Fabrication and Characterization of Highly Porous Fe (OH) 3

Research Article pubs.acs.org/journal/ascecg

Fabrication and Characterization of Highly Porous Fe(OH)3@Cellulose Hybrid Fibers for Effective Removal of Congo Red from Contaminated Water Jiangqi Zhao,† Zhixing Lu,§ Xu He,† Xiaofang Zhang,† Qingye Li,† Tian Xia,† Wei Zhang,*,†,‡ and Canhui Lu*,†,‡ †

State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute at Sichuan University, No. 24 South Section 1, Yihuan Road, Chengdu 610065, China ‡ Advanced Polymer Materials Research Center of Sichuan University, Port Avenue, Shishi 362700, China § Key Laboratory of Organic Optoelectronics and Molecular Engineering of the Ministry of Education, Department of Chemistry, Tsinghua University, No. 30 Shuangqing Road, Beijing 100084, China S Supporting Information *

ABSTRACT: Research on water purification has recently revolved around nanomaterials because of their large specific surface area. However, there are still some problems associated with the preparation, application, and recovery of nanomaterials. Herein, we report for the first time a novel approach for one-step synthesis of porous hybrid fibers (PHFs), which can be used as an effective adsorbent for dye removal from polluted water. A lowcost biopolymer cellulose was chosen as the matrix of the fibers, whereas a NaOH solution was applied as the coagulation bath for the cellulose spinning dope that contained a certain amount of FeCl3. The obtained Fe(OH)3@Cellulose PHFs exhibited a multiscaled pore structure, with the in situ generated Fe(OH)3 nanoparticles uniformly distributed on the regenerated cellulose nanofibrous network of the fibers. These structural attributes are quite advantageous for an efficient adsorbent. The maximum Congo red removal capacity of the Fe(OH)3@Cellulose PHFs reached 689.65 mg/g, which was much higher than many early reported values. Importantly, the Fe(OH)3@Cellulose PHFs could favorably remove the dye at natural pH through filtration adsorption with excellent reusability. This approach, with the desired characteristics of simplicity, high efficiency, low cost, and being environmentally friendly, demonstrated a great potential for industrial applications. KEYWORDS: Porous fibers, Cellulose, Fe(OH)3 nanoparticles, Water purification, Filtration adsorption

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A large number of adsorbents, such as clays,10 metal oxides,11 activated carbon,7 and fly ash12 have been applied to remove dyes from wastewater. However, owing to the high cost and the difficulties related to disposal and regeneration, most of these adsorbents cannot be widely used. Comparatively, iron oxides/ hydroxides have been extensively explored for water treatments since they are low in cost and environmentally friendly.13 As is well recognized, the performance of adsorbents depends mainly on the specific surface area of the materials and the amount of active sites responsible for adsorption.14 A lot of metal oxides/ hydroxides nanomaterials which have higher specific surface area and better adsorption properties, including Fe2O3,15 Fe3O4,16 and α-FeOOH nanoparticles17 have been developed for water treatment in recent years.

INTRODUCTION

The environmental and health issues in regard to contaminated waters have led to increasing public concern as the population density and industrialization have increased globally. Dyes are extensively used in different industries, such as paper and plastics, leather, pharmaceuticals, food, cosmetics, dyestuffs, textiles, etc.1 As a result, considerable amount of colored wastewater is generated and many organic dyes are harmful to human beings and toxic to microorganisms.2 Hence, the removal of various dyes from industrial waste waters before discharging into natural water bodies is extremely important for environmental safety. Over the years, various methods for the removal of dyes from the wastewater have been developed, such as membrane separation,3 oxidation,4 chemical precipitation,5 biological process,6 adsorption,7 etc. And adsorption is one of the most commonly used methods because of its simplicity, effectiveness, ease of operation, and reusability.8,9 © 2017 American Chemical Society

Received: April 17, 2017 Revised: June 17, 2017 Published: July 24, 2017 7723

DOI: 10.1021/acssuschemeng.7b01175 ACS Sustainable Chem. Eng. 2017, 5, 7723−7732

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. (a) Schematic illustration of the preparation process of Fe(OH)3@Cellulose PHFs. (b and c) Photos of the Fe(OH)3@Cellulose PHFs before and after freeze-drying.

