ABSTRACT
Objective: The main objective of the present work was to develop a multi-stage, sequential, and mild pretreatment method for the efficient extraction of lignin from sugarcane bagasse (SCB), aiming to optimize lignin removal and reduce the environmental impact commonly associated with more aggressive processes. In addition, a goal was to develop kinetic modeling of the process to obtain information for process optimization. Finally, the work also aimed to characterize the extracted lignin, which provides information on the material's structure for future higher value-added applications.
Theoretical Framework: The main concepts in this work are centered on chemical kinetics and reactor modeling for process optimization. The research also addresses the physical chemistry of materials, exploring the structure of lignocellulosic biomass and the effectiveness of each pretreatment step , and analytical techniques such as SEM, EDS, and XRD to elucidate the structure of the extracted lignin. Mass transfer is an underlying concept, in which temperature, and solvent concentration influence the extraction of lignin from the solid matrix to the liquid phase.
Method: The methodology of the work consisted of a three-stage sequential pretreatment process for the extraction of lignin from sugarcane bagasse (SCB). The process consisted of an acid extraction phase with an aqueous 0.5% (v/v) HCl solution, followed by an alkaline extraction with 0.5% (v/v) NH4OH, and finally, a solvent (organosolv) extraction using 95% (v/v) ethanol. The experiments were conducted in a batch reactor, where parameters such as temperature (varying between 50°C and 70°C), initial SCB mass, and time were controlled to evaluate the process kinetics. Lignin extraction was quantified by UV-Vis spectroscopy, and the kinetic data were fitted to a mathematical model that describes the batch reactor operation to determine the reaction rate constant and activation energy. The extracted lignin was characterized by various techniques, such as SEM, EDS, XRD, TGA/DTG, and UV-Vis, to analyze its morphology, elemental composition, crystallinity, and thermal stability.
Results and Discussion: The study demonstrated the effectiveness of a sequential and mild pretreatment methodology, achieving efficiencies greater than 80%. The process revealed a notable synergy between the stages, resulting in a 30% increase in extraction efficiency compared to single-stage treatments. Kinetic analysis allowed for the determination of activation energies for the acid (10.82 kJ·mol-1), alkaline (9.55 kJ·mol-1), and organosolv (4.41 kJ·mol-1) stages. The extracted lignin showed an irregular plate-like morphology, an amorphous nature, and a high sulfur content. These results validate a more sustainable and economically viable route for lignin valorization, providing crucial data on process kinetics and the properties of the final product for higher valueadded applications in biorefineries.
Research Implications:This study contributes to the literature in several ways. Firstly, it presents a sequential and mild pretreatment methodology for biomass, establishing a more sustainable route. Secondly, the research offers a kinetic analysis that underpins process optimization, which is a key differentiator, as many studies focus on only one or two treatment stages. Finally, the study characterizes the removed lignin, providing an understanding of its properties at the end of the process. The work's relevance lies in the reduction of costs and environmental impact associated with the extraction, which makes lignin valorization more attractive.
Originality/Value: The work contributes to biomass delignification studies by demonstrating the effectiveness of a sequential pretreatment under mild conditions, achieving high efficiency without the need for severe conditions, and generating easier-to-treat effluents. An important insight is the synergy between the stages, with the alkaline phase being the most important for lignin removal and the acid phase being essential to expose the biomass structure for lignin removal. The kinetic study, with the evaluation of the activation energies for each stage, provides information for future process optimizations. The characterization of the extracted lignin provides information to direct the material towards future higher value-added applications, whose separation occurred via a more sustainable and economically viable route.
Keywords: Delignification, Sugarcane Bagasse, Lignin, Kinetic.
RESUMO
Objetivo: O presente trabalho teve como principal objetivo o desenvolvimento de um método de pré-tratamento em múltiplas etapas, sequencial e sob condições brandas, para a extração eficiente de lignina a partir do bagaço de cana-de-açúcar (SCB), visando otimizar a remoção de lignina e diminuir o impacto ambiental comumente associado a processos mais agressivos.Além disso, o desenvolvimento da modelagem cinética do processo, como forma de obter-se informações para otimização do processo. Por fim, o trabalho também teve como objetivo a caracterização da lignina extraída, o que promove informações da estrutura do material para futuras aplicações de maior valor agregado.
Referencial Teórico: Os principais conceitos neste trabalho estão centrados na cinética química e na modelagem de reatores para otimização do processo. A pesquisa também aborda a fisico-química dos materiais, explorando a estrutura da biomassa lignocelulósica e eficácia de cada etapa do pré-tratamento, e técnicas analíticas como MEV, EDS e DRX, para elucidação da estrutura da lignina extraída. A transferência é um conceito subjacente, na qual a temperatura e a concentração do solvente influenciam a extração de lignina da matriz sólida para a fase líquida.
