Effect Of Microwave On Extraction And Depolymerization Of Chitin And Chitosan From Crayfish Shells

Crayfish is an important economic breeding species in the aquatic industry of China.The processing or consumption of crayfish produces lots of shell waste containing a large amount of chitin.Chitin and chitosan,derivative of chitin,are widely used in food,biology,materials and other fields.The separation and extraction of chitin from crayfish shells is the first step of its application.Due to the traits of high efficiency,energy saving,and environmental protection,microwaves have been widely used in the extraction,purification and depolymerization of biomass.Taking these into consideration,the effects of microwaves on the extraction of chitin from crayfish shells were investigated in this study,and water bath heating with the same heating rate as microwave heating were taken as a control.The effects of further preparation of chitosan was also explored.Then the mechanism of the difference of reactions caused by microwave mentioned above was excavated.Furthermore,the depolymerization of chitin and chitosan by the synergy between microwave irradiation and graphene oxide(GO)was further studied,providing an environmentally friendly way for the depolymerization of nitrogencontaining biomass.The main research contents and results are as follows:(1)Effects of microwaves on the extraction of chitin from crayfish shellsThe dielectric properties of the chitin extraction reaction systems were detected.It was found that the reaction systems exhibited a high dielectric response and were suitable for microwave heating.The decalcification and deproteinization reaction systems had loss tangents of 4.06 and 3.90 at 2450 MHz,so they can absorb microwave energy and convert it into heat effectively.The heating rates of microwave treatment were controlled the same as that of water bath heating,and the effects of them to the quality of chitin extracted at different time were studied.The results showed that the extraction of chitin assisted by microwave heating had higher decalcification rate(88.85%)and deproteinization rate(82.96%).Scanning electron microscope(SEM)results showed that the surface fiber structure of chitin extracted by microwave-assisted was more dense than that of water bath heating,indicating that microwave heating can promote the reaction rate of the chitin extraction process effectively.(2)Effects of microwaves on the preparation of chitosan from chitinTo further prepare chitosan from chitin extracted by microwave,the effects of microwave heating and water bath heating on the quality of chitosan at the same heating rate was studied.After 10 min of the deacetylation reaction,the deacetylation degree(DD)of chitosan prepared by microwave(MCS)was 59.34%,which was much higher than that of chitosan prepared by water bath(WCS)(38.08%).The SEM images showed more pores and fiber breakage of MCS than WCS.After 60 min,the microwave-assisted deacetylation reaction basically completed,and MCS always had lower molecular weight and viscosity than WCS.At 240 min,the DD of MCS and WCS were close.The X-ray diffraction patterns showed that the crystallinity of MCS(28.74%)was slightly higher than WCS(27.45%),indicating that microwave heating can not only accelerate the initial reaction rate,but also promote the overall uniformity and thoroughness of deacetylation reaction.(3)Studies on the depolymerization of chitin and chitosan by the synergy between microwave and GOChitin and chitosan extracted by microwave heating were chosen as the experimental materials.The depolymerization effects of solid catalyst GO and graphene(GR)on chitin and chitosan under microwave field were investigated.The results showed that the catalysts GO and GR could improve the microwave absorption properties of the reaction systems to varying degrees,and significantly increased the heating rates.The depolymerization degree of chitosan was higher than that of chitin.The total organic carbon contents in the depolymerization products of chitin and chitosan catalyzed by GO were 77.55 mg/L and 694.70 mg/L,respectively.There was no reducing sugar in the depolymerization product without catalyst or with GR as the catalyst.The total reducing sugar content in the reaction of the depolymerization of chitosan catalyzed by GO was 37.22 mg/L,indicating that GO could synergize microwave to catalyze the depolymerization of chitosan to produce monosaccharide.The difference in catalytic capabilities between GO and GR indicated that oxygen-containing functional groups played a key role in the breakage of molecular chains.In addition,the difference in the degree of depolymerization between chitin and chitosan could be attributed to the stronger intramolecular and intermolecular hydrogen bonding of chitin than chitosan.Therefore,the weakening of hydrogen bonds by decrystallization is of great significance to the progress of the depolymerization reaction.