Fabrication And Adsorption Properties Of Magnetic Aerogels Based On Amphiprotic Cellulose

Magnetic aerogel is a macroscopical porous three-dimensional（3D） material with significant magnetic properties under the influence of external magnetic field, which possesses low density, high porosity and high specific surface area. It is widely used in many fields such as shock-resistant, heat insulation, anti noise, energy storage device and super adsorption material. In the field of wastewater treatment, magnetic aerogel can be easily separated and recycled by the magnets except as the advanced adsorption capacities. Graphene oxide（GO） and cellulose are the dominant resources possess respective advantages. GO nanosheets have the advantages of high dispersion, high specific surface area and high surface activity, haved been used as a kind of structural unit materials for the construction of various macroscopical aerogels by self-assembly. By contrast, the cellulose aerogel has the advantages of low density, high strength, high porosity and high specific surface area, compared to the inorganic aerogel or synthetic polymer aerogel, it has been widely concerned for its better toughness besides of the same excellent adsoption capacity. Furthermore, cellulose is a renewable and environmental friendly natural polymer with abundant resources, has been regarded as an ideal candidate of scaffold materials owing to its high strength. Especially microcrystalline cellulose（MCC）, a kind of modified cellulose with small size, high strength and high modulus, whose surface exposed with many hydroxyl groups. Thus, it shows high high hydrophilic, high specific surface area and high reaction activity, possessing both the advantages of natural cellulose and micro/nano scale materials. Moreover, MCC is not soluble in water, dilute acid, organic solvents and oils, so that it can adapt to the various environment of adsorption, which is a promising carrier material for strengthenning the GO aerogel system and constructing aerogel adsorbent with excellent properties.This study first proposed a modified method for preparing amphiprotic cellulose, and cotton fiber was choosed to verify the feasibility of this method, the resulting amphiprotic cotton（AP-cotton） exhibit good adsorption capacity and significant electrostatic property because of it contains both anionic groups and cationic groups, possessing good conditions for constructing the aerogels. Therefore, we further treat the cotton fiber successively by oxidtion and amphiprotic modification to obtain the amphiprotic microcrystalline cellulose（AP-MCC）. Then it was used as carrier for loading the homemade large size GO nanosheets and ferroferric oxide（Fe₃O₄） nanoparticles aggregates, utilizing the electrostatic interaction of positive and negative groups on the surface of AP-MCC to induce the combination system construct the AP-MCC/GO magnetic aerogel by self-assembly. The structure, strength and magnetic properties of AP-MCC/GO aerogel were characterized. The effects of AP-MCC/GO mass ratio on the microstructure and adsorption performance of aerogel were analyzed. We further studied the adsorption capcacities of AP-MCC/GO aerogel for the anionic dyes（congo red, CR）, cationic dyes（methylene blue, MB） and heavy metal ions(Pb²⁺, Cd²⁺, Cu²⁺, and Cr³⁺), and the corresponding adsorption mechanism was investigated. In addition, the aerogel loading with contaminants were separated by magnets, following by desorption based on the adsorption mechanism to recycle the AP-MCC/GO magnetic aerogel, then which was reemployed to remove the dyes and heavy metal ions for studying its regeneration.The results show that the cationic monomer（3-chloro-2-hydroxypropyl trimethyl ammonium chloride, CTA） and anionic grafting monomers（2-acrylamido-2-methyl propane sulfonic acid, AMPS） were grafted successively in cotton fiber can obtain the amphiprotic cotton（AP-cotton） successfully. Of which, The CTA was grafted in C₆ of cellulose by etherification, and AMPS was introduced to the C₂ or C₃ of cellulose by free radical copolymerization. AP-cotton possess a nano/micro porous surface, which was empolyed for the removal of CR, MB, Cu²⁺, and Pb²⁺. The results show that the adsorption equilibrium time was 2 h⁴ h, the saturation adsorption capacity was 175.1 mg/g, 113.1 mg/g, 88.9 mg/g, and 70.6, 6 mg/g, respectively, there are 90% of adsorption capacity was maintained after 6 times regeneration. Besides, the adsorption process of AP-cotton on CR, MB, Cu²⁺, and Pb²⁺ is in accordance with the pseudo-second-order kinetics model, and the adsorption isotherm model conforms to the Langmuir model. In addition, the adsorption process strongly depends on the solution p H values due to eletrostatic interation（ionic bond） is the dominant mechanism during the adsorption process.GO suspension was prepared by chemical oxidation intercalation method. AFM shows the thickness of GO nanosheets is about 1.1nm and the lateral size is about 1⁴ μm, SAED and HRTEM indicate the GO have good quality because of its almost intact carbon skeleton, which is easily to construct into aerogel with openness and orientation porous structure. In order to avoid the uncontrollable aggregation of common Fe₃O₄ nanoparticles in the process of self-assembly fabrication of aerogel, we utilize the unimolecular or multimolecular nanocapsules with core-shell structure based on amphiphilic hyperbranched