Immobilization of Zerovalent Iron Nanoparticles into Electrospun

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J. Phys. Chem. C 2009, 113, 18062–18068

Immobilization of Zerovalent Iron Nanoparticles into Electrospun Polymer Nanofibers: Synthesis, Characterization, and Potential Environmental Applications Shili Xiao,†,‡ Mingwu Shen,§ Rui Guo,§ Shanyuan Wang,‡ and Xiangyang Shi*,†,§ State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Textiles, College of Chemistry, Chemical Engineering and Biotechnology, Donghua UniVersity, Shanghai 201620, People’s Republic of China ReceiVed: June 13, 2009; ReVised Manuscript ReceiVed: September 6, 2009

We present a facile approach to immobilizing zerovalent iron nanoparticles (ZVI NPs) into electrospun polymer nanofibrous mats. Electrospun poly(acrylic acid) (PAA)/poly(vinyl alcohol) (PVA) nanofibrous mats were treated at an elevated temperature to render them water stable. The water-insoluble nanofibrous mats were then used as nanoreactors to complex ferric iron for subsequent formation and immobilization of ZVI NPs. Scanning electron microscopy (SEM) studies show that the smooth, uniform morphology of the electrospun nanofibrous mats does not significantly change after immobilization with ZVI NPs. Energy-dispersive spectroscopy (EDS), Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), selected area electron diffraction (SAED), and thermogravimetric analysis (TGA) were used to characterize the polymer nanofibers before and after the immobilization of ZVI NPs. We show that the formed ZVI NPs are uniformly distributed into the electrospun nanofibers with a mean particle size of 1.6 nm. The produced ZVI NP-containing polymer nanofibrous mats exhibit a superior capability to decolorize acid fuchsine solution, a model dye in wastewater of printing and dyeing industry. Findings from this study suggest a significant potential of using the electrospun nanofibers as nanoreactors to synthesize reactive iron NPs for a broad range of environmental remediation applications providing a foundation for further rational design of various composite nanofibrous materials for various applications. Introduction In recent years, zerovalent iron nanoparticles (ZVI NPs) have received tremendous scientific and technological interest because of their superior capacity of environmental remediation. Many efforts have been devoted to the uses of ZVI NPs and their nanobimetallic systems (Fe/Pd,1,2 Fe/Ni,3-5 Fe/Ag6) in the dechlorination of chlorinated organic contaminants such as trichloroethylene (TCE) and polychlorinated biphenyls (PCBs),1,7,8 in the sequestration of toxic metal ions such as As(V)9 and Cr(VI),10 in the stabilization of biosolids,11 in the decoloration of dyes,12,13 and in the degradation of nuclear wastes,14 explosives,15 and herbicides.16 ZVI NPs are commonly prepared by reducing Fe(II) or Fe(III) ions in aqueous solution using sodium borohydride. This is because the method is simple and can be safely performed in most chemistry labs.8 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.17,18 To mitigate the agglomeration of ZVI NPs and thus improve their transport and delivery capacity through porous media, some promising new synthetic methods have been developed to produce more dispersible and stable ZVI NPs or to immobilize the NPs onto microsized particles without compromising the surface reactivity of ZVI NPs. For instance, Sun et al.19 prepared * To whom correspondence should be addressed. E-mail: [email protected]. † State Key Laboratory for Modification of Chemical Fibers and Polymer Materials. ‡ College of Textiles. § College of Chemistry.

