One-Step Fabrication of Fe(OH)3

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One-Step Fabrication of Fe(OH)3@Cellulose Hollow Nanofibers with Superior Capability for Water Purifications Jiangqi Zhao, Zhixing Lu, Xu He, Xiaofang Zhang, Qingye Li, Tian Xia, Wei Zhang, Canhui Lu, and Yulin Deng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07038 • Publication Date (Web): 10 Jul 2017 Downloaded from http://pubs.acs.org on July 10, 2017

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ACS Applied Materials & Interfaces

One-Step Fabrication of Fe(OH)3@Cellulose Hollow Nanofibers with Superior Capability for Water Purifications Jiangqi Zhao,† Zhixing Lu,‡ Xu He,† Xiaofang Zhang,† Qingye Li,† Tian Xia,† Wei Zhang,*,† Canhui Lu*,† and Yulin Deng§ †

State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute at

Sichuan University, Chengdu 610065, China ‡

Key Laboratory of Organic Optoelectronics and Molecular Engineering of the Ministry of

Education, Department of Chemistry, Tsinghua University, Beijing 100084, China §

School of Chemical and Biomolecular Engineering and IPST at GT, Georgia Institute of

Technology, Atlanta 30332-0620, GA USA

ABSTRACT The conventional strategies employed to synthesize hollow nanofibers (HNFs) require either multi-step treatments or the special design of the equipment. An additional annealing process is always required, which inevitably consumes more energy and raises the manufacturing cost. In addition, the annealing process may also cause a waste of the matrix materials and the release of toxic gases. Herein, we report for the first time a novel one-step synthesis of hollow hybrid nanofibers via electrospinning. Cellulose was chosen as the polymer matrix, and Fe(OH)3 nanoparticles was in situ grown on the nanofibers during electrospinning. There was no need to remove cellulose to create the hollow nanofiber structure. This can significantly simplify the fabrication process without any negative influence to the air. The obtained Fe(OH)3@Cellulose HNFs membranes exhibited great mechanical properties and an extremely high water flux of 11200 L m-2 h-1 bar-1. They could effectively remove various pollutants in water, including phosphate, heavy metal ions and organic dyes with excellent reusability. Importantly, this approach could also be applied to fabricate other hybrid HNFs, which may serve in a broad range of scientific and engineering applications, including water purification, energy conversion and storage, catalysts, sensors, and so on.

KEYWORDS cellulose, electrospinning, hollow nanofiber, high water flux, water purification

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INTRODUCTION Hollow micro-/nanostructured materials are gaining popularity owing to their promising applications, such as sensors, catalysis, biomedical engineering, water purification, energy storage and conversion.1,2 Recently, as a special branch of nanomaterials, one-dimensional hollow nanofibers (HNFs) have been extensively developed because of their unique structure, high porosity and large specific surface area.3 So far, several approaches such as templating, hydrothermal synthesis, electrospinning, and so forth have been developed for preparing HNFs.1 Notably, the electrospinning has been extensively explored as an advantageous, low-cost and easy method to produce fibers with diameters ranging from tens of nanometers to several micrometers. 3,4 The strategies employed to synthesize HNFs via electrospinning could be classified into three major ways: fiber templating method,1 coaxial electrospinning5 and single nozzle coelectrospinning.6 However, these methods require either multi-step treatments or the special design of the equipment with accurate control over the electrospinning parameters. In recent years, single spinneret electrospinning was reported to fabricate various kinds of HNFs.7,8 Nevertheless, it is not possible for this method to directly obtain HNFs. An additional annealing process is always required, which inevitably consumes more energy and raises the manufacturing cost. The annealing process, in particular for those synthetic polymer matrices, may also cause some adverse results, such as a waste of the expensive materials and the release of toxic gases. Therefore, it is urgent to develop a simple and more environmentally friendly method for the fabrication of HNFs. Herein, we report a novel one-step synthesis of hollow hybrid nanofibers via electrospinning. Cellulose, one of the most abundant biopolymers in the world with plenty of extraordinary availability,

9-11

properties

such

as

biodegradability,

sustainability

and

widespread

was chosen as the polymer matrix. The cellulose solutions in LiCl/DMAc,

which contained a certain amount of FeCl3, were prepared as the spinning dope (see Figure S1, Supporting Information), while an aqueous alkali was used as the coagulation bath to solidify the electrospun nanofibers. During this process, Fe(OH)3 nanoparticles could be in situ grown in the regenerated cellulose matrix. A schematic of the fabrication process was illustrated in Figure 1a and b. There was no need to remove the cellulose matrix in order to create the hollow nanofiber structure. This may significantly simplify the fabrication process without any negative influence to the air. In addition, the presence of cellulose in the hybrid would also provide the HNFs membranes with a high mechanical strength and excellent flexibility. 2


