific surface area of rice straw was obtained based on the BET sorption method. Each sample was measured in duplicate.
2.3.3. X-Ray Diffraction (XRD)
XRD analysis of the rice straw was conducted using a D8 Advance X diffractometer (Bruker, Karlsruhe, Germany) equipped with a Cu Kα radiation source operated at 40 kV and 40 mA. The diffraction patterns were recorded over a 2θ range from 5° to 40°, using a step size of 0.2° and a scan rate of 2° per minute. The crystallinity index (CrI) of the samples was calculated from the X-ray diffraction data utilizing the formula presented in the following equation, as previously reported [30]. Each sample was measured in duplicate.
CrI (%) = [(I002 − Iam)/I002] × 100
where I002 is the maximum intensity of the 002 peak, and Iam is the intensity at approximately 2θ = 18.0°.2.3.4. X-Ray Photoelectron Spectroscopy (XPS)
XPS measurement of the rice straw was conducted using a AXIS ULTRA DLD photoelectron spectrometer (Shimadzu, Kyoto, Japan). The surface elemental composition was determined by quantifying the areas under the corresponding photoelectron peaks for each element. The carbon spectra (C1s) were deconvoluted utilizing the XPSPEAK 4.1 software to resolve the complex peak structures. Each sample was measured in duplicate.
2.3.5. Particle Size Distribution Measurement
The particle size distribution of the rice straw was determined using a laser diffraction particle analyzer, the Mastersizer 3000 (Malvern Instruments Ltd., Warwickshire, UK). The measurements were taken to define the D10, D50, and D90 values, which correspond to the particle sizes at the 10th, 50th, and 90th percentiles of the cumulative volume distribution, respectively. Each sample was measured in duplicate.
2.4. Enzymatic Hydrolysis of Rice Straw
The cellulase preparation Cellic CTec 2, sourced from Novozymes in Copenhagen, Denmark, was applied for the enzymatic hydrolysis of rice straw. This enzyme preparation exhibited a cellulase activity of 190.33 FPU/mL, as determined by the NREL/TP-510-42628 protocol [31]. For the enzymatic treatment, 1 g of rice straw sample (on a dry basis) was introduced into 50 mL centrifuge tubes. The enzymatic hydrolysis was initiated by introducing 1 g of rice straw (dry weight basis) into 50-mL centrifuge tubes. The tubes were then filled with 0.1 M sodium citrate buffer adjusted to pH 4.8 and enriched with 20 FPU of Cellic CTec 2 per gram of rice straw, creating a suspension with a 5% (w/v) solid consistency. To prevent microbial contamination, 8 mg of tetracycline was added to each tube. The hydrolysis reactions were meticulously controlled at a temperature of 50 °C with continuous agitation at 250 rpm for durations of 2, 4, 8, 12, 24, 48, and 72 h.
After the enzymatic hydrolysis process, the concentration of total sugars in the samples was determined using a UV–visible spectrophotometer (Unico, Shanghai, China), which measured the absorbance at a wavelength of 540 nm, employing the 3,5-dinitrosalicylic acid (DNS) method. The DNS method entails an alkaline redox reaction between DNS and the reducing sugars, culminating in the formation of 3-amino-5-nitro salicylic acid. Upon boiling, this compound imparts a brownish-red hue, the intensity of which correlates directly with the concentration of reducing sugars present in the sample. Each sample was measured in duplicate. The total sugar yield was calculated as follows:
Total sugar yield (mg/g) = c × V/m
where c (mg/mL) is the concentration of total sugars in the enzymatic solution, V (mL) is the volume of the enzymatic solution, and m (g) is the mass of the rice straw sample subjected to enzymatic hydrolysis on a dry basis. Additionally, the percentage of total sugar yield (%) was determined based on cellulose and hemicellulose in the rice straw.3. Results
3.1. Particle Size Distribution
Particle size distributions of RS, A-RS, BM-RS, and AAMD-RS are shown in Figure 1. The RS sample exhibits a broad particle size range from 2 to 2000 μm, with a median particle size of 279.3 μm (Table S1). The multimodal distribution observed for RS indicates the heterogeneous nature of rice straw particle sizes, attributed to the varying mechanical properties of different rice straw components, such as nodes and internodes [32]. The acid impregnation treatment does not significantly affect the particle size distribution, as evidenced by the nearly identical curve for A-RS when compared to that of RS. In contrast, ball milling for 1 h leads to a pronounced leftward shift in the particle size distribution curve, resulting in a unimodal distribution. This shift indicates that ball milling could effectively disrupt the structure of rice straw, reduce particle size, and enhance the homogeneity of the straw powder, with a median particle size diminished to 20.1 μm.
The AAMD-RS exhibits an even more pronounced leftward shift, with a more concentrated range, and the median particle size further decreased to 11.8 μm (Table S1). This observation suggests a synergistic interaction between the acid catalyst and mechanical forces during the acid-assisted mechanocatalytic degradation process, which accelerates the structural disintegration of rice straw and leads to a substantial reduction in particle size.