a stable dispersion of ZVI NPs using a nontoxic, biodegradable polymeric surfactant poly(vinyl alcohol-co-vinyl acetate-coitaconic acid) (PV3A) as a dispersant. The PV3A-stabilized ZVI NPs had a relatively smaller mean size of 15.5 nm, whereas in the absence of PV3A, the formed ZVI NPs had a mean size of 105.7 nm. The batch-settling experiment demonstrated that there was no noticeable sedimentation or flocculation with PV3Astabilized ZVI NPs over six months. He and Zhao20 used carboxymethyl cellulose (CMC) as stabilizers to synthesize ZVI NPs with different sizes to prevent the NPs from agglomeration through electrosteric stabilization. Ponder et al.21 have successfully synthesized the ZVI NPs with a diameter of 10-30 nm on a nonporous, hydrophobic polymer resin support, which exhibits a high reactivity to remove metal ion contaminants in aqueous solution. Recently, we reported a tunable synthesis and immobilization of ZVI NPs into polyelectrolyte (PE) multilayers constructed via a layer-by-layer (LbL) self-assembly approach.22 In this approach, poly(acrylic acid) (PAA)/polyallylamine hydrochloride (PAH) multilayers assembled onto micrometer-sized particle surfaces were used to complex Fe(II) ions in aqueous solution through an electrostatic interaction with the carboxylic acid groups of PAA. Followed by an in-situ chemical reduction reaction, ZVI NPs can be formed and immobilized onto the microparticles. The ZVI NPs immobilized onto the microparticle surfaces displayed an excellent reactivity for the dechlorination of trichloroethylene (TCE). In general, for environmental applications, it is important to immobilize ZVI NPs onto a carrier that can be easily separated from the contaminated water solution. The ZVI NPs stabilized by PAA,23 starch,1 and polyglycol24 are difficult to recycle once they are used to treat contaminants. To avoid secondary water contamination, im-

10.1021/jp905542g CCC: $40.75  2009 American Chemical Society Published on Web 09/29/2009

Immobilization of Iron Nanoparticles mobilization of ZVI NPs onto solid supports, for example, polymeric membranes25 and activated carbon,26 could be an additional option. However, the solid supports reported in literature have low specific surface area, which could significantly affect the reactivity of ZVI NPs. Therefore, 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 promising and low-cost method for synthesizing various polymeric nanofibers and nanostructured materials with a high aspect ratio and a specific surface area.27-29 Through this technology, various inorganic/polymer hybrid composite nanofibers can be synthesized.30-32 For example, Son et al.33 fabricated cellulose acetate (CA) nanofibers containing silver NPs by UV irradiation of electrospun CA nanofibers mixed with small amounts of silver nitrate. Ding et al.34 and Hong et al.35 prepared titania nanofibrous mats by applying an LbL self-assembly method and a sol-gel process onto electrospun polymer nanofibers, respectively. Herein, we report a facile approach to immobilizing ZVI NPs onto a continuous medium with a high surface area to volume ratio though electrospining technology, which is expected to yield a desirable material for environmental applications. First, PAA/PVA nanofibers were fabricated by electrospinning, and thermal treatment was introduced to prepare water-insoluble nanofibrous mats. Then, ZVI NPs were synthesized and immobilized into the electrospun polymer nanofibers through insitu reducing of Fe(III) ions complexed with the water-insoluble nanofibrous mats. The decoloration efficiency of the formed ZVI NPs/polymer nanofibrous mats was examined using acid fuchsine as a model contaminant. The developed ZVI NP-containing polymer nanofibers display an excellent reactivity toward the decoloration of the model dye. To our best knowledge, this is the first report related to the use of electrospinning polymer nanofibrous mats as nanoreactors to synthesize and simultaneously immobilize ZVI NPs for environmental applications. Experimental Section Materials. PAA (average Mw ) 240 000, 25% in water) was obtained from Aldrich. PVA (88% hydrolyzed, average Mw ) 8800) and sodium borohydride were from J&K Chemical. FeCl3 · 6H2O (ACS reagent grade) was purchased from Sinopharm Chemicial Reagent Co., Ltd. Water used in all experiments was purified using a Milli-Q Plus 185 water purification system (Millipore, Bedford, MA) with resistivity higher than 18 MΩ cm. Synthesis of ZVI NP-Containing Polymer Nanofibrous Mats. PVA solution was prepared by dissolving PVA powder into water at 80 °C for 3 h under magnetic stirring, and then the solution was cooled to room temperature. The measured PVA and the PAA aqueous solutions were mixed to achieve a mass ratio of 1:1 at a constant total polymer concentration. The 1:1 mass ratio is proved to achieve the highest viscosity of the mixture solution according to literature.36 The mixed solution was stirred for 30 min before use. Freshly prepared PAA/PVA mixture solution (15 mL) was loaded into a syringe with a needle having an inner diameter of 0.8 mm, the feed rate of which was controlled by a syringe pump (JZB-1800, Jian Yuan Medical Technology Co., Ltd., China) at 0.5 mL/h. The highvoltage power supplier (BGG40/2, Institute of Beijing High Voltage Technology, China) was connected to the needle by a high-voltage insulating wire with two clamps at the ends. An aluminum board was used as the collector and was connected to the ground. The electrospinning setup can be found in our