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As is well known, iron oxides/hydroxides are extensively explored for water purification.12,13 Compared with many other adsorbent materials, such as polyamine polymers and lanthanum hydroxide, Fe(OH)3 is lower in cost and more environmentally friendly, and demonstrates chemical stability over a wide pH range.14 However, the adsorption performance of conventional Fe(OH)3 is limited owing to its large particle size. In addition, the powdery Fe(OH)3 particles are difficult for separation after water treatment particularly for those nanosized ones. In this study, the numerous Fe(OH)3 nanoparticles were in situ grown and anchored on the electrospun HNFs. As a result, a large specific surface of the material could be obtained, giving rise to significantly improved adsorption properties. In the meantime, the electrospun membranes were easy for separation and recycling, which may remarkably lower the cost for water purification. The membranes were capable to efficiently remove various kinds of pollutants in water, including phosphate, heavy metal ions and organic dyes, with a very high water flux and excellent reusability. These features consistently suggested their great potential for real world practice in water purifications. It is also important to note that this approach can be applied to fabricate other kinds of HNFs, which may open up new opportunities for the widespread applications of these unique nanomaterials.

EXPERIMENTAL SECTION Preparation of Fe(OH)3@Cellulose HNFs The cellulose solution was prepared according to our previous work.15 Briefly, 1 g cotton was “activated” and then dissolved in 100 g LiCl/DMAc (8:92 in weight) solution under magnetic stirring until the solution became transparent (Figure S1a, Supporting Information). Then, 1 g FeCl3 was added to the cellulose solution and stirred at ambient conditions until completely dissolved (Figure S1b, Supporting Information). After that, the as-prepared solution was loaded in a plastic syringe for electrospinning. The voltage and distance between a needle tip and drum collector were set as 20 kV and 20 cm, respectively, and the feeding rate of the solutions was kept at 0.03 mL min-1. The rotating collector was partly immersed in an aqueous alkali coagulation bath with different concentrations of NaOH. Finally, the electrospun nonwoven membranes were thoroughly washed and freeze-dried in a lyophilizer (FD31A350, Biocool, China) at -30 oC. According to the NaOH concentration in the coagulation bath, the resultant Fe(OH)3@Cellulose HNFs membranes were coded as 0.01M, 0.1M and 1M. Moreover, some other factors including different cellulose concentrations (0.7 wt.% and 1.5 wt.%) and applied voltages (15 kV and 25 kV) were investigated to reveal the effect of electrospinning conditions 3



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on the morphology of electrospun fibers. Additionally, in order to examine the universal applicability of this approach, a cellulose solution containing a certain amount of LaCl3 (1wt.%) was also prepared for electrospinning La(OH)3@Cellulose HNFs in the same way. For comparison purposes, neat cellulose nanofibers (CelluNFs) were electrospun at the same conditions but with a water coagulation bath, while Fe(OH)3 powders was synthesized following a conventional method that is to add a NaOH solution into a FeCl3 solution directly. Batch Adsorption Experiments Three model pollutants, KH2PO4, K2Cr2O7 and Congo red (CR) were dissolved in deionized water and then diluted to the required concentrations before use. To investigate the influence of pH on the adsorption behaviors, the adsorbents were placed in contact with the pollutant solution in the pH range of 2 to 10, which were adjusted with 0.1 M HCl or NaOH solutions. For adsorption isotherm studies, the adsorption was conducted on each pollutant with a series of initial concentrations, and the pH values of the solutions were adjusted to 3 for phosphate, 3 for Cr(VI), and 6 for CR. The adsorbents were added into the three pollutant solutions and constantly shaken for 4 h at the room temperature. The amount of adsorbate adsorbed per unit mass of adsorbent was calculated by Equation 1. The equilibrium adsorption data were fitted with the Langmuir model (Equation 2) and the Freundlich model (Equation 3), respectively:16

(1)

(2)

(3) -1

Where Co and Ce are the initial and equilibrium concentrations (mg L ), respectively, qe (mg g-1) is the equilibrium adsorption capacity, qm (mg g-1) is the maximum adsorption capacity, KL and KF are the constants for Langmuir and Freundlich isotherms, respectively, n is a Freundlich constant related to adsorption intensity of the adsorbents. Membrane Filtration Adsorption Experiments The filtration adsorption experiments were carried out with a dead-end filtration device as shown in Figure 1e and Movie S2, Supporting Information. The membranes were customized into a circular shape with a diameter of 25 mm, and then placed in the dead-end cells (the effective permeation area was 3.8 cm2). A CR solution with an initial concentration of 20 mg L-1 was filtered through the membrane at various flow rates (1-5 mL min-1) using an injection 4


