Preparation Of In-situ Doped Nano-Calcium Carbonate Chitin Hydrogel And Its Adsorption Performance For Heavy Metal Ions

As a natural polysaccharide,chitin has a wide range of sources,and it usually exists in the shells of shrimps and crabs,the epidermis of insects and the bones of some mollusks.And chitin has many excellent properties,such as easy biodegradation,non-toxic,low price,high viscosity and strong permeability.The hydrogel is a kind of high molecular polymer with highly hydrophilic groups in the molecule,which has strong water retention capacity.Through research using industrial-grade chitin as the raw material and using in-situ synthesis to simulate the shell structure,nano-calcium carbonate is doped into the chitin gel to obtain the advantages of both natural and synthetic polymers.The composite material makes the gel play a better comprehensive performance.Chitin gels use the abundant hydrophilic functional groups（hydroxy-and amino-）to make them a highly absorbent hydrogel system.The preparation process does not require the introduction of hydrophilic groups,the process is simplified,and it can also be used for biology.Degradation avoids pollution problems.Chitin hydrogel has the advantages of green adsorption,treatment and large adsorption capacity,which provides a certain theoretical basis for chitin as a functional material for water treatment carriers.Based on the above considerations,the content of this paper mainly includes the following four aspects:1.Preparation of chitin hydrogel and CaCO₃/chitin hydrogel.In order to improve the insolubility of chitin in acid and alkali at room temperature,the formed gel has poor mechanical properties,so that the chitin has better solubility and the formed gel has stronger mechanical properties.Firstly,improve the solvent system of alkali urea（11wt%Na OH/4wt%urea）,using a new type of solvent（28wt%Na₂CO₃/20wt%Ca（OH）₂/4wt%urea）at ultra-low temperature（-40℃）.Then use the principle of displacement reaction in the new solvent to generate nanometer calcium carbonate in situ,use the crosslinking agent epichlorohydrin to chemically crosslink it,and physically crosslink it under the condition of 20wt%absolute ethanol.Use scanning electron microscopy,infrared spectroscopy,Raman spectroscopy,BET,mechanical property testing and other means to characterize and analyze materials.The characterization results showed that nano-calcium carbonate and chitin were successfully compounded,and the material performance after compounding was significantly better than that of chitin hydrogel,which was beneficial to the application of adsorption.2.Adsorption of Cu²⁺,Cd²⁺and Ni²⁺on chitin hydrogel and CaCO₃/chitin hydrogel.The adsorption of the two materials under different factors（initial concentration,amount of adsorbent,p H,temperature）was studied.The experimental results show that the saturated adsorption capacity of the two materials is affected by four factors.The nonlinear fitting kinetic correlation coefficients showed that the adsorption process followed the Langmuir model,and the adsorption process was dominated by monolayer adsorption.3.Adsorption of Cu-Cd,Cu-Ni and Cd-Ni by chitin hydrogel and CaCO₃/chitin hydrogel.The influence of different environmental factors（p H,temperature,ion strength）on different ionic systems was studied.The results show that under the influence of different environmental factors,different ion adsorption order has a certain difference,there is competitive adsorption under double ion extraction,adsorption capacity also has a certain difference.The kinetic studies show that the competitive adsorption conforms to the extended Langmuir model in the dual-ion system following the second-order kinetic model.4.Chitin hydrogel and CaCO₃/chitin hydrogel adsorption on multi-metal Cu-Cd-Ni system combined with density functional theory was studied.The influence of different environmental factors on the multi-ion system was studied.The adsorption order of several ions was Cu²⁺>Cd²⁺>Ni²⁺.In order to further study the adsorption mechanism,the nature of adsorption of copper,cadmium and nickel ions by chitin hydrogel was revealed from the molecular and microscopic scale by combining experiment and density functional theory calculation.The chitin hydrogel functional monomer model was established,and the structure of chitin hydrogel functional monomer and adsorbed Copper,cadmium and nickel ions were optimized by density functional method at B3LYP/6-311g（D,P）level.By surface electrostatic potential analysis,it was found that acetyl,methyl hydroxyl,hydroxyl and amino were the adsorption active sites in the functional monomer model of chitin hydrogel.However,by charge decomposition analysis,molecular orbital analysis and interaction energy comparison,chitin hydrogel functional monomer adsorption copper ion is more stable.This is consistent with the results of adsorption experiment that chitin hydrogel has a greater adsorption capacity of copper ions.