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Unfortunately, some serious problems emerged when using the metal oxides/hydroxides nanomaterials in adsorption technologies. First, these nanomaterials have to be thoroughly removed after the water treatments through filtration,18 centrifugation,19 or magnetic separation.20 Such separation processes impart complexity and increase the cost for water treatment. More seriously, any failure in removing the nanoparticles will increase the risk of bringing nanoparticles into the treated water, and these nanoparticles may have chronic or acute health effects when ingested.13 In addition, because of the nanosize effect, the individual nanoparticle is easy to aggregate into large particles via the interparticle dipolar forces, leading to the loss of size effect and the decrease of specific surface area.21 To reduce the agglomeration, some additional materials, such as graphene oxide,22 poly(ether sulfone),23 polyvinyl chloride,24 etc., have been used as the template to disperse metal oxides/hydroxides nanoparticles. However, most of these approaches are complicated and costly, and the materials’ adsorption properties are also not satisfactory. Therefore, it is urgent to develop simple and more efficient methods to prepare high-performance adsorbent materials. In this work, a novel approach to fabricate porous hybrid fibers (PHFs) for dye adsorption was demonstrated. Cellulose, one of the most abundant and renewable biopolymers in the world, was chosen as the fiber matrix. The cellulose solutions in LiCl/DMAc, which contained a certain amount of FeCl3, were prepared as the spinning dope, while an aqueous alkali was used as the coagulation bath to solidify the spun fibers. A schematic of the fabrication process was illustrated in Figure 1. During cellulose regeneration process, numerous Fe(OH)3 nanoparticles were in situ generated in the cellulose matrix, resulting in a multiscaled pore structure and uniform dispersion of Fe(OH)3 nanoparticles. This not only significantly simplified the fabrication process but also made it more efficient, less costly, and more environmentally friendly. The morphology, structure, and adsorption properties of the hybrid fibers were analyzed and discussed. In addition, filtration adsorption experiments were carried out to evaluate the possibility of hybrid fibers in practical use.

EXPERIMENTAL SECTION

Materials. Purified cotton (medical level, Health Materials Co., Ltd., Xuzhou, China) was used as the raw material. Analytical grade N,N-dimethylacetamide (DMAc), LiCl, and FeCl3 were purchased from Kelong Chemicals Co., Ltd. (Chengdu, China). Distilled water was used throughout the experiment. Preparation of Fe(OH)3@Cellulose PHFs. The cellulose solution was prepared according to our previous work.25 First, the cotton was activated for better dissolution. One gram of cotton was successively immersed in 100 mL of water, methanol, and DMAc for 1 h at room temperature. Then, the activated cotton was dissolved in 100 g of LiCl/DMAc (8:92 in weight) solution under magnetic stirring until the solution became transparent. Finally, 1 g of FeCl3 was added to the cellulose solution and stirred at ambient conditions until completely dissolved. After that, the as-prepared solution was loaded in a plastic syringe for the spinning. The syringe was secured to an injection pump (TYZ5810, TSY, China), and the feeding rate of the solutions was kept at 60 mL/h (Movie S1). The coagulation bath was 0.1 mol/L NaOH solution. The obtained fibers were thoroughly washed and freeze-dried using a lyophilizer (FD31A350, Biocool, China). For comparison purposes, conventional Fe(OH)3 powders were synthesized by adding a NaOH solution into a FeCl3 solution directly. Characterization. The morphologies of fibers were observed using scanning electron microscope (SEM, Inspect F50, FEI, USA) and transmission electron microscopy (TEM, JEOL JEM-100CX, Japan). Fiber diameters and particle sizes were measured from SEM images with the software ImageJ (at least 100 fibers or particles were randomly selected from the images). The chemical structure of samples was characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). XRD analysis was performed on an X-ray diffractometer (X’Pert Pro MPD, Philips, Holland) with nickel filtered Cu Kα radiation (λ = 0.1540 nm) at 40 kV and 35 mA. FTIR analysis was performed from 4000 to 500 cm−1 at a resolution of 2 cm−1 using a Nicolet 560 FTIR spectrometer. XPS spectra were recorded on a Kratos XASAM 800 spectrometer with an Al Ka X-ray source (1486.6 eV) and an X-ray beam of around 1 mm. The content of Fe in hybrid fibers before and after adsorption was characterized by energy dispersive spectrometry (EDS, Inspect F50, FEI, USA). The N2 adsorption and desorption isotherms were obtained using a Quantachrome Instruments (Autosorb AS56B, USA) and the specific surface area and the pore size distribution were calculated by Brunauer−Emmett−Teller (BET) and Barrett−Joyner−Halenda (BJH) methods, respectively. The mechanical properties of the fibers 7724