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pump (TYZ5810, TSY, China). Neat CelluNFs membranes and Fe(OH)3 powders were also examined to make a comparison, and the Fe(OH)3 powders were spread on the filter paper for the testing. To evaluate the reusability of the membrane, a CR solution with an initial concentration of 20 mg L-1 was loaded at 1 mL min-1 until the outlet solution became colored. After that, 120 mL desorption solution (0.1 M NaOH in 50% ethanol aqueous solution) was loaded at the same flow rate to elute the dye, thereby regenerating the membrane (Movie S3, Supporting Information). Then 100 mL deionized water was passed through the cell to wash out the residual NaOH and ethanol. This process was repeated for 5 consecutive runs. Characterization The morphologies of products were observed by SEM (Inspect F50, FEI, USA) at 20 kV and TEM (JEOL, JEM52010) at 120 kV. The membranes were fractured in liquid nitrogen for SEM imaging on the cross-section. For TEM observations, the electrospun fibers in the membranes were mechanically isolated and uniformly dispersed in distilled water by a high-shear homogenizer (T18, IKA, Germany). The membranes were also embedded in epoxy resin for ultrathin sectioning to observe their cross-section. XRD measurements were carried out on an X-ray diffractometer (X’Pert Pro MPD, Philips, Holland) using Cu Kα (1.5406 Å) radiation. XPS spectra were recorded on a Kratos XASAM 800 spectrometer with an Al Ka X-ray source (1486.6 eV). Thermogravimetric analysis (TGA) was conducted on a synchronous thermal analysis system (STA 6000, Perkin Elmer, USA) from 25 to 600 oC (10 oC min-1) under air atmosphere (40 mL min-1). The N2 adsorption and desorption isotherms were obtained from a Quantachrome Instruments (Autosorb AS56B, USA) and the specific surface area and the pore volume were calculated by Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods, respectively. The mechanical properties of the membranes were measured on a universal testing machine (Instron 5567, USA) at a speed of 1 mm min-1. The water contact angle of the membranes was analyzed using a contact angle goniometer (Krüss, DSA 100, Germany). The zeta potential of the membranes was measured by a Zetasizer Nano ZS90 (Malvern Instruments Co., Britain). A dead-end filtration device (see Figure 1e) was employed to measure the water flux driven by nitrogen gas. The concentrations of the pollutants in the solutions were measured using a UV-vis spectrophotometer (UV-1600, MAPADA, China). The concentration of phosphate was measured at 700 nm using the molybdenum-blue ascorbic acid method.17 The concentration of Cr(VI) was analyzed at 540 nm after complexation with 1,5-diphenylcarbazide,16 and the concentration of CR was tested at 500 nm.18

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RESULTS AND DISCUSSION Characterization of Materials The electrospinning process and the obtained wet membranes were demonstrated in Figure S2, Supporting Information. It is clear that the colors of membranes changed from white for the CelluNFs to red-brown for the hybrids, in accordance with the in situ generation of Fe(OH)3. The surfaces of these Fe(OH)3@Cellulose HNFs were not smooth. The higher the NaOH concentration, the more agglomerates the membranes had. This could be possibly ascribed to the inherent lubricating properties of the NaOH solution, which made the rotating metal collector too slippery for the adhesion of the electrospun nanofibers, leading to fiber aggregations. The wet nonwoven membrane was then freeze-dried and the resultant sample was shown in Figure 1d-f. It is noteworthy that the strong capillary forces due to water surface tension could be eliminated during the freeze-drying process so that the pore structure as well as the specific surface area of the membrane could be largely retained, representing a distinct advantage for water treatment.19 Compared with those fragile annealed HNFs, the hybrid membrane produced in this study was easily bendable and could be tailored into any desired shape. To be more vivid, the membrane was arbitrarily twisted but without any cracks (see Movie S1, Supporting Information), indicating its excellent flexibility. The morphologies of the electrospun nanofibers were observed by SEM coupled with TEM. Figure 2a showed a top view SEM image of the neat electrospun CelluNFs. These nanofibers were randomly oriented with a rather smooth surface. And the diameter of these fibers varied from 60 to 180 nm with an average diameter of 143 nm (Figure S3a and Table S1, Supporting Information). Figure 2b-d displayed the morphologies of Fe(OH)3@Cellulose HNFs generated from alkali solutions with different concentrations. A well-defined fiber texture was still retained for all the samples. However, these fibers had a much rougher surface and a larger diameter as compared with neat CelluNFs. The hybrid nanofibers bore numerous tiny pores on them (indicated by red arrows). And the Fe(OH)3 nanoparticles well-positioned on the fibers’ surface could be clearly seen (indicated by green arrows). Importantly, the size of those Fe(OH)3 nanoparticles was much smaller than that of conventional Fe(OH)3 particles (Figure S4, Supporting Information). With the increase of NaOH concentration of the coagulation bath, both of the fiber diameter of and their surface pore size were gradually increased (Figure S3 and Table S1, Supporting Information). The cross sectional SEM images of nanofibers were shown in Figure 2e-h. The neat CelluNFs had a solid interior (Figure 2e), whereas the Fe(OH)3@Cellulose HNFs displayed 6


