Valorization of crab shell waste into chitin, chitosan, and carboxymethyl chitosan: Extraction, chemical modification, and characterization

Crustacean exoskeleton waste derived from blue swimming crab ( Portunus pelagicus ) in Bangladesh represents an abundant yet underutilized bioresource. This study investigates the conversion of this waste into chitin, chitosan, and carboxymethyl chitosan (CMCh) using established acid-alkali extraction and chemical modification protocols, including demineralization, deproteinization, alkaline deacetylation, and carboxymethylation. Chitin was obtained at a yield of 16.4 ± 0.6 % relative to dried shell biomass, while chitosan and CMCh yields were 80.0 ± 2.1 % and 57.3 ± 1.8 %, respectively, calculated on a stepwise basis relative to their immediate precursors (n = 3). Physicochemical characterization indicated effective mineral removal, with moisture contents ranging from 3.36 ± 0.18 % to 9.61 ± 0.42 % and ash contents from 0.71 ± 0.05 % to 0.88 ± 0.06 %. FTIR spectroscopy confirmed the characteristic chitin structure as well as successful deacetylation and carboxymethylation, yielding a high degree of deacetylation for chitosan (DDA = 79.57 ± 1.12 %, FTIR-based) and a moderate degree of substitution for CMCh (DS = 0.40 ± 0.03, titrimetric). These structural modifications resulted in a substantial increase in solubility in 1 % acetic acid, from 7.50 ± 0.41 % for chitin to 93.23 ± 1.27 % for chitosan and 96.81 ± 0.98 % for CMCh. Viscosity-average molecular weights of (2.48 ± 0.17) × 10⁵ g mol⁻¹ for chitosan and (1.92 ± 0.14) × 10⁵ g mol⁻¹ for CMCh were obtained from intrinsic viscosity measurements. Although the processing routes employed are conventional and no life-cycle assessment was conducted, the study provides a coherent physicochemical dataset for chitin and its derivatives derived from Portunus pelagicus shell waste sourced in Bangladesh, supporting its potential as a local feedstock for functional biopolymer production within conventional processing frameworks.