Environmentally friendly pretreatment of chitin using relatively low concentration KOH/urea mixture for enhanced nanofiber preparation

Traditional methods to prepare chitin nanofibers require circular chemical treatment to further purify the chitin from the seller and the morphology of the prepared nanofibers is commonly rod-like one. This study introduces an approach utilizing low-temperature freeze pretreatment of chitin in a relatively low concentration KOH/urea mixture. The crystallinity of β-chitin reduced as the concentration of KOH increased in the pretreatment solvent. The concentration of urea had little effect on the crystallinity of chitin. The pretreated chitin was then oxidized in a 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO)/NaBr/NaClO system to prepare nanofibers. The morphologies of the nanofibers can be regulated by oxidizing the chitin pretreated with different concentration of KOH. The nanofibers obtained by oxidizing chitin pretreated by 3 wt% KOH/ 1 wt% urea mixture showed rod-like morphologies with length about 150 nm. The carboxyl content of nanofibers prepared with KOH/urea mixture freeze treated chitin increased compared to that of nanofibers prepared with solely freeze treated chitin. In addition, the pretreatment solvent can be used to treat α-chitin and can be reused. The freeze KOH/urea mixture pretreatment methods can be extended to treat other biomasses for nanofiberization.