Fibrillated Nanocellulose Obtained by Mechanochemical Processes from Coconut Fiber Residue

The study addresses the need for sustainable materials to replace fossil-derived products, focusing on cellulose and particularly nanocellulose (CNFs and CNCs) for its renewability, versatility, and role in the bioeconomy. It proposes valorizing agro-industrial waste—specifically green coconut (Cocos nucifera L.) residue—due to its global abundance and composition (~35% cellulose, ~35% hemicellulose, ~25% lignin), as a low-cost feedstock for nanocellulose. Hybrid routes combining mild alkaline hydrolysis with mechanical defibrillation are highlighted to preserve crystallinity, remove hemicellulose and lignin efficiently, reduce aggressive reagents and energy consumption, and improve yield and quality. Prior works demonstrate diverse applications of coconut-derived nanocellulose in EMI shielding, antibacterial activity, hydrogel reinforcement, drug delivery, and biodegradable films. The research hypothesis is that cleaner, minimized chemical and mechanical routes—using a colloidal mill—can yield a high-quality nanofibrillated cellulose gel with industrially relevant physicochemical properties, offering an environmentally responsible and economically viable pathway aligned with circular economy principles. The study aims to validate this hypothesis by investigating critical processing parameters and resulting material characteristics to provide a sustainable extraction method for nanocellulose from coconut waste.

The paper situates coconut-derived nanocellulose within the broader context of sustainable nanomaterials, referencing: advances in agro-industrial waste valorization for nanocellulose production; functional outcomes such as antibacterial behavior and electromagnetic interference attenuation in coconut-based nanopapers; reinforcement of thermo-responsive hydrogels enabling controlled release; and biodegradable films exhibiting high crystallinity and mechanical performance. Reviews on nanocellulose production emphasize integrating alkaline treatments with mechanical processes to enhance extraction efficiency, yield, and structural quality while reducing reliance on harsh acids. The literature also notes the importance of preserving cellulose I crystallinity, surface OH group abundance for functionalization, and rheological gel behavior characteristic of nanofibril networks. Collectively, prior studies support the sustainability and application potential of nanocellulose from lignocellulosic biomass, while indicating gaps in NFC production routes specifically optimized for coconut biomass using cleaner mechanochemical methods.

Materials: Green coconuts were collected in João Pessoa, PB, Brazil. Water was removed and shell residue crushed and sieved (40 mesh). 200 g of fibers were washed with 500 mL purified water (1:5 m/v) for 2 h at 50 °C under 200 rpm agitation; the filtrate was discarded.

Alkaline pre-treatment: Fibers treated in 2% (w/v) NaOH under mechanical stirring (200 rpm) for 2 h at 80 °C. Fibers were filtered, washed twice with deionized water, and oven-dried at 50 °C for 24 h. Depolymerization conditions were tested at 2% and 5% NaOH; no significant yield differences were found, so 2% NaOH and 2 h were selected.

Bleaching: 100 g of alkali-treated fibers dispersed in 300 mL solution containing 30 g NaClO₂ and 8–10 drops glacial acetic acid in deionized water. Suspension stirred for 1 h at 70 °C, cooled in an ice bath, then washed with cold water and acetone.

Mechanochemical grinding (Supermasscolloider): Cellulose pulp from green coconut biomass at 2% m/v (60 g cellulose in 3 L water) was processed using a Supermasscolloider colloid mill. Shear forces degraded cell walls and exposed microfibrils. Operating conditions: flow rate 1 L/min, speed ~1500 rpm, 15 passes. Gel behavior appeared from the 10th pass; 15 passes were used to obtain a viscous CNF gel.

Characterization:

• FTIR: IR Prestige-2 (Shimadzu). Dried, crushed samples mixed with KBr at 1:100 (sample/KBr) to make pellets. Scans 400–4000 cm⁻¹, resolution 4 cm⁻¹, 20 scans per sample (raw, delignified, CNF).
• Elemental analysis (CHNS): UNICUBE elemental analyzer. 1.0 g freeze-dried CNF (24 h) analyzed.
• XRD: Shimadzu XRD-6000 with Cu Kα (λ = 1.5418 Å), 40 kV, 30 mA, scan rate 0.5° min⁻¹; samples on glass holders. Crystallinity index (CI) from peak fitting as CI = Σ(area crystalline peaks)/Σ(area crystalline+amorphous peaks) × 100%.
• TG/DTG: TA Instruments SDT 2960, nitrogen atmosphere (50 mL/min), heating rate 10 °C/min, 25–900 °C, ~10 mg in alumina crucible.
• SEM: FE-SEM MIRA3 LMH (TESCAN), 20 kV, with EDS; and TM4000PLUS II under ambient conditions. Samples freeze-dried (24 h), mounted on carbon tape, sputter-coated with Au for 35 s at 20 mA under Ar (except where noted no metallization). Various magnifications acquired.
• AFM: Samples diluted in water at 1:100, deposited on mica, dried overnight in vacuum oven at 60 °C. Tapping mode imaging using Mikromasch NSC-35 cantilever (force constant 8.9 N m⁻¹, resonance 200 kHz). Height used to assess fibril dimensions due to tip convolution affecting width.
• Rheology: Discovery Hybrid Rheometer HR-10 (TA Instruments), cone-and-plate geometry (40 mm diameter, 2° cone angle, 0.100 mm gap), Peltier at 25 °C. Stress sweep at 10 Hz from 0.001–500 Pa to determine LVR. Frequency sweep within LVR from 0.01–10 Hz, recording G′, G″, tan δ. Flow behavior via controlled shear rate ramp, 1–100 s⁻¹ (300 s each ramp). Tests at 25 °C with 60 s rest; five points per decade.