Método: A metodologia do trabalho consistiu em um processo de pré-tratamento sequencial em três etapas para a extração de lignina do bagaço de cana-de-açúcar (SCB). O processo consistiu em uma fase de extração ácida com solução aquosa de HCl 0,5% (v/v), seguida por uma extração alcalina com NH4OH 0,5% (v/v), e, finalmente, uma extração com solvente (organosolv) utilizando etanol 95% (v/v). Os experimentos foram realizados em um reator em batelada, onde parâmetros como temperatura (variando entre 50∘C - 70∘C), massa inicial do SCB e tempo foram controlados para avaliar a cinética do processo. A extração de lignina foi quantificada por espectroscopia UV-Vis, e os dados cinéticos foram ajustados a um modelo matemático que descreve a operação do reator em batelada para determinar a constante de velocidade e a energia de ativação. A lignina extraída foi caracterizada por diversas técnicas, como MEV, EDS, DRX, TGA/DTG e UV-Vis, para analisar sua morfologia, composição elementar, cristalinidade e estabilidade térmica.
Resultados e Discussão: O estudo demonstrou a eficácia de uma metodologia de pré-tratamento sequencial e brando, alcançando eficiências superiores a 80%. O processo revelou uma sinergia notável entre as etapas, resultando em um aumento de 30% na eficiência de extração em comparação com tratamentos de etapa única. A análise cinética permitiu a determinação das energias de ativação para as etapas ácida (10,82 kJ·mol-1), alcalina (9,55 kJ·mol-1) e etanólica (4,41 kJ·mol-1). A lignina extraída apresentou morfologia de placas irregulares, natureza amorfa e um alto teor de enxofre. Esses resultados validam uma rota mais sustentável e economicamente viável para a valorização da lignina, fornecendo dados cruciais sobre a cinética do processo e as propriedades do produto final para aplicações de maior valor agregado em biorrefinarias.
Implicações da Pesquisa: Este estudo contribui para a literatura em algumas formas. Primeiramente, ele apresenta uma metodologia de pré-tratamento sequencial e branda para a biomassa, estabelecendo uma rota mais sustentável. Em segundo, a pesquisa oferece uma análise cinética, o que está por trás da otimização do processo, o que é um diferencial, visto que muitos trabalhos se concentram em apenas um ou duas etapas de tratamento. Por fim, o estudo caracteriza a lignina removida, proporcionando um entendimento das suas propriedades ao final do processo. A relevância do trabalho reside na redução de custos e do impacto ambiental associados à extração, o que torna a valorização da lignina mais atrativa.
Originalidade/Valor: O trabalho contribui nos estudos de deslignificação de biomassa ao mostrar a eficácia de um pre-tratamento sequencial em condições brandas, com alta eficiência sem a necessidade de condições severas, gerando efluentes de mais fácil tratamento. Um insight importante é a sinergia entre as etapas, sendo a alcalina a mais importante na remoção da lignina e a ácida essencial para expor a estrutura da biomassa para a remoção da lignina. O estudo cinético, com a avaliação das energias de ativação de cada etapa, dá informações para futuras otimizações de processo. A caracterização da lignina extraída fornece informações para direcionar o material para futuras aplicações de maior valor agregado, cuja separação se deu de uma rota mais sustentável e economicamente viável.
Palavras-chave: Deslignificação, Bagaço de Cana-de-Açúcar, Lignina, Cinética.
RESUMEN
Objetivo: El objetivo principal de este trabajo fue desarrollar un método de pretratamiento secuencial, multietapa y suave para la extracción eficiente de lignina del bagazo de caña de azúcar (SCB). Este método busca optimizar la remoción de lignina y reducir el impacto ambiental comúnmente asociado con procesos más agresivos. Además, se desarrolló el modelado cinético del proceso para obtener información para su optimización. Finalmente, el trabajo también buscó caracterizar la lignina extraída, lo que proporciona información sobre la estructura del material para futuras aplicaciones de mayor valor.
Marco teórico: Los conceptos principales de este trabajo se centran en la cinética química y el modelado de reactores para la optimización de procesos. La investigación también aborda la fisicoquímica de los materiales, explorando la estructura de la biomasa lignocelulósica y la efectividad de cada etapa del pretratamiento. Se utilizan técnicas analíticas como SEM, EDS y XRD para dilucidar la estructura de la lignina extraída. La transferencia es un concepto subyacente, en el que la temperatura y la concentración del disolvente influyen en la extracción de lignina de la matriz sólida a la fase líquida.
Método: La metodología consistió en un proceso de pretratamiento secuencial de tres pasos para la extracción de lignina del bagazo de caña de azúcar (SCB). El proceso consistió en una fase de extracción ácida con una solución acuosa de HCl al 0,5 % (v/v), seguida de una extracción alcalina con NH4OH al 0,5 % (v/v) y, finalmente, una extracción con disolventes (organosolv) con etanol al 95 % (v/v). Los experimentos se realizaron en un reactor discontinuo, donde se controlaron parámetros como la temperatura (entre 50 °C y 70 °C), la masa inicial de SCB y el tiempo para evaluar la cinética del proceso. La extracción de lignina se cuantificó mediante espectroscopía UV-Vis, y los datos cinéticos se ajustaron a un modelo matemático que describe el funcionamiento del reactor discontinuo para determinar la constante de velocidad y la energía de activación. La lignina extraída se caracterizó mediante diversas técnicas, como SEM, EDS, XRD, TGA/DTG y UV-Vis, para analizar su morfología, composición elemental, cristalinidad y estabilidad térmica.