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Figure 2. Surface (a−c) and cross-sectional (d−f) SEM images of the Fe(OH)3@Cellulose PHFs.

and red-brown. After freeze-drying, the obtained fibers became khaki, and could be arbitrarily knotted without any cracks (Figure 1c), indicating their excellent flexibility. Furthermore, the Fe(OH)3@Cellulose PHFs also exhibited great mechanical properties with a tensile strength of 10.34 MPa (Figure S1 and Table S1). The SEM images of the hybrid fibers were shown in Figure 2. The diameter of the Fe(OH)3@Cellulose PHFs was mainly distributed between 500 and 800 μm, and the surface of the fibers was not smooth (Figure 2a). In the high magnification SEM images (Figure 2c), a network structure was clearly visible on the surface of the fiber. The cross sectional SEM images of fibers were shown in Figure 2d−f. The Fe(OH)3@Cellulose PHFs displayed an obvious porous microstructure. The sizes of these pores were in the range of 20−50 μm. Furthermore, the pores were neatly arranged and interconnected through sheet-like “walls”, which seemed like the honeycomb structure (Figure 2e). Moreover, the zooming in image on a single “wall” revealed that the “wall” was constructed by a three-dimensional network consisting of nanofibers with diameter distribution of 20−30 nm and minor pores with sizes of 20−200 nm (Figure 2f). Such a multiscaled pore structure had special significance for adsorption. The major pores were conducive for mass diffusion, while the minor pores provided a large surface area. More interestingly, as shown in Figure 2f, the in situ generated Fe(OH)3 nanoparticles were uniformly distributed on the surface of the nanofibers. The regenerated cellulose could provide nucleation sites for the in situ growth of Fe(OH)3 nanoparticles. Meanwhile the cellulose matrix also acted as the “spacer” to reduce agglomeration of Fe(OH)3 nanoparticles and promoted their dispersion along the nanofibers. As shown in Figures 3b and d and S2, the sizes of the Fe(OH)3 nanoparticles were distributed between 15 and 50 nm, which were much smaller than those of conventional Fe(OH)3 particles (5−80 μm, see Figure 3a and c). The uniformly distributed Fe(OH)3 nanoparticles gave rise to increased specific surface area and active sites of the materials, which helped improve their adsorption properties significantly. Structure of Materials. The specific surface area and porosity of the products were evaluated by nitrogen

were measured on a universal testing machine (Instron 5567, USA) at a speed of 1 mm/min. The atomic absorption spectroscopy (AAS, HITACHI, Z-5000) was used to detect the iron ion concentration of the CR solution after adsorption. Batch Adsorption Experiments. Congo red (CR) were dissolved in deionized water and then diluted to the required concentrations before use. All adsorption experiments were carried out in a temperature-controlled oscillator at 25 °C under a shaking speed of 120 rpm in 100 mL flasks. The influence of pH on the adsorption behaviors was studied in the pH range of 2 to 10 adjusted by 0.1 mol/ L HCl or NaOH solutions with an initial CR concentration of 200 mg/L. The effect of the initial CR concentrations (from 10 to 1000 mg/L) on the adsorption performance was investigated at natural pH for 10 h. For adsorption kinetics studies, the effect of contact time was examined up to 24 h and the initial CR concentration was 200 mg/L. For comparison, the adsorption of CR by pure spun cellulose fibers and Fe(OH)3 was analyzed under the same conditions. Moreover, the adsorption capacities of Fe(OH)3@Cellulose PHFs on some other typical dyes, such as methyl orange (MO) and rhodamine B (RhB), were also studied. The concentrations of dye in the solutions were measured using a UV−vis spectrophotometer (UV-1600, MAPADA, China). Filtration Adsorption Experiments. The filtration adsorption experiments were carried out with a dead-end filtration device as shown in Figure S9 and Movie S2. The obtained long PHFs were cut into short ones with a length of about 2 cm, and then neatly arranged in the filtration device. A CR solution, with an initial concentration of 20 mg/L at natural pH, was forced to filter through the filtration device at a flow rate of 5 mL/min using an injection pump. To evaluate the stability and reusability of Fe(OH)3@Cellulose PHFs, 100 mL of CR solution with an initial concentration of 20 mg/L was loaded at 5 mL/min. After that, 100 mL of desorption solution (0.1 mol/L NaOH in 50% ethanol aqueous solution) was loaded at the same rate to elute the dye, thereby regenerating the fibers (Movie S3). Then 100 mL of deionized water was passed through the device to wash out the residual NaOH and ethanol. The above cycle was repeated in five consecutive runs.

