Preparation And Adsorption Properties Of Thermosensitive Poly (N-isopropylacrylamide) Composite Hydrogels

Currently,water pollution caused by organic phenolic pollutants is a formidable challenge in the field of water treatment.Adsorption method,among the many technologies developed to date,has received widespread attention due to its simple operation,low effective cost,and high recovery rate.Hydrogels are considered the optimal potential candidates for adsorption materials because of their high water absorption,porous network structures,abundant functional groups,and high stability under loading.However,the desorption after reaching adsorbent saturation usually requires the chemical reagents,which lead to secondary pollution and increase the cost.Thermosensitive poly（N-isopropylacrylamide）（PNIPAM）hydrogels not only can recover pollutants quickly and efficiently,but also will undergo volume phase transition under the influence of temperature,thereby promoting the desorption process after adsorption saturation.However,the poor mechanical properties of pure PNIPAM hydrogels limit application to a great extent.Currently,some methods for constructing PNIPAM hydrogels with high mechanical properties and functionalization response（e.g.introducing reinforcing materials,constructing multiple crosslinking networks）reduce the number of hydrogel active groups,and high crosslinking density will also decrease the swelling ratio of the hydrogel,thereby reducing its adsorption capacity.Therefore,using thermosensitive PNIPAM hydrogels as matrix materials,graphene oxide（GO）and MXene inorganic two-dimensional materials as the reinforcing materials.The structure and properties of PNIPAM composite hydrogels can be adjusted by different modifications of inorganic materials,and then thermosensitive PNIPAM composite hydrogels with high adsorption capacity,excellent mechanical properties,high chemical stability and good reusability were developed to achieve the research goal of efficient treatment of phenol-containing wastewater.The multifunctional adsorption of the hydrogel in phenols,dyes and metal ions wastewater will achieve by growing metal organic frames in situ on the pore walls of composite hydrogels to satisfy multiple interactions between the hydrogel and different adsorbent substances（including hydrogen bonding,π-π,and chelation interactions）.The mechanisms of composite hydrogels adsorption processes were analyzed via thermodynamics,isotherms and kinetic models.The main research contents are as follows:The double-modified GO/PNIPAM composite hydrogel was investigated.The silane coupling agent 2-cyanoethyltriethoxysilane（CTES）was grafted onto the GO surface through a solvothermalreaction,andthen single（6-tetraethylenepentamine-6-deoxy）-β-cyclodextrin（NH-β-CD）was further grafted onto the GO surface after oxidation of cyano group in order to obtain modified GO（GO-CTES-β-CD）.The zeta potential value of GO-CTES-β-CD was significantly decreased from-27.15 m V to-33.81 m V.A double-modified GO-CTES-β-CD/PNIPAM composite hydrogels was prepared by in-situ polymerization of MXene-CTES-β-CD incorporated into NIPAM monomer solution.As-prepared hydrogels showed excellent mechanical properties,swelling and water retention ratios,and high thermal stability under the dual action of chemical crosslinking and physical entanglements.Simultaneously,the introduction of NH-β-CD compensated for the active sites consumed by GO participation in the in-situ polymerization,which significantly improved the adsorption capacity of the composite hydrogel for p-nitrophenol（4-NP）.Among them,the GO-CTES-β-CD_15/PNIPAM composite hydrogel achieved an adsorption capacity of131.64 mg·g^-1for 4-NP at room temperature.After five repeated adsorption–desorption cycles,the hydrogel balanced and maintained at 74%of the initial 4-NP removal ratio.The adsorption results followed pseudo-first-order kinetics,which was regulated by a combination of surface adsorption and intra-particle diffusion mechanisms,and the adsorption process corresponding to the heterogeneous multilayer spontaneous adsorption based on hydrogen bonding.The PNIPAM hydrogels were enhanced with MXene,a two-dimensional material with stronger negative charges,to improve the adsorption capacity of the composite hydrogels.MXene nanosheets are vulnerable to oxidative degradation in moist environments.Therefore,similar to the research idea in the previous chapter,the silane coupling agent CTES was designed as a bridge,and NH-β-CD was used to encapsulate MXene to obtain MXene-CTES-CD.The encapsulated MXene exhibited high chemical/thermal