Fabrication Of The Polyacrylonitrile Electrospun Composite Fibers For Cr(Ⅵ) Adsorption

In recent years,the water pollution has become a serious issue,thereinto,the chromium is defined as one of toxic and non-degradable pollutants.Among the above methods,adsorption is widely used for the removal of heavy metals due to its advantages of multiple adsorbents,high efficiency,easy operation,good reversibility and low cost.Electrospun polymer based composite fibers with high surface area and excellent mechanical properties,was an appropriate candidate for adsorption of contaminants from waste water.Based on to this background,electrospun polyacrylonitrile（PAN）fibers was chosen as substrate to fabricate functional composite fibers by hydrophilic modification and nanoparticles loading,and further investigated the Cr（Ⅵ）adsorption capacity of the obtained composite fibers.In order to improve the Cr（Ⅵ）adsorption capacity of PAN fibers,microporous zeolitic imidazolate frameworks-8（ZIF-8）with excellent thermal/chemical stability,high surface area and abundant active sites was chosen to prepare composite fibers.In this work,we report a therapy to realize the efficient loading for carrier and the uniform distribution for adsorbent by in-situ growth of ZIF-8 onto ZIF-8/polyacrylonitrile（PAN）coelectrospun nanofibers,thereinto ZIF-8 crystals in ZIF-8/PAN nanofibers were used as seeds for the subsequent growth.The abundant micropores and active sites of ZIF-8 supplied the effective adsorption and reduction to heavy metal ions of Cr（Ⅵ）.The obtained novel composite fibers overcame the crystal agglomeration of ZIF-8 effectively and maintained the porous structures by the nanofibrous morphology of PAN and the evenly distributed ZIF-8,and thus exhibited relative higher surface area(871.0 m²·g^-1)and excellent loading efficiency of ZIF-8（82.9%）.As target adsorbate,Cr（Ⅵ）could be removed from aqueous effectively with the maximum adsorption capacity（q_m）of 39.68 mg·g^-1,and the good recyclability of composite fibers was affirmed,the influence of adsorption time,temperature,initial concentration and p H value of the solution,was also investigated for the Cr（Ⅵ）adsorption capacity.Besides,according to the results of adsorption kinetics,adsorption isotherms and XPS analysis,there were various interactions in the process of Cr（Ⅵ）adsorption by ZIF-8@ZIF-8/PAN composite fiber,including electrostatic attraction,pore filling,hydrogen bond and reduction reaction,thereinto chemical interaction was regarded as the main factor in this process.On this basis,a feasible mechanism for Cr（Ⅵ）adsorption-reduction was proposed,due to the abundant active sites of composite fibers,toxic Cr（Ⅵ）ions could be adsorbed and further reduced partially to the relative harmless Cr（III）.Polydopamine（PDA）was used to improve the hydrophilicity of PAN fibers,moreover,the existence of abundant amino and phenolic hydroxy within PDA molecules served as efficient adsorptive sites towards the adsorption of heavy metal ions.For PDA coated PAN fibers by the polymerization of dopamine（DA）,layer of PDA was covered on the surface of PAN nanofibers,which resulted in the disappearance of the fibrous morphology to some extent and the porous structures were blocked with the appearance of relative rough surfaces.Therefore,in order to avoid such phenomenon,and guarantee the distribution of DA uniformly across single fiber and maintain the fibrous morphology of PAN,combinations of the coelectrospinning of dopamine with PAN（DA/PAN）and the following self-polymerization of DA were adopted to fabricate PDA/PAN fibers.Then,the test results showed that the obtained composite fibers as adsorbent owned both the hydrophilic surface and almost evenly distributed active sites of PDA to realize the application of fibers within aqueous media and the efficient adsorption of removing metal ions.Benefiting from the existence of active groups（amino and hydroxyl）of PDA coating and the porous and fibrous morphology by electrospinning,the composite fibers showed better hydrophilicity（water contact angle of zero at room temperatures）and relative excellent adsorption capacity(q_mof 61.65 mg·g^-1)towards Cr（Ⅵ）.The further theoretical and experimental studies demonstrated that,the adsorption process was an endothermic and spontaneous process and increasing the temperature favored the adsorption,and the adsorption of Cr（Ⅵ）were better adapted to the pseudo second-order model and the Langmuir model and introducing the protonated forms governed the adsorption.Benefiting from abundant functional groups（including amino and hydroxyl）of PDA coating,the ZIF-8 particles primordially growth and aggregation on the fiber surface,and thicker ZIF-8 layer was obtained by prolonging the reaction time.Thorough characterization confirmed the successful growth of ZIF-8 on the fiber,and the obtained composite ZIF-8@PDA/PAN fibers exhibited fibrous structure with uniform and orderly ZIF-8 nanoparticles on the whole surfaces.In addition,the ZIF-8@PDA/PAN fibers retained high surface area(299.9 m²·g^-1)and appropriate pore size,and exhibited super-hydrophilicity and well water-stability,which were vital for Cr（Ⅵ）removal.Then,the adsorption ability,adsorption kinetics and isotherms of ZIF-8@PDA/PAN fiber were explored systematacially.The composite fiber exhibited significant removal efficiency（more than 90%at lower initial Cr（Ⅵ）concentrations）,and showed good adsorption capacity(212.7 mg·g^-1)with excellent recyclability.