Resultados y discusión: El estudio demostró la eficacia de una metodología de pretratamiento secuencial y suave, logrando eficiencias superiores al 80 %. El proceso reveló una notable sinergia entre las etapas, lo que resultó en un aumento del 30 % en la eficiencia de extracción en comparación con los tratamientos de una sola etapa. El análisis cinético permitió determinar las energías de activación para las etapas ácidas (10,82 kJ·mol-1), alcalinas (9,55 kJ·mol-1) y etanólicas (4,41 kJ·mol-1). La lignina extraída presentó una morfología irregular en placas, naturaleza amorfa y un alto contenido de azufre. Estos resultados validan una ruta más sostenible y económicamente viable para la valorización de la lignina, proporcionando datos cruciales sobre la cinética del proceso y las propiedades del producto final para aplicaciones de mayor valor añadido en biorrefinerías.
Implicaciones de la investigación: Este estudio contribuye a la literatura de varias maneras. En primer lugar, presenta una metodología de pretratamiento secuencial y suave para la biomasa, estableciendo una ruta más sostenible. En segundo lugar, la investigación ofrece un análisis cinético que sustenta la optimización del proceso, una característica distintiva, dado que muchos estudios se centran en solo uno o dos pasos de tratamiento. Finalmente, el estudio caracteriza la lignina extraída, lo que permite comprender sus propiedades al final del proceso. La relevancia del trabajo radica en la reducción de costos e impacto ambiental asociados a la extracción, lo que hace más atractiva la valorización de la lignina.
Originalidad/Valor: El trabajo contribuye a los estudios de deslignificación de biomasa al demostrar la eficacia del pretratamiento secuencial en condiciones suaves, con alta eficiencia sin necesidad de condiciones severas, generando efluentes más fáciles de tratar. Un aspecto importante es la sinergia entre las etapas, siendo la etapa alcalina la más importante para la remoción de lignina y la etapa ácida esencial para exponer la estructura de la biomasa para su posterior remoción. El estudio cinético, que evalúa las energías de activación de cada etapa, proporciona información para futuras optimizaciones del proceso. La caracterización de la lignina extraída proporciona información para orientar el material hacia futuras aplicaciones de mayor valor, cuya separación se realizó mediante una ruta más sostenible y económicamente viable.
Palabras clave: Deslignificación, Bagazo de caña de azúcar, Lignina, Cinética.
(ProQuest: ... denotes formulae omitted.)
1 INTRODUCTION
Lignocellulosic biomass represents a vast and promising source of renewable resources, strategically positioning itself as a sustainable alternative to fossil fuels to produce biofuels, such as ethanol, and various other value-added products (Haq et al., 2020; Nanda et al., 2015). Composed primarily of cellulose, hemicellulose, and lignin, the polysaccharides and lignin within this biomass can serve as substrates for a wide range of bioproducts (Haq et al., 2020; Singh et al., 2014a). Given the high availability of agricultural residues, such as sugarcane bagasse, the importance of developing efficient technologies for their processing is observed.
Sugarcane bagasse (SCB), an abundant co-product of the sugar and ethanol industry, stands out as a lignocellulosic biomass with great potential due to its widespread availability (Kininge & Gogate, 2022). Its typical composition (Fig. 1) includes cellulose (40-43%), hemicellulose (21-25%), and lignin (15-21%), along with other extractive compounds (Haq et al., 2020; Singh et al., 2014).
Lignin, a complex phenolic polymer, acts as a binding matrix, impeding access to and conversion of the polysaccharides. This intricate lignin structure, characterized by ether (C-O- C) and carbon-carbon (C-C) linkages, confers a natural recalcitrance to chemical and biological degradation, directly impacting the efficiency of enzymatic hydrolysis of cellulose and hemicellulose (Kim et al., 2015; Kleinert & Barth, 2008; Yáñez-S et al., 2014).
Despite the challenges of extraction and purification resulting from its structural complexity, lignin holds potential for various industrial applications, including the production of phenolic resins, binding and dispersing agents, polyurethane foams, antioxidants, aromatic monomers, and biofuels (Kim et al., 2015; Yáñez-S et al., 2014). The valorization of lignin is therefore crucial for the economic and environmental optimization of biorefineries.
Pretreatment strategies for lignocellulosic biomass are primarily aimed at the efficient separation of lignin from polysaccharides to enhance the accessibility of sugars for subsequent processes. Several methods have been reported in the literature, including acid extraction (Zhao & Liu, 2013; Zhuo et al., 2024), alkaline extraction (Marcelo Fuertez-Córdoba et al., 2021; Maryana et al., 2014), and organosolv extraction (Agnihotri et al., 2015; de Vasconcelos et al., 2013). Chemical extraction using acids and bases is considered effective and economical, as acids hydrolyze hemicellulose while alkalis disrupt the rigid lignin structure and enhance the accessibility of polysaccharides by dissolving hemicellulose, lignin, and silica (Pham et al., 2022; Rezende et al., 2011). However, acid-induced corrosion limits large-scale implementation. Alkaline extraction, while offering the possibility of alkali recovery to reduce process costs, requires longer extraction times and is limited to stronger alkalis. The primary mechanism of organosolv extraction involves the penetration of the substrate and the separation of lignin from the polysaccharide matrix through the solubilization of hemicellulose (Nair et al., 2023). Nonetheless, the high pressures and temperatures required for this method still make its large-scale adoption unattractive.
