Adsorption Properties Of NO₂^-by Modified Chitosan Resin And Its Application In The Fish Sauce

Because the environment such as surface water and soil accumulate a large amount ofNO2-and NO3-, also naturally occurring or artificial additives NO2-in food processing, lots ofNO2-exits in the raw materials and processing of the food. NO2-has a strong toxic effect tothe human being. It not only can cause methemoglobinemia, making direct poison and evendeath, but also can act as a precursor of the strong carcinogenic N-nitrosamines. Therefore,removal of NO2-from the food has become one of the most important issues to be solved inthe current food industry.Chitosan is the only one marine basic polysaccharide in nature, a natural, safe andnon-toxic cationic polymer compound produced by the deacetylation of chitin, having lots ofadvantages such as biodegradability, biocompatibility, and adsorption. Modified chitosanresin by crosslinking has been widely applied in the removal of heavy metals, the clarificationof fruit and vegetable juice, the purification of drug, the extraction of active substances and soon, due to its characteristics such as larger porosity, stronger mechanics, undissolve in water,dilute acid and alkali solution, etc.In this topic, two kinds of chitosan resins, including modified chitosan resin andnano-Fe2O3loaded chitosan resin, were prepared based on chitosan power by cross-linkingand loading modification. Their adsorption properties for NO2-from the water wereinvestigated, the structures were preliminary characterized and the adsorption properties ofthe resins for NO2-were deeply discussed from aspects of adsorption thermodynamics andkinetics. Moreover, the adsorption capabilities for NO2-in the fish sauce by nano-Fe2O3loaded chitosan resin was studied, which provide powerful theoretical basis and experimentsupport for further researching and applying of nano-Fe2O3loaded chitosan resin in the NO2-adsorption areas for liquid food. The results of the whole paper were as follows:(1) Modified chitosan resin (MCR) was prepared by cross-linking with glutaraldehydeand its structure was characterized. The adsorption of MCR for NO﹣2from aqueous solutionwas investigated at different pH and temperatures. Meanwhile, the adsorption properties of MCR for NO2-were deeply discussed from two aspects of adsorption thermodynamics andkinetics. The results showed that the thermal stability and crystalline of MCR had nochanging during the adsorption process and the active amino groups of chitosan participatedin the adsorption. The adsorption capacity of MCR for NO2-was saturated at acidic mediumand313～328K. The adsorption process of MCR for NO2-followed Freundlich andLangmuir isotherm models and appeared to be the monomolecular type, it was favorableadsorption. Meanwhile, the adsorption process followed pseudo-second order kinetics modeland controlled by both intraparticle diffusion and liquid membrane diffusion, and adsorptionrate depends on the former.(2) The adsorption capacity of different kinds of undissolved power materials includingmetal hydroxides, hydroxides, metal oxides and metal simple substance for NO2-wasinvestigated. The results showed that metal hydroxides and hydroxides had no adsorptioncapacity for NO2-and the others had obviously adsorption capacity for NO2-. Considered onthe stability and adsorption capacity, the ideal adsorbent was nano-Fe2O3.(3) Nano-Fe2O3loaded chitosan resin (NFCR) was prepared to be investigated theadsorption property for NO﹣2from aqueous solution. The adsorption conditions wereoptimized and the adsorption properties of NFCR for NO2-were deeply discussed from twoaspects of adsorption thermodynamics and kinetics. The results showed that the adsorptioncapacity of NFCR for NO2-was saturated at acidic environment and higher than308K. Theadsorption process of MCR for NO2-followed Freundlich and Langmuir isotherm models andappeared to be the monomolecular type, it was favorable adsorption. Meanwhile, theadsorption process followed pseudo-second order kinetics model and it was mainly controlledby intraparticle diffusion. The thermal stability of NFCR had no changing during theadsorption process and the adsorption capacity of NFCR for NO2-was selective adsorptionand reusability.(4) NFCR was applied to remove NO2-from the fish sauce. The optimum adsorptionconditions and the changing of the fish sauce quality were investigated. The results showedthat the optimized adsorption conditions were pH4～6,308K,8h and material ratio1:200.The physical and chemical properties and the contents of major elements, organic acids andamino acids of the fish sauce had no significantely changing during the adsorption process. But the color and the contents of trace elements and harmful metal elements reduced. And theclarity of the fish sauce improved after the adsorption treatment.