3.2. SEM Images and SSA
The RS sample is characterized by predominantly intact sheet-like structures in Figure 2, indicating that the plant structure of rice straw remains relatively intact without significant damage. Although there is no apparent change in particle size in the A-RS sample, the surface structure is partially disrupted under the impregnation of the acid catalyst, exposing the underlying network-like fiber structure. Due to the limited amount of acid catalyst and the mild temperature conditions during impregnation, hemicellulose cannot be dissolved, and the overall fiber structure of rice straw remains relatively intact [33]. The structure of BM-RS is disrupted by mechanical force, and intact sheet-like structures are no longer observed, instead presenting a state of particle aggregation. The SEM image of AAMD-RS reveals further disintegration of particles, with the acid catalyst moderating inter-particle aggregation and enhancing dispersion uniformity. These observations are consistent with the particle size distribution curves, underscoring the synergistic impact of acid catalysts and mechanical forces on the degradation process.
SSA of RS, A-RS, BM-RS, and AAMD-RS are 1.14, 1.98, 2.98, and 3.16 m2/g, respectively (Table 1). RS, with its structural integrity largely preserved, displays a comparatively lower specific surface area. After acid impregnation, there is no significant change in particle size; however, the internal pore structure becomes exposed on the surface, leading to a 73.68% increase in SSA supported by SEM imaging. BM-RS experiences a 1.6-fold increase in SAA after ball milling. It is worth noting that compared to the decrease in particle size, the increase in SSA is relatively limited. This may be due to the fact that the surface of rice straw samples is not smooth but contains a large number of pores. The contribution of these pores to the specific surface area should not be ignored, which is consistent with other research [30,34]. AAMD-RS demonstrates a further escalation in SSA, with SEM images revealing improved particle dispersibility under the synergistic influence of the acid catalyst and mechanical forces. This results in the exposure of more internal pores to the particle surface, culminating in a specific surface area 2.8 times greater than that of RS.
3.3. XRD
The X-ray diffraction patterns of RS, A-RS, BM-RS, and AAMD-RS are presented in Figure 3. It can be observed that the RS sample exhibits three distinct cellulose diffraction peaks near 16°, 22°, and 35°, corresponding to the (101), (10), (020), and (040) crystallographic planes [35]. The first two peaks are closely aligned, forming a broad peak at 16°. The cellulose crystallinity of RS is calculated to be 43.05%. The XRD curve of the rice straw after acid catalyst impregnation does not exhibit any changes in the crystalline peaks, suggesting that the cellulose crystal structure remains unaltered under room temperature acid impregnation. The crystallinity of A-RS is determined to be 43.91%, which is close to RS. The XRD pattern of BM-RS reveals a notable reduction in the intensity of all three cellulose peaks, indicating that ball milling disrupts the crystalline structure of cellulose within the cell walls. The cellulose crystallinity of BM-RS is calculated to be 22.73%, representing a 47.20% decrease from the RS sample. This finding agrees with previous research on the impact of mechanical pretreatment on cellulose crystallinity [36,37].
The XRD pattern of AAMD-RS exhibits a significant reduction in the intensities of the (101) and (10) peaks, to the point of near disappearance. This observation indicates that the synergistic action of mechanical force and the acid catalyst significantly enhances the degradation of the cellulose crystalline structure. Despite this, the intensities of the (020) and (040) peaks are comparable to those of BM-RS, leading to a cellulose crystallinity index of 22.71% for AAMD-RS.
3.4. XPS
Figure 4 displays the XPS full spectrum data of different rice straw samples, revealing two prominent peaks corresponding to the oxygen (O) peak at around 532 eV binding energy and the carbon (C) peak at around 284 eV [38]. The main chemical composition of rice straw includes cellulose, hemicellulose, and lignin, with the surface primarily composed of carbon (C), hydrogen (H), and oxygen (O) elements. The surface oxygen-to-carbon (O/C) ratio for each sample is derived from the relative peak areas of O and C, considering their sensitivity factors, as detailed in Table 1. The O/C ratio of RS is 0.24, which slightly increases to 0.28 after acid impregnation of the rice straw. Theoretical O/C ratios for pure cellulose/hemicellulose, lignin, and extractives are reported to be 0.83, 0.33, and 0.10, respectively [39]. Therefore, the slight increase in O/C ratio observed in A-RS could be attributed to the dissolution or removal of surface extractives during the acid catalyst impregnation process. BM-RS exhibits a substantial increase in O/C ratio to 0.35, indicating a higher number of polysaccharides in the substance. This can be attributed to the disruption of the plant cell structure, exposing the internal cellulose/hemicellulose to the surface [30]. The O/C ratio of AAMD-RS further increases to 0.42 closely with the values for polysaccharides, representing a 75% improvement compared to RS. This suggests that the synergistic effect of the acid catalyst and mechanical force significantly degrades straw components, not only exposing the internal cellulose/hemicellulose to the surface but also potentially generating numerous α(1,6) branched structures during the degradation process [24,40].