Seeking to improve the efficiency of extraction methods, one study developed an ultrasound-assisted alkaline extraction method to reduce both extraction time and alkali concentration (Kininge & Gogate, 2022). By using 1M NaOH for 1 hour at 70°C, with a 100W ultrasound power, a delignification of 67.30% was achieved. Despite this efficiency, large-scale application is limited by the need for an additional piece of equipment.
The effectiveness of each delignification method is intrinsically linked to the biomass's characteristics and the operational conditions, including particle size, solid-to-liquid ratio, temperature, time, and agitation (Lima Filho, 1991). Thus, the choice of a delignification method is fundamental for the valorization of biomass in a biorefinery, as it impacts the composition and purity of the obtained fractions, as well as the overall economic viability of the process.
In this work, a three-stage methodology for the extraction of lignin from SCB was developed to enhance lignin recovery efficiency while minimizing the generation of aggressive waste streams. The proposed methodology involves a sequence of mild treatments: an initial acid stage, followed by an alkaline stage, and an ethanol extraction. Diluted concentrations of acid (0.5% v/v) and alkali (0.5% v/v) were employed in all phases to mitigate the environmental impacts associated with using concentrated reagents and reduce the complexity of subsequent effluent treatment. The study also includes a kinetic analysis of each extraction stage, along with mathematical modeling to obtain a predictive model of the system's behavior and to optimize operational conditions. The extracted lignin was subsequently characterized using SEM, EDS, XRD, TGA/DTG, and UV-Vis techniques.
2 MATERIALS AND METHODS
2.1 MATERIALS
SCB was donated by Usina José Agro-industrial S.A. (Pernambuco, Brazil). The chemicals used for lignin extraction (HCl 37% P.A., NaOH 97% P.A.- pearls, and ethanol P.A) were obtained from VETEC, Brazil. Ammonium hydroxide (28-30% P.A) was purchased from NEON, Brazil.
2.2 EXPERIMENTAL METHODOLOGY
2.2.1 Preparation and characterization of sugarcane bagasse
The SCB was washed with distilled water to remove impurities and then oven-drying at 60 °C for 72 h. After this, the material was reduced (12, 28, 32 mesh). Then, its chemical composition was analyzed by the NREL method (Sluiter et al., 2008). The overall composition is described in Table 1.
2.2.1.1 Moisture
To determine the moisture, 2.0 g of moist material was weighed in an infrared moisture analyzer scale (Gehaka IV 2002). Eq. 1 quantifies the sample's moisture content Mo (%).
... (1)
Where:
wws and mds are the weights of wet and dry samples (105°C), respectively.
2.2.1.2 Ash
The ash was determined by the residue content of the complete burning of SCB, according to the modified method of the TAPPI T211 om-02 standard (TAPPI, 2002). 1 g of SCB was put into a crucible, then heated in a muffle at 525°C for 4 h. After cooling, the solid was weighed. The Equation 2 was used to calculate the ash content.
... (2)
Where:
wc+a is the weight of the crucible and the ash together, mc is the weight of the crucible alone, and wd is the weight of the dry sample.
2.2.1.3 Extractives content
The percentage of SCB extracts was determined in 5.0 g of the dry and crushed raw material, with an average particle diameter of 0.595 mm. The weighed solid was placed in a cartridge, and the extraction was carried out in a Soxhlet with 250 mL of ethanol P.A., proceeding for 4 hours, counted after the first cycle at 80 °C.
The percentage of extractives was calculated using Eq. 3.
... (3)
Where:
E (%) is equivalent to the percentage of extractives in the sample,
wca+e corresponds to the mass of the cartridge with extractives,
Mc to the mass without extractives,
and Mdb the dry mass of the SCB.
2.2.2 SCB delignification
The delignification of SCB was conducted in three stages: acid extraction, followed by alkaline extraction, and organosolv (ethanolic) extraction, in which step parameters were evaluated. The methodology is presented in Fig. 2 and described as follows.
The acid extraction was conducted at 50°C, 60°C, and 70°C using a 0.5% (v/v) hydrochloric acid aqueous solution. Nine erlenmeys (125 mL), each containing 2 g of SCB and 75 mL of the HCl solution, were stirred at 150 rpm for four hours in a shaker (Marconi, MA- 420). Kinetic evaluation was performed by collecting nine samples at 30 min intervals, with each sample being removed while the system remained under agitation.
After the given time, the suspension was filtered. A sample of the liquor was diluted (1:5) in a 0.5% (v/v) NH4OH aqueous solution and analyzed in a UV-Vis spectrophotometer. The solid content was washed with 50 mL of distilled water, which was then analyzed using UV-Vis spectroscopy (section 2.2.3). After washing, the solids were dried in an oven at 60°C for 24 h and then weighed.
After drying and weighing, the solid was mixed with 75 ml of aqueous NH4OH 0.5 (v/v). The extractive procedures were kept identical to those applied in the previous step (acid extraction), during a four-hour extraction. Then, vacuum filtration and washing (50 mL water) were followed by drying for 24 h at 60°C. The filtrate was also analyzed by UV-Vis.
A third stage was carried out with the resulting solid dried from the previous step, with the ethanol solvent (95% v/v), proceeding with the analysis of the filtrates and washing water, and carrying out the final drying of the SCB residue (24 h, 60°C), mainly containing cellulose and hemicellulose.