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RESULTS AND DISCUSSION Morphologies of Materials. The spinning process was demonstrated in Movie S1. It is clear that the colors of fibers changed from transparent to red-brown within a short period, in accordance with the in situ generation of Fe(OH)3. As shown in Figure 1b, the resulting wet fibers were translucent 7725


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Figure 3. SEM images of the conventional Fe(OH)3 powders (a) and Fe(OH)3@Cellulose PHFs (b). Size distribution of the conventional Fe(OH)3 powders (c) and Fe(OH)3 nanoparticles in Fe(OH)3@Cellulose PHFs (d). N2 adsorption−desorption isotherms and corresponding bimodal pore size distributions of the conventional Fe(OH)3 powders (e) and Fe(OH)3@Cellulose PHFs (f).

broad peaks for Fe(OH)3 powder at 2θ = 35° and 63°, implying that the conventional Fe(OH)3 was also an amorphous material. In the XRD curve of Fe(OH)3@Cellulose PHFs, both the characteristic peaks for neat cellulose and Fe(OH)3 appeared, suggesting their hybrid and amorphous structures. The EDS spectrum in Figure 4b further confirmed the presence of Fe in the obtained products, with an atomic concentration of 4.17%. And the content of Fe(OH)3 in the hybrid fibers was calculated to be about 28.13%. XPS analysis of the Fe(OH)3 @Cellulose PHFs was conducted to elucidate the surface elements of the products, and the results were shown in Figure 4c and d. In the wide scan XPS spectrum, the photoelectron lines at binding energies of about 285.5 and 534.5 eV are attributed to C 1s and O 1s, respectively.27 Compared with neat cellulose, a new shoulder peak was found at around 720 eV in the spectrum, which is assigned to the Fe 2p.28 There were two distinct peaks at the binding energies of 711.5 and 725.1 eV in the high-resolution Fe 2p spectrum (Figure 4d), which are ascribed to Fe 2p 3/2

adsorption−desorption measurements. As shown in Figure 3e and f, the very low N2 uptake of conventional Fe(OH)3 powder indicated its nearly nonporous structure. In contrast, the Fe(OH)3@Cellulose PHFs displayed much-enhanced N2 uptake. The Fe(OH)3@Cellulose PHFs exhibited a high BET surface area of 203.68 m2/g and a large pore volume of 0.65 cm3/g, both of which were much higher than those for conventional Fe(OH)3 powder (33.68 and 0.03 cm3/g, respectively). The pore size of the Fe(OH)3@Cellulose PHFs was mainly distributed in the range of 2−70 nm, with an average pore size of 29 nm (the inserted image in Figure 3f). These results further confirmed the porous structure of Fe(OH)3@Cellulose PHFs. It can be deduced that the Fe(OH)3@Cellulose PHFs with a multiscaled porous structure and a large specific surface area should have excellent adsorption performance. The crystalline structures of the products were examined by XRD and the results were shown in Figure 4a. The curves for neat cellulose exhibited a characteristic peak at 20.5, indicating the regenerated cellulose was amorphous.26 There were two 7726


