Preparation Of Chitosan-based Composite Hydrogels And Study Of Its Adsorption Performance On Heavy Metal Ions

Heavy metal ions are hard to degrade,highly toxic and carcinogenic,etc.They will cause great harm to the ecosystem,human health and life safety if being direct discharged into rivers and lakes.Therefore,the study of heavy metal wastewater treatment has important research significance.In recent years,hydrogels,as a kind of hydrophilic but insoluble three-dimensional network porous polymers,have been favored by researchers for their excellent performance in the removal of heavy metal ions due to their simple preparation,low cost and high adsorption efficiency.In this paper,two environmentally friendly CISP and CIAP chitosan-based composite hydrogels,which can be recycled multiple times and have good mechanical properties,were prepared by the synergistic method of free radical polymerization-freeze-thaw cycle and applied to the removal of Cu²⁺and Cd²⁺.The main research contents and results are as follows:（1）Two chitosan-based composite hydrogels,CISP and CIAP,were prepared by a free radical polymerization-freeze-thaw cycle synergistic method using IA and SMAS（or AM）as modified grafting agents for CS,followed by PVA as the physical crosslinking backbone.The results showed that the removal efficiencies of both CISP and CIAP for Cu²⁺and Cd²⁺were optimal when n（IA）:n（SMAS）=2:1 or n（IA）:n（AM）=2:1.（2）The structures of CISP and CIAP composite hydrogels were characterized by FTIR,XRD,and XPS.SEM and EDS characterization showed that both CISP and CIAP have three-dimensional porous network structures.TGA characterization showed that both CISP and CIAP have good thermal stability.SEM,EDS,FTIR,and XPS characterization indicated that CISP and CIAP had good adsorption effects on Cu²⁺and Cd²⁺.（3）The results of the swelling properties of CISP and CIAP showed that the water absorption and swelling rates of CISP and CIAP were 339.47%and 225%,respectively.And the swelling mechanisms of both composite hydrogels were in accordance with the Schott secondary kinetic model.（4）Both CISP and CIAP had good removal effects on Cu²⁺and Cd²⁺.The removal rates of Cu²⁺and Cd²⁺by both composite hydrogels could reach more than 90%,when operated for24 h,at 25°C,the dosage of 3 g/L and p H 3-5.The equilibrium adsorption time of CIAP was smaller than that of CISP,but the saturation adsorption amount was larger.Increasing the concentration of heavy metal ions in the system and the introduction of interfering ions both affected the adsorption of Cu²⁺and Cd²⁺by CISP and CIAP.（5）The adsorption processes of both CISP and CIAP were consistent with the pseudo-secondary kinetic model and the Langmuir isothermal adsorption model,and the adsorption processes are spontaneous,irreversible,exothermic,and dominated by the chemisorption of a single molecular layer.In addition,the fitted parameters of the Langmuir model indicated that the maximum adsorption capacity of CIAP for Cd²⁺was higher than that of CISP,and as for Cu²⁺,CISP was higher,which depended on the ionic radius,electronegativity and the type of reactive groups of Cu²⁺and Cd²⁺.（6）The results of adsorption-desorption experiments showed that both CISP and CIAP had good regeneration after seven cycles of adsorption-desorption,but the regeneration performance of CIAP was better than that of CISP,which was attributed to the more ordered three-dimensional network pore structure and the nature of active adsorption sites of CIAP.After seven cycles of recycling,both chitosan-based composite hydrogels were able to maintain their original state without being broken,indicating that both have good mechanical properties.In addition,they showed superior removal effects in the adsorption of Cu²⁺and Cd²⁺compared to the maximum adsorption capacity fitted by the Langmuir model of other adsorbents reported in the literature.Therefore,both CISP and CIAP composite hydrogels will have broad development prospects in the direction of heavy metal ion removal from wastewater.