Conversion of sugarcane bagasse into carboxymethl cellulose
Authors:   , Makhanya Fezokuhle Mfundo 
Published: January 2022

The process of converting sugarcane into sugar has a high percentage of dry residue that remains after the juice has been extracted, the dry residue is referred to as sugarcane bagasse (SCB). The focus of this study has been on using sugarcane bagasse to extract cellulose from the matrix and converting it into carboxymethylcellulose (CMC). This has been achieved via a two-step synthesis process. Mill run sugarcane bagasse was used as received and was pre-treated using NaOH for the purpose of extracting cellulose from the sugarcane bagasse. The extractant was refluxed separately using two different nitric acid (HNO3) concentrations namely 8 M (cellulose sample 1) and 4 M (cellulose sample 2) in 20 % (v/v) ethanol to obtain cellulose. The extracted cellulose was obtained in yields of 37 % and 40 % for the 8 M and 4 M HNO3 concentrations. The extracted cellulose was converted into carboxymethyl cellulose. The synthesis was done by a carboxymethylation reaction where the cellulose was reacted with different NaOH concentrations (m/v %) namely 20 %, 25 % and 30 %. The CMC yield (m/m %) %) was 120 %, 125 % and 140 %, respectively at the different NaOH concentrations (20 %, 25 % and 30 %). The higher concentration of NaOH facilitates greater carboxymethylation. The extracted cellulose, synthesised CMC and the commercial samples of cellulose and CMC were characterized using Fourier transform infra-red spectroscopy (FTIR), transmission electron microscopy (TEM), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) techniques. The FTIR spectrum of the commercial cellulose exhibited peaks at 3334 cm-1, this peak was characteristic of the –OH stretching vibration. Cellulose samples 1 and 2 showed peaks at 3306 cm-1 and 3331 cm-1, respectively for the similar vibration. The commercial CMC sample showed FTIR peaks at 1400 cm-1 and 1600 cm-1, which corresponded to the carboxymethyl substituent. The extracted CMC from sugarcane bagasse showed the similar peaks at 1439 and 1631 cm-1, respectively. The TEM and SEM images for all cellulose samples showed that the spherical shape of commercial and extracted cellulose were similar in length and width, the extracted cellulose samples appear to be longer in length compared to the commercial cellulose. The TEM results for all cellulose samples appear to be similar from the images. Both commercial and extracted CMC sample TEM images showed highly dispersed and crystalline particles that are consistent with those observed for carboxymethylated cellulose. The CMC particles observed appear to be dark spots that are spherical. SEM images for CMC samples showed a contrast to cellulose samples, the surface was smoother in appearance that correlated strongly with CMC SEM images observed in literature and the commercial sample. XRD diffraction patterns showed two significant peaks at 2Ɵ = 15° and 22.5° that confirmed the presence of cellulose I and cellulose II, respectively, in both the commercial and extracted cellulose samples. Both commercial and synthesised CMC samples had a single peak at approximately 2Ɵ = 20°, characteristic cellulose peaks are no longer visible on the diffractograms. The TGA scans showed that the cellulose sample degraded similarly to the commercial cellulose sample. The TGA scans of synthesised CMC and the commercial CMC samples showed similar degradation patterns. DSC scans also showed similar trends for the commercial and synthesised CMC samples. The DSC curves showed that all samples had two major peaks: a small peak for moisture loss between 50 °C - 90 °C and a more significant peak at approximately 350 °C due to decarboxylation and CO2 bond breakage. The degree of substitution (DS) for the commercial CMC sample was 0.420 and for the extracted CMC samples there was an increase in DS to 0.357, 0.366 and 0.420 which correlated with an increase in NaOH concentration (20 %, 25 % and 30 % (w/v)), respectively. Characterization for this study confirmed the successful delignification of sugarcane bagasse as confirmed by the similar properties of commercial cellulose. Furthermore, the carboxymethylation was successfully achieved at various NaOH concentrations. The study gave insight on how each of the parameters optimized affected the production of the bio-derived cellulose and CMCs. A comparision of the commercial cellulose and CMCs with the bio-derived cellulose and CMCs showed that they were successfully extracted and synthesised, respectively.