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Figure 4. (a) XRD patterns of the neat cellulose, conventional Fe(OH)3 powder and Fe(OH)3@Cellulose PHFs. (b) EDS analysis of Fe(OH)3@ Cellulose PHFs. (c, d) XPS spectra of Fe(OH)3@Cellulose PHFs: (c) wide scan and (d) Fe 2p spectrum.

and Fe 2p 1/2, respectively.28 This was in agreement with the reported values for Fe3+ compounds.29,30 Influence of Solution pH on the Adsorption. The pH value is one of the most important factors that can affect the adsorption behavior remarkably.31 The effect of pH on CR removal over the range of 2−10 was investigated, and the results was shown in Figure 5. For Fe(OH)3@Cellulose PHFs,

As the pH increased, the number of positively charged sites decreased due to the successive deprotonation of hydroxyl groups on the adsorbent, thereby retarding the adsorption of the anionic dye molecules. Furthermore, in the alkaline conditions, there existed a competition between OH− and dye anions for the positively charged adsorption sites. In addition, under strong acid conditions, the amine groups on the CR molecules could also be protonated to form positively charged sites, resulting in electrostatic repulsion between the protonated hydroxyl groups and protonated amino groups. Thus, the adsorption properties of the material also declined in strong acid conditions. The results were in consistent with some other reports.1,34 It is important to note that the natural pH of CR solution is between 6 and 7.19,35 Hence, the Fe(OH)3@Cellulose PHFs can favorably adsorb the dye without the need to adjust pH of the polluted water. This will make the water treatment simpler and less costly. The subsequent adsorption experiments, including filtration adsorption, were all conducted at natural pH conditions without additional pH adjustment. Adsorption Isotherm Study. To examine the relationship between the adsorbent and the adsorbate at equilibrium and to estimate the maximum adsorption capacity of the adsorbent, the effect of the CR concentration on adsorption was analyzed. The equilibrium adsorption capacity was obtained using eq 1. And, the equilibrium adsorption data were fitted with Langmuir (eq 2) and Freundlich models (eq 3), respectively35

Figure 5. Effect of pH values on CR adsorption of Fe(OH)3@ Cellulose PHFs.

the adsorption capacity of CR reached 378.13 mg/g at pH 6 and then decreased with the increased pH values. The effect of solution pH could be tentatively explained based on the surface charge of the adsorbent and the ionic forms of the adsorbate. CR is a benzidine-based anionic diazo dye. In the aqueous solution, this anionic diazo dye dissolves and ionizes out an anion of sulfonate group. The surface charge of the CR molecule changes at the solution pH about 6.8,32 and the pHzpc (the pH of zero point charge) of iron hydroxide is around 6,1,33

qe =

qe = 7727

(Co − Ce) × V m

(1)

qmKLCe 1 + KLCe

(2) DOI: 10.1021/acssuschemeng.7b01175 ACS Sustainable Chem. Eng. 2017, 5, 7723−7732

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Figure 6. (a) Langmuir and Freundlich isotherms for CR adsorption. (b) CR adsorption curve with contact time. (Inset: Pseudo-second-order kinetics plot.)

qe = KFCe1/ n

respectively, suggesting that the versatile Fe(OH)3@Cellulose PHFs are quite promising for treating real world waste waters. Such outstanding adsorption properties of Fe(OH)3@ Cellulose PHFs were highly dependent on their unique structures, which were summarized as follows: (1) Nanosized Fe(OH)3 particles and their uniform distribution could provide more active sites for adsorption. (2) The specific surface area of Fe(OH)3@Cellulose PHFs was much larger than that of conventional Fe(OH)3 powder. This is of particular importance for adsorbents, since adsorption mainly occurs at the materials’ surface. (3) The Fe(OH)3@Cellulose PHFs had a high porosity coupled with a multiscaled pore structure, leading to a fast adsorption rate and a high adsorption capacity. Adsorption Kinetics Study. The study of adsorption kinetics is significant as it provides valuable insights into the adsorption rate and the mechanism of adsorption. The experimental data were analyzed using the pseudo-secondorder equation (eq 4) as follows:18