J. Phys. Chem. C, Vol. 113, No. 42, 2009 18063 previous report.37 The distance of tip to collector was set at 20 or 25 cm, and the electrospinning voltage was kept at 16.6 kV. The electrified polymer jets generated by the applied high voltage field was elongated to ultrafine fibers and was whipped continuously by the electrostatic repulsion until it was deposited as a nanofiber mat onto the aluminum foil glued on an aluminum board. Freshly prepared PAA/PVA nanofibrous mats were crosslinked upon heat treatment at 145 °C for 30 min in the oven and were cooled to room temperature. Then, the nanofibrous mats were immersed into an aqueous solution of ferric trichloride (0.18 mol/L) for 3 h to allow ferric cations to complex with available free carboxyl groups on PAA through ion exchange followed by rinsing with deionized water three times. Sodium borohydride solution (0.94 mol/L) was dropped onto the nanofibrous mats gradually until there was no hydrogen gas to reduce the ferric iron to zerovalent iron according to eq 1. The formed ZVI NP-containing fibrous mats were rinsed three times with water followed by vacuum drying at room temperature for 24 h and then were stored in desiccator before characterization and application.

4Fe3+ + 3BH4- + 9H2O f 4Fe0 V + 3H2BO3- + 12H+ + 6H2v

(1)

For a control experiment, ZVI NPs were also synthesized in solution. Ferric salt solution (0.18 mol/L, 5 mL) was directly reduced by adding a solution of sodium borohydride (0.94 mol/ L, 5 mL) under vigorous stirring for 20 min to yield ZVI NPs.8 The particles were rinsed, freeze-dried, and stored in a vial before use. Characterizations. Morphologies of the electrospun nanofibrous mats were observed using scanning electron microscope (SEM) (JSM-5600LV, JEOL Ltd., Japan) with an operating voltage of 10 kV. Prior to SEM measurements, samples were sputter-coated with 10 nm thick Pt films. The elemental composition of the samples was analyzed by X-ray energydispersive spectroscopy (EDS) detector (IE 300X, Oxford, United Kingdom) attached to the SEM at an operating voltage of 15 kV. To observe the distribution of iron particles in the nanofibers, the ZVI NP-immobilized polymer nanofibrous mats were embedded in epoxy resin and were cut into ultrathin sections with ultramicrotome equipped with a diamond knife. The cross-sectional image of the fibers containing ZVI NPs was imaged using a transmission electron microscope (TEM) (JEM2100, JEOL Ltd., Japan) with an operating voltage of 200 kV. Selected area electron diffraction (SAED) was performed to analyze the crystal structure of the formed ZVI NPs. The diameters of nanofibers and particle sizes were measured using image analysis software ImageJ 1.40G (http://rsb.info.nih.gov/ ij/download.html). At least 200 randomly selected nanofibers or ZVI NPs in different SEM or TEM images were analyzed for each sample to acquire the diameter/size distribution histograms. Thermogravimetric analysis (TGA) was carried out on a TG 209 F1 (NETZSCH Instruments Co., Ltd., Germany) thermogravimetric analyzer with a heating rate of 10 °C/min in air. Fourier transform infrared (FTIR) spectra were recorded using a Nicolet 5700 FTIR spectrometer (Thermo Nicolet Corporation, United States) in a wavenumber range of 4000-500 cm-1 at ambient conditions. The apparent density and porosity of electrospun fibrous mats before and after treatment were calculated using eqs 2 and 3,38 where the thickness of the fibrous mats was measured by a micrometer and the bulk density of