2.2.3 Insoluble and soluble lignin analysis method
At the end of each step extraction (acid, alkaline, ethanolic), the suspension was filtered, treated (solid), and dried at 60°C for 24 h. The filtrate contains soluble lignin, as the residue contains insoluble lignin. The insoluble lignin was determined by the gravimetric method (Equation 4).
... (4)
Where:
LE, wi, and wt are the percentage of lignin extraction (% wt.) and the initial dry mass of bagasse (g) and bagasse mass after extraction (g), respectively.
The soluble lignin in the filtrates was determined using UV-Vis spectroscopy, following the steps of filtration, reaction, and washing. After each extraction, 1 mL of the filtrate was diluted to 5 mL with NH4OH (0.5% v/v) and analyzed using UV-Vis spectrophotometer at 280 nm. The soluble lignin concentration was determined using an analytical calibration curve at a concentration range 0.001 to 0.125 g L-1.
2.3 CHARACTERIZATION OF EXTRACTED LIGNIN AND STANDARD LIGNIN
A scanning electron microscope (SEM) (TESCAN, Vega3), coupled with an energydispersive X-ray detector (EDS), was used to analyze the morphology of lignin. The samples were metallized with gold using a DENTOM VACUUM. The crystallinity of lignin was determined using a Rigaku SmartLab X-ray diffractometer equipped with Cu Kα radiation (wavelength 1.54 Å), operating at 40 kV and 30 mA. The analysis was performed over a 2θ angle range of 10° to 70°, with a step size of 0.02° and an acquisition time of 2 sec/step. The average size of ordered domains was estimated from the Scherrer Eq. (Eq. 5).
... (5)
Where:
D is the average size of ordered domains in the same unit of λ (Å or nm),
λ is the wavelength of x-ray radiation 1.54 Å (0.154 nm),
K is shape factor (assumed to be 0 .90), β is the width at half height [36].
Thermogravimetric analysis (TGA) and derivative thermogravimetric analysis (DTG) were conducted using the SHIMADZU TA-50WSI TGA50 module (Shimadzu, Japan). The measurements were carried out under a N2 flow of 10 cm3 min-1, with a heating rate of 20°C min-1, up to a maximum temperature of 900°C. The optical properties of the lignin were investigated using a Varian Cary 50 UV-vis spectrophotometer in the wavelength range 200- 700 nm. A wavelength of 280 nm was used to determine the soluble lignin concentration, as described above (section 2.2.3).
3 RESULTS AND DISCUSSION
3.1 DELIGNIFICATION PROCESS OPTIMIZATION
Lignin extraction was evaluated according to several parameters, which were adjusted for each stage of the process (Fig. 1), including temperature, initial SCB mass, and solvent concentration (acidic, alkaline, and ethanolic). In all phases, lignin removal was monitored over time to provide kinetic data for the process.
3.1.1 Effect of Temperature
Lignin extraction was assessed by the kinetic evolution of each stage under the influence of temperature (50 °C, 60 °C, 70 °C). Mixing and extraction operations were performed in batches with a 1:37.5 ratio of SCB to solution volume and agitation at 150 RPM.
The concentration of lignin in the liquid phase at different temperatures for each extraction stage is shown in Fig. 4. An increase in lignin concentration in the liquid phase was observed over time in all three stages during the 240 minutes of operation, indicating that longer reaction times resulted in greater lignin removal from the SCB structure.
The enhancement of lignin extraction in the acidic, alkaline, and ethanolic steps (Fig. 4) increased with the operation time, up to 240 minutes. This occurred at higher levels at 70 °C compared to those at 50 °C. In the presence of NH4OH, a lignin concentration of 0.83 g L-1 was reached, approximately twice as high as concentrations extracted with HCl or ethanol (0.20 - 0.30 g L-1). However, no significant enhancement was observed when comparing 60 °C with 70 °C in terms of lignin extraction. Thus, the extraction operation at 60 °C produces satisfactory results for extracting lignin from sugarcane bagasse.
The main advantage of multi-stage lignin extraction lies in the improved total lignin removal compared to a single-stage process (Rezende et al., 2011). Da Silva et al. (2024), aiming for second-generation ethanol production, achieved a delignification of approximately 59% of sugarcane bagasse in a two-stage process (acid-alkaline). The authors also reported a lignin purity of 95-97%, concluding that the process preserved most of the lignin structure. However, in contrast to the present study, they employed more severe conditions, such as higher temperatures (120 °C) and higher concentrations (H3PO4 at 1.5% w/v).
3.1.2 Effect of the initial mass of sugarcane bagasse and dilution
The effect of the initial mass on the amount of soluble lignin in each extraction stage (acidic, alkaline, and ethanolic) was determined as a function of the initial SCB mass (Fig. 5). For this stage, the extraction temperature for each phase was maintained at 60 °C, based on the results from the previous section.