The deconvoluted C1s spectra of the four rice straw samples are presented in Figure 5. The C1s peak is typically resolved into four component peaks: C1, C2, C3, and C4, located at approximately 284.8 eV, 286.3 eV, 287.8 eV, and 289.0 eV, respectively. C1 represents C-C/C-H, primarily originating from lignin and extractives. C2 represents C-O, mainly derived from cellulose/hemicellulose. C3 and C4 correspond to C=O/O-C-O and O=C-O functionalities, respectively [39]. The subdued intensities of C3 and C4 suggest a scarcity of O=C-O groups on the straw surface. Table 1 provides the specific peak deconvolution results for C1s. The area ratios of A-RS and RS in the four peaks remain unchanged, indicating that the acid impregnation treatment with trace amounts of acid catalyst has minimal effect on altering the carbonaceous groups on the sample surface. In the case of BM-RS, there is a significant decrease in the C1 proportion and an increase in the C2 proportion, concluding an increased relative content of cellulose/hemicellulose on the surface. AAMD-RS exhibits a further reduction in C1 and an elevation in C2, indicating that the synergistic effect of the acid catalyst and mechanical force results in enhanced degradation of the straw. AAMD treatment facilitates the contact between cellulase and polysaccharides, promoting an improvement in enzymatic hydrolysis [30]. Moreover, the diminished relative content of lignin/extractives on the surface, known to harmful affect cellulose hydrolysis through non-productive enzyme adsorption, further contributes to the conversion efficiency.
3.5. Enzymatic Hydrolysis
All four rice straw samples undergo an initial rapid phase of enzymatic hydrolysis, followed by a subsequent slower rate (Figure 6). The total sugar yield of RS after 72 h of enzymatic hydrolysis is 187.6 mg/g, corresponding to a hydrolysis yield of 36.24% based on the cellulose and hemicellulose content. The A-RS sample exhibits a slight improvement in hydrolysis yield, reaching a total sugar yield of 234.0 mg/g after 72 h hydrolysis. This enhancement is attributed to the partial removal of surface extractives and the expansion of the surface area, which facilitates the exposure of internal pores and enhances the interaction between cellulase enzymes and the straw substrate. Nevertheless, the overall enzymatic conversion efficiency is only 45.20%. In contrast, BM-RS exhibits a more pronounced increase in total sugar yield, reaching 258.4 mg/g and indicating a significant enhancement in enzymatic conversion efficiency. It is inferred that ball milling not only diminishes the crystallinity of cellulose but also increases the specific surface area and surface polysaccharide content of the straw, thereby substantially intensifying the enzymatic hydrolysis reaction. This observation is supported by the findings of Ji et al., who reported that as ball milling time increased from 0 to 8 h, cellulose crystallinity decreased from 44.80% to 13.29%, and the glucose yield correspondingly increased from 95.55 mg/g to 287.07 mg/g [41].
AAMD-RS shows a remarkable increase in total sugar yield compared to the other three samples, reaching 508.8 mg/g after only 12 h, with a hydrolysis yield of 98.28% (Figure 7). This suggests that the cellulose and hemicellulose in AAMD-RS are nearly entirely converted into reducing sugars. Additionally, the initial hydrolysis rate of AAMD-RS is also significantly higher than the other samples, with a total sugar yield of 364.4 mg/g after just 2 h of hydrolysis, comparable to the 72 h yield of BM-RS. This demonstrates that the AAMD process is a highly effective pretreatment method that significantly enhances hydrolysis yield and shortens hydrolysis time. During the AAMD process, the synergistic degradation by mechanical force and acid catalyst leads to significant changes in the microstructure of rice straw, such as reduced particle size and crystallinity, destruction of cellulose crystal structure, increased surface polysaccharides, and decreased extractives and lignin. These results suggest that mechanical force significantly reduces the energy barrier required for the cleavage of β-1,4 glycosidic bonds in an acidic environment [25].
4. Discussion
In this study, we employed a novel acid-assisted mechanocatalytic depolymerization (AAMD) pretreatment for rice straw, synergistically combining the advantages of acid treatment and ball milling to achieve an effect that surpasses the sum of its individual components. Compared to the well-established conventional liquid acid pretreatment under high temperature and pressure, the AAMD technology requires minimal acid as a catalyst, and no large amounts of solvents are added during the entire process, which is beneficial for reducing wastewater discharge and protecting the environment. An increased amount of acidic catalyst favors greater degradation of the straw in the AAMD process, enhancing the conversion efficiency of cellulose. However, an excessively high acid concentration may lead to excessive degradation of hemicellulose in the straw into by-products such as furfural, which is detrimental to the subsequent enzymatic hydrolysis [27]. Conversely, if the amount of acid catalyst added is too low, the degradation degree of the straw in the AAMD process may be insufficient, with the long chains of cellulose in the straw remaining intact, affecting the subsequent enzymatic hydrolysis efficiency and failing to achieve the desired outcome. Therefore, the amount of acidic catalyst usage is a critical parameter in the AAMD process.