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obvious hollow structures (indicated by purple arrows). This hollow structure was further verified by TEM. As shown in Figure 2i and Figure S5a, the neat CelluNFs exhibited a uniform contrast over the entire area, indicative of the dense structure of CelluNFs. On the other hand, the Fe(OH)3@Cellulose HNFs (Figure 2j-l and Figure S5b) showed a strong contrast between the fiber edge (dark) and its inner area (bright), which agreed with its hollow structure. These images also suggested that the morphology and structure of HNFs could be adjusted by changing the concentration of NaOH in the coagulation bath. It is important to note that by substituting FeCl3 with other metal salts in the spinning dope, some other kinds of hybrid nanofibers with similar porous and hollow structures could be prepared as well. For example, the La(OH)3@Cellulose HNFs was successfully fabricated under the same conditions (Figure S6), demonstrating the versatility of this novel approach. The electrospinning process can be influenced by quite a lot of factors. Herein, the major factors including cellulose concentration and voltage were investigated.20 As shown in Figure S7, the concentration of cellulose had a significant effect on the morphologies of the resultant HNFs. The diameter of Fe(OH)3@Cellulose HNFs obtained from the 0.7 wt.% solution was more uniform with a narrower diameter distribution compared with others, but the fibers were more likely to adhere together (Figure S7a). The average diameter of the obtained HNFs increased from 187 nm to 273 nm with the increase of cellulose concentrations from 0.7 wt.% to 1.5 wt.% (Figure S7e and f). The applied voltage also had a great impact on the electrospun HNFs. From Figure S8, it is obvious that the diameter distribution of HNFs gradually narrowed with the increase of applied voltage. And the average fiber diameter decreased from 291 nm to 173 nm as the applied voltage increased from 15 kV to 25 kV (Figure S8e and f). When processed at a higher voltage, the electric-field intensity increases, and the charged jet can be split into more jets, leading to the decrease of fiber diameter.19 The continuous hollow structure formation of the electrospun fibers was revealed by SEM and the representative images were shown in Figure S9. For the electrospun fibers without solvent exchange in NaOH solution, a solid interior was observed (Figure S9a). Whereas, after the solvent exchange in NaOH solution for 1 min, the cross-section of electrospun fibers become hollow (Figure S9b). And if a longer time of solvent exchange was applied (10 min), the hollow fiber structure was more visible (Figure S9c). Based on the above results, a possible mechanism for the formation of the hollow structure of Fe(OH)3@Cellulose HNFs was deduced. As schemed in Figure 1b and c, this unique structure was created mainly due to the reverse gel growth caused by the solvent exchange and the Kirkendall effect which had been previously suggested to govern the hollow crystal formation 7



