Preparation Of Functionalized Cellulose-based Aerogel Microspheres And Their Cr(Ⅵ) Adsorption Performance Study

Compared with Cr（Ⅲ）,Cr（Ⅵ）threatens the ecosystem and human health due to its high mobility and toxicity.Thus,the reduction-sequestration of Cr（Ⅵ）is of great significance for protecting the ecological environment system.Chemical reduction is a commonly used method to remove Cr（Ⅵ）in industry,but it has many drawbacks such as high energy consumption and large sludge production.Adsorption has great application prospects in the field of Cr（Ⅵ）removal because of its operation flexibility and high removal efficiency.However,the development of efficient,eco-friendly,and cost-effective adsorbents is a key challenge of this method.In this thesis,cellulose was used as the starting material for surface modification before aerogel preparation.Cellulose-based aerogel microspheres were designed with the assistance of liquid nitrogen freeze-forming and freeze-drying techniques and their Cr（Ⅵ）adsorption performance was investigated.The detailed research work was as follows:By grafting hyperbranched polyethyleneimine（PEI）on cellulose nanofibers（CNF）using（3-glycidyloxypropyl）trimethoxysilane（GPTMS）as the crosslinker,aminated cellulose based aerogel microspheres（CGP）were synthesized.Scanning electron microscope,Fourier transform infrared spectrometer,and Electronic universal testing machine were adopted to evaluate the morphology,functional groups,and mechanical strength of CGP.The Cr（Ⅵ）adsorption-reduction-sequestration performances of CGP were examined through batch adsorption experiments.The results revealed that the Cr（Ⅵ）adsorption by CGP was an endothermic process.The maximum adsorption capacity achieved 386.66 mg/g when the solution p H was 2 and the adsorption temperature was 45℃.Meanwhile,the adsorption isotherm study demonstrated that the adsorption process was fitted to the Langmuir adsorption isotherm model.The kinetic study showed that the adsorption process was in accordance with the pseudo-second order kinetic model,which suggested that the adsorption was driven by pore diffusion and electrostatic attraction.The Cr（Ⅵ）adsorption-sequestration mechanism of CGP is proposed as follows:（i）at p H=2,HCr O₄^-is adsorbed by the protonated amino groups;（ii）the adsorbed Cr（Ⅵ）ions could be reduced to Cr（Ⅲ）via electron donors,such as amino and oxygen containing groups;（iii）the majority of Cr（Ⅲ）ions were immobilized on CGP by chelation with amine and hydroxyl groups.Furthermore,CGP displayed good recycling performance and Cr（Ⅵ）adsorption-sequestration performances in the presence of various interfering substances.The Cr@CGP did not leach Cr（Ⅲ）and Cr（Ⅵ）in neutral,acidic and alkaline water systems,showing good immobilization stability.Finally,fixed bed column dynamic adsorption experiment demonstrated that the residual TCr concentration（200 ppb）in the solution was below the wastewater discharge limit of China（500 ppb）when CGP was used to treat 1 L of 1 ppm Cr（Ⅵ）solution.To further develop more greener and efficient adsorbent for Cr（Ⅵ）adsorption,natural macromolecule,tannic acid（TA）was used to modify CNF via the Michael addition reaction.Then,plant polyphenol functionalized cellulose based aerogel microspheres（CPT）was prepared by low temperature freezing bath and freeze-drying techniques.Fourier transform infrared spectrometer,Field emission scanning electron microscopy,X-ray photoelectron spectrometer,and Electronic universal testing machine were adopted to evaluate the morphology,functional groups,and mechanical strength of CPT.The Cr（Ⅵ）adsorption-reduction-sequestration performances of CPT were studied through batch adsorption experiments.Results demonstrated that the Cr（Ⅵ）adsorption by CPT was an endothermic process.When the solution p H was at 2.0 and the adsorption temperature was 45℃,the maximum adsorption capacity achieved 414.00 mg/g.The dynamic adsorption curve complied with the pseudo-second order kinetic model,indicating that electrostatic attraction and pore diffusion accounted for the adsorption process.Similarly,the adsorption isotherm study showed that the adsorption process accorded with the Langmuir adsorption isotherm model.The Cr（Ⅵ）adsorption mechanism by CPT is proposed as follows:（i）protonated amino groups attracted Cr（Ⅵ）through electrostatic interaction;（ii）a large number of phenolic hydroxyl groups on CPT provided electrons for Cr（Ⅵ）reduction;（iii）Cr（Ⅲ）was immobilized by chelating with the oxygen-containing groups on CPT.In addition,CPT demonstrated excellent recycling performance and immobilization stability in different simulated neutral,acidic,organic and high-temperature water systems.Finally,CPT showed excellent Cr（Ⅵ）adsorption selectivity in the simulated Cr⁶⁺/Cr³⁺ mixture.