Obtaining Nanolignin from Green Coconut Shell and Fiber by the Acetosolv Method with Subsequent Ultrasonication

The growing concern about replacing fossil materials with renewable resources that have less environmental impact has been a central theme in numerous scientific articles over the years. Within this context, lignocellulosic biomass has gained prominence due to its renewability and wide availability. This type of biomass can be used as a raw material for the development of the bioeconomy, leading to the synthesis of a wide range of chemical products [1]. Among the various sources of lignocellulosic biomass, coconut cultivation (Cocos nucifera) stands out for generating large quantities of waste that are often not properly discarded or used [2]. Brazil, being the world’s fifth largest producer of coconut, focuses mainly on removing and drying the pulp to obtain copra. The main byproducts of copra are coconut oil and coconut flour [3], resulting in approximately 15% of the fruit being used. This generates around 850 kg of waste for each ton of fruit produced [4,5]. Despite being an organic waste, the large amount of lignocellulosic material in coconuts means that it takes much longer to decompose in the environment than other organic materials. Given the high consumption of fruit and the consequent generation of waste, it is important to develop reuse techniques that not only reduce the environmental impact but also add economic value to by-products and waste, fostering a sustainable bioeconomy [6]. Lignin is the second most abundant biopolymer in nature. The proportion of lignin present in lignocellulosic biomass can vary between 15 and 40%, which corresponds to a significant percentage of the lignocellulosic material, thus justifying the intensification of research aiming at its reuse [7]. Lignin is currently used in a variety of products, including cosmetics as a UV-protective agent, nutraceuticals, and biocides. Its benefits extend to biostabilizers and coating materials, as well as flame retardants in the paper industry. Lignin is also valued for its antimicrobial and antioxidant properties and is used in the preparation of biofertilizers, epoxy resins, and various fine chemicals [8]. Traditional methods for extracting lignin from lignocellulosic biomass are acid or alkaline hydrolysis, enzymatic processes, cleaning with ionic liquids, the use of organic solvents, or combinations of these techniques [9]. The acetosolv pulping technique is an organosolv method for lignin extraction, where a combination of organic solvents is used, such as acetic acid (in higher concentration), hydrochloric acid as a catalyst, and water, with the solution being subjected to a temperature ranging from 125 to 170 °C and kept under reflux for a period of 60 to 180 min [10]. This process selectively dissolves lignin from plant biomass, which is of high purity, has little degradation, low levels of inorganic solids, and is free of sulfur compounds. These characteristics make acetosolv lignin an attractive option for several industrial applications, such as in the manufacture of chemicals, advanced materials, and bioplastics. Additionally, acetosolv pulping is considered environmentally friendly, as it allows for the recovery of approximately 70% of the solvent used and its reuse, reducing the environmental impact and improving the economic viability of the process [11]. Although the removal of extractives is commonly considered an essential step to improve the yield and quality of lignin, this research is distinguished by its systematic investigation of the impact of this pretreatment step on the characteristics and yield of the extracted lignin. By comparing the results with and without the removal of extractives, it was possible to identify that eliminating this step did not compromise the extraction process performance, presenting significant operational advantages. Among these advantages, the reduction in energy consumption and chemical products, making the process more efficient and sustainable, simplifying extraction, and lowering operational costs can be mentioned. Despite its promise, lignin, as a macromolecule, is a difficult material to apply due to some characteristics, such as its resistance to water, which makes it difficult to incorporate into aqueous mixtures. However, when reduced to a nanometric size, this resistance decreases, facilitating the incorporation process, and for this reason nanolignin has been standing out and gaining increasingly more space in scientific research and in the industrial sector [12]. Nanolignin is widely recognized as one of the most effective strategies for improving the properties of lignin. Its reduced size facilitates interactions with polymeric matrices, while its spherical shape contributes to greater stability and dispersion in water, ensuring homogeneous and stable dispersion. Moreover, the increased surface area of the nanoparticles enhances their chemical modification due to the presence of various functional groups, such as thiols, aliphatic hydroxyls, and phenolics, making these functional groups more accessible. This increased accessibility facilitates chemical reactions. Nanolignin also exhibits antimicrobial activity, further expanding its applications [13]. These characteristics make lignin nanoparticles multifunctional, enabling their use as surfactants, plasticizers, emulsifiers, or additives in polymer matrices, such as epoxy and thermoplastics. Furthermore, they may contribute to sustainability by serving as nanocarriers for environmentally sustainable pesticides [14,15]. There are several approaches to reducing lignin to the nanometric scale, including chemical methods, such as hydrolysis and acid precipitation, and mechanical methods, such as homogenization and ultrasonication [16]. In ultrasonic treatment, high-frequency sound waves generate cavitation in the liquid, fragmenting the lignin into nanostructures. This method is useful for maintaining desirable properties of lignin, such as its interaction with materials and its antioxidant characteristics [17]. This work aimed to obtain nanolignin from the shell and fiber of green coconut, promoting more effective use of these residues using the acetosolv method (applying different temperatures and with or without extractives in the biomass) and ultrasonic treatment (with different contact times).