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with reversed crystal growth.21,22 At the very beginning, when the electrospun fibers contacted with the coagulation bath, a gel layer of regenerated cellulose was formed on the surface of the precursor fibers (Figure 1c). This process was accomplished through solvent exchange with a rapid phase separation, leading to the formation of thin gel shell and solution core structured nanofibers. Afterwards, the continuous solvent exchange further thickened the gel layer. During the solvent exchange process, two different types of forces in opposite directions might greatly influence the morphology of electrospun fibers. One was the inward diffusion of water molecules, which thickened the gel layer. Another effect was the outward diffusion of solvent molecules, leading to the swelling of electrospun fibers (Figure 1c). For neat cellulose, the diffusion rate of outward solvent molecules was similar to that of the inward water molecules, resulting in dense cellulose nanofibers. But for Fe(OH)3@Cellulose HNFs, apart from the mutual diffusion of solvent and water, the outward diffused Fe3+ ions were able to react with the inward diffused OH- ions to form Fe(OH)3 precipitates (Fe3+ + 3OH- = Fe(OH)3↓). The concentration gradient of Fe3+ irons in the electrospun fibers became more evident. It would significantly accelerate the outward diffusion rate of Fe3+, which could be much faster than those of inward water molecules and the gel layer growth. As a result, voids would be formed and ultimately a hollow fiber structure was generated (Figure 1c) due to the Kirkendall effect,9,23,24 though this mechanism was not fully understood. N2 gas adsorption-desorption experiments were performed to evaluate the specific surface area and the porosity of the electrospun nanofibers. As shown in Figure 3a, the very low N2 uptake of CelluNFs and conventional Fe(OH)3 powders indicated their nearly nonporous structure. In contrast, all the Fe(OH)3@Cellulose HNFs displayed much-enhanced N2 uptake. And the continuous increase of the N2 uptake with the increase of NaOH concentration implied more pores were created in the HNFs. The pore size of the nanofibers was mainly distributed in the range of 2-10 nm, with an increase at a higher concentration of NaOH. As shown in Figure 3b and Table S1, the BET surface area and the pore volume of the Fe(OH)3@Cellulose HNFs (265.33 m2 g-1 and 0.48 cm3 g-1 for 1M) were much higher than those of neat CelluNFs (11.59 m2 g-1 and 0.07 cm3 g-1) and conventional Fe(OH)3 (33.68 m2 g-1 and 0.03 cm3 g-1). These features could be attributed to several reasons. First, a hollow structure could double the surface area of the nanofibers.10 Second, compared with neat CelluNFs, there were plenty of uniformly distributed

Fe(OH)3

nanoparticles

as

well

as

numerous

mesopores

along

the

Fe(OH)3@Cellulose HNFs (Figure 3a), which provided the membrane with a large specific surface area.3,22 The inorganic content in the membranes was further evaluated via TGA under air atmosphere,25 and the results were shown in Figure S10 and Table S1. It was estimated that 8


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the content of Fe(OH)3 in the hybrid membranes varied between 15.30 wt.% and 19.35 wt.%. The crystalline structures of the products were examined by XRD. Figure 3c showed the XRD patterns of Fe(OH)3, neat CelluNFs and Fe(OH)3@Cellulose HNFs. The CelluNFs was almost amorphous as characterized by the broad peak centered around 20.5°.13 The two broad peaks for Fe(OH)3 at 2θ=35o and 63o implied that the as-prepared sample was an amorphous material.26 For Fe(OH)3@Cellulose HNFs, both the characteristic peaks for CelluNFs and Fe(OH)3 appeared, suggesting its hybrid structures. XPS was carried out to analyze the surface composition of the products, and the results were shown in Figure 3d. Compared with neat CelluNFs, a new peak was found in the spectrum of Fe(OH)3@Cellulose HNFs, which was assigned to Fe element. In the high-resolution Fe 2p spectrum (the inserted image in Figure 3d), the two distinct peaks at the binding energies of 711.5 eV and 725 eV were ascribed to Fe 2p3/2 and Fe 2p1/2, respectively. This was in accordance with the reported values for Fe3+compounds.27,28 Water flux and mechanical properties are of great importance for the nanofiber membranes when applied in water purifications. And it is still a challenge to achieve desired performance in both aspects simultaneously. As shown in Figure 4a, the water flux of Fe(OH)3@Cellulose HNFs could be as high as 11200 L m-2 h-1 bar-1, which was two to three orders of magnitude higher than that for commercial nano- or ultrafiltration membranes.29,30 Moreover, the Fe(OH)3@Cellulose HNFs also exhibited excellent mechanical properties owing to the presence of the cellulose matrix (see the tensile testing curves in Figure S11, Supporting Information). For example, the tensile strength of sample 0.01M was up to 9 MPa. Actually, during the water filtration adsorption experiment, the Fe(OH)3@Cellulose HNFs membrane could be used alone without any additional supporting membrane. A comparison of water flux and tensile strength of the Fe(OH)3@Cellulose HNFs membrane with some early reported materials was depicted in Figure 4b. It was evident that the Fe(OH)3@Cellulose HNFs had outstanding while balanced properties in regard to water flux and mechanical strength. The extremely high water flux could be ascribed to the unique hierarchical pore structure of the Fe(OH)3@Cellulose HNFs membranes. As shown in Figure 4d, the thickness of the membrane was nearly 180 µm. And this membrane was indeed composed of numerous nanofibers, which had created its loose and porous structure. Figure 4e-h presented the typical SEM images of Fe(OH)3@Cellulose HNFs (0.1M) at different magnifications, and the scales of relevant structures was shown in Figure 4i. It was discovered that the membrane had major cellular pore sizes of 10-50 µm (Figure 4e), while these cells were interconnected by many minor cellular 9



