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This study focuses on the isolation, chemical and crystalline structures, and the kinetics of heterogeneous deacetylation of chitin from mud crab shells ( Scylla serrata). The independent variables in this work are temperature, NaOH concentration, and reaction time. The degree of deacetylation (DD) of the resulting chitosan was determined for different reaction times. It was found that the DD increases nonlinearly with the reaction time. The deacetylation of the chitin followed pseudo first-order kinetics and it occurred in two stages, which are respectively attributed to the reaction of acetamide groups in the amorphous region on the external layer, and in the crystalline region inside the chitin particles. The rate constants for the latter stage of the deacetylation at 35°C, 75°C, and 105°C were 4.67×10−4 min−1, 3.00×10−3 min−1, and 4.61×10−3 min−1, respectively. Based on this temperature-dependent rate constant, the activation energy of the slow deacetylation stage was estimated to be 32.6 kJ mol−1.

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In this study, adsorption characteristics of a negatively charged dye, Acid Blue 25 (AB25), on pomelo pith (PP) was studied by varying the adsorption parameters, with the aim of evaluating the adsorption mechanism and establishing the role of hydrogen bonding interactions of AB25 on agricultural wastes. The kinetics, intraparticle diffusion, mechanism, and thermodynamics of the AB25 adsorption were systematically evaluated and analyzed by pseudo-first-order and pseudo-second-order kinetic models, the Weber–Morris intraparticle and Boyd mass transfer models, the Langmuir, Freundlich, Dubinin–Radushkevich, and Temkin isotherm models, and the Van’t Hoff equation. It was found that AB25 adsorption followed pseudo-second-order kinetics, governed by a two-step pore-volume intraparticle diffusion of external mass transfer of AB25 onto the PP surface. The adsorption process occurred spontaneously. The adsorption mechanism could be explained by the Langmuir isotherm model, and the maximum adsorption capacity was estimated to be 26.9 mg g−1, which is comparable to many reported adsorbents derived from agricultural wastes. Changes in the vibrational spectra of the adsorbent before and after dye adsorption suggested that AB25 molecules are bound to the PP surface via electrostatic and hydrogen bonding interactions. The results demonstrated that the use of pomelo pith, similar to other agricultural wastes, would provide a basis to design a simple energy-saving, sustainable, and cost-effective approach to remove negatively charged synthetic dyes from wastewater.

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