The ball-milling process is highly effective in disrupting the structure of straw, but its major drawback is the high energy consumption, which hinders its industrial application. Research on traditional ball milling has shown that extending the milling time from 10 min to 8 h can increase the enzymatic glucose yield from rice straw from 35.29% to 81.71% [41]. This indicates that achieving satisfactory enzymatic hydrolysis efficiency through mechanical force alone requires a very long ball-milling time and substantial energy input. In contrast, AAMD in this study, which introduces an acidic catalyst to the traditional ball-milling process, could significantly accelerate the degradation of straw and enhance the efficiency of mechanical force, thereby reducing the grinding time and energy consumption. To further validate the feasibility of this technology for broader application, future studies should consider the impact of different ball-milling durations during the AAMD process on the degree of straw degradation and enzymatic conversion efficiency. This research direction will provide valuable insights into optimizing the AAMD process for energy efficiency and maximizing the enzymatic hydrolysis performance of straw.
The comparative results of AAMD on various biomass types are presented in Table 2. Meine et al. [42] pioneered the application of two archetypal acid catalysts, hydrochloric acid (HCl) and sulfuric acid (H2SO4), to perform AAMD on cellulose, sugarcane bagasse, and beechwood biomass. Their findings indicated that cellulose in different biomass types was degraded from an insoluble state to a soluble state, primarily in the form of soluble oligosaccharides (with a degree of polymerization greater than 3). Building upon this, Kaufman et al. [43] employed the identical AAMD methodology on a kilogram scale, achieving glucose yields of 70–80% through acid hydrolysis. Wang et al., focusing on eucalyptus biomass, subjected it to ball milling under 2.5% acetic acid conditions for 1 h prior to enzymatic hydrolysis [23]. They discovered that the AAMD approach significantly enhanced the yields of glucose and xylose after 48 h of enzymatic hydrolysis, reaching 86.6% and 57.8%, respectively. In a recent study, Liu et al. [44] used P2O5 as an acid catalyst to mechanically catalyze corn stover and subsequently conducted acid hydrolysis at 185–215 °C, resulting in a glucose yield of 75%. The AAMD treatment combined with enzymatic hydrolysis in this study does not require high-temperature and high-pressure conditions yet achieves close to 100% enzymatic conversion efficiency, presenting significant advantages over other methods. These studies collectively highlight the effectiveness of AAMD in enhancing the enzymatic hydrolysis of various biomass types, demonstrating its potential for improving sugar yields in biofuel production.
5. Conclusions
The AAMD pretreatment combines the effects of acid catalyst and mechanical force on the cellulose chain β-1,4 glycosidic bonds and the cell wall structure. This synergistic approach can effectively degrade the long-chain fibers in rice straw, leading to significantly different microstructural characteristics, such as a substantial reduction in particle size and cellulose crystallinity and a significant increase in specific surface area and surface polysaccharide content. The total sugar yield of AAMD-RS achieves 95% within just 12 h of enzymatic hydrolysis, representing a 171% improvement in enzymatic hydrolysis efficiency and a 5-fold increase in enzymatic hydrolysis rate compared to untreated straw. These findings underscore the efficacy of the AAMD pretreatment method in enhancing the enzymatic hydrolysis and conversion efficiency of rice straw.
Conceptualization, P.Y. and H.Y.; funding acquisition, J.C. and G.J.; investigation, Q.Y.; methodology, J.N.; supervision, G.J.; validation, Y.L. (Yuxin Liu), Q.N. and G.J.; writing—original draft, P.Y.; writing—review and editing, J.C., Y.L. (Yucheng Liu) and G.J. All authors have read and agreed to the published version of the manuscript.
The original contributions presented in the study are included in the article/
The authors declare no conflicts of interest.
Footnotes
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Enlarge this image.Figure 1. Particle size distribution of rice straw samples. RS denotes raw rice straw, BM-RS denotes ball milled rice straw, A-RS denotes acid impregnated rice straw, AAMD-RS denotes acid-assisted mechanocatalytic depolymerization pretreated rice straw.
Enlarge this image.Figure 2. SEM images of rice straw samples at 500-fold magnification: (a) RS; (b) BM-RS; (c) A-RS; (d) AAMD-RS.
Enlarge this image.Figure 5. Deconvoluted peaks of C1 spectra for rice straw samples: (a) RS; (b) BM-RS; (c) A-RS; (d) AAMD-RS. C1, C2, C3, C4 represent four component peaks of the deconvoluted C1s spectra at approximately 284.8 eV, 286.3 eV, 287.8 eV, and 289.0 eV, respectively.
Enlarge this image.Figure 6. Total sugar yield of rice straw samples. Enzymatic hydrolysis experiments were performed at a 5% (w/v) concentration with a Cellic Ctec 2 loading of 20 filter paper activity (FPU)/g rice straw for 2, 4, 8, 12, 24, 48 and 72 h. Error bars represent the standard deviation of the measurements for the total sugar yield.
