| The rapid growth of modern industry and agriculture have caused the indiscriminate discharge of heavy metal ions into water bodies without adequate treatment,and thus the problems of water scarcity and water pollution are increasingly serious.Meanwhile,with the excess exploitation and unreasonable utilization of mineral resources,the global reserves have been gradually depleted in the past decades.Especially the rare earth element,as an important national strategic resource,has been widely used in mundane and high-tech fields.Therefore,how to separate and recover metal ions from wastewater has gained considerable attention in current research.It is noteworthy that the adsorption process has been proven to be a well-established and economical technology for wastewater treatment in virtue of its high efficiency,simple design,easy operation,and low energy consumption.Electrospun nanofibers,possessing remarkable advantages like large surface-to-volume ratio,high porosity,uniform fiber morphology,interpenetrating open pore structure,good membrane formation and convenient recyclability,have been usually considered as a promising candidate for highly efficient adsorbents.Herein,functional polymers including polyacrylonitrile-co-acrylic acid(PAN-AA),polyacrylic acid(PAA)and sodium alginate(SA)were used as electrospinning raw materials,and a series of high-efficiency nanofiber adsorbents with carboxyl affinity ligands were prepared by one-step electrospinning and simple post-treatment.Various preparation conditions of electrospinning and post-treatment were studied systematically to optimize the morphology and properties of nanofibers.Finally,the obtained functional nanofibers adsorbents were applied to remove metal ions from the wastewater.The optimal adsorption conditions were obtained by comprehensively investigating the influences of p H values,initial solution concentration,contact time and adsorbents dosage on the adsorption capacity of the functional nanofibers adsorbents,and the regeneration capacity of the adsorbents were also evaluated.The adsorption isotherm and adsorption kinetics of the functional nanofibers adsorbents were completely examined,and the adsorption mechanism between the adsorbents and metal ions also analyzed in detail.1.The functional polymer poly(acrylonitrile-co-acrylic acid)(PAN-AA)was prepared by a free radical polymerization of acrylic acid and acrylonitrile,and then PAN-AA blended with PVP was used for electrospinning.The obtained PAN-AA/PVP nanofibrous membrane was immersed into hot water bath for leaching pore-forming agent PVP to fabricate the multi-scale PAN-AA/PVP nanofibrous affinity membrane(NAM),and then used for adsorption of La3+.The systematic adsorption experiments showed that the optimized p H value for adsorption of La3+ was 5,the adsorption equilibrium time was about 5 h,and the removal rate of La3+ was up to 99.9% with the membrane dosage of 1.2 g/L.The curve-fitting results of the adsorption isotherm and adsorption dynamic demonstrated that the adsorption process of La3+ onto multi-scaled PAN-AA/PVP NAM were well-fitted to Langmuir isotherm model and pseudo-second-order model,and the theoretical maximum equilibrium adsorption capacity calculated from the Langmuir equation was 57.4 mg/g,which was agreement with the experiment data.Finally,the multi-scaled PAN-AA/PVP NAM adsorbed with La3+ could be desorbed in 0.1 mol/L HCl and reused for the next adsorption process after washing with water.The regeneration rate of the NAM still remained at 85.7% after 4 repeated adsorption-desorption experiments.The XPS and FT-IR analysis showed that the adsorption mechanism of PAN-AA/PVP NAM was the coordination between La atom and O atom in carboxyl groups,instead of the N atom in nitrile groups.Thus,the multi-scaled PAN-AA/PVP NAM exhibited certain removal efficiency towards trace rare earth ions,and it also proved that the oxygen atom from carboxyl group in the affinity membrane has a strong coordination with rare earth ions.2.Given three dimensional macromolecular network of hydrogel can promote the diffusion of molecules or ions,we proposed a new idea for the