The paper reviews lignocellulosic biomass valorization with emphasis on coconut residues due to high waste volumes and slow environmental degradation. It surveys lignin applications (cosmetics, biocides, stabilizers, coatings, flame retardants, antimicrobial and antioxidant uses, biofertilizers, epoxy resins) and extraction methods (acid/alkaline hydrolysis, enzymatic routes, ionic liquids, organosolv approaches). The acetosolv method is highlighted for producing high-purity, sulfur-free lignin with solvent recovery (~70%), improving environmental and economic viability. The rationale for studying the removal of extractives is framed by common assumptions in literature that pretreatment improves yield and purity; this work questions its necessity under mild conditions. For nanolignin formation, prior approaches include chemical hydrolysis/precipitation and mechanical methods (homogenization, ultrasonication). Comparative literature cited reports wide yield and purity variability across biomasses and processes (e.g., mango tegument up to 98% yield; ethanosolv on sugarcane bagasse up to 97.3% purity but at higher temperatures and cost). Prior DLS-reported lignin nanoparticle sizes range from ~250–500 nm (hybrid organosolv/steam explosion with ethanol) to ~340 nm (1 h sonication), indicating process parameters and biomass type strongly influence nanoparticle size.

Materials: Green coconut residues (Cocos sp.) were sourced in Maceió, Brazil (Sept 2022). The outer shells were washed, almonds removed, shells and fibers cut, oven-dried (forced air, 72 h), milled (micro knife mill MA680), sieved (40 mesh), and stored dry. Biomass composition (% w/w): lignin 40.0 ± 0.5% (72% H2SO4 digestion at 45 °C for 7 min, diluted, autoclaved 121 °C for 30 min), cellulose 15.9 ± 1.5% (TAPPI T 203 cm-99), hemicellulose 15.86 ± 0.33%, extractives 8.17 ± 1.43% (Soxhlet, hexane/ethanol 2:1, 5 h), ash 3.28 ± 0.13% (550 °C, 5 h), moisture 9.59 ± 0.13% (105 °C gravimetric). Acetosolv pulping: Two biomass types were used: in natura coconut flour (FCIN) and coconut flour without extractives (FCSE). For each run, 8 g biomass and 120 mL acetosolv solution were combined (1:15 w:v). Liquor composition: acetic acid 93% w/w, hydrochloric acid 0.3% w/w (catalyst), distilled water 6.7% w/w. After reaching boiling, reflux was maintained for 3 h at two temperature conditions: 100 °C and 120 °C. Internal pressure likely increased due to volatility but was not directly measured. Post-reaction, pulp was vacuum filtered to separate cellulose fibers and acidic black liquor (lignin). Black liquor was rotary evaporated at 60 °C to concentrate volume by ~10× and recover solvent. Lignin was precipitated by adding hot distilled water (80 °C) at 1:10 to the evaporated liquor volume, then stood at room temperature for 24 h. Precipitated lignin was vacuum filtered, washed to near-neutral pH, and oven-dried at 60 °C to constant mass. Lignin yield on a dry basis was calculated as YL = (ML/MIL) × 100, where ML is recovered dry lignin mass and MIL is initial lignin mass in the fibers (corrected for moisture and lignin percentage). Lignin characterization: Moisture, ash, and Klason lignin analyses followed Section 2.1 methods. SEM: Samples metallized (platinum 3 min, gold 5 min at 10 mA), Tescan Vega3 operated at 5 kV, magnifications 300–4000×, fields of view 454–115 µm (scales 100 µm and 20 µm). TG: Shimadzu TGA-51/51H, nitrogen atmosphere, 10 °C/min heating to 900 °C; data captured at 0.5 s intervals using TA-60 WS v2.21. FTIR: Shimadzu IRAffinity-1, 4000–600 cm⁻¹, 64 scans; spectra for raw biomass, extractive-free biomass, and extracted lignin. Nanolignin preparation: Lignin extracted via acetosolv at 100 °C from FCIN was suspended in water and ultrasonicated at 850 W for 10 and 20 min using an Ecosonics ultrasonic sonicator to reduce particle size and improve dispersion. Dynamic Light Scattering (DLS): Samples were diluted 500 µL into 25 mL, then tip-ultrasonicated for 3 min at 200 W. Measurements were performed at 25 °C using a Zetasizer Nano (Malvern Instruments, model ZS) to determine hydrodynamic diameter and polydispersity.

