Scalable Process to Obtain Highly Charged Cellulose Nanofibrils from Sugarcane Bagasse

TEMPO-oxidized Cellulose nanofibrils (TOCNFs) are negatively charged polyelectrolytes with advanced applications in several fields. However, methods to isolate them from most plant-based biomasses are often time, energy, and water-intensive, mainly due to lignin removal steps and high-energy mechanical defibrillation. As so, large-scale commercial utilization of TOCNFs is still at an early stage.

Previous studies showed that TEMPO-catalyzed oxidation with sodium hypochlorite (NaClO) leads to a simultaneous removal of lignin while still chemically defibrillating cellulose. Additionally, using sugarcane bagasse as the initial material enables fibers to reach nanoscale without requiring mechanical treatments. Herein, we adapted the one-pot TEMPO oxidation to sugarcane bagasse using a low-energy mechanical treatment (Ultra-Turrax). High CNF yields (up to 30%) with different carboxylate contents (from 0.37 to 1.1 mmol/g) were obtained by modulating the NaClO content (10-50 mmol of NaClO/g of fibers) and the bagasse granulation. Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and acetyl-bromide lignin determination showed a decrease in lignin content compared to the initial biomass, with a lignin content of 6 wt.% for 50 mmol NaClO/g. Atomic force microscopy (AFM) images revealed the presence of flexible nanostructures with high aspect ratios (diameters smaller than 5 nm and lengths around 1 µm) when using NaClO contents higher than 20 mmol/g. Aqueous suspensions (0.5 wt.%) of highly charged CNFs (30 and 50 mmol/g) showed highly viscous shear-thinning power-law behavior, as observed for CNFs prepared by conventional methodologies.

Additionally, the product obtained by this newly developed process was tested as a potential water decontamination agent. A 1% wt. % dispersion of the TOCNF and sodium alginate (1:1 mass ratio) were crosslinked with calcium trough ionotropic gelation, to form hydrogel beads that were used to remove transition metal ions from water via adsorption. Batch and column tests using Cu (II) as a model ion were performed, showing a high maximum adsorption capacity of 195 mg/g.

Overall, this project obtained a simplified methodology for biomass delignification and TOCNF extraction, showing great potential for applicability on an industrial level.

Acknowledgment: FAPESP (Grant Number: 2023/02955-0 and 2024/07351-8)

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