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pores with sizes of 0.1-10 µm (Figure 4f). In the zoom-in images, the HNFs had their inner tunnel sizes of 50-300 nm (Figure 4g) and bore numerous mesopores of 2-50 nm in size on their surface (Figure 4h). The combination of these multi-scaled pores provides the membrane with tremendous open and interconnected channels for water (Figure 4c), which substantially decreased the water transfer resistance and gave rise to its super-high water flux. In addition, the water contact angle of the membrane was measured to be 0o, and the total spreading and penetration time (TSPT) of a water drop was less than 0.195 s (Figure S12). These results strongly suggested the superhydrophilic characteristic of the membrane,40,41 which was also crucial for its high water flux.41 Batch Adsorption Experiments In order to evaluate the adsorption performance of this membrane towards water pollutants, phosphate, Cr(VI) and Congo red (CR) were chosen as the representatives of eutrophication pollutants, heavy metal ions and organic dyes, respectively. The influence of pH value on the adsorption of these pollutants was studied, and the results were given in Figure 5a-c. The pH value could remarkably influence the adsorption of phosphate and Cr(VI), and the highest adsorption capacity was observed at pH 3 for both of them. Meanwhile, the adsorption of CR seemed less dependent on the pH, and it reached the highest at pH 6. The effect of solution pH could be tentatively explained by considering the surface charge of the adsorbent and the ionic forms of the adsorbate. The zeta potentials of Fe(OH)3@Cellulose HNFs membrane at different pH were illustrated in Figure S13. As we can see, the pHzpc (pH of zero-point charge) of the membrane was around 6.4. Hence, the membrane surface would become positively charged when pH < 6.4, which could promote the electrostatic interaction between the membrane and anionic adsorbates in the solution. The pKa values of phosphate and Cr(VI) were around 3 42,43 so that the adsorption capacities for both of them reached the highest at pH 3. While the CR molecules could be converted to anionic ions at natural solution pH of 6.8.44 When pH decreased, the amine groups on CR molecules would also be protonated to form positively charged sites, resulting in electrostatic repulsion between the membrane surface and protonated amino groups. As a consequence, the highest adsorption of CR was achieved at natural solution pH. To reveal the adsorption mechanism and describe the relationship between adsorbents and adsorbates, the influence of adsorbate concentration on the adsorption was analyzed and the obtained data were fitted by Langmuir and Freundlich models, respectively. As shown in Figure 5e-f and Table 1, compared with the Freundlich isotherm, the Langmuir isotherm could better 10


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describe the adsorption behaviors with higher correlation coefficients, indicating the monolayer adsorption of three pollutants onto the Fe(OH)3@Cellulose HNFs.14 The maximum adsorption capacities for phosphate, Cr(VI) and CR from the Langmuir model were 172.41, 63.29, and 735.29 mg g-1, respectively. These values were significantly higher than those of many other materials as reported previously (Table 2). Such excellent adsorption properties of the Fe(OH)3@Cellulose HNFs were well in line with their highly porous and hollow structures, large surface area, and uniformly dispersed Fe(OH)3 nanoparticles. Membrane Filtration Adsorption Experiments In addition, a CR solution (20 mg L-1) was selected as the model polluted water to intuitively evaluate the filtration adsorption performance of the Fe(OH)3@Cellulose HNFs membrane (0.1 M). As demonstrated in Figure 6a and Movie S2, the CR solution could be completely decolored after filtration, and the filtrated solution displayed no obvious peak on its UV-vis spectrum. Figure 6b compared the breakthrough curves for the CR solutions against the Fe(OH)3@Cellulose HNFs membrane, neat CelluNFs membrane and Fe(OH)3 powders that were spread on filter paper, at a constant flow rate of 1 mL min-1. The Fe(OH)3@Cellulose HNFs membrane was able to remove CR thoroughly for over 180 mL. Whereas a high concentration of residue CR was detected in the filtrates even at the beginning of filtration for both the CelluNFs membrane and the Fe(OH)3 powder layer though all three had similar thicknesses. During the filtration of the CR solution, water permeated rapidly through the open and interconnected channels in the Fe(OH)3@Cellulose HNFs membrane (see the scheme in Figure 4c). In the meantime, CR molecules would be predominantly captured by those numerous accessible Fe(OH)3 nanoparticles to release purified water. The residence time may have a profound impact on the filtration adsorption capacity. As shown in Figure 6c, a higher flux resulted in a slightly earlier breakthrough for the Fe(OH)3@Cellulose HNFs membrane. This was due to the fact that a higher flux cannot offer adequate residence time for the diffusion of CR molecules in the hierarchically structured membrane. It is noteworthy that this Fe(OH)3@Cellulose HNFs membrane was capable to completely remove CR from more than 160 mL solution even at a high flux of 5 mL min-1. One of the advantages for membrane adsorption is the easy scale-up by stacking more membranes together. Since the Fe(OH)3@Cellulose HNFs membrane had a high water flux, stacking multiple membranes would improve the filtration adsorption capacity while still maintain a low pressure drop. The filtration adsorption performance of multi-layered membrane was assessed, and the corresponding breakthrough curves were shown in Figure 6d. The membranes with two layers and three layers could remove CR entirely from more than 480 11