(3)

where Co and Ce (mg/L) are the initial and equilibrium concentrations of CR in solution, qe is the equilibrium adsorption capacity, qm is the maximum adsorption capacity, KL and KF are constants for Langmuir and Freundlich isotherms, respectively, and n is the Freundlich constant relating to the adsorption intensity of the adsorbents. The adsorption isotherms and fitting results are shown in Figures 6a, S3, and S4 and Table S2. Compared with the Freundlich isotherm, the Langmuir isotherm could better describe the adsorption behaviors with all R2 > 0.99, implying the monolayer adsorption of CR onto the adsorbents surface.36 The maximum adsorption capacity of Fe(OH)3@Cellulose PHFs was calculated to be 689.65 mg/g, much higher than those of conventional Fe(OH)3 powder (44.11 mg/g, Figure S3) and pure spun cellulose fibers (200.81 mg/g, Figure S4). Furthermore, this adsorption capacity was also higher than many reported values for other adsorbents (Table 1). It is

⎛1⎞ 1 t ⎜⎜ ⎟⎟t = + qt k 2qe 2 ⎝ qe ⎠

Table 1. Comparison of CR Adsorption Capacities of Different Adsorbents absorbent samples

qmax (mg/g)

ref

g-GG/SiO2 hybrid nanocomposite CoFe2−xGdxO4 nanoparticles nanocrystalline Fe3O4 xerogel eucalyptus wood saw dust hollow nestlike α-Fe2O3 nanostructures pTSA-Pani@GO-CNT polyaniline/Fe0 composite nanofibers bamboo hydrochars NiO−Al2O3 nanocomposite GO/Fe3O4/PEI Fe(OH)3@Cellulose PHFs

357 263.2 251.89 66.67 160 66.666 142.69 33.7 233.24 574.7 689.65

1 2 34 35 45 46 47 48 49 50 This study

(4)

Where qe is the adsorption capacity (mg/g) at equilibrium, qt is the adsorption capacity at time t, k2 (g/mg·h) is the rate constant of pseudo-second-order adsorption. As shown in Figure 6b, the adsorption rate of CR was initially quite high and then gradually reached equilibrium in 2 h. The rapid adsorption rate at the incipient stage could be attributed to the driving force provided by the concentration gradient of CR in aqueous solutions and the existence of a great number of available active sites on the surfaces of Fe(OH)3@Cellulose PHFs. The linear fitting of experimental data and correlation kinetics parameters are shown in the inset image of Figure 6b and Table S4, respectively. The plots appeared in good linearity with a high correlation coefficient (R2 = 0.9997), and the theoretical qe value (369.68 mg/g) was very close to the experimental data (368.13 mg/g). These results indicated that the adsorption kinetics followed well the pseudo-second-order model, and the removal of CR was a chemisorption process.37 Adsorption Mechanism. To better understand the interaction between CR and Fe(OH)3@Cellulose PHFs, the FTIR spectra of Fe(OH)3@Cellulose PHFs before and after adsorption were investigated. As shown in Figure S7, the absorption peaks at 3460 (Figure S7a), and 3435 cm−1 (Figure S7b) were attributed to the stretching vibration of N−H on CR and O−H on Fe(OH)3@Cellulose PHFs, respectively.3,38 After

interesting to note that the effective adsorption capacity for the Fe(OH)3 nanoparticles was as high as 1887.05 mg/g (the content of Fe(OH)3 in PHFs was about 28.99%), more than 40 times that of the conventional Fe(OH)3 powder. The waste waters from industry generally contain several kinds of dyes. Hence, the adsorption capacities of Fe(OH)3@ Cellulose PHFs on some other typical dyes, including MO and RhB, were further evaluated. The obtained results were shown in Figures S5 and S6 and Table S3. The maximum adsorption capacities for MO and RhB were 322.58 and 125.31 mg/g, 7728


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Figure 7. XPS C 1s, O 1s, and Fe 2p spectra of Fe(OH)3@Cellulose PHFs before and after CR adsorption.