The results showed a positive linear effect on the extracted lignin amount with increasing SCB mass. This indicates that, under constant conditions, a greater initial mass of SCB resulted in a higher quantity of extracted lignin. In the acidic stage (Fig. 5a), increasing the SCB mass led to an increase in the extracted lignin concentration up to 5.53 g of SCB. Beyond this mass, no significant increase in lignin extraction was observed. An HCl solution at 0.5% (v/v) was used in this stage, which removed lignin without significantly affecting the carbohydrate content of the polysaccharide fractions. Furthermore, the acid is believed to promote the breakdown of lignin bonds within the material's structure, making it more exposed to subsequent alkaline and ethanolic treatments, as lignin removal is known to increase hemicellulose digestibility (Tian et al., 2021). This was further confirmed by the increase in lignin concentration with the increased NH4OH concentration in the second step.
The use of alkali in pretreatment involves the saponification of cross-linking ester bonds and increases porosity as these bonds are removed. Thus, the efficiency of this process is dependent on the lignin content of the biomass (Binod et al., 2010; Singh et al., 2014b). Among the available alkalis, Rezende et al. (2011) studied a two-stage delignification process (H2SO4 1% + NaOH 2%), achieving nearly 100% lignin removal at 120 °C and a solid-to-liquid ratio of 1:10 (g bagasse/mL solution). However, this process resulted in a considerable loss of cellulose (approximately 30%). Additionally, the morphology of the bagasse changed throughout the treatment stages, transitioning from plates to hollow and fibrous elements, which suggests a degradation of the bagasse's structural integrity.
In a study of another alkali, Fuertez-Córdoba et al. (2021) investigated the delignification of four types of biomass (sugarcane bagasse, palm fiber, coffee pulp residue, and kikuyo grass) using Ca(OH)2. They observed the highest lignin removal from kikuyo grass (28.31%), with agitation speed identified as a crucial factor in the process. Although a lowaggressivity alkali was used, the process time was long (72 hours).
In the third delignification stage (Fig. 5c), a linear relationship was also observed between the amount of lignin removed and the increase in the SCB mass. For a given mass, the effect of ethanol dilution on lignin removal was not conclusive. For 2 g of SCB, the intermediate dilution (1:100), while in the same order of magnitude, appeared to be slightly more efficient. In contrast, when 6 g of SCB was used, the 1:50 ethanolic solution was the most efficient, with a concentration difference of 0.4 g L-1. Across all trials, the 1:200 dilution ratio consistently showed the lowest efficiency, although the difference in liquid-phase lignin concentration was smaller for lower masses. Based on these findings, it can be inferred that for masses smaller than 4 g, more diluted ethanol solutions may be used.
The literature presents various studies on organosolv delignification using acid (Brenelli et al., 2016; Zhao & Liu, 2013), glycerol (Meighan et al., 2017; Terán Hilares et al., 2017) and ionic liquids (Saha et al., 2017a). In general, while high delignification rates were achieved, higher temperatures (>120 °C) were typically required. This study, however, demonstrates that with milder temperatures (60 °C) and diluted solutions, it is possible to achieve satisfactory delignification rates with longer contact times (approximately 120 minutes). This approach generates less aggressive waste, which is easier and less costly to treat.
3.2 KINETIC EVALUATION OF DELIGNIFICATION
The objective of the kinetic study is to evaluate how lignin is removed from the SCB over time, translating these processes into rates derived from mathematical models. As the process is carried out in three stages, each stage will have its own kinetic model developed.
The reaction rates of the solid-liquid lignin extraction process were expressed by rLIGN = MLCMkLIGNCi(CLIGN,eq-CLIGN)n, where the kinetic parameters were the specific reaction rate kLIGN and the reaction order (n). Considering the batch operations taking place in a pseudo-homogeneous system, the mass balance of the extraction process can be formulated as (Equation. 6):
... (6)
Where:
CLIGN and CLIGN,eq , are the concentrations of lignin (g L-1) dissolved in the liquid extract over time and in the liquid-solid balance, respectively.
The specific reaction rate is defined as k'LIGN (K'LIGN = kLIGNCi; i = HCl, NH4OH Oor EtOH,) with Ci considered a constant in each extraction condition. At the beginning of the operation with t = 0, CLIGN = CLIGN0 = 0.
The results of the model adjustment, including the orders of magnitude of the parameters obtained for the acid (HCl), alkaline (NH4OH), and organosolv (ethanol - EtOH) extraction steps, are presented in Tables 3, 4, and 5, respectively. The kinetics of the extraction processes showed rate constants ranging from 0.035 to 0.406 Ln g-n min-1, with values increasing as a function of temperature. At 70 °C, the highest specific extraction rate (4.06 x 10-2 Ln g-n min-1) was observed when operating with NH4OH. The predictions based on the solution of the model equations and the experimental concentration evolution are shown in Figs. 6, 7, and 8, respectively.
For the processes, the temperature effect (Fig 6d, 7d, 8d) was evaluated using the Arrhenius equation (Equation 7):
... (7)
Where:
k'i is the rate constant,
k'0i' is a pre-exponential factor,
Eai is the activation energy,
R is the universal gas constant and T is the absolute temperature.
The concentration profiles predicted and validated against the experimental values show that the concentration of extracted lignin reached maximum levels of approximately 0.80 g L-1 after 240 minutes of operation with NH4OH at 70 °C. This value was roughly three times higher than the concentrations extracted with HCl in the first stage and with ethanol in the complementary stage under the same operating conditions. The predictions of the kinetic behavior for the three extraction operations (Figs. 6, 7, and 8), based on the model equations (Eq. 7), showed a good representation of the experimental data, with reaction orders in the range of 1.4 to 1.8. Specific reaction rates ranging from 5.20 × 10-2 to 2.72 × 10-2 Ln g-n min-1 contributed to representing the profiles across the three operating temperatures, where the average deviations from the experimental values were smaller than 4.82%.
