Electrospun Zero-Valent Iron Nanoparticles/Polymer Materials: Synthesis, Characterization And Environmental Application

In recent years, nanotechnology has received much attention in many fields, and the synthesis and application of nanomaterials are the hot spot in the field of materials science in particular. For environmental remediation materials, the unique properties of nanostructured materials such as high surface area to volume ratio can improve the degradation effects of materials to the pollutants. Zero-valent iron nanopaticles (ZVI NPs), a representative nanomaterial used in environmental remediation, have exhibited excellent capacity because of their superior physical and chemical properties in the dechlorination of chlorinated organic contaminants, in the sequestration of toxic metal ions, and in the decoloration of dyes and so on. However, the formed particles are prone to agglomeration during the process of contaminant degradation and their transport process in the subsurface environment. This often leads to a reduced reactivity, which is a critical drawback in the environmental application of ZVI NPs. Although some promising new synthetic methods have been developed to produce more dispersible and stable ZVI NPs, ZVI NPs dispersed in water could cause second pollution. So, immobilizing ZVI NPs onto a continuous medium with a high surface area to volume ratio and good porosity is anticipated to meet the requirements for environmental remediation applications.Electrospinning technology has recently emerged as a straightforward method for synthesizing various polymeric nanofibers and nanostructured materials. In this study, polyacrylic acid (PAA)-containing nanofibers were first fabricated using the electrospinning method. Then, ZVI NPs were synthesized and immobilized in the electrospun polymer nanofibers through in situ reducing of ferric (or ferrous) ions complexed with nanofibrous mats, forming functional nanostructured materials for wastewater treatment. We focused on the study of synthesis, characterization and the processing parameter optimization of ZVI NP-containing composite nanofibrous mats. Meanwhile, we also evaluated the environmental remediation capability of ZVI NP-immobilized composite nanofibrous mats through decoloration of representative dyes in printing and dyeing wastewater (acid fuchsine, methyl blue, and acridine orange), removal of copper ions in the model electroplating wastewater, and degradation of chlorinated organic contaminant trichloroethylene (TCE).Electrospun cellulous acetate (CA) nanofibers were LbL-assembled with multilayers of poly(diallyldimethylammonium chloride) (PDADMAC) and PAA through electrostatic interaction. Then, the PAA/PDADMAC multilayers coated onto the CA nanofibers were used as a nanoreactor to complex Fe(II) ions through the binding with the free carboxyl groups of PAA for subsequent reductive formation and immobilization of ZVI NPs. We studied the effect of electrospinning parameters such as solvent, polymer concentration, flow rate, collection distance, and applied voltage on the formation of CA nanofibers. Smooth and uniform CA nanofibers were produced with a mean diameter of 295±145 nm under the optimized experimental conditions. To render the final formed PE multilayer-coated CA nanofibrous mats with porous structure, the number of PE multilayers deposited onto the CA nanofibrous mat was also optimized based on SEM characterization. Six bilayers of PAA/PDADMAC were chosen to coat onto the CA nanofibers and were used as a nanoreactor. Through the subsequent chemical reduction, ZVI NPs were successfully immobilized onto the (PAA/PDADMAC)6-CA nanofibrous mats. Combined EDS, TEM, and FTIR studies demonstrate that the synthesized ZVI NPs are uniformly distributed into the PE multilayers assembled onto the CA nanofibers with a mean size of 1.4 nm. The ZVI NP-immobilized nanofibrous mats were demonstrated to be able to decolorize an organic dye, acid fuchsin, a model contaminant in dyeing wastewater, and the decoloration percentage could be up to 86.8% within 40 min. The remarkable remediation capability of composite nanofibrous mats is solely attributed to the presence of ZVI NPs (1.4 nm).In order to further simplify the fabrication process, electrospun PAA nanofibers were directly used as a nanoreactor according to our previous study. For generating ultrafine, uniform, and stable PAA nanofibers, we systematically investigated the influence of processing parameters (namely solution concentration, applied voltage, flow rate, and collection distance) on the morphology of PAA nanofibers. We showed that the concentration of PAA solution was crucial to the fiber formation and nanofiber morphology. Meanwhile, longer