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Figure 1. SEM images of the electrospun PAA/PVA nanofiber (a) and ZVI NP-containing PAA/PVA nanofibers (c). (b, d) The diameter distribution histograms of the electrospun PAA/PVA nanofiber and ZVI NP-containing PAA/PVA nanofibers, respectively.

mixture was calculated according to their weight ratio in the mixture.

apparent density (g/cm3) ) mat mass (g) mat thickness (cm) × mat area (cm2)

(2)

mat porosity ) mat apparent density (g/cm3) 1× 100% (3) bulk density of mixture (g/cm3)

(

)

Acid Fuchsine Decoloration Experiments. Acid fuchsine was selected as the model species to prepare the dyecontaminated water. Decoloration of acid fuchsine was performed in a 20 mL vial under air condition. Each reactor was filled with an aliquot of acid fuchsine solution (60 mg/L) dissolved in water with a pH value of 6.8. ZVI NPs synthesized with two different methods were added to each reactor under vigorous stirring at room temperature to reach a final Fe concentration of 0.15 g/L. At each time interval, 1 mL of the aqueous sample was withdrawn. The sample was diluted to 2.5 mL with deionized water for the analysis of acid fuchsine decoloration efficiency by a Lambda-25 UV-vis spectrometer (Perkin-Elmer, United States). Results and Discussion Preparation and Characterization of Electrospun PAA/ PVA Nanofibers. Both PAA and PVA are environmentally benign, water-soluble polymers, which are excellent candidate materials for environmental remediation.39,40 The PAA/PVA polymer nanofibers were generated by electrospinning a mixture solution of PAA and PVA. To obtain uniform PAA/PVA nanofibers, we figured out the electrospinning conditions through the variation of the polymer concentration and the collection

distance. The morphology of PAA/PVA nanofibers electrospun at a concentration of 8.5 wt % (Figure S1a of the Supporting Information) shows that continuous nanofibers with a diameter of 134 ( 39 nm (Figure S1b of the Supporting Information) can be fabricated under the applied voltage (16.6 kV) and collection distance (20 cm). However, there was some adhesion between the nanofibers. To prepare uniform nanofibers with reduced adhesion between nanofibers, we increased the polymer concentration to 10 wt % and prolonged the collection distance to 25 cm. In general, increasing the polymer concentration can increase the viscosity of polymer solution thus favoring the formation of uniform fibers.41 In addition, extending the collection distance is beneficial for the solvent evaporation and nanofiber solidification. Figure 1a and 1b shows a typical SEM image and the diameter distribution histogram of the PAA/PVA nanofibers electrospun under the above conditions, respectively. Smooth and uniform nanofibers with a diameter of 170 ( 27 nm were produced, and there was nearly no adhesion observed between nanofibers (Figure 1a). PAA and PVA are water-soluble polymers. Therefore, the produced PAA/PVA nanofibers can be dissolved in water immediately, which is impractical for them to be used as wastewater filtration materials. To retain the porous nanofibrous mat structure in water, PAA/PVA nanofibrous mats were heated at 145 °C for 30 min. The cross-linking reaction was induced by esterification between the PAA carboxylic acid and the PVA hydroxyl groups.36 The obtained mats became insoluble in water, and the nanofibrous structure was perfectly retained after immersing in water for a week (Figure S2 of the Supporting Information), although extended water immersion caused the fibrous mats to be a little swollen. Synthesis and Characterization of ZVI NPs/Polymer Hybrid Nanofibrous Mats. The carboxyl groups of PAA in the PAA/PVA nanofibrous mats can be used to complex Fe(III) ions for the subsequent formation of ZVI NPs similar to the PAA-based nanoreactor systems reported in the literature.22,42-44

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Figure 2. (a) A cross-sectional and (b) a high-magnification cross-sectional TEM image of ZVI NP-containing polymer nanofibers. (c) The size distribution histogram of the formed ZVI NPs.

