Efficient extraction and purification of cellulose from almond shell powder using natural deep eutectic solvents

Introduction

One potential use of cellulose, abundant in lignocellulosic residues such as almond shells, is for bioplastic manufacturing. Natural deep eutectic solvents (NaDESs), non-toxic solvents, could be used to develop more sustainable methods for obtaining this polysaccharide.

Methods

The objective of this study was to purify the almond shell powder (<0.5 mm) (ASP) into cellulose using an innovative process with a NaDES (chlorine chloride/urea 1:2) (4 h, 100 °C), followed by two NaClO₂ 1.3% (2 h, 75 °C) bleaching treatments and one H₂O₂ 6% (1.5 h, 60 °C) bleaching treatment, to obtain enriched cellulose concentrates. Two control experiments were conducted: one using NaOH instead of NaDES as a conventional method and another applying only the bleaching treatments (blank). The ASP and the concentrates obtained were characterised in terms of composition (cellulose and lignin), colour (white index, WI), and FTIR spectra.

Results

The initial composition of ASP consisted of 19% cellulose and 33% lignin. The NaDES treatment yielded the highest cellulose (45%, p<0.05) and lowest lignin content (7%, p<0.05). Control experiments showed NaDES was more effective in delignification than NaOH (38% cellulose, 10% lignin) and underscored the necessity of the 4 h treatment as the blank showed the lowest cellulose (28%, p<0.05) and highest lignin content (13%, p<0.05). The WI was higher (p<0.05) for NaDES (88%) compared to NaOH (70%) and the blank (67%). Additionally, the FTIR spectra for the NaDES and NaOH experiments showed the fading of the 1700 cm^-1 peak corresponding to C=O bonds between lignin and hemicellulose.

Conclusions

The innovative use of NaDESs could represent an effective method for extracting and purifying cellulose from almond shell powder, offering a valuable approach for lignocellulosic waste materials upcycling.