collection distance was beneficial for the evaporation of solvent and fiber solidation. To retain the nanofibrous structure of PAA, we electrospun PAA and polyvinyl alcohol (PVA) mixture solution to form PAA/PVA nanofibers. Then thermal treatment was introduced to crosslink PAA/PVA nanofibers, resulting in water-stable PAA/PVA nanofibers. SEM results showed that PAA/PVA nanofibers exhibited smooth and uniform morphology with a mean diameter of 170±27 nm. ZVI NPs were directly immobilized into the PAA/PVA nanofibers after chemical reducing ferric ions complexed with PAA/PVA nanofibers. We show that the ZVI NP-immobilized composite nanofibers still retain uniform fibrous structure with a smooth surface similar to the electrospun PAA/PVA nanofibers without ZVI NPs. The mean diameter of composite nanofibers was 205±27 nm. TEM morphological studies show that the ZVI NPs are uniformly distributed in the cross-sections of PAA/PVA nanofibers with a mean size of 1.6 nm. Acid fuchsine decoloration experiment demonstrated that the red color of acid fuchsin solution was obviously decolorized after 5 min exposure of ZVI NP-immobilized composite nanofibrous mats. The decoloration percentage approached 95.8% within 40 min. The composite nanofibrous mats were easy reusable and recyclable. Inductively coupled plasma atomic emission spectroscopy studies showed that no iron was released into the water solution after 1 month exposure of the fibers, thereby avoiding second pollution of ZVI NPs.Nanofibrous mats deposited with randomly orientated nanofibers were always not as strong as desired due to the intrinsic weak mechanical properties of the nanofibers. In order to improve the mechanical properties of composite nanofibrous mats containing ZVI NPs, we doped MWCNTs into the PAA/PVA mixture solution before electrospining. The mechanical property testing suggested that the incorporation of MWCNTs could effectively improve the tensile strength and Young's modulus before and after immobilizing ZVI NPs. With only 1 wt% MWCNTs incorporation, the tensile strength could be increased up to 10.1 MPa from 6.21 MPa, about 62.6% higher than that of pure PAA/PVA nanofirous mats. In addition, the Young's modulus of PAA/PVA nanofibrous mats also increased up to 114 MPa from 88.3 MPa. After immobilization of ZVI NPs, the mechanical properties of nanofibrous mats exhibited a decrease compared to their counterparts, but still higher than those ZVI NP-immobilized PAA/PVA nanofibrous mats without MWCNTs, which demonstrated that it was an effective way to improve the mechanical properties of electrospun nanofibrous mats through electrospinning MWCNT-doped polymer solution.To systematically evaluate the environmental remediation capability of MWCNT-reinforced PAA/PVA composite nanofibrous mats containing ZVI NPs, we further investigated the efficiency of the fibers toward the decoloration of commonly used dyes such as acridine orange and methyl blue, the removal of copper ions, and the degradation of TCE. We showed that (1) MWCNT-reinforced PAA/PVA composite nanofibrous mats containing ZVI NPs could decolorize acridine orange and methyl blue rapidly. At the equilibrium state, the decoloration percentage of acridine orange and methyl blue could reach up to 98% and 96% respectively, obviously higher than that of ZVI NP-immobilized PAA/PVA nanofibrous mats without MWCNTs; (2) MWCNT-reinforced PAA/PVA nanofibrous mats with and without ZVI NPs could effectively remove the copper ions in the model electroplated wastewater. Mechanistic analysis showed that copper ions was removed via reduction by ZVI NPs to form Fe-Cu bimetal nanoparticles in the presence of ZVI NP-containing composite nanofibrous mats. While the removal of copper ions by PAA/PVA nanofibrous mats without ZVI NPs was mainly due to the complexation of free carboxyl group of PAA. Equilibrium isotherm investigation indicated that the Langmuir model was appropriate to describe the interaction between MWCNT-reinforced nanofibrous mats (with and without ZVI NPs) and copper ions. Meanwhile, the kinetics of copper ion removal was found to follow a pseudo second-order rate behavior for MWCNT-reinforced nanofibrous mats with and without ZVI NPs; (3) likewise, the same nanofibrous mats could effectively remove a chlorinated contaminant of TCE with a removal efficiency approaching 93% within 3.5h.In summary, we report a facile approach to synthesizing and immobilizing ZVI NPs onto or into nanofibrous mats, preventing the agglomeration and second pollution of the ZVI NPs. Meanwhile, the concept of using electrospun nanofibers as nanoreactors might open a new avenue in the fabrication of various three-dimensional porous nanostructured hybrid materials for various applications.