Enlarge this image.Figure 7. Total sugar yield of rice straw samples after 72 h of enzymatic hydrolysis. Total sugar yield by percentage is calculated based on the cellulose and hemicellulose content of raw rice straw. Error bars represent the standard deviation of the measurements for the total sugar yield.
Microstructure properties of rice straw samples.
| Sample | SSA (m2/g) | CrI (%) | O/C | C1 (%) | C2 (%) | C3 (%) | C4 (%) |
|---|---|---|---|---|---|---|---|
| RS | 1.14 | 53.36 | 0.24 | 57.4 | 31.63 | 7.69 | 3.21 |
| BM-RS | 2.98 | 28.25 | 0.35 | 45.97 | 39.07 | 13.04 | 1.92 |
| A-RS | 1.98 | 28.16 | 0.28 | 57.74 | 32.84 | 6.41 | 3.01 |
| AAMD-RS | 3.16 | 20.44 | 0.42 | 43.53 | 43.67 | 9.75 | 3.04 |
Data are shown as their average values. Each sample was measured in duplicate. RS represents raw rice straw, BM-RS represents ball milled rice straw, A-RS represents acid impregnated rice straw, AAMD-RS represents acid-assisted mechanocatalytic depolymerization pretreated rice straw. CrI represents the crystallinity index. O/C represents the surface oxygen-to-carbon (O/C) ratio. C1, C2, C3, C4 represent four component peaks of the deconvoluted C1s spectra, respectively.
Comparison of acid-assisted mechanocatalytic depolymerization pretreatment of different biomass with technologies developed in this study.
| Biomass Type | Pretreatment Conditions | Hydrolysis Conditions | Sugar Yield | Ref. |
|---|---|---|---|---|
| Cellulose | 0.61 mmol/g HCl | Acid hydrolysis at 130 °C for 1 h | Glucose yield 3.7% | [ |
| Sugarcane bagasse | 0.88 mmol/g H2SO4 | Acid hydrolysis at 130 °C for 1 h | Glucose yield 4.6% | |
| Beechwood | Glucose yield 2.5% | |||
| Poplar wood | 0.88 mmol/g H2SO4 | Acid hydrolysis at 145 °C for 70 min | Glucose yield 70–80% | [ |
| Cellulose | 0.88 mmol/g H2SO4 | Glucose yield 70–80% | ||
| Corn stover | 0.70 mmol/g P2O5 | Acid hydrolysis at 185–215 °C for 30 min | Glucose yield 75% | [ |
| Sugarcane bagasse | 0.15 mmol/g AlCl3 | Enzymatic hydrolysis at 50 °C for 48 h | Glucose yield 79.7% | [ |
| Holocellulose | Glucose yield 85.1% | |||
| Eucalyptus | 2.5% acetic acid | Enzymatic hydrolysis at 50 °C for 48 h | Glucose yield 86.6% | [ |
| Rice straw | 1.00 mmol/g H2SO4 | Enzymatic hydrolysis at 50 °C for 8 h | Total sugar yield 91.2% | This study |
| Enzymatic hydrolysis at 50 °C for 72 h | Total sugar yield 98.3% |
Supplementary Materials
The following supporting information can be downloaded at:
References
1. Chakraborty, P.; Kumar, R.; Chakrabortty, S.; Saha, S.; Chattaraj, S.; Roy, S.; Banerjee, A.; Tripathy, S.K.; Kumar Ghosh, A.; Jeon, B.-H. Technological advancements in the pretreatment of lignocellulosic biomass for effective valorization: A review of challenges and prospects. J. Ind. Eng. Chem.; 2024; 137, pp. 29-60. [DOI: https://dx.doi.org/10.1016/j.jiec.2024.03.025]
2. Ren, M.; Fakayode, O.A.; Kong, F.; Zhou, C.; Chen, L.; Fan, X.; Liang, J.; Li, H. Characterization of cellulose nanocrystals prepared by different delignification methods and application of ultra-light, hydrophobic aerogels as oil absorbent in food systems. Ind. Crops Prod.; 2023; 197, 116653. [DOI: https://dx.doi.org/10.1016/j.indcrop.2023.116653]
3. Ding, S.Y.; Liu, Y.S.; Zeng, Y.; Himmel, M.E.; Baker, J.O.; Bayer, E.A. How does plant cell wall nanoscale architecture correlate with enzymatic digestibility?. Science; 2012; 338, pp. 1055-1060. [DOI: https://dx.doi.org/10.1126/science.1227491] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/23180856]
4. Xie, S.; Li, Z.; Luo, S.; Zhang, W. Bioethanol to jet fuel: Current status, challenges, and perspectives. Renew. Sustain. Energy Rev.; 2024; 192, 114240. [DOI: https://dx.doi.org/10.1016/j.rser.2023.114240]
5. Abdul Kareem Joyia, M.; Ahmad, M.; Chen, Y.-F.; Mustaqeem, M.; Ali, A.; Abbas, A.; Ashraf Gondal, M. Trends and advances in sustainable bioethanol production technologies from first to fourth generation: A critical review. Energy Convers. Manag.; 2024; 321, 119037.