polyglycerols（AP-HPG） to design a Fe₃O₄ nano-aggregates with uniform size about 400 nm, which show good dispersion and stability in the polar and non polar solution. The saturation magnetization（Ms）, coercive force（Hc）, and residual magnetization（Mr） were 86.07 emu/g, 260.87 A/m, and 18.09 emu/g, respectively. Therefore, it would not aggregate under the interference of GO and AP-MCC in the construction process self assembled aerogels, which can be uniformly dispersed and attached to the inner/outer surface of the aerogel, providing a homogeneous magnetic properties for the aerogel.AP-MCC has good water dispersion with uniform particle size about 900 nm, possesses a isoelectric point about p H 3.9 and an bsolute value of zeta potential about 36, which shows a strong electrostatic behavior in the large range of p H values, exhibiting good adsorption capacity for both anionic and cationic contaminaments that is beneficial to fabricate the high-performance aerogels. Thus, GO and Fe₃O₄ nanoparticles were introduced into AP-MCC that as the carrier, then the composite system formed into a AP-MCC/GO magnetic aerogel by electrostatic induction of AP-MCC under ice-bath. It is found that the magnetic aerogel was composed of AP-MCC ribs and giant GO cell walls covered by Fe₃O₄ nano-aggregates, which exhibits an interconnected, porous 3D architecture of randomly oriented, crinkly sheets with continuous macropores ranged from several microns to about one hundred micron. It helps furthest preserve the abundant active sites, open-porous network structure, and excellent structural stability. The microstructure of magnetic aerogel strongly depends on the mass ratio of AP-MCC/GO, which shows the layered macroporous structure, orientation and porous structure, and randomly oriented porous structure with the increase of AP-MCC content. Moreover, magnetic aerogel possesses openness porous, good thermal stability and high mechanical strength when AP-MCC/GO was 5/5, and the Fe₃O₄ nano-aggregates still exist on the inner/outer surface of aerogel with previous size and morphology. As a result, AP-MCC/GO aerogel exhibits weak ferromagnetic properties with saturation magnetization（Ms） of 8.61 emu/g, which because it shows coercive forces（Hc） of 260.87 A/m as well as remanent magnetization（Mr） of 1.81 emu/g. Therefore, it can be easily manipulated by a magnet.Magnetic aerogel shows the best adsorption capacity for dyes and heavy metal ions when its density was 2 mg/cm3 and 8 mg/cm3 based on the mass ratio of AP-MCC/GO was 5/5（m/m）, respectively. On this basis, we further studied the adsorption performance, adsorption mechanism, and regeneration performance of AP-MCC/GO magnetic aerogel. The results show that AP-MCC/GO magnetic aerogel exhibits good removal effect for both anionic contaminants and cationic contaminants in the acidic or alkaline environment. The adsorption process of magnetic aerogel for heavy metal ions(Cu²⁺、Pb²⁺、Cd²⁺、Cr³⁺) and dyes（CR and MB） reached equilibrium in 150 min and 240 min, respectively. Besides, the adsorption process of magnetic aerogel on both dyes and heavy metal ions are in accordance with the pseudo-second-order kinetics model. However, the adsorption isotherm model of heavy metal ions(Cu²⁺、Pb²⁺、Cd²⁺、Cr³⁺) and dyes conform to the Langmuir model and Freundlich medel, respectively. In the process of adsorption, besides of the electrostatic interaction based on ionic bond is dominant mechanism, other chemical（hydrogen bond, complexation, etc.） and physical forces also play a supporting role, as a result, the adsorption process strongly depends on the solution p H values. The result of adsorption mechanism confirms that the openness porous structure and sulfonic acid groups of magnetic aerogel play the main role for the adsorption of heavy metal ions(Cu²⁺、Pb²⁺、Cd²⁺、Cr³⁺). By contrast, the dyes adsorption process more depends on electrostatic interaction of anions and cations in groups based on aerogel. Of course, No matter what type of interaction is the leading role, it is important to maintain the open porous three-dimensional structure of AP-MCC/GO magnetic aerogel due to which is a prerequisite for ensuring the free diffusion of pollutants in the channels of aerogel. Moreover, after the process of adsorption, AP-MCC/GO magnetic aerogel can be separated effective by the magnet and be recycled by desorption. After 5 repititions of desorptionre-adsorption loop, the residual adsorption capacity of AP-MCC/GO magnetic aerogel maintained at about 80%, and the adsorption capacity on dyes（CR and MB） is slightly higher than 70%.The above results indicate that AP-MCC/GO magnetic aerogel is a multifunctional, highly efficient and recyclable adsorbent material, which can be reused for many times. The electrostatic interaction and other chemical adsorption action（hydrogen bond, complexation, etc.） are the dominant mechanism. meanwhile, the open oriented porous structure of aerogel is more important to promote the diffusion and transfer of pollutants, ensuring the full contact with the adsorption site. Therefore, the abundant surface active sites, open pore morphology and excellent structural stability are the essential factors for the versatile and efficient adsorption properties of AP-MCC/GO magnetic aerogel. It is expected to be used for the removal of many kinds of pollutants, which also provide technical reference for fabrication of other high-performance adsorbent materials.