Abstract:

Significance Chitosan is a natural polymeric alkaline polysaccharide derived from a wide range of sources, and its molecular chain is rich of reactive groups, which can be used as adsorbent in the field of water treatment. However, conventional chitosan adsorbents have the disadvantages of small specific surface area, poor stability and difficulties in secondary recovery, resulting in low adsorption rate and poor economic efficiency, which seriously limits the industrial applications. Chitosan nanofibers are functional biomass regeneration fibers with large specific surface area, high porosity, flexible surface function and certain strength prepared by a series of spinning methods with chitosan as the main component, and fibrillation of chitosan can significantly eliminate the defects in the conventional chitosan adsorbents. Fibers can be formed by various techniques such as electrostatic spinning, wet spinning and chemical vapor deposition spinning, among which electrostatic spinning is the most common method for preparing chitosan-based nanofibers with uniform morphology. This paper presents a review of domestic and international studies on the preparation of chitosan-based nanofibers using electrostatic spinning technology, aiming to provide guidance for improving the spinnability of chitosan and the physical morphology and mechanical properties of chitosan-based nanofibers.

Progress In order to enhance the spinnability of chitosan and improve the physical morphology and chemical properties of chitosan-based nanofibers, researchers have carried out a lot of studies in the aspect of preparing chitosan nanofibers using electrostatic spinning technology, and found that the parameters of spinning liquid and process parameters of electrostatic spinning device are the important factors determining the properties of nanofibers. First of all, only the spinning solution with good viscosity and conductivity can make chitosan nanofibers with uniform diameter and good mechanical properties by electrostatic spinning technology. In recent years, researchers have prepared ideal spinning solution by modifying chitosan through cross-linking, grafting and derivatization, but this still falls short of the standard for industrial application. Researchers have used natural/synthetic polymers to further enhance the viscosity and conductivity of the spinning solution, but synthetic polymers such as polylactic acid, polycaprolactone, polyurethane and other synthetic polymers have a certain degree of toxicity leading to the final production of fibers with a limited range of applications, while natural polymers such as cellulose, collagen and so on, have become a hotspot of the research on the preparation of excellent chitosan spinning solution in recent years because of their non-toxic and non-hazardous advantages. Secondly, in addition to the preparation of spinning solution with good viscosity and conductivity, suitable process parameters are also important prerequisites for the preparation of excellent chitosan nanofibers. For example, the appropriate voltage value in the electrostatic spinning process is an important guarantee to ensure that the fibers have good morphology and excellent performance, and it is found that the fiber diameter decreases with the increase of voltage, but the fiber diameter starts to increase when the voltage is higher than the critical range. Finally, this paper summarizes the effectiveness of chitosan-based nanofibers as adsorbents for the treatment of heavy metal ions such as Ni²⁺, Cu²⁺, Cr⁶⁺ and U⁶⁺ and dyes such as Congo Red, methylene blue and carmine in wastewater, and finds that the resulting fibers can be used for the simultaneous adsorption of a variety of heavy metal ions, anionic and cationic dyes as the spinning technology improves, and elucidates the repetitive regeneration properties of chitosan-based nanofibers in the adsorption of different pollutants.

Conclusion and Prospect Chitosan-based nanofiber is a new type of adsorbent material with the advantages of easy separation, large specific surface area and flexible surface function, which can effectively improve the economic efficiency and avoid secondary pollution, and it is of great significance to help the early realization of "double carbon". Chitosan fibrillation based on electrostatic spinning technology can be divided into two steps: preparing of spinning solution and spinning formation. The preparation of spinning solution by dissolving chitosan in acid is the first step to enable chitosan spinnable, and changes in parameters such as spinning solution, process and environment during spinning formation ultimately change fiber morphology by affecting the ease of jet stretching. In addition, modification methods such as cross-linking, graft copolymerization, derivatization and blending can not only improve the spinnability of chitosan, but also enhance the acid resistance, thermal stability, antibacterial properties and adsorption of chitosan-based nanofibers. In the co-blending spinning process, the electrostatic interaction between chitosan and natural/synthetic polymers and the entanglement resulting from the reaction of different groups can improve the spinnability of chitosan. The search for new green, non-toxic and post-treatment-free solvents in the preparation of spinning solution, the search for new natural/synthetic polymers as co-spinning agents for improving chitosan spinnability during spinning and forming, and the use of multi-template molecular imprinting technology to enhance the adsorption for contaminants are the future trends of chitosan-based nanofibers.

Key words: chitosan-based nanofiber, electrostatic spinning technology, heavy metal, dye, adsorbent, water treatment