Figure 8. Proposed mechanism for CR adsorption on Fe(OH)3@Cellulose PHFs.

molecules (as shown in Figure 8). Since the adsorption happened at the surface of adsorbents, the O 1s and Fe 2p intensities were attenuated as shown in Figure 7b and c. In addition, there was no noticeable chemical shifting in both of the O 1s and Fe 2p spectra. As is well-known, the sulfonic (−SO3−) binds to metal oxide/hydroxide in three complexation forms: chelating bidentate, bridging bidentate and unidentate bonding.13 Since chelating bidentate and bridging bidentate could alter the oxidation state of oxygen and Fe, it was therefore deduced that the unidentate complex between Fe and −SO3− via an oxygen was main complexation form in this study.13 Overall, based on the results described above, a possible adsorption mechanism of CR on Fe(OH)3@Cellulose PHFs was proposed. As shown in Figure 8, under weak acid conditions, a part of hydroxyl groups on the surface of the materials were protonated and positively charged. When the CR molecules contacted with the materials’ surface, the electrostatic interaction happened between the negatively charged −SO3− groups on CR and the positively charged hydroxyl groups. The oxygen atoms of the −SO3− donated electron pairs to form monodentate complexes by exchange with the positively charged hydroxyl groups through Lewis acid−base interactions. At the same time, the hydrogen

adsorption, the peak was broadened and shifted to a lower wavenumber (3405 cm−1), which was indicative of hydrogen bond formation between the hydroxyl groups of Fe(OH)3@ Cellulose PHFs and the amine groups of dye molecules.39 Furthermore, the peak at 1582 cm−1 attributed to NN stretching38 was diminished after adsorption. Meanwhile, the strong bands at the 1225, 1171, and 1065 cm−1 regions, attributed to SO stretching,40 also diminished after the adsorption, indicating that the −NN− and −SO3− groups of CR were involved in the adsorption process. These results implied that the adsorption of CR onto Fe(OH)3@Cellulose PHFs was governed by chemical activation or chemisorption.41 The chemisorption process was further examined in respect to the chemical bonding between CR and Fe(OH)3@Cellulose PHFs through XPS analysis. As shown in Figure S8, two new peaks at around 167.7 and 399.5 eV were detected after adsorption, which were assigned to S 2p and N 1s, respectively.42 This confirmed the existence of CR on Fe(OH)3@Cellulose PHFs after adsorption. The highresolution spectra were shown in Figure 7. The peaks at 284.7, 286.5, and 288.1 eV were attributed to C−C, C−O, and CO bonds, respectively.43,44 After adsorption, the intensity of the peak at 284.7 eV increased (Figure 7a). This was due to the presence of a large amount of C−C bonds in the CR 7729


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Figure 9. (a) Absorption spectra of the initial CR solution (20 mg/L) and the treated solution after filtration adsorption (the inset shows a photo of the corresponding solutions). (b) Breakthrough curve for passage of CR solutions through the Fe(OH)3@Cellulose PHFs at a flow rate of 5 mL/ min. (c) 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 5 mL/min. (d) Adsorption and desorption ratio during consecutive adsorption−desorption cycles for CR.

bonding between −NH2 groups on CR molecules and unprotonated hydroxyl groups on the surface of Fe(OH)3 and cellulose also played a remarkable role in the adsorption. Filtration Adsorption Experiments. The above batchadsorption results indicated that the Fe(OH)3@Cellulose PHFs had a high adsorption rate and a large adsorption capacity for CR, which made it possible to use these fibers for a fast and highly efficient water purification. Additionally, one of the advantages for fiber adsorbents is that they can be arbitrarily cut and placed in customized filtration devices. Herein, a CR solution (20 mg/L) was selected as the model pollutant to intuitively evaluate the filtration adsorption performance of the Fe(OH)3@Cellulose PHFs. As shown in Figure 9a, the CR solution could be completely decolored after filtration, and the filtrated solution displayed no visible peak on its UV−vis spectrum. Figure 9b showed the breakthrough curve for the permeation of CR solution through Fe(OH)3@Cellulose PHFs at a constant flow rate of 5 mL/min. About 200 mL of the solution was purified with a dye removal percentage higher than 99%. The complete adsorption of CR at the early filtration stage could be possibly ascribed to the rapid mass diffusion through the multiscaled pores of the fibers and the effective adsorption of CR molecules onto the Fe(OH)3 nanoparticles. Movie S2 more vividly demonstrated the filtration adsorption process. The stability and recyclability of adsorbents are of great importance for practical applications as they will lower the material costs for water treatment. After the adsorption, various kinds of desorption solutions have been tested for the regeneration of the adsorbents, and 0.1 mol/L NaOH in 50%