• Biomass composition: lignin 40.0 ± 0.5%, cellulose 15.9 ± 1.5%, hemicellulose 15.86 ± 0.33%, extractives 8.17 ± 1.43%, ash 3.28 ± 0.13%, moisture 9.59 ± 0.13%. • Acetosolv lignin yields: Average yield for raw coconut flour (FCIN) 13.86%. Condition-specific yields (mean ± SD): FCIN 100 °C: 13.89 ± 4.35%; FCIN 120 °C: 11.39 ± 0.69%; FCSE 100 °C: 14.09 ± 1.08%; FCSE 120 °C: 16.07 ± 1.07%. ANOVA (95% confidence, duplicate runs) indicated no statistically significant differences across temperatures or pretreatment (extractive removal). • Purity: Ash content low (0.45–0.58%), indicating minimal inorganic impurities. Klason lignin purity 78.20–80.94%. Reported representative values include ash 0.55 ± 0.26% and Klason lignin 78.82 ± 0.81% (FCIN 100 °C). • FTIR: Characteristic lignin bands observed, including C=O stretching (~1710 cm⁻¹) indicative of lignin–carbohydrate complexes; aromatic C=C stretching (1609 and 1510 cm⁻¹); methyl/methylene (1460, 1426 cm⁻¹); guaiacyl/syringyl units (1270, 1220, 1164 cm⁻¹); C–O stretching in ethers/alcohols (~1120 cm⁻¹); out-of-plane C–H (~850 and ~770 cm⁻¹). Removal of extractives reduced peak intensity at 1030 cm⁻¹ without altering essential features. • TG: Lignin showed thermal stability with initial decomposition events near ~200 °C (lighter compounds) and ~400 °C (heavier components), followed by curve stabilization, indicating minimal further mass loss. • SEM: Lignin exhibited spherical agglomerates and amorphous structure; LCSE surfaces were smoother/more uniform than LCIN, consistent with extractive removal. Porosities likely arose from precipitation and nucleation effects; no significant morphological differences across 100–120 °C pulping conditions. • Nanolignin (DLS): Ultrasonication at 850 W reduced particle size with time. After 10 min: 500–750 nm (average 656.25 nm). After 20 min: 400–600 nm (average 533.75 ± 15.12 nm). Longer sonication enhanced fragmentation and dispersion.

The study demonstrates that acetosolv pulping of green coconut shell and fiber yields lignin of acceptable purity with low ash content under mild temperatures (100–120 °C), and that eliminating the conventional pretreatment step of extractive removal does not significantly affect yield or quality within the tested range. This directly addresses the research question of process simplification and sustainability: omitting pretreatment reduces chemical and energy inputs and operational complexity, while solvent recovery inherent to acetosolv further enhances environmental and economic performance. Structural characterization (FTIR) confirmed preservation of key lignin functionalities (aromaticity, carbonyls, guaiacyl/syringyl units), supporting suitability for downstream applications. TG analysis indicated sufficient thermal stability for processes involving elevated temperatures. Morphological observations showed spherical agglomeration and amorphous character, with surface smoothing when extractives were removed, but no temperature-induced morphological changes between 100 and 120 °C. Ultrasonication effectively reduced lignin particle size into the submicron range, improving dispersion and potentially enabling better interaction with polymer matrices and aqueous systems. While the obtained average hydrodynamic diameters (~534–656 nm) are larger than some literature values achieved at longer sonication times or with additional fractionation steps, the results validate the feasibility of producing nanolignin under higher power and shorter durations, supporting energy-efficient processing. Overall, the findings suggest a more streamlined, sustainable route to nanolignin from coconut residues without compromising key material properties.

This research investigated the production of nanolignin from the shell and fiber of green coconuts, using acetosolv pulping, with the aim of transforming waste into high-value-added products and promoting sustainability. The results obtained during lignin extraction revealed that the biomass pretreatment step to remove extractives is not necessary since this process does not significantly alter the lignin yields. The percentage yield of lignin obtained in the process without the pretreatment step was 13.89 ± 4.35%, contributing to a reduction in chemical and energy resource consumption during the extraction process. The acetosolv method preserves the chemical structure of lignin and, at the same time, removes cellulose and hemicellulose with high quality. This is essential to ensure purity, as confirmed by methods such as the corrected calculation of Klason lignin and ash, presenting high purity values and indicative of the quality of the product obtained, with FTIR and thermograms also identifying characteristic peaks of lignin and thermal stability, important for its applications. Furthermore, the ultrasonic treatment was essential to obtain lignin at a nanometric scale, with the average size of the obtained nanolignin particles being 533.75 nm.

• Internal pressure during acetosolv pulping was not directly measured; while characteristic of the apparatus and temperatures, this limits precise replication of pressure conditions. • Nanoparticle characterization relied on DLS for hydrodynamic diameter; complementary techniques (SEM/TEM for morphology and size verification, zeta potential for colloidal stability) were identified as necessary but not reported. • ANOVA was conducted at a 95% confidence level with duplicate experiments; broader replication could strengthen statistical conclusions regarding the non-significance of temperature and pretreatment effects. • The study focused on green coconut residues; results may vary for other biomasses with different structural properties. • Ultrasonication parameters were limited to two time points at high power; a more extensive parameter sweep (time, power, solvent systems) could optimize particle size and polydispersity further.