The relationship between the estimated values for the lnk'i (for i = LIGN,HCl; LIGN,NH4OH; LIGN, EtOH) and the extraction temperature characterized by the linearized Arrhenius equation (...) provided the orders of magnitude of the activation energies and frequency factors of reaction steps of the lignin extraction (Table 3).
The activation energy values indicated that the action of ethanol required 55% less energy than the reactions with HCl and NH4OH. This justifies the physical nature of ethanol's participation, where extraction/dissolution is the primary mechanism for removing lignin. The activation energy value reported here for the rate-determining step of alkaline extraction with the weak alkali NH4OH was half the value found by Macfarlane et al. (al., 2009) and Rashid et al (Rashid et al., 2018). , who used a strong base (1% wt. NaOH) for extraction at temperatures from 60 °C to 120 °C. Compared to the extraction of lignin from corn SCB obtained by steam explosion, which was also performed with a strong base, the activation energy of the process with NH4OH was 21.14% lower.
3.3 CHARACTERIZATION OF EXTRACTED LIGNIN
The micrographs of standard lignin and extracted lignin are shown in Fig 9. The morphology of the extracted lignin appears as irregular plates with fewer agglomerated portions, larger dimensions, and a less porous structure and fewer roughness compared to standard lignin. The extraction processes used may have contributed to these differences. (Ibrahim et al., 2010) showed that a steam-exploded process followed by bleaching results in different morphological structures, like sponge-like and rock-like. Another possible explanation lies in the source of the lignin. Lignin obtained from wood (softwood and hardwood) and corn stalks were fibrous (Diop et al., 2014; Gea et al., 2020), while alkaline extraction of lignin from palm empty fruit bunches resulted in flake and sphere morphologies, depending on the concentration of the alkali used (Evdokimov et al., 2018).
Lignin has several function groups, where hydroxyl groups stand out for their reactivity (Singh et al., 2005). The energy-dispersive X-ray spectroscopy (EDS) method was performed to measure the carbon and the sulfur content on booth lignin (standard and extracted). The results (Table 4) indicate that the extracted lignin contains fewer hydroxyl groups and a higher sulfur content, which could be due to origin of raw material. Further investigation is required to obtain a lignin reduced sulfur content, thereby enhancing its versatility for various applications (Saha et al., 2017b). The carbon contained has good agreement with literature (Singh et al., 2005), although a different technique was used.
The diffraction patterns of standard lignin and lignin extracted from sugarcane bagasse (extracted) are shown in the diffractogram in Fig 10, and were similar. In both, the characteristic peak at 22.5° of cellulose was present. A peak of lower intensity, at 35.7°, was also present, indicating that the lignin originated from a cellulosic material. The vast area below the curve shows the amorphous characteristic of lignin (Goudarzi et al., 2014; Ibrahim et al., 2010). The peaks were adjusted using the Gaussian, Loretzian, Pearson VII, and Pseudo-Vigot models to determine the average size of the ordered domains. For lignin extracted from sugarcane bagasse, the Pearson VII model was the best fit (R2 = 0.9478), with the average peak at 22.48°, while for standard lignin, the best model was Loretzian. (R2=0.8958) with an average peak of 22.2°. For the extracted lignin, d=0.43 nm was found, while for the standard lignin, it was 0.29 nm.
The UV-vis absorption spectra of lignin and the standard were analyzed over 200 to 700 nm. As shown in Fig.11, a band is observed at 200-250 nm, corresponding to non-conjugated phenolic groups. The band at 250-300 nm is attributed to the presence of etherified alcohols, resulting from electronic transitions within the aromatic groups (Saha et al., 2017b). Absorption in the range of 340-400 nm is associated with conjugated phenolic groups, involving electronic transitions related to the Cα=Cβ linkage and C=C bonds. This indicates the presence of pcoumaric acid and ferulic acid in lignin (Li, 2010). These findings align well with the literature (Li et al., 2010; Maziero et al., 2012; Saha et al., 2017b), despite the lignin being sourced differently (De et al., 2020).
Fig. 12 and 13 present the TGA and DTG curves for sugarcane bagasse before and after undergoing the three pre-treatment steps. The thermogravimetric profiles exhibit three distinct stages of mass loss.
The first stage occurs between 30°C and 100°C and is attributed to the evaporation of adsorbed water on the surface. The second stage takes place between 200°C and 350°C, corresponding to the decomposition of organic matter, such as carbohydrate components and lignin, which generate carbonized material. At temperatures above 350°C, lignin begins to decompose, releasing gaseous products such as CO, CO2, and CH4 (Moubarik et al., 2013).
The thermogram also indicates that degradation starts only after the material absorbs a certain amount of energy. This process initiates with the breaking of chemical bonds, leading to molecular chain ruptures. Above 500°C, no solid residue remains, as the material is completely burned.
