Study On Preparation Of Functionalized PVDF Electrospun Fiber Membranes And The Adsorption Properties

The rapid development of modern industry provides people with convenience,but inevitably exacerbates the problem of water pollution.Contaminants in wastewater,such as organic dyes and heavy metal ions,which have complex structure,difficult degradation and high toxicity,cause great harm to the ecological environment and human health.In recent years,the membrane adsorption and separation technology has attracted great attention,due to its advantages of high efficiency,none pollution,easy operation and low cost,which has been widely used in the field of wastewater treatment.Among some polymer membrane materials,PVDF?polyvinylidene fluoride?has become one of the most widely studied membrane materials because of its good mechanical property,chemical stability and processing property.PVDF nanofiber membranes based on electrospinning technology have a wider application range for their large specific surface area,high porosity and controllable morphology.However,because of weak functionality and inherent hydrophobicity during the using process,the PVDF nanofiber membranes are easily contaminated with oils,resulting in the blockage of membrane pores,the decrease of water flux and membrane life.Therefore,it's of great significance to improve their hydrophilicity and adsorption properties through the functional modification of PVDF nanofiber membranes,which will expand the application of PVDF membranes in the field of wastewater treatment.In this thesis,aiming at solving the problem of strong hydrophobicity and weak functionality of PVDF electrospun fiber membranes,it is proposed to introduce functional components to improve their hydrophilicity and adsorption performance.Firstly,PVDF nanofibers were modified by co-spinning and self-polymerization polydopamine?PDA?.The effects of PDA on the hydrophilicity,the adsorption and separation performance of PVDF nanofiber membranes were systematically investigated.Secondly,PVDF nanofiber membranes were modified with polypyrrole?PPy?and utilized PDA as a binder.The adsorption capacity for various contaminants and the recycling performance were investigated.Finally,stable PVDF/GO composite nanofiber membranes were prepared through anchoring graphene oxide?GO?on hydrophobic PVDF nanofiber membranes by ultrasonic treatment,and their dye adsorption behavior and recycling performance were studied.The main research results are as follows:?1?PVDF/PDA nanofiber membranes with core-shell structure were successfully prepared by introducing PDA into PVDF nanofiber membranes by the method of co-spinning and self-polymerization.The microstructure characterization revealed that the fibrous membrane exhibited a good core-shell structure and high porosity?above 85%?.The contact angle test showed that the hydrophilicity of the film increased with the increase of the PDA polymerization time.When the PDA polymerization time was 30 h,the fibrous membrane exhibited superhydrophilicity because of the PDA with a large number of hydrophilicity groups such as hydroxyl and amino groups.The adsorption consequences showed that the maximum experimental adsorption capacities of PVDF/PDA nanofiber membranes for Methylene blue?MB?and Cu²⁺were 172.3 mg/g and 26.7 mg/g,respectively.It's attributed to the fact that PDA with catechol groups,amino groups,and imino groups could bind to MB and Cu²⁺.Further by adsorption kinetics and adsorption isotherm and thermodynamic parameters of simulated calculation,PVDF/PDA membrane adsorption for MB followed the pseudo-second-order kinetics model and Langmuir model,which was a spontaneous endothermic process,the maximum theoretical adsorption value was as high as 917.4 mg/g.In addition,the oil/water separation test showed that the separation effect was related to porosity and hydrophilicity,and PVDF/PDA-24 membrane had the best separation effect that the water flux was up to729.3 L m^-2 h^-1.The cyclic test found that the water flux could still maintain a high value after 20 cycles,indicating that PVDF/PDA membrane had a huge potential application in oil-water separation.?2?The in-situ polymerization and deposition of polypyrrole?PPy?on the PVDF electrospinning fiber surface were realized through utilizing PDA as binder to construct the second modification platform.The high load and uniform distribution of PPy modified PVDF nanofibers were successfully prepared.The microscopic morphology showed that the presence of PDA significantly improved the deposition of PPy particles on the surface of PVDF fibers.The PPy particles on the fibers increased with the increase of Pyrrole?Py?monomer concentration.However,when the Py concentration was too high,the PPy nanoparticles were easily aggregated,resulting in the reduced fiber membrane porosity.The contact angle test showed that the PPy nanoparticles augmented the roughness of the fiber surface,making the water contact angle gradually decrease and greatly improving the hydrophilicity of the membranes.The adsorption tests showed that the adsorption of PVDF/PDA/PPy fiber membranes for MB and Congo red?CR?had an obvious dependence on pH.The alkaline condition was in favor of the adsorption of MB,and the acidic condition was in favor of the adsorption of CR.This was ascribed to that PPy in acid and alkali conditions underwent protonation or deprotonation,thus exhibiting different charge-interaction results for anionic or cationic dyes.Further theoretical simulations showed that the adsorption behavior of the composite fiber membranes was consistent with the pseudo-second-order kinetic model and the Langmuir model.Continuous adsorption-desorption cycling tests showed that PVDF/PDA/PPy-3 membranes still retained a high amount of adsorption after 12 cycles,indicating that the membranes had excellent regenerative cycle property.In addition,the adsorption test for metal ion revealed that PVDF/PDA/PPy membranes could effectively remove Cr?VI?and the maximum adsorption capacity was 126.7 mg/g.?3?The PVDF/GO composite nanofiber membranes were prepared through GO anchored on the PVDF nanofiber membrane accompanying with ultrasonic treatment.The microstructure,chemical structure and dye adsorption properties of the membranes were systematically characterized,and the adsorption behavior was simulated.The results showed that GO could be stably loaded on PVDF nanofiber membrane and endowed PVDF with good adsorption properties.The adsorption behavior of PVDF/GO fiber membrane on MB was consistent with the pseudo-second-order kinetic model and Langmuir model,and the maximum adsorption value of the theory was as high as 769.2 mg/g.The adsorption cycle test showed that the PVDF/GO composite nanofiber membrane had excellent circulatory usability and stability.After 10 cycles,the adsorption capacity of the membrane could still reach more than 85%of the initial adsorption capacity.Therefore,the PVDF/GO composite nanofiber membranes have a promising application in the wastewater treatment.