• Yield: From 200 g initial fibers, ~60 g bleached fiber obtained (~30% yield), aligning with literature (20–40%) and exceeding traditional acid hydrolysis methods (8–20%).
• FTIR: O–H stretching bands near 3400 cm⁻¹ (raw), 3355 cm⁻¹ (delignified), 3350 cm⁻¹ (CNF) indicative of hydrogen bonding. C–H stretches at 2922 cm⁻¹ (raw) and 2899 cm⁻¹ (delignified) absent in CNF, consistent with dewaxing and reduced aliphatic fractions. Carbonyl (C=O) at 1734 cm⁻¹ (lignin/hemicellulose esters); increased absorption around 1639 cm⁻¹ in treated samples linked to cellulose OH bending/moisture. Lignin-associated bands (e.g., aromatic features around ~1058 cm⁻¹; phenolic C–O/C–H at ~1369–1371 cm⁻¹; carbonyl at ~1514 cm⁻¹) diminish or disappear after delignification and in CNF, corroborating removal of lignin/hemicellulose.
• CHNS elemental composition (CNF): C 40.18%, H 5.24%, N 0.00%, S 0.00%; O by difference ~54.58%. Absence of N and S indicates removal of proteins and sulfate groups; high cellulose purity.
• XRD: Progressive increase in crystallinity and sharpening of cellulose peaks. CI increased from 30% (in natura) to 40% (delignified) and 49% (CNF). Main cellulose I peak shifted from 2θ ≈ 21.3° (raw) to ≈22.4° (CNF), consistent with cellulose I.
• TG/DTG: Initial mass loss (30–130 °C) due to moisture: raw 5%, delignified 7%, CNF 10%. Raw fiber shows additional degradation (150–224 °C, ~7% mass) attributed to hemicellulose depolymerization; DTG peak at ~265 °C present in raw but absent in purified samples, indicating removal of xylan/lignin. Major cellulose degradation occurs between ~200–360 °C; purified samples exhibit improved thermal stability and reduced late-stage mass loss, consistent with efficient lignin removal.
• Morphology (SEM/AFM): CNFs form elongated, interwoven fibrillar networks. SEM-derived individual fibril widths ~72–82 nm; single fibril measured at ~82.25 nm. AFM height ~8 ± 2 nm, with micrometric lengths; nanoscale surface shows irregularities due to residual amorphous regions.
• Rheology: CNF suspension exhibits gel-like, predominantly elastic behavior. In stress sweep (LVR), G′ ≫ G″ and is nearly constant; frequency sweep shows G′ and G″ increase with frequency, with G′ > G″ across the range, indicating a robust, hydrogen-bonded fibrillar network. Gel behavior evident from ~10th pass in grinding.

The results validate the hypothesis that a cleaner, minimized chemical–mechanical route can produce high-quality nanofibrillated cellulose from green coconut waste. Mild alkaline pretreatment combined with colloid mill grinding effectively removes lignin and hemicellulose while preserving and enhancing cellulose crystallinity (CI up to 49%) and thermal stability. FTIR and XRD confirm the reduction of amorphous constituents and emergence of cellulose I features, while CHNS demonstrates high purity without nitrogen or sulfur. Morphological analyses show nanoscale fibrils capable of forming entangled networks, which underpin the observed gel-like rheology with dominant elastic responses (G′ > G″). Compared to acid hydrolysis routes, the method improves yield (~30%) and reduces use of aggressive reagents, aligning with circular economy principles. These structural and rheological properties make the CNF gel suitable for applications requiring stable, elastic networks and tunable functionality through surface OH groups, supporting its potential in packaging, coatings, composites, gels, and biomedical systems.

Mechanochemical processing coupled with mild alkaline and bleaching treatments successfully converted green coconut fibers into nanofibrillated cellulose with increased crystallinity (30% → 49%), enhanced thermal stability, and gel-like rheological behavior. SEM and AFM confirmed nanometric fibril dimensions (widths ~72–82 nm; heights ~8 ± 2 nm). The approach achieves higher yield than traditional acid hydrolysis and minimizes aggressive chemicals, demonstrating a sustainable pathway to valorize agro-industrial residues. Future work could: optimize grinding parameters and pass numbers to tailor fibril dimensions and rheology; scale-up and assess energy and cost metrics; explore surface functionalization and composite formulations for targeted applications (e.g., barrier films, hydrogels, EMI shielding); and perform comprehensive lifecycle and environmental impact analyses to quantify sustainability benefits.

• Quantitative rheological parameters (absolute values of G′, G″ across frequency/stress) are described qualitatively; detailed numerical data are not reported.
• AFM width measurements are affected by tip convolution; height was used, but precise cross-sectional shapes remain uncertain.
• Oxygen content was calculated by difference in CHNS, not measured directly.
• Residual lignin/hemicellulose fractions are inferred from spectroscopic/thermal profiles without direct compositional quantification.
• Industrial scalability, energy consumption, and process economics were not experimentally assessed.
• Nanocellulose yield after grinding (from bleached pulp to CNF gel) is not explicitly quantified beyond fiber yield after bleaching.