Extraction, characterization, and kinetics of N-deacetylation of chitin obtained from mud crab shells

Abstract

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.

Mechanism of Heterogeneous Alkaline Deacetylation of Chitin: A Review

“…Data published in the scientific literature [ 3 , 7 ] and our own experimental data [ 44 , 47 , 62 , 63 ] show that α-chitin obtained from various raw materials (crab shells, shrimp, Antarctic krill, as well as the internal organs of crustaceans, for example, the gills of the king crab [ 64 ]), is almost identical both in its chemical composition and in physicochemical properties. A similar conclusion was made in [ 65 ].…”

“…It has been suggested that the presence on the kinetic curve ( Figure 3 ) of areas of fast and slow deacetylation is explained by the presence of amorphous and crystalline [ 80 ] regions in chitin. This hypothesis was put forward in [ 74 ] and continues to be used in various works [ 59 , 65 , 72 ]. It is assumed that in the amorphous regions, the deacetylation reaction proceeds rapidly (the first section of the kinetic curve), while in the crystalline regions, the reaction rate is lower due to the slow diffusion of the reacting particles (alkali molecules or hydroxide ions) to the chitin molecules.…”

“…This is primarily due to the excess of one of the reactants of the reaction, namely, alkali. In the literature, the deacetylation of the acetamide groups is considered to be a first-order process [ 43 , 58 , 65 , 72 ]. The slow deacetylation rate is the same order as those reported for the deacetylation of chitin derived from mud crab shells [ 65 ], shrimp shells [ 56 ], and squid [ 73 ], and is in the range (1.6–5.9) × 10 −3 min −1 .…”

“…In the literature, the deacetylation of the acetamide groups is considered to be a first-order process [ 43 , 58 , 65 , 72 ]. The slow deacetylation rate is the same order as those reported for the deacetylation of chitin derived from mud crab shells [ 65 ], shrimp shells [ 56 ], and squid [ 73 ], and is in the range (1.6–5.9) × 10 −3 min −1 . The estimated activation energy [ 59 , 60 , 65 ] for heterogeneous deacetylation of chitin derived from different crustacean shells is in the range (16.2–56.0 kJ mol −1 ).…”

The Use of Chitin from the Molts of Mealworm (Tenebrio molitor) for the Removal of Anionic and Cationic Dyes from Aqueous Solutions

“…The two discussed peaks suggest that CHSC has the crystalline form of α-chitin, which is typical of marine crustaceans. Contrary to the crab chitin spectrum, the mealworm chitin spectrum in the 1700–1600 cm −1 range has only one distinct peak at 1626 cm −1 indicating a hydrogen bond between the carbonyl groups of amide I and amide II of the same chain [ 21 , 22 ]. This may suggest that a significant part of the chitin coming from the mealworms has the form of β-chitin [ 23 ], typical of the highly elastic structures.…”