stability.The as-prepared composite hydrogels displayed a three-dimensional porous structure,and the pore size and wall thickness increased as MXene was modified.When the content of MXene-CTES-CD increased to 15 mg,the MXene-CTES-CD₁₅/PNIPAM composite hydrogel showed a more uniform three-dimensional porous structure,and its rheological properties,compressive strength,chemical stability and swelling ratio were significantly higher than other hydrogels.Besides,the MXene-CTES-CD₁₅/PNIPAM composite hydrogel achieved an adsorption capacity of 162.03 mg·g^-1for 4-NP at room temperature.After five repeated adsorption–desorption cycles,the hydrogel maintained at 82%of the initial4-NP removal ratio.The adsorption results followed pseudo-second-order kinetics,which was dominated by a combination of chemical adsorption and intra-particle diffusion mechanisms,and the adsorption process corresponding to the heterogeneous multilayer spontaneous adsorption based on hydrogen bonding.Imidazolium salts,room-temperature ionic liquids（ILs）,were introduced on the surface of MXene by a one-step method to shorten the modification cycle of inorganic materials,whilst improving the chemical stability and the adsorption capacity for phenolic contaminants of MXene.The ILs bind to the surface of MXene flakes through electrostatic interactions,and the MXene modified with different ILs showed good dispersion,and high thermal/chemical stability.Meanwhile,the composite hydrogels prepared by different ILs-modified MXene demonstrated excellent mechanical properties,reusability,and high adsorption capacity.Particularly,the modification of MXene with 1-Ethyl-3-methylimidazolium tetrafluoroborate（EMIMBF4）was the optimal choice.The MXene-EMIMBF4 suspension had no significant changes even after 45 days of aging,and its composite hydrogel showed excellent mechanical properties,without damage even after ten repeated compression.The maximum adsorption capacity of the MXene-EMIMBF4/PNIPAM hydrogel for4-NP at room temperature was 200.29 mg·g^-1,and its maximum adsorption capacity maintained at 74%of the initial 4-NP removal ratio even after five repeated adsorption–desorption cycles.The adsorption results followed pseudo-second-order kinetics,and the entire adsorption process was guided by collaborative adsorption processes,including spontaneous boundary layer diffusion,intra-particle diffusion,or both simultaneously.4-NP was randomly adsorbed on the heterogeneous surface of the MXene-EMIMBF4/PNIPAM hydrogel,and the adsorption process corresponding to the heterogeneous multilayer spontaneous adsorption based on hydrogen bonding andπ-πinteractions.The ZIF-8 was designed to grow on the pore walls of the IL-MXene/PNIPAM composite hydrogel by in situ to satisfy multiple interactions between the hydrogel and different adsorbent substances（including hydrogen bonding,π-π,and chelation interactions）,thereby enabling the multifunctional adsorption of the hydrogel in phenols,dyes,and metal ions wastewater.In situ growth of ZIF-8 by soaking IL-MXene/PNIPAM composite hydrogel in Zn²⁺and 2-methylimidazole solutions sequentially,growing appropriate amount of ZIF-8 significantly increased the specific surface area of ZIF-8@IL-MXene/PNIPAM composite hydrogel and provided additional chelation sites.The ZIF-8@IL-MXene/PNIPAM composite hydrogels displayedexcellentmechanicalproperties.Particularly,the ZIF-8₂@IL-MXene/PNIPAM composite hydrogel returns to its original state after ten repeated compressions without damage,and exhibits the best compressive performance and microscopic morphology.Compared with the maximum capacity of the IL-MXene/PNIPAM composite hydrogels for 4-NP,crystal violet（CV）and copper ion(Cu²⁺),which were 200.23,187.63 and 47.12 mg·g^-1,respectively,the maximum adsorption capacities of ZIF-8₂@IL-MXene/PNIPAM were increased to 198.40,325.03 and 285.65 mg·g^-1,respectively.The maximum adsorption capacities of the composite hydrogels for 4-NP,CV,and Cu²⁺maintained at 79%,91%,and 29%of its initial value after five adsorption-desorption cycles.The adsorption results of CV and Cu²⁺onto the composite hydrogel followed pseudo-second-order kinetics,and the entire adsorption process was guided by collaborative adsorption processes,including spontaneous boundary layer diffusion,intra-particle diffusion,or both simultaneously.CV and Cu²⁺were randomly adsorbed on the heterogeneous surface of the ZIF-8₂@IL-MXene/PNIPAM composite hydrogel,and the adsorption process corresponding to the heterogeneous multilayer spontaneous adsorption based on hydrogen bonding,π-πand chelation interactions.