Upon an addition of the reducing agent NaBH4, ZVI NPs were simultaneously produced and immobilized into electrospun nanofibrous mats. SEM, EDS, TEM, FTIR, and TGA were used to characterize the ZVI NPs/polymer hybrid nanofibrous mats. Figure 1c shows a typical SEM image and the diameter distribution histogram of the ZVI NPs/polymer composite nanofibrous mats. It is clear that the composite nanofibrous mats still retain uniform fibrous structure with a smooth surface similar to the electrospun PAA/PVA polymer nanofibers without ZVI NPs (Figure 1a). This suggests that the formed ZVI NPs are uniformly distributed in the nanofibers. The diameter of ZVI NP-immobilized nanofibers (205 ( 27 nm) was larger than that of the PAA/PVA polymer nanofibers without ZVI NPs (Figure 1d), which may be caused by the swelling of nanofibers after immersing into the aqueous solution and the loading of ZVI NPs.36 Figure 2a shows a cross-sectional TEM image of ZVI NPcontaining polymer nanofibers. Individual ZVI NPs with a relatively dense and uniform distribution along the cross section of nanofibers can be clearly observed in a magnified TEM image (Figure 2b). The sizes of 300 randomly selected ZVI NPs at different TEM images were measured using ImageJ software as described in the Experimental Section. The size distribution histogram of the ZVI NPs (Figure 2c) on the basis of the measurement of 300 ZVI NPs suggests that the formed ZVI NPs have a size ranging narrowly from 1 to 3 nm. The size (mean ( std) of the ZVI NPs immobilized in the polymer nanofibers was estimated to be 1.6 ( 0.4 nm (Martin’s diameter). An SAED pattern (Figure S3 of the Supporting Information) confirmed the formation of an RFe-core/γFe2O3shell structure, which is consistent with literature data.22,45-47 EDS was used to characterize the composition of the ZVI NP-containing polymer nanofibers (Figure S4a of the Supporting Information). The presence of element iron and oxygen is consistent with an iron/iron oxide core-shell structure reported in the literature.22,47 The oxygen is also partly attributable to the PAA and PVA polymers in the nanofibers. Carbon was detected as a main component of polymer nanofibers. Aluminum was from the aluminum foil acting as a nanofiber collector. Sodium was probably due to a residue from the sodium borohydride used for ferric reduction. The distribution of ZVI NPs in the polymer nanofibers was also observed through the EDS elemental mapping. Red and green colors represent the

Figure 3. FTIR spectrum of electrospun PAA/PVA nanofibers (a), cross-linked PAA/PVA nanofibers (b), and ZVI nanoparticles immobilized PAA/PVA nanofibers (c).

distribution of oxygen and iron elements, respectively. We can clearly see that the ZVI NPs are uniformly distributed inside the nanofibers (Figure S4b of the Supporting Information). Figure 3 shows the FTIR spectra of PAA/PVA nanofibrous mats with different treatments. It is clear that the absorption peaks at 3360 cm-1 and 1710 cm-1 represent the hydroxyl groups of PVA and carboxyl groups of PAA, respectively. There is still a considerable amount of carboxyl groups left in the PAA/ PVA nanofibers after heat treatment, which is available for the following complexation of ferric cations. However, it is hard to detect the formation of an ester bond in the FTIR spectrum of cross-linked PAA/PVA nanofibers (Figure 3, curve b), which is consistent with literature data,48 although esterification was the only proper explanation for the insoluble nanofiber mats. After the ZVI NPs were formed and immobilized into the electrospun nanofibers, there were some significant differences in the FTIR spectra of PAA/PVA nanofibers and ZVI NPimmobilized PAA/PVA nanofibers in the 1710-1560 cm-1 and the 1410-1150 cm-1 regions, which is similar to the PAA-Fe spectrum reported in the literature.49 The absorption peak

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TABLE 1: Porosity of Electrospun Nanofibrous Mats nanofibrous mats

porosity (%)

PAA/PVA nanofibrous mats cross-linked PAA/PVA nanofibrous mats ZVI NP-containing PAA/PVA nanofibrous mats