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mL and 860 mL solution, respectively. This manifested that by simply stacking individual membranes, the adsorption performance would be largely improved. Moreover, the stability and recyclability of adsorbent are of great importance for cost effective water purifications. As demonstrated in Movie S3, Supporting Information, by applying a desorption solution (0.1 M NaOH in 50% ethanol), the CR dye on the used Fe(OH)3@Cellulose HNFs membrane could be easily washed off. The desorption solution from the outlet turned red, while the color of the membrane changed from red to its original (Figure 7d). It was worth mentioning that just after the permeation of the initial 20 mL desorption solution, the desorption percentage could be higher than 80%, indicating the regeneration

of

the

membrane

is

quite

simple

and

easy.

The

continuous

Adsorption-Desorption-Washing test of the membranes was conducted for 5 cycles to evaluate the reusability of the membrane. As shown in Figure 7a, the membrane could not only remove CR at a nearly constant amount in all adsorption stages, but also achieve an almost complete dye desorption in the elution stages. The adsorption capacity of the membrane changed slightly during the recycling test and 92% capacity was maintained after 5 cycles (Figure 7b). The breakthrough curves (Figure 7c) also suggested that even after 5 cycles, the breakthrough performance was still almost the same as that of the fresh membrane. Those results consistently confirmed the excellent reusability of the Fe(OH)3@Cellulose HNFs membrane in water purifications.

CONCLUSIONS In summary, we have demonstrated a new strategy for a one-step fabrication of HNFs through electrospinning. Cellulose was chosen as the polymer matrix, and Fe(OH)3 nanoparticles was in situ grown on the nanofibers during electrospinning. The Kirkendall effect was considered to play a key role on the formation of HNFs. The Fe(OH)3@Cellulose HNFs membranes had a large surface area with numerous Fe(OH)3 nanoparticles uniformly dispersed/embedded along the nanofibers. They could effectively remove various pollutants in water, including phosphate, heavy metal ions and organic dyes with excellent reusability. In addition, the membranes also exhibited great mechanical properties and an extremely high water flux of 11200 L m-2 h-1 bar-1. These extraordinary characteristics promised the membranes outstanding performances in water purification applications. Importantly, this approach could also be applied to fabricate other hybrid HNFs, which may serve in a broad range of scientific and engineering applications, including water purification, energy 12


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conversion and storage, catalysts, sensors, and so on.

ASSOCIATED CONTENT Supporting Information Supporting figures and tables. Videos of the twisting test, adsorption and desorption process of the Fe(OH)3@Cellulose HNFs membranes.

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected]. * E-mail: [email protected].

ORCID Wei Zhang: 0000-0001-5778-9936 Notes The authors declare no competing financial interest

ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (51303112, 51473100 and 51433006), Excellent Young Scholar Fund of Sichuan University (2015SCU04A26) and State Key Laboratory of Polymer Materials Engineering (sklpme20165 3509). The authors declare no competing financial interests.

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18


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Table 1. Isotherm parameters for phosphate, Cr(VI) and CR adsorption on Fe(OH)3@Cellulose HNFs. Langmuir model Absorbent samples q (mg/g) K (L/mg) m L

Freundlich model R2

n

KF (L/mg)

R2

phosphate

172.41

0.045

0.9886 3.348

29.424

0.9575

Cr(VI)

63.29

0.047

0.9891 2.500

7.729

0.9631

CR

735.29

0.010

0.9991 1.747

18.368

0.9445

19



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Table 2. Comparison of phosphate, Cr(VI) and CR adsorption capacities of different adsorbents. Phosphate (mg/g)