6. Ji, G.; Xing, Y.; You, T. Biochar as adsorbents for environmental microplastics and nanoplastics removal. J. Environ. Chem. Eng.; 2024; 12, 113377. [DOI: https://dx.doi.org/10.1016/j.jece.2024.113377]
7. Himmel, M.E.; Ding, S.Y.; Johnson, D.K.; Adney, W.S.; Nimlos, M.R.; Brady, J.W.; Foust, T.D. Biomass recalcitrance: Engineering plants and enzymes for biofuels production. Science; 2007; 315, pp. 804-807. [DOI: https://dx.doi.org/10.1126/science.1137016]
8. Hollenbach, R.; Ochsenreither, K. Mechanoenzymatic Reactions—Challenges and Perspectives. ChemCatChem; 2023; 15, e202300656. [DOI: https://dx.doi.org/10.1002/cctc.202300656]
9. Khan, I.; Iqbal, B.; Khan, A.A.; Inamullah,; Rehman, A.; Fayyaz, A.; Shakoor, A.; Farooq, T.H.; Wang, L.-X. The Interactive Impact of Straw Mulch and Biochar Application Positively Enhanced the Growth Indexes of Maize (Zea mays L.) Crop. Agronomy; 2022; 12, 2584. [DOI: https://dx.doi.org/10.3390/agronomy12102584]
10. Karimipour-Fard, P.; Chio, C.; Brunone, A.; Marway, H.; Thompson, M.; Abdehagh, N.; Qin, W.; Yang, T.C. Lignocellulosic biomass pretreatment: Industrial oriented high-solid twin-screw extrusion method to improve biogas production from forestry biomass resources. Bioresour. Technol.; 2024; 393, 130000. [DOI: https://dx.doi.org/10.1016/j.biortech.2023.130000]
11. Nazar, M.; Xu, Q.; Zahoor,; Ullah, M.W.; Khan, N.A.; Iqbal, B.; Zhu, D. Integrated laccase delignification with improved lignocellulose recalcitrance for enhancing enzymatic saccharification of ensiled rice straw. Ind. Crops Prod.; 2023; 202, 116987. [DOI: https://dx.doi.org/10.1016/j.indcrop.2023.116987]
12. Pan, S.; Zabed, H.M.; Wei, Y.; Qi, X. Technoeconomic and environmental perspectives of biofuel production from sugarcane bagasse: Current status, challenges and future outlook. Ind. Crops Prod.; 2022; 188, 115684. [DOI: https://dx.doi.org/10.1016/j.indcrop.2022.115684]
13. Ren, M.; Kong, F.; Zhou, C.; Fakayode, O.A.; Liang, J.; Li, H.; Zhou, M.; Fan, X. Green, one-pot biomass hierarchical utilization strategy for lignin-containing cellulose nanofibrils and fractionated lignin preparation. Ind. Crops Prod.; 2023; 203, 117193. [DOI: https://dx.doi.org/10.1016/j.indcrop.2023.117193]
14. Sun, L.; Han, J.; Wu, J.; Huang, W.; Li, Y.; Mao, Y.; Wang, L.; Wang, Y. Cellulose pretreatment with inorganic salt hydrate: Dissolution, regeneration, structure and morphology. Ind. Crops Prod.; 2022; 180, 114722. [DOI: https://dx.doi.org/10.1016/j.indcrop.2022.114722]
15. Ma, G.; Chen, X.; Liu, Y.; Hu, J.; Han, L.; Mao, H. Effects of Compound Biochar Substrate Coupled with Water and Nitrogen on the Growth of Cucumber Plug Seedlings. Agronomy; 2022; 12, 2855. [DOI: https://dx.doi.org/10.3390/agronomy12112855]
16. Kaabel, S.; Arciszewski, J.; Borchers, T.H.; Therien, J.P.D.; Friscic, T.; Auclair, K. Solid-State Enzymatic Hydrolysis of Mixed PET/Cotton Textiles. ChemSusChem; 2023; 16, e202201613. [DOI: https://dx.doi.org/10.1002/cssc.202201613]
17. Zhai, R.; Hu, J.; Jin, M. Towards efficient enzymatic saccharification of pretreated lignocellulose: Enzyme inhibition by lignin-derived phenolics and recent trends in mitigation strategies. Biotechnol. Adv.; 2022; 61, 108044. [DOI: https://dx.doi.org/10.1016/j.biotechadv.2022.108044]
