Preparation Of Temperature Sensitive Hydrogel And Its Application On Adsorption Of Copper Ions

As one kind of polymer material with three-dimensional network structure,hydrogels have been used as adsorbents for the adsorption of heavy metal ions in wastewater.The temperature sensitivity of poly(N-isopropylacrylamide)(PNIPAM)hydrogel has been employed to make the adsorption process stimuli-responsive under temperature control.By utilizing the volume shrinkage around the lower critical solution temperature(LCST),thermo-sensitive hydrogels can be used for water purification and accumulation of heavy metal ions.In order to ensure thermo-sensitive hydrogels can meet the requirements of practical application,comonomers with functional groups have been introduced into the polymerization to modify the polymer network of hydrogels.Through modification,the mechanical property and adsorption capacity of hydrogels have been improved.Furthermore,the modified hydrogels can be applied on selective adsorption or recognition of heavy metal ions,which expand the application prospect of thermo-sensitive hydrogels.In the theoretical aspect,synchrotron radiation small-angle X-ray scattering(SAXS)has been used to study the effect of temperature and heavy metal ions on the microstructure of hydrogels,and explain the mechanism of phase transition and adsorption behavior.Maleic acid(MA)was introduced into the PNIPAM hydrogel to prepare chemically crosslinked P(NIPAM-MA)hydrogel.The synthetic conditions(e.g.,monomercompositions,monomer concentrations,crosslinker concentrations,initiatoror concentrations and polymerization temperatures)had different influence on the structures and properties(e.g.,optical transmittances,swelling ratios,LCST,comperssion curves and Cu2+adsorption)of P(NIPAM-MA)hydrogel.To ensure the priority of temperature sensitivity and high adsorption capacity,the suitable synthetic conditions provided a reference for the subsequent formulation design of chemically crosslinked hydrogels.Under proper synthetic conditions,acrylamide(AM)was introduced into the copolymer network to synthesize pH-and temperature-sensitive P(NIPAM-AM-MA)hydrogel.P(NIPAM-AM-MA)hydrogel could meet the requirements for repeated use,therefore,it was used as an adsorbent to remove Cu2+ in the aqueous solution.The adsorption performance and reutilization of P(NIPAM-AM-MA)hydrogel were studied through the adsorption kinetic,adsorption isotherm and influencing factors(temperature and pH).The maximum amount of Cu2+ adsorption capacity was 24.4 mg/g dry hydrogel in the CuSO4 solution(500 mg/L)at 30 ?.P(NIPAM-AM-MA)hydrogel,which had reached the adsorption equilibrium,was immerged in deionized water at 50 ?.The temperature-dependent property made it possible to release almost 90%of absorbed water containing few heavy metal ions after the volume phase transition.The Cu2+-loaded hydrogel could be easily regenerated and reused almost without loss of adsorption capacity in the next three cycles of adsorption-desorption process.SAXS results showed that the volume phase transition increased the size of cross-linked domains and the fractal dimension,while the presence of Cu2+ had an opposite.To further improve the adsorption capacity and mechanical property,GO was introduced into the polymerization of NIPAM and 1-vinylimidazole(?).The Cu2+-imprinted GO/P(NIPAM-?)cryogel was synthesized by ion imprinting method and freezing polymerization.Compared with non-imprinted traditional hydrogel,GO/P(NIPAM-?)cryogel had faster deswelling rate,higher compression elastic modulus and higher Cu2+ adsorption capacity.In the mixed heavy metal ion solution containing Cu2+,Zn2+,Ni2+,and Pb2+,GO/P(NIPAM-?)cryogel exhibited an obvious selective adsorption of Cu2+ after volume phase transition.Effects of pH,initial concentration,adsorption time,and adsorption temperature on the Cu2+ adsorption of GO/P(NIPAM-?)cryogel were mainly discussed.The maximum amount of Cu2+ adsorption capacity reached 30.4 mg/g dry hydrogel in the CuSO4 solution(500 mg/L)at 30 ?.GO/P(NIPAM-?)cryogel could also be reused without loss of Cu2+ adsorption capacity in the three cycles of adsorption-desorption process.In addition to the chemically crosslinked hydrogels above,the thermo-,pH-,and metal ion-responsive P(NIPAM-MA-?)physical hydrogel was also synthesized.Effects of temperature,pH,monomer ratio,copolymer concentration,Cu2+ concentration and heavy metal ions on the phase transition and adsorption performance of P(NIPAM-MA-?)were studied.Upon heating,P(NIPAM-MA-?)solution with heavy metal ions transformed into gel and shrunken gel,heavy metal ions were adsorbed by the carboxylate and imidazole units of polymer network and aqueous solution was expelled out of the gel,which made it possible for P(NIPAM-MA-?)to be applied on water purification and recovery of heavy metal ions.Heavy metal ions had different effects on phase transition behavior,therefore,P(NIPAM-MA-?)could recognize the metal ion species from phase transition temperatures.XPS,SEM and SAXS were used to verify the proposed mechanisms of adsorption behavior and phase transition.