CR (mg/g)

refs

Porous hollow γ-Al2O3 nanofibers

24.16

24

Fe3O4/Bi2S3 microspheres

92.24

45

160

46

Hierarchically porous NiO–Al2O3

357

47

g-GG/SiO2

233.24

48

Absorbent samples

Cr(VI) (mg/g)

58.5

Hollow Nestlike α-Fe2O3

Zeolitic imidazolate framework-67

13.34

49

MnFe2O4@SiO2−CTAB

25.044

50

Modified iron oxide-based sorbents

38.8

41

Chitosan biosorbent modified with zirconium

60.6

52

Fe3O4@SiO2 core/shell magnetic nanoparticles

27.8

53

Fe(OH)3@Cellulose HNFs

172.41

20


63.29

735.29

This study

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Figure 1. Schematic illustrations of a) the electrospinning process and b,c) the formation of the HNFs. d-f) Photos of the Fe(OH)3@Cellulose HNFs membranes and the dead-end filtration device.

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Figure 2. SEM images and corresponding TEM images of the as-spun a,e,i) neat CelluNFs and the Fe(OH)3@Cellulose HNFs generated from NaOH solutions with different concentrations: b,f,j) 0.01M, c,g,k) 0.1M, d,h,l) 1M.

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Figure 3. a) N2 adsorption-desorption isotherms and the corresponding bimodal pore size distributions of neat CelluNFs, Fe(OH)3 particles, and Fe(OH)3@Cellulose HNFs. b) Specific surface areas and the corresponding pore volume of neat CelluNFs, Fe(OH)3 particles, and Fe(OH)3@Cellulose HNFs. c) XRD patterns of the neat CelluNFs, conventional Fe(OH)3 particles, and Fe(OH)3@cellulose HNFs. d) XPS spectra of neat CelluNFs and Fe(OH)3@Cellulose HNFs (the inset shows the Fe 2p spectrum).

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Figure 4. a) Water flux versus pressure applied on the Fe(OH)3@Cellulose HNFs membranes. b) Comparisons of water flux and mechanical properties among reported materials in different coordinates with a star representing the results in this study. c) Schematic illustration of the water permeation and pollutant adsorption process. d) A cross-sectional SEM image of the Fe(OH)3@Cellulose HNFs membrane. e-h) Microscopic architecture of Fe(OH)3@Cellulose HNFs at different magnifications, which shows the hierarchical cellular and the porous and hollow structures. i) Schematic representation of the scales of relevant structures.

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Figure 5. The effect of pH values on the adsorption performance of a) phosphate, b) Cr(VI), and c) CR by Fe(OH)3@Cellulose HNFs. Adsorption isotherms of d) phosphate, e) Cr(VI), and f) CR by Fe(OH)3@Cellulose HNFs. The adsorption tests were conducted at pH 3, 3, and 6 for phosphate, Cr(VI), and CR, respectively.

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Figure 6. a) Absorption spectra of the initial CR solution (20 mg L-1) and the treated solution after filtration adsorption (the inset shows a photo of the corresponding solutions). b) Breakthrough curves for passage of CR solutions through the Fe(OH)3 powder, neat CelluNFs and Fe(OH)3@Cellulose HNFs membranes at a flow rate of 1 mL min-1. c) Breakthrough curves for passage of CR solutions through the Fe(OH)3@Cellulose HNFs membranes at various flow rates. d) Breakthrough curves for passage of CR solutions through different layers of Fe(OH)3@Cellulose HNFs membranes, flow rate 1 mL min-1.

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Figure 7. a) Adsorption and desorption of CR through filtration from the aqueous solutions, CR concentrations in the outlet solutions are plotted as a function of permeated volume, flow rate 1 mL min-1. b) Adsorption and desorption ratio during consecutive adsorption-desorption cycles for CR. c) Breakthrough curves for adsorption of CR by fresh Fe(OH)3@Cellulose HNFs membrane and the membrane after 5 adsorption-desorption washing cycles, flow rate 1 mL min-1. d) The photos of the Fe(OH)3@Cellulose HNFs membrane before adsorption (left), after adsorption (middle) and after desorption (right), and the corresponding CR solution (20 mg L-1) and desorption solution, respectively.

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Table of Contents/Abstract Graphic

A novel strategy for the fabrication of Fe(OH)3@Cellulose HNFs through one step electrospinning. The obtained HNFs exhibited great mechanical properties and an extremely high water flux, which could efficiently remove various kinds of pollutants in water with excellent reusability. This strategy, with a wide range of applicability, may open up new opportunities for the widespread application of environmentally friendly HNFs.

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One-Step Fabrication of Fe(OH)3

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