first time that taking full advantage of the interior adsorption sites of nanofibers to enhance the adsorption capacity.Thus,a novel choreographed poly(acrylic acid)-silica hydrogel nanofibers(PAA-Si O2 HNFs)scaffold was fabricated by a facile route consisting of colloid-electrospinning of PAA/Si O2 precursor solution,moderate thermal crosslinking of PAA-Si O2 nanofiber matrix and fully swelling under the water circumstance.To achieve excellent swelling rate and remain the intrinsic fibrous morphology of the resultant PAA-Si O2 HNFs,the optimized precursor solution for electrospinning was prepared by mixing 10 wt % PAA and 15% Si O2 with weight ratio of 2:1,and the obtained nanofibers were thermal cross-linked at 150℃ for 5 h.The swelling rate and gel fraction of the PAA-Si O2 HNFs was 4.0 and 95.1%.The PAA-Si O2 HNFs with loose and spongy porous network structure exhibited a remarkable adsorption capacity of lanthanide ions(Ln3+)triggered by the penetration of Ln3+ from the nanofiber surface to interior through the abundant water channels.The maximum equilibrium adsorption capacities for La3+,Eu3+ and Tb3+ are 232.6,268.8 and 250.0 mg/g,respectively,and the adsorption equilibrium time was 3 h.The adsorption behavior of Ln3+ onto PAA-Si O2 HNFs at equilibrium were well-fitted by the Langmuir isothermal model and the D-R model.Moreover,the pseudo-second-order model and intra-particle diffusion model were more suitable to explain the adsorption kinetics of Ln3+ on PAA-Si O2 HNFs.In addition,after four cycles of adsorption-desorption process,the regeneration rate of PAA-Si O2 HNFs was still kept at above 90%,suggesting that PAA-Si O2 HNFs exhibited excellent reuse performance.Furthermore,benefiting from the unique 4f layer electronic structure of Ln3+,the Ln-PAA-Si O2 HNFs simultaneously exhibited versatile advantages including good photoluminescent performance,tunable emission color and excellent flexibility and processability.Therefore,the Ln-PAA-Si O2 HNFs could be used as a flexible nano-scaled photoluminescence device,and hold great potential for applications in luminescent patterning,underwater fluorescent devices,sensors and biomaterials,etc,which expand the application scope of nanofibrous affinity membrane complexed with Ln3+.3.Nanofibrous hydrogels with high-water-content and robust mechanical performances have emerged as attractive candidates for multifunctional adsorbent materials,however,the major obstacle of nanofibrous hydrogels is their intrinsic lamellar deposition structure with the resultant nanofibers generally swelling to closely packed two-dimensional(2D)hydrated membranes rather than three dimensional(3D)bulk hydrogels,and it is a great challenge to construct 3D bulk nanofibrous hydrogels.Here we report a facile top-down approach to construct water-rich and superelastic 3D nanofiber-reconfigured spongy hydrogels(NRSHs)with hierarchically cellular architectures by combining sodium alginate/polyacrylamide(SA/PAM)electrospun nanofibers and the freeze-drying process of homogenized nanofiber dispersion,followed by chemically cross-linked with pyromellitic dianhydride(PMDA).The SA/PAM NRSHs possessed strong hydration capacity;it could quickly adsorb water to recover its original shape and maintained the excellent water adsorption capacity,even after several cycles of water adsorption and extrusion dehydration procedure.The SA/PAM NRSHs exhibited superior elasticity and super recyclable compressibility due to the synergistic effects of its cellular structures,well-hydrated and robust bonded nanofibrous skeleton.Furthermore,the well-designed NRSHs,possessing uniform distribution of abundant carboxyl groups on cell wall and robust mechanical properties,performed superior adsorption behavior towards Ln3+ and excellent regenerability.Under the optimal p H of 6,the maximum equilibrium adsorption capacities for Eu3+ and Tb3+ reached up to 492.9 and 472.9 mg/g.The experimental data were fitted with the typical adsorption isotherm curve and adsorption kinetic curve.It was found that the adsorption process exhibited better compliance with Langmuir isothermal model and pseudo-second-order