Production of Chitosan from Crab Shells Using an Aqueous Extract of Wood Ash for the Deacetylation of Chitin: An Innovative, Eco-friendly, and Low-cost Method

Abstract

The importance of chitosan has grown significantly over the last years due to its medicinal, agricultural, and industrial applications, as well as the biodegradability properties. The present study aimed to describe for the first time, the preparation of chitosan from chitin using wood ash aqueous extract as an alkaline medium. Chitin was obtained from crab shells in a microbial fermentation process using fish viscera for protease. Pilled and milled sweet potatoes and cassava water extracts were used separately in the deproteinization and demineralization steps as sources of carbon. The obtained chitin samples were then converted into the respective chitosan samples (B-C2 and M-B2) by an innovative, low-cost, and eco-friendly method using an aqueous extract (pH=12.4) prepared from wood ash, instead of the chemical process using sodium hydroxide solution in the deacetylation of chitin. The yield of the obtained chitosan was found to be 11.5% for B-C2 and 9.4% for M-C2. The presence of chitosan was determined by the analysis of the FTIR spectrum of both samples. The degree of acetylation was found to be 77.3% and 83.2% for B-C2 and M-B2, respectively, based on the IR analysis of absorption bands at 3456 and 1636 cm-1, and 75.4% and 84.1% based on the potentiometric titration. The average molecular weight (192.1 kDa for B-C2 and 194.3 kDa for M-B2) was determined using the viscometer method. The solubility of B-C2 and M-B2 in 1% acetic acid was found to be 77.8% and 63.9%, respectively. The analysis of SEM photographs of B-C2 showed an amorphous morphology while M-B2 showed a crystalline morphology. The TGA and DSC curves obtained in a dynamic air atmosphere showed three degradation stages. In the main event (the second stage), an exothermic peak was observed at 355°C for B-C2 coupled with the weigh-loss of 7%, while for M-C2 was observed at 365°C coupled with weight-loss of 10%. Due to their high degree of deacetylation, low molecular weight, and viscosity, both chitosan samples could be utilized unambiguously in the agri-food field as a biofertilizer or as a bio-adsorbent in the wastewater treatment.