ethanol aqueous solution was found to be an ideal desorption solution. As demonstrated in Movie S3, the adsorbed CR could be quickly and effectively washed off, and almost 100% desorption was achieved. The continuous adsorption− desorption−washing test of Fe(OH)3@Cellulose PHFs was conducted at a flux of 5 mL/min for 5 cycles to evaluate the reusability of the membrane and the results were shown in Figure 9c. The Fe(OH)3@Cellulose PHFs could not only remove the CR dye almost completely in all the adsorption stages, but also achieve a nearly complete dye desorption in the elution stages. Note that even after 5 cycles, the filtration adsorption capacity just decreased slightly to 97%. Compared with many other nanomaterials and powders which often require complicated separation procedures for reuse, the regeneration of Fe(OH)3@Cellulose PHFs is quite simple and easy. The morphology and the composition of Fe(OH)3@ Cellulose PHFs before and after the successive adsorption− desorption−washing tests were characterized by SEM and EDS, and the results were displayed in Figure S10. It is evident that the microstructure of PHFs, including their multiscaled pore structure and the uniformly distributed Fe(OH)3 nanoparticles, were well maintained after the tests. As estimated from the EDS spectra, the atomic concentration of Fe in the fibers after the successive tests was about 4.11%, which was very similar to that in the fresh ones (4.17%). In addition, AAS was used to analyze the Fe concentration in the treated solution, but no Fe was detected. These results indicated that the Fe(OH)3 nanoparticles could hardly fall from the hybrid fibers during the adsorption process. Such a strong structural integrity ensured 7730


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(2015SCU04A26), and State Key Laboratory of Polymer Materials Engineering (sklpme2016-3-09). The authors declare no competing financial interests.

the excellent reusability of Fe(OH)3@Cellulose PHFs in treating waste waters.

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CONCLUSIONS In summary, we have demonstrated a new strategy to fabricate Fe(OH)3@Cellulose PHFs via a one-step pathway. Cellulose matrix acted as the “spacer” to reduce agglomeration of the in suit generated Fe(OH)3 nanoparticles and promoted their uniform dispersion along the cellulose nanofibrous network in the fibers. The obtained Fe(OH)3@Cellulose PHFs exhibited a multiscaled pore structure with a large specific surface area. A high CR adsorption capacity of 689.65 mg/g was achieved at natural pH conditions. Through the adsorption kinetics and mechanism investigations, it was elucidated that the Fe(OH)3@ Cellulose PHFs essentially immobilized the dye via hydrogen bond and unidentate chemisorption. Filtration adsorption studies indicated that Fe(OH)3@Cellulose PHFs could remove the CR dye in water easily and effectively with excellent reusability. This approach is proved to be simple, highly efficient, low-cost, and environmentally friendly, suggesting a promising potential for its industrial applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01175. Stress−strain curve for the Fe(OH)3@Cellulose PHFs, results of the tensile testing, TEM image of the Fe(OH)3@Cellulose PHFs, Langmuir and Freundlich isotherms for CR adsorption onto conventional Fe(OH)3 powders and pure cellulose fibers, isotherm constants for the adsorption of CR, MO and RhB, Langmuir and Freundlich isotherms for MO and RhB adsorption onto Fe(OH)3@Cellulose PHFs, kinetics parameters for CR removal, FTIR spectra of CR and Fe(OH)3@Cellulose PHFs, XPS spectra of Fe(OH)3@ Cellulose, photos of dead-end filtration devices and Fe(OH)3@Cellulose PHFs neatly arranged in the filtration device, and SEM images and EDS analysis of Fe(OH)3@Cellulose PHFs (PDF) Spinning process of the Fe(OH)3@Cellulose PHFs (AVI) Filtration adsorption process of the Fe(OH)3@Cellulose PHFs (AVI) Filtration desorption process of the Fe(OH)3@Cellulose PHFs (AVI)

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REFERENCES

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.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51473100, 51303112, and 51433006), Excellent Young Scholar Fund of Sichuan University 7731


Research Article

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Fabrication and Characterization of Highly Porous Fe (OH) 3

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