86.6 69.6 73.5

attributed to dissociated carboxylic acid groups at 1710 cm-1 disappeared and was replaced by a new strong band at 1560 cm-1 attributed to CdO stretching in the carboxylate. Changes at ∼1400 cm-1, in the region of 1330-1150 cm-1 (C-O stretching) and 980-808 cm-1 are assigned to the interactions between ZVI NPs and carboxyl groups. TGA was used to estimate the iron-loading capacity of the cross-linked PAA/PVA nanofibrous mats (Figure S5 of the Supporting Information). At the high temperature of 600 °C, the polymer component of composite nanofibrous mats is burned out, and iron oxide (Fe2O3) is left. On the basis of this, we estimate the content of ZVI NPs immobilized in the polymer nanofibrous mats to be approximately 22.3%. The iron content in the nanofibers could be increased by increasing the number of the Fe(III) ion complexation/reduction cycles, which is similar to other nanoreactor systems reported in the literature.22,44 Through the increase of the number of Fe(III) ion complexation/ reduction cycles, the size of the ZVI NPs could also be increased. This work is currently being undertaken to confirm our hypothesis. Porosity is one of the most important parameters to evaluate the property of electrospun nanofibrous mats. Knowing the bulk densities of PAA and PVA polymers, along with the calculated apparent density of the nanofibers, we were able to calculate the porosities of PAA/PVA, cross-linked PAA/PVA, and ZVI NP-containing PAA/PVA nanofibrous mats (Table 1). The porosity of PAA/PVA nanofibrous mats (86.6%) without any treatment decreased to 69.6% after heat-induced cross-linking and fiber contraction thus forming the dense fibrous mats. However, after loading with ZVI NPs, the porosity of ZVI NPcontaining polymer nanofibrous mats increased back to 73.5%, which is presumably due to the swelling of the nanofibers after the chemical processing.

Acid Fuchsine Decoloration. As an example of potential environmental applications, the ZVI NP-containing polymer nanofibrous mats were used to decolorize dyes in wastewater. Acid fuchsine, a common organic dye containing benzene rings in textile industry, which cannot be easily decomposed using traditional chemical and biological methods,50 was selected as a model contaminant. The ZVI NP-containing polymer nanofibrous mats were able to decolorize about 95% of the acid fuchsine (concentration ) 60 mg/L) without any additives at room temperature (Figure 4a and 4b). The red color of acid fuchsine solution was suddenly decolorized after 5 min and then gradually faded within a time frame of 40 min (Figure 4a). This was also confirmed by UV-vis spectroscopic measurements. We show that the intensity of the characteristic absorption peak of acid fuchsine at 544 nm significantly decreased after 5 min exposure of the ZVI NP-containing polymer nanofibrous mat to the acid fuchsine solution (Figure 4b). When the exposure time was extended to 40 min, only gradual decoloration was observed. The decoloration effect of the ZVI NP-containing polymer nanofibrous mats is solely related to the reactive nature of ZVI NPs immobilized into the polymer nanofibers. When the PAA/PVA fibrous mats without ZVI NPs were exposed to the same acid fuchsine solution, no decoloration was observed. This suggests that the adsorption of the dye onto the polymer nanofibrous mats does not contribute to the decoloration effect. Detailed molecular mechanism regarding the decoloration of acid fuchsine using ZVI NPs is still unclear now. We think that the chromophore of acid fuchsine is able to be destroyed after the iron reduction process. For comparison, ZVI NPs synthesized using the literature method8 was also used to decolorize acid fuchsine. ZVI NPs with similar weight to those immobilized into the polymer nanofibers were added into the acid fuchsine water solution (60 mg/L). It is clear that the decoloration capacity of ZVI NPs is lower than those immobilized into polymer nanofibers (Figure 4c and 4d). We do not observe the sudden decoloration effect after 5 min implying that the ZVI NP-containing polymer nanofibrous mats have better decoloration ability than the ZVI NPs prepared using the literature method. This is presumably

Figure 4. Photographs of the acid fuchsine solution treated with ZVI NP-containing polymer nanofibrous mats (a) and ZVI NPs synthesized using literature method (c) at different time intervals. UV-vis spectra of a solution of acid fuchsine (60 mg/L, 20 mL) in the presence of ZVI NPcontaining polymer nanofibrous mats (b) and ZVI NPs synthesized using literature method (d) at a time interval of 0 min, 5 min, 10 min, 20 min, 30 min, and 40 min.

