Table 3.

Pre-treatment processes of lignocellulosic fibre before nanocellulose isolation.

Pre-treatment method	Chemical/Enzyme/Solvents	Purpose	Advantageous	Drawback	References
Alkali	•Aqueous NaOH •KOH •Ca(OH)2 •NH4OH	•Promote depolymerization and lignin solubilization. •Hydrolysis of the highly branched hemicellulose content to expose cellulose and improve accessibility for subsequent downstream processes •Effectively removes the acetyl groups and uronic acid substrate from hemicellulose. The procedure stimulates solvation and saponification reactions, which cause the fibre to swell.	•Non-corrosive •Non-toxic •Easy to recover	•Does not entirely remove the lignin and hemicellulose constituents. •The use of a large amount of sodium hydroxide, which is the most effective and the involvement of the heating process increases the capital expenditure, treatment cost and potential environmental threat.	[[60], [61], [62], [63], [64]]
Enzymatic hydrolysis	•Celluclast •Cellic CTec3 •Cellic® CTec2	•Endocellulases catalyse the cleavage of internal bonds in the disordered domains of the cellulose chains, resulting in the formation of new chain ends and swelling of the cell wall, which aids in the fibrillation of the feedstock.	•More effective •Requires a lesser water usage •Generates no chemical residues	•Slower reaction rates than chemical treatments •Cost-to-benefit ratio needs to be conducted for each specific cellulose feedstock	[[65], [66], [67]]
Bleaching/delignification	•H2O2 •ClO2 •NaClO •NaClO2 •CH3CO3H •O3	•Bleaching is required to remove any remaining lignin if the alkali treatment is insufficient.	•Bleaching significantly improves the surface characteristics of purified cellulose fibres and is regarded as a critical pretreatment step.	•Bleaching lignocellulosic fibre with chlorine and chlorine derivatives results in the formation of highly toxic and hazardous pollutants. •The degradation of cellulose during ozone bleaching	[56,[68], [69], [70]]
TEMPO-mediated oxidation	•2, 2, 6, 6- tetramethylpiperidine – 1 - oxyl	•Used in addition to alkali and enzymatic pre-treatment for surface modification of nanocellulose. •Under mild conditions, TEMPO converts the primary hydroxyl groups (OH6) in cellulose's glucose units to carboxyl derivatives.	•The weakening of the inter fibril hydrogen during the reaction aid in lowering the energy cost of mechanical defibrillation during CNF's production. •Commercially available and stable nitroxyl radicals.	•Reaction pH should be strictly controlled. •Low uniformity of the final product.	[4,71,72]
Ionic liquids (IL) pretreatment	•1-ethyl-3-methylimidazolium acetate ([emim] [CH3COO]) •1-butyl-3-methylimidazolium chloride [bmim][Cl] •1-ethyl-3-methylimidazolium acetate [emim][CH3CO2]	•Ionic liquids are relatively new solvents made up of a mixture of large organic cations and small inorganic/organic anions that disrupt non-covalent interactions.	•Recovered cellulose at high purity and yield.	•Poor biodegradability •High production costs. •Toxic behaviour	[4,73,74]
Deep eutectic solvents (DES) pretreatment	•Choline chloride-lactic acid (ChCl-LA) •choline chloride-oxalic acid (ChCl-OA)	•Deep eutectic solvents (DESs) were developed as a new generation of ionic fluids. They are composed of at least two eutectic compounds linked together via hydrogen bonds to form low-cost eutectic mixtures with physical and chemical properties comparable to those of an ionic liquid.	•Save the energy consumption associated with the production of nanocellulose and increase the homogeneity of nanocellulose. •Renewable nature.	•Limited research for broad application •DES has a higher density and viscosity than other common solvents. These aspects are disadvantages of using DES in continuous flow reactions.	[74]
Organosolv	•The use of organic solvents or their aqueous mixture during delignification to obtain highly pure cellulose.	•Ethanol •Acetone •Ethylene glycol •Formic acid •Acetic acid	•Reusable solvents	•Expensive organic solvents. •Solvents are highly flammable and volatile.	[59,75]