The valorization of industrial fishery waste through the hydrothermal-chemical transformation of chitosan
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 25oC and 50oC. The comparison of the results using these two temperatures indicated that the most favourable demineralization occurred after 6 h at 25oC and after 2 h at 50oC 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 25oC 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 60oC to 120oC at intervals of 20oC. Second, the reaction time was changed from 30 to 180 min at 100oC 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 100oC. As a result, after one hour at 110oC, 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 110oC. 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.