18. Yoo, C.G.; Meng, X.; Pu, Y.; Ragauskas, A.J. The critical role of lignin in lignocellulosic biomass conversion and recent pretreatment strategies: A comprehensive review. Bioresour. Technol.; 2020; 301, 122784. [DOI: https://dx.doi.org/10.1016/j.biortech.2020.122784]
19. Yang, J.; Gao, C.; Yang, X.; Su, Y.; Shi, S.; Han, L. Effect of combined wet alkaline mechanical pretreatment on enzymatic hydrolysis of corn stover and its mechanism. Biotechnol. Biofuels Bioprod.; 2022; 15, 31. [DOI: https://dx.doi.org/10.1186/s13068-022-02130-0]
20. Hick, S.M.; Griebel, C.; Restrepo, D.T.; Truitt, J.H.; Buker, E.J.; Bylda, C.; Blair, R.G. Mechanocatalysis for biomass-derived chemicals and fuels. Green Chem.; 2010; 12, pp. 468-474. [DOI: https://dx.doi.org/10.1039/b923079c]
21. Schneider, L.; Haverinen, J.; Jaakkola, M.; Lassi, U. Solid acid-catalyzed depolymerization of barley straw driven by ball milling. Bioresour. Technol.; 2016; 206, pp. 204-210. [DOI: https://dx.doi.org/10.1016/j.biortech.2016.01.095] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/26859328]
22. Kuga, S.; Wu, M. Mechanochemistry of cellulose. Cellulose; 2019; 26, pp. 215-225. [DOI: https://dx.doi.org/10.1007/s10570-018-2197-1]
23. Wang, H.-M.; Wang, B.; Wen, J.-L.; Wang, S.-F.; Shi, Q.; Sun, R.-C. Green and efficient conversion strategy of Eucalyptus based on mechanochemical pretreatment. Energy Convers. Manag.; 2018; 175, pp. 112-120. [DOI: https://dx.doi.org/10.1016/j.enconman.2018.09.002]
24. Dornath, P.; Cho, H.J.; Paulsen, A.; Dauenhauer, P.; Fan, W. Efficient mechano-catalytic depolymerization of crystalline cellulose by formation of branched glucan chains. Green Chem.; 2015; 17, pp. 769-775. [DOI: https://dx.doi.org/10.1039/C4GC02187H]
25. Lukin, S.; Germann, L.S.; Friscic, T.; Halasz, I. Toward Mechanistic Understanding of Mechanochemical Reactions Using Real-Time In Situ Monitoring. Acc. Chem. Res.; 2022; 55, pp. 1262-1277. [DOI: https://dx.doi.org/10.1021/acs.accounts.2c00062]
26. Hronec, M.; Fulajtárová, K.; Klempová, T.; Čertík, M. Fractionation of Wheat Straw Catalyzed by Recyclable Terephthalic Acid. ChemistrySelect; 2019; 4, pp. 6060-6065. [DOI: https://dx.doi.org/10.1002/slct.201900562]
27. Schneider, L.; Haverinen, J.; Jaakkola, M.; Lassi, U. Pretreatment and fractionation of lignocellulosic barley straw by mechanocatalysis. Chem. Eng. J.; 2017; 327, pp. 898-905. [DOI: https://dx.doi.org/10.1016/j.cej.2017.06.175]
28. Sluiter, A.; Hames, B.; Ruiz, R.; Scarlat, C.; Sluiter, J.; Templeton, D.; Crocker, D. Determination of Structural Carbohydrates and Lignin in Biomass Laboratory Analytical Procedure; Laboratory Analytical Procedure (LAP)/National Renewable Energy Laboratory: Golden, CO, USA, 2008.
29. Barakat, A.; Mayer-Laigle, C.; Solhy, A.; Arancon, R.A.D.; de Vries, H.; Luque, R. Mechanical pretreatments of lignocellulosic biomass: Towards facile and environmentally sound technologies for biofuels production. RSC Adv.; 2014; 4, pp. 48109-48127. [DOI: https://dx.doi.org/10.1039/C4RA07568D]
30. Ji, G.; Gao, C.; Xiao, W.; Han, L. Mechanical fragmentation of corncob at different plant scales: Impact and mechanism on microstructure features and enzymatic hydrolysis. Bioresour. Technol.; 2016; 205, pp. 159-165. [DOI: https://dx.doi.org/10.1016/j.biortech.2016.01.029]
31. Adney, B.; Baker, J. Measurement of Cellulase Activities Laboratory Analytical Procedure; NREL/TP-510-42628 National Renewable Energy Laboratory: Golden, CO, USA, 1996.