4 CONCLUSIONS
This study demonstrated the effectiveness of a sequential, multi-stage extraction methodology for lignin from sugarcane bagasse using diluted acid, alkaline, and ethanolic treatments. The overall performance, based on lignin concentration in the final solution, reached levels 2.5 to 3.5 times higher than those achieved with single-step extractions using HCl and ethanol, respectively. Furthermore, a remarkable 30% increase in extraction efficiency was observed when the alkaline and ethanolic steps were combined, suggesting a synergistic effect that streamlines the process. Kinetic analysis of the extraction processes revealed a direct relationship between reaction rate and temperature, with the highest specific extraction rate recorded at 70°C for the ammonium hydroxide stage. This indicates that the process is kinetically favored at higher temperatures.
The structural characterization of the extracted lignin provided critical insights. Scanning electron microscopy (SEM) showed that the lignin particles were plate-like with a smooth surface and minimal agglomeration, an important factor for downstream processing. Energy-dispersive X-ray spectroscopy (EDS) analysis confirmed the presence of a low number of hydroxyl groups and a high sulfur content, a characteristic potentially linked to the biomass's origin. X-ray diffraction (XRD) patterns were consistent with the amorphous nature of the extracted lignin, while thermogravimetric analysis (TGA) indicated thermal stability up to 200°C. UV-Vis spectroscopy further corroborated these findings by identifying characteristic chromophore groups, such as phenolic and non-conjugated groups.
This comprehensive characterization is fundamental for directing the lignin towards high-value applications, such as the production of aromatic compounds, bioplastics, and various additives. By demonstrating an efficient, multi-stage process under mild conditions and providing a detailed analysis of the resulting lignin, this work contributes significantly to the sustainable valorization of lignocellulosic biomass in a biorefinery context.
ACKNOWLEDGMENTS
This work was supported by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).
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Abstract
Objective: The main objective of the present work was to develop a multi-stage, sequential, and mild pretreatment method for the efficient extraction of lignin from sugarcane bagasse (SCB), aiming to optimize lignin removal and reduce the environmental impact commonly associated with more aggressive processes. In addition, a goal was to develop kinetic modeling of the process to obtain information for process optimization. Finally, the work also aimed to characterize the extracted lignin, which provides information on the material's structure for future higher value-added applications. Theoretical Framework: The main concepts in this work are centered on chemical kinetics and reactor modeling for process optimization. The research also addresses the physical chemistry of materials, exploring the structure of lignocellulosic biomass and the effectiveness of each pretreatment step , and analytical techniques such as SEM, EDS, and XRD to elucidate the structure of the extracted lignin. Mass transfer is an underlying concept, in which temperature, and solvent concentration influence the extraction of lignin from the solid matrix to the liquid phase. Method: The methodology of the work consisted of a three-stage sequential pretreatment process for the extraction of lignin from sugarcane bagasse (SCB). The process consisted of an acid extraction phase with an aqueous 0.5% (v/v) HCl solution, followed by an alkaline extraction with 0.5% (v/v) NH4OH, and finally, a solvent (organosolv) extraction using 95% (v/v) ethanol. The experiments were conducted in a batch reactor, where parameters such as temperature (varying between 50°C and 70°C), initial SCB mass, and time were controlled to evaluate the process kinetics. Lignin extraction was quantified by UV-Vis spectroscopy, and the kinetic data were fitted to a mathematical model that describes the batch reactor operation to determine the reaction rate constant and activation energy. The extracted lignin was characterized by various techniques, such as SEM, EDS, XRD, TGA/DTG, and UV-Vis, to analyze its morphology, elemental composition, crystallinity, and thermal stability. Results and Discussion: The study demonstrated the effectiveness of a sequential and mild pretreatment methodology, achieving efficiencies greater than 80%. The process revealed a notable synergy between the stages, resulting in a 30% increase in extraction efficiency compared to single-stage treatments. Kinetic analysis allowed for the determination of activation energies for the acid (10.82 kJ·mol-1), alkaline (9.55 kJ·mol-1), and organosolv (4.41 kJ·mol-1) stages. The extracted lignin showed an irregular plate-like morphology, an amorphous nature, and a high sulfur content. These results validate a more sustainable and economically viable route for lignin valorization, providing crucial data on process kinetics and the properties of the final product for higher valueadded applications in biorefineries. Research Implications:This study contributes to the literature in several ways. Firstly, it presents a sequential and mild pretreatment methodology for biomass, establishing a more sustainable route. Secondly, the research offers a kinetic analysis that underpins process optimization, which is a key differentiator, as many studies focus on only one or two treatment stages. Finally, the study characterizes the removed lignin, providing an understanding of its properties at the end of the process. The work's relevance lies in the reduction of costs and environmental impact associated with the extraction, which makes lignin valorization more attractive. Originality/Value: The work contributes to biomass delignification studies by demonstrating the effectiveness of a sequential pretreatment under mild conditions, achieving high efficiency without the need for severe conditions, and generating easier-to-treat effluents. An important insight is the synergy between the stages, with the alkaline phase being the most important for lignin removal and the acid phase being essential to expose the biomass structure for lignin removal. The kinetic study, with the evaluation of the activation energies for each stage, provides information for future process optimizations. The characterization of the extracted lignin provides information to direct the material towards future higher value-added applications, whose separation occurred via a more sustainable and economically viable route.