Exploring totally chlorine-free bleaching methods to produce sugar beet pulp cellulose nanofibers

A simple, one-pot process can be applied to sugar beet pulp (SBP), a low-lignin secondary biomass material, to produce cellulose nanofibers (CNF). However, this process uses sodium chlorite (SC) for a key bleaching step. This work explores both the replacement of SC with the environmentally friendly peracetic acid (PAA), both exogenous and in situ generated, and how the choice of peroxyl or free-radical bleaching mechanism affects this reaction. It was found that PAA-bleached CNFs (PAA-CNF) were very close to SC-bleached CNF (SC-CNF) in terms of colour (~67% relative brightness index), and that whilst the peroxyl mechanism brighten the pulp, free-radical bleaching darkened the pulp. PAA and Fenton-oxidised CNFs were more enriched in lignin due to loss of matrix polysaccharides than SC-oxidised CNF (9, 10 and 4%) but were in turn more viscous (4500, 2500 and 2000 cP). The CNFs all showed stability incipiently, with zeta potentials from − 17 to − 23 mV, and increased the aspect ratio of the fibrils in comparison to the unoxidised CNF. Moreover, the in situ generation of PAA-CNFs was of similar quality to CNF bleached by exogenous PAA. In this study, we demonstrate three successful alternatives for replacing SC in our CNF production method. After PAA oxidation the lignin remains, yet the resultant CNF suspensions are more viscous and equal in brightness to SC-CNF. Overall, this work shows how we can innovate an entirely chlorine-free, enzyme-mediated production processing that represents an advance for the safety and reduction of toxicity of the process, and furthers the development of upcycling of secondary biomass into sustainable alternatives for biomaterials.