model.Confirmed by FT-IR and XPS analysis,the proposed adsorption mechanism between Ln3+ and SA/PAM NRSHs was bidentate coordination,including the chelating and bridging.Similarly,benefitting from the typical 4f transition photoluminescence features of rare earth elements and favourable processability of SA/PAM NRSHs,the Ln-SA/PAM NRSHs could be processed into three-dimensional photoluminescence devices with various shapes.The photoluminescence color of the Ln-SA/PAM NRSHs could be fine-tuned by reasonably controlling the concentration ratio of Tb3+ to Eu3+.Meanwhile,the outstanding photoluminescent performance of Ln-SA/PAM NRSHs was independent of compression deformation,and the tunable mechanical strength could be achieved by simply adjusting the Ln3+ concentration.The fabrication of such fascinating NRSHs will provide new prospects to design and develop multifunctional 3D bulk hydrogels for various applications,such as adsorbents,luminescent patterning,underwater fluorescent devices,sensors and bioengineering.4.A thorough cleanup of heavy metal ions in water bodies is of vital importance for the urgent requirement of clean portable water.In this study,combining the prominent structural characteristics of electrospun nanofibers and the special property of hydrogels,we developed a green strategy to construct an eco-friendly poly(acrylic acid)-sodium alginate nanofibrous hydrogels(PAA-SA NFHs)with highly efficient Cu2+ remediation by electrospinning PAA-SA nanofibers,and followed by a facile thermal cross-linking treatment at 150℃ for 3 h.Comprehensive consideration of the surface morphology,swelling ratio and gel fraction,and the adsorption performance of PAA-SA NFHs,the optimized concentration ratio of PAA to SA used for the construction of PAA-SA NFHs was set as 4:1.The PAA-SA NFHs possessed a good resistance to the corrosive environment with p H of 0~14,and also exhibited satisfactory mechanical performance.The PAA-SA NFHs showed prominent adsorption performance towards Cu2+ at optimal p H of 5.5,and the maximum equilibrium adsorption capacity reached 591.7 mg/g within 3 h for 500 mg/L Cu2+ solution.After 7 adsorption-desorption cycles,the regeneration rate was remarkably stable and still remained at above 90%,and the nanofibrous morphology of the regenerated NHFs was almost the same as the freshly prepared sample.The extraordinary reusability of such PAA-SA NFHs was ascribed to the well-distributed carboxyl acid groups on the nanofibers that were beneficial to bind and elute adsorbates.The excellent stability in the corrosive environment and the satisfactory mechanical property prevented the NFHs from breaking up during the adsorption-desorption process.Notably,by virtue of the synergy of porous network structure,superhydrophilicity and faster adsorption rate,such PAA-SA NFHs membrane permitted extremely high water permeation flux solely driven by gravity and remarkable removal efficiency,when acted as a free-standing filter for the dynamic removal of Cu2+.Such PAA-SA NFHs membrane could treat 120 m L of Cu2+ contaminated water(20 mg/L)at the high flux of 868.5 L m-2 h-1,while the Cu2+ concentration of the filtrate could maintain the WHO drinking water limit standard(<1 mg/L).In addition,we also demonstrated a sustainable strategy of converting the toxic Cu2+ pollutants to valuable Cu nanoparticles for additional catalytic functionality,and the resultant PAA-SA NFHs with uniformly distributed Cu nanoparticles possessed an excellent catalytic activity.Consequently,such attractive nanofiber adsorbents fabricated by electrospinning functional polymers and simple post-treatment not only exhibited high efficiency remediation for metal ions contaminated water,but also provide new insights into the design and exploitation of novel adsorbents or filters for the practical water purification.On the basis of the characteristics of metal ions,we exploit the further applications of such nanofiber adsorbents complexed metal ions,it not only realizes the sustainable utilization of nature resources,but also expands the application field of the adsorbents. |
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