Immobilization of Iron Nanoparticles

Figure 5. Removal percentage of acid fuchsine using (a) freshly prepared ZVI NP-containing polymer nanofibrous mats (ZVI NP-Mat1) and (d) ZVI NPs synthesized using the literature method (ZVI NPs). (b, c) The second time (ZVI NP-Mat-2) and third time (ZVI NP-Mat3) remediation capability of the ZVI NP-containing polymer nanofibrous mat used in a.

due to the much smaller size of the ZVI NPs (1.6 nm) uniformly distributed in the polymer nanofibers, which favors the reactive degradation of the dye molecules. In contrast, the ZVI NPs synthesized using the literature method display a wider size distribution (1-100 nm) and are prone to aggregation during the treatment.8 The better remediation capability of ZVI NP-containing polymer nanofibrous mats was also demonstrated by plotting the percentage of decoloration as a function of exposure time (Figure 5). In addition, the ZVI NP-containing polymer nanofibrous mats are easily reusable and recyclable. After treatment with sodium borohydride for 10 min, the renewed ZVI NPcontaining polymer nanofibrous mats still perform well for the second and third decoloration experiments (Figure 5, curves b and c). Furthermore, the red color of acid fuchsine almost completely faded after 40 min treatment with ZVI NP-containing polymer nanofibrous mats (Figure 4a), while the color of acid fuchsine solution treated with ZVI NPs changed to light brown (Figure 4c), which is associated with the existence of ZVI NPs dispersed in solution. For practical environmental remediation applications, it is ideal to separate the ZVI NPs from contaminated water after the remediation process instead of leaving the particles dispersed in water to introduce secondary contamination. In our case, the ZVI NP-immobilized fibrous mats can be easily separated from the contaminated water, and the ZVI NPs are quite stable and do not escape from the fibrous mats during the remediation process. Inductively coupled plasma atomic emission spectroscopy studies show that no iron is released even if the ZVI NPimmobilized fibrous mats are exposed to water for a month. This further demonstrates that the developed ZVI NP-containing fibrous mats are very useful for practical environmental applications. We chose to expose our fibrous mats in water for decoloration experiment in this study. Our aim is to demonstrate the capability of ZVI NP-immobilized fibrous mats for environmental remediation. In our future efforts, we will develop the composite fibrous mats as membrane filtration materials, which are expected to be conveniently used for remediation of various contaminants in wastewater. Conclusions In summary, we have developed a facile approach to simultaneously synthesize and immobilize ZVI NPs using electrospun polymer nanofibrous mats as nanoreactors. The ZVI NPs with a mean diameter of 1.6 nm are uniformly distributed

