Valorisation des résidus industriels de pêches pour la transformation de chitosane par technique hydrothermo-chimique The valorization of industrial fishery waste through the hydrothermal-chemical transformation of chitosan

Résumé

Le chitosane est une substance biodégradable d’origine naturelle obtenue par la désacétylation de la chitine, qui se trouve dans l’exosquelette des crustacés. Une de leurs applications plus récentes est le traitement des eaux. Pour favoriser l’utilisation du chitosane des exosquelettes de crustacés (crevettes, crabes, etc.) dans le traitement des eaux usées, les procédés de transformation du chitosane ont été simplifiés et optimisés. Dans la présente étude, les crustacés de crevette grise (Palaemodiae) ont été utilisés pour extraire le chitosane. Les expériences ont porté sur les effets des interactions de la concentration de la solution alcaline, du temps et de la température de réaction sur le degré de désacétylation. En plus de simplifier le processus de transformation de trois à deux étapes, les résultats montrent l’influence de la concentration d’hydroxyde de sodium (7,5‑ 12,5 M), du temps de réaction (30 ‑ 180 min) et de la température (80 ‑ 120 ^oC) sur le degré de désacétylation. La méthode de la spectroscopie infrarouge en film mince a été utilisée pour analyser le degré de désacétylation du chitosane. Un degré de désacétylation de 90 % a été atteint avec une concentration de la solution alcaline de 12,5 M, un temps de réaction de 120 min et une température de 110 ^oC. Par ailleurs, en réduisant le nombre d’étapes de transformation et la consommation de réactifs, le procédé développé est plus économique et a une meilleure performance environnementale.

Mots clés:

• Exosquelettes/carapaces de crevettes,
• Chitosane,
• N-désacétylation,
• Chitine,
• Hydrothermo-chimique,
• Valorisation

Summary

Chitosan is a natural biodegradable biopolymer produced from chitin, a polysaccharide derived from the shells of shrimp, crab and lobster. The development of commercial applications of chitin and chitosan in different fields such as biomedicine, nutrition, food processing, agriculture, cosmetics, and wastewater treatment has rapidly expanded in recent years. This paper investigates the hydrothermal production of chitosan from the carapace of gray shrimp (Palaemodiae) for use as a coagulant in wastewater treatment.

To obtain chitosan from shrimp exoskeletons, they were treated following two different steps. The first step was the demineralization of the shrimp’s exoskeleton, where calcium was removed using dilute HCl. The second deacetylation step completely dissolved the shrimp exoskeleton using a NaOH solution. In both steps, the ratio between solution digestive and exoskeleton was studied (1:10 (w:v)). For the demineralization process, the concentration of HCl was varied from 0.5 to 3.5 M in 0.5 M intervals at constant temperatures of 25^oC and 50^oC. The comparison of the results using these two temperatures indicated that the most favourable demineralization occurred after 6 h at 25^oC and after 2 h at 50^oC at a HCl concentration of 2 M. In this case, it was not necessary to use a HCl concentration greater than 2 M due to the fixed reaction time. When the HCl solution was heated, the reaction time of the demineralization process was reduced by a factor of three compared to that when room temperature HCl was used under the same conditions. Moreover, this reaction followed a pseudo-second-order equation with approximate rate constant of 2.38 L g^‑1 min^‑1 at 25^oC in 1.5 M HCl.

The effectiveness of the transformation to chitosan depends on the interaction among the sodium hydroxide concentration, the reaction time and the temperature at which the deacetylation process occurs. The influence of the concentration of the alkaline solution, the reaction temperature, and the reaction time on the degree of deacetylation (DD) was investigated. The DD obtained was quantitatively analyzed by thin film infrared spectroscopy (IR). Film thickness was measured by using a micrometer with the smallest possible unit measurement count of 0.01 mm. First, exoskeletons were exposed to NaOH concentrations of 7.5 M, 10 M and 12.5 M for 60 min at various temperatures ranging from 60^oC to 120^oC at intervals of 20^oC. Second, the reaction time was changed from 30 to 180 min at 100^oC at 30 min intervals using the same NaOH concentrations previously mentioned. The results show that the factors that influence the DD values were the reaction temperature and the concentration of NaOH. However, based on these experiments, the concentration of NaOH influences the DD values the most. The DD values of chitosan production began to reach a constant level when the reaction temperature was greater than 100^oC. As a result, after one hour at 110^oC, chitosan production was obtained with different DD values of 60%, 67%, and 78% at NaOH concentrations of 10 M, 11.25 M and 12.5 M respectively. Nevertheless, chitosan could not be formed at 7.5 M NaOH, even though the reaction time was 3 h. Furthermore, when the reaction time was longer than 120 min, the DD values of chitosan increased slowly.

Therefore, the optimum conditions required for the deacetylation production of chitosan from gray shrimp carapace (DD of 90%), to be applied in wastewater treatment, are as follows: the exoskeleton should be exposed to a solution of 12.5 M NaOH (45%) for 2 h at 110^oC. This product is generally termed chitosan when it has greater than 65% of the acetylic groups removed.

By diminishing the number of steps and reducing the chemical reagents needed, this study demonstrates the economical and environmental advantages of using chitosan as a coagulant to treat wastewater.

Key words:

• Shrimp exoskeleton,
• Carapace,
• Chitosan,
• N-deacetylation,
• Chitin,
• Hydrothermal-chemistry,
• Valorization