32. da Silva, A.S.; Inoue, H.; Endo, T.; Yano, S.; Bon, E.P. Milling pretreatment of sugarcane bagasse and straw for enzymatic hydrolysis and ethanol fermentation. Bioresour. Technol.; 2010; 101, pp. 7402-7409. [DOI: https://dx.doi.org/10.1016/j.biortech.2010.05.008]
33. Boissou, F.; Sayoud, N.; De Oliveira Vigier, K.; Barakat, A.; Marinkovic, S.; Estrine, B.; Jerome, F. Acid-Assisted Ball Milling of Cellulose as an Efficient Pretreatment Process for the Production of Butyl Glycosides. ChemSusChem; 2015; 8, pp. 3263-3269. [DOI: https://dx.doi.org/10.1002/cssc.201500700] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/26346950]
34. Zhang, Q.; Chen, Y.; Su, W. Highly efficient preparation of cellulose nanocrystals by mechano-enzymatic hydrolysis: A mechanism study. Catal. Sci. Technol.; 2023; 13, pp. 618-623. [DOI: https://dx.doi.org/10.1039/D2CY01904C]
35. Park, S.; Baker, J.O.; Himmel, M.E.; Parilla, P.A.; Johnson, D.K. Cellulose crystallinity index: Measurement techniques and their impact on interpreting cellulase performance. Biotechnol. Biofuels; 2010; 3, 10. [DOI: https://dx.doi.org/10.1186/1754-6834-3-10]
36. Wu, Y.; Ge, S.; Xia, C.; Mei, C.; Kim, K.-H.; Cai, L.; Smith, L.M.; Lee, J.; Shi, S.Q. Application of intermittent ball milling to enzymatic hydrolysis for efficient conversion of lignocellulosic biomass into glucose. Renew. Sustain. Energy Rev.; 2021; 136, 110758. [DOI: https://dx.doi.org/10.1016/j.rser.2020.110442]
37. Zhang, H.; Han, L.; Dong, H. An insight to pretreatment, enzyme adsorption and enzymatic hydrolysis of lignocellulosic biomass: Experimental and modeling studies. Renew. Sustain. Energy Rev.; 2021; 140, 110758. [DOI: https://dx.doi.org/10.1016/j.rser.2021.110758]
38. Zhao, L.; Boluk, Y. XPS and IGC characterization of steam treated triticale straw. Appl. Surf. Sci.; 2010; 257, pp. 180-185. [DOI: https://dx.doi.org/10.1016/j.apsusc.2010.06.060]
39. Dorris, G.M.; Gray, D.G. The surface analysis of paper and wood fibers by ESCA I. Cellul. Chem. Technol.; 1978; 12, pp. 9-23.
40. Amirjalayer, S.; Fuchs, H.; Marx, D. Understanding the Mechanocatalytic Conversion of Biomass: A Low-Energy One-Step Reaction Mechanism by Applying Mechanical Force. Angew. Chem.; 2019; 58, pp. 5232-5235. [DOI: https://dx.doi.org/10.1002/anie.201811091]
41. Ji, G.; Han, L.; Gao, C.; Xiao, W.; Zhang, Y.; Cao, Y. Quantitative approaches for illustrating correlations among the mechanical fragmentation scales, crystallinity and enzymatic hydrolysis glucose yield of rice straw. Bioresour. Technol.; 2017; 241, pp. 262-268. [DOI: https://dx.doi.org/10.1016/j.biortech.2017.05.062]
42. Meine, N.; Rinaldi, R.; Schuth, F. Solvent-free catalytic depolymerization of cellulose to water-soluble oligosaccharides. ChemSusChem; 2012; 5, pp. 1449-1454. [DOI: https://dx.doi.org/10.1002/cssc.201100770]
43. Kaufman Rechulski, M.D.; Käldström, M.; Richter, U.; Schüth, F.; Rinaldi, R. Mechanocatalytic Depolymerization of Lignocellulose Performed on Hectogram and Kilogram Scales. Ind. Eng. Chem. Res.; 2015; 54, pp. 4581-4592. [DOI: https://dx.doi.org/10.1021/acs.iecr.5b00224]
44. Liu, X.; Liu, H.; Yan, P.; Mao, L.; Xu, Z.; Zhang, Z.C. Mechanocatalytic Synergy for Expedited Cellulosic Ethanol Production Compatible with Integrated Biorefinery. ACS Sustain. Chem. Eng.; 2020; 8, pp. 2399-2408. [DOI: https://dx.doi.org/10.1021/acssuschemeng.9b06018]
45. Zhang, Y.; Huang, M.; Su, J.; Hu, H.; Yang, M.; Huang, Z.; Chen, D.; Wu, J.; Feng, Z. Overcoming biomass recalcitrance by synergistic pretreatment of mechanical activation and metal salt for enhancing enzymatic conversion of lignocellulose. Biotechnol. Biofuels; 2019; 12, 12. [DOI: https://dx.doi.org/10.1186/s13068-019-1354-6] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/30647772]
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; Yang, Qizhi 1 ; Liu, Yucheng 4 ; Ji, Guanya 1 1 Key Laboratory of Modern Agriculture Equipment and Technology, School of Agriculture Engineering, Jiangsu University, Zhenjiang 212013, China;
2 Anhui Undergrowth Crop Intelligent Equipment Engineering Research Center, West Anhui University, Lu’an 237012, China;
3 Jingjiang College, Jiangsu University, Zhenjiang 212013, China;
4 Key Laboratory of Modern Agriculture Equipment and Technology, School of Agriculture Engineering, Jiangsu University, Zhenjiang 212013, China;