J. Phys. Chem. C, Vol. 113, No. 42, 2009 18067 in the nanofibrous mats. The composite nanofibrous mats containing ZVI NPs are stable in water and have a porosity of 73.5%. Acid fuchsine decoloration experiments demonstrate that ZVI NP-containing polymer nanofibrous mats are able to quickly decolorize an organic dye, acid fuchsine, a model contaminant in dyeing wastewater, and the decoloration percentage approaches 95.8% within 40 min. We anticipate that the asprepared ZVI NP-immobilized polymer nanofibrous mats can be useful in the remediation of many other contaminants such as TCE, PCB, and toxic metal ions (e.g., arsenic). Related experiments are currently being carried out in our laboratory. Furthermore, the developed approach to immobilizing ZVI NPs into polymer nanofibers may also open a new avenue in the fabrication of various three-dimensional porous nanostructured inorganic/organic materials for various applications. Acknowledgment. This research is financially supported by the State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, and the National Basic Research Program of China (973 Program, 2007CB936000). Supporting Information Available: Additional SEM, SAED, EDS, and TGA characterization data for PAA/PVA nanofibers and ZVI NP-containing PAA/PVA nanofibers. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) He, F.; Zhao, D. EnViron. Sci. Technol. 2005, 39, 3314. (2) Zhang, W.-X.; Wang, C.-B.; Lien, H.-L. Catal. Today 1998, 40, 387. (3) Schrick, B.; Blough, J. L.; Jones, A. D.; Mallouk, T. E. Chem. Mater. 2002, 14, 5140. (4) Tee, Y. H.; Grulke, E.; Bhattacharyya, D. Ind. Eng. Chem. Res. 2005, 44, 7062. (5) Xu, J.; Bhattacharyya, D. EnViron. Prog. 2005, 24, 358. (6) Xu, Y.; Zhang, W.-X. Ind. Eng. Chem. Res. 2000, 39, 2238. (7) Varanasi, P.; Fullana, A.; Sidhu, S. Chemosphere 2007, 66, 1031. (8) Wang, C.-B.; Zhang, W.-X. EnViron. Sci. Technol. 1997, 31, 2154. (9) Kanel, S. R.; Greneche, J.-M.; Chol, H. EnViron. Sci. Technol. 2006, 40, 2045. (10) Ponder, S. M.; Darab, J. G.; Mallouk, T. E. EnViron. Sci. Technol. 2000, 34, 2564. (11) Li, X.-Q.; Brown, D. G.; Zhang, W.-X. J. Nanopart. Res. 2007, 9, 233. (12) Epolito, W. J.; Yang, H.; Bottomley, L. A.; Pavlostathis, S. G. J. Hazard. Mater. 2008, 160, 594. (13) Fan, J.; Guo, Y.; Wang, J.; Fan, M. J. Hazard. Mater. 2009, 166, 904. (14) Darab, J. G.; Amonette, A. B.; Burke, D. S. D.; Orr, R. D.; Ponder, S. M.; Schrick, B.; Mallouk, T. E.; Lukens, W. W.; Caulder, D. L.; Shuh, D. K. Chem. Mater. 2007, 19, 5703. (15) Naja, C.; Halasz, A.; Thiboutout, S.; Ampleman, G.; Hawari, J. EnViron. Sci. Technol. 2008, 42, 4364. (16) Kim, G.; Jeong, W.; Choe, S. J. Hazard. Mater. 2008, 155, 502. (17) Li, X.-Q.; Elliott, D. W.; Zhang, W.-X. Crit. ReV. Solid State Mater. Sci. 2006, 31, 111. (18) Schrick, B.; Hydutsky, B. W.; Blough, J. L.; Mallouk, T. E. Chem. Mater. 2004, 16, 2187. (19) Sun, Y.-P.; Li, X.-Q.; Zhang, W.-X.; Wang, H. P. Colloids Surf., A 2007, 308, 60. (20) He, F.; Zhao, D. EnViron. Sci. Technol. 2007, 41, 6216. (21) Ponder, S. M.; Darab, J. G.; Bucher, J.; Caulder, D.; Craig, I.; Davis, L.; Edelstein, N.; Lukens, W.; Nitsche, H.; Rao, L.; Shuh, D. K.; Mallouk, T. E. Chem. Mater. 2001, 13, 479. (22) Huang, G.; Shi, X.; Pinto, R. A.; Petersen, E.; Weber, W. J., Jr. EnViron. Sci. Technol. 2008, 42, 8884. (23) Yang, G. C. C.; Tu, H.-C.; Hung, C.-H. Sep. Purif. Technol. 2007, 58, 166. (24) Wang, W.; Jin, Z.-H.; Li, T.-L.; Zhang, H.; Gao, S. Chemosphere 2006, 65, 1396. (25) Kim, H.; Hong, H.-J.; Lee, Y.-J.; Shin, H.-J.; Yang, J.-W. Desalination 2008, 223, 212.

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