Highly flexible magnesium silicate nanofibrous membranes for effective removal of methylene blue from aqueous solution

Accepted Manuscript Highly flexible magnesium silicate nanofibrous membranes for effective removal of methylene blue from aqueous solution Rui Zhao, Yanzi Li, Bolun Sun, Shen Chao, Xiang Li, Ce Wang, Guangshan Zhu PII: DOI: Reference:

S1385-8947(18)32242-3 https://doi.org/10.1016/j.cej.2018.11.011 CEJ 20313

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

18 August 2018 11 October 2018 2 November 2018

Please cite this article as: R. Zhao, Y. Li, B. Sun, S. Chao, X. Li, C. Wang, G. Zhu, Highly flexible magnesium silicate nanofibrous membranes for effective removal of methylene blue from aqueous solution, Chemical Engineering Journal (2018), doi: https://doi.org/10.1016/j.cej.2018.11.011

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Highly flexible magnesium silicate nanofibrous membranes for effective removal of methylene blue from aqueous solution

Rui Zhao,b Yanzi Li,a Bolun Sun,a Shen Chao,a Xiang Li,*,a Ce Wang*,a and Guangshan Zhub

a

Alan G. MacDiarmid Institute, College of Chemistry, Jilin University, Changchun

130012, PR China b

Key Laboratory of Polyoxometalate Science of the Ministry of Education, Faculty of

Chemistry, Northeast Normal University, Changchun 130024, PR China

*

Corresponding authors：

Tel.: +86-431-85168292; Fax: +86-431-85168292. Email address: [email protected] (X. Li).

1

Abstract Dye pollution in water environment is a significant threat to ecological and human health, and its removal is very important yet challenging. Magnesium silicate-based materials have proven to be good adsorbents for dye wastewater treatment. In consideration of recycling and practicability, flexible and robust magnesium silicate fiber membrane (MgSiFM) is prepared for the first time via hydrothermal reaction using electrospun flexible SiO2 fiber membrane as silicon template. The preparation process is efficient, low cost and easy to scale-up. MgSiFM shows high tensile strength (5.52 MPa) and good mechanical stability. Moreover, the BET surface area of MgSiFM is measured as 463.4 m2/g, which is similar to the reported powdery magnesium silicate-based materials. As expected, MgSiFM shows excellent adsorption capacity of 609.75 mg/g for cationic dye methylene blue (MB) with good recyclability. In particular, unlike powdery MgSi adsorbents, MgSiFM could act as a filter membrane for continuous and fast MB removal. The filtration performance is better than commercial nylon-6 microfiltration membrane and MgSi coated electrospun polymer fiber membrane. This work demonstrates a novel and facile strategy for the preparation of flexible magnesium silicate fiber membrane, which exhibits promising potential in practical wastewater treatment.

Keywords: magnesium silicate; fiber membrane; flexibility; adsorption; filtration

2

1. Introduction With the accelerated urbanization and agriculture/industry developments, water pollution problems have become more and more serious in the 21st century [1-3]. As one of the most toxic pollutants in wastewater, organic dyes are widely used in a large number of industries, including paper, textile, leather, pharmaceutical, food, cosmetic and so on [4]. Most organic dyes show complex molecular structures, low biodegradability and stable nature; in addition, due to their tendency to accumulate in living and food chain transmission, dye pollutants in aquatic environment would be a great threat toward human health. More seriously, they could be mutagenic and carcinogenic [5, 6]. Therefore, various methods, such as catalysis [7], biological processes [8], ozonization [9], adsorption [10], electrochemical [11], etc., have been applied to purify dye contaminated wastewater. Among the many treatment methods, adsorption is promising for the removal of dyes because of its easy operation, high efficiency and low cost [12, 13]. As an important part of adsorption method, adsorbents determine the feasibility and efficiency of the adsorption processes. The performance of an adsorbent is mainly dependent on its own structure and chemical composition [14, 15]. Traditional adsorbents, such as activated carbon, natural zeolite, activated alumina, etc, have been employed in practical wastewater treatment industry [16-18]. However, novel and non-conventional adsorbents with high adsorption performance still need to be explored and developed. In the past decade, silicate materials have received unprecedented attention as adsorbents in water treatment due to their low cost, stability, environmentally friendly, 3

and interesting structures [19, 20]. The superior adsorption capacity of silicate materials toward cationic species attributes to the replaceable metal ions existing between layers or among chains of silicon-oxygen tetrahedron frameworks [21]. Among the metal silicates, magnesium silicate (MgSi), which is fabricated via hydrothermal reaction from the precursor SiO2, is a common candidate for the removal of pollutants. Many nano-sized magnesium silicate materials with different structure have been prepared [22, 23]. For example, MgSi spheres [24], MgSi nanotubes [22], MgSi short nanofibers [25] and mesoporous MgSi [26] were developed and the obtained adsorbents showed high adsorption capacities toward heavy metals or dyes (Pb2+, Cd2+ and methylene blue). The present research focuses on the synthesis of magnesium silicate-based adsorbents with unique structure and special morphologies. To date, the synthesized magnesium silicate adsorbents are in the form of powder, which is difficult to recycle and can lead to the secondary pollution. Although some improved studies have reported to recycle magnesium silicates by an external magnetic field through the combination with magnetic particles [27], the processes are complex and time-consuming. Flexible and robust membrane form can be separated more easily and is a necessity in practical applications. Over the past few years, electrospinning technique is considered as an easy, effective and versatile method to produce one-dimensional fibrous membrane composed by continuous organic, inorganic, or organic/inorganic fibers with diameters from micrometer down to nanometer range [28, 29]. Owning to the high 4

porosity, relatively high specific surface area, and high pore interconnectivity, electrospun fibers show great potential in adsorption field [30]. Chen’ group fabricated flexible silica/mesoporous alumina core-shell fibers through a facile and applicable one-step coaxial electrospinning technique [31]. The core-shell fiber adsorbents exhibited good adsorption performance toward Congo red with an adsorption of 115 mg/g within 48 h. Moreover, the form of the silica/mesoporous alumina fibers was kept well throughout the reuse process. In another study, Najafabadi et al. developed a novel electrospun chitosan/GO nanofibrous adsorbent to combine the adsorption capacities of chitosan and GO [32]. The adsorption behaviors toward Cu(II), Pb (II) and Cr (VI) metal ions from aqueous solutions were investigated. The adsorption processes followed double-exponential mechanism and Redlich–Peterson isotherm model. The maximum adsorption capacities toward Cu(II), Pb (II) and Cr (VI) onto chitosan/GO composite nanofibers based on Langmuir isotherm were 461.3, 423.8 and 310.4 mg/g at 45 °C. In our previous study, magnesium silicate was coated onto electrospun polyacrylonitrile (PAN) fibers to prepare flexible magnesium silicate-based adsorbent [33]. However, PAN was the main body of the composite fibers and the content of magnesium silicate in composite fibers was only 17.30%. Recently, flexible electrospun inorganic fibrous membranes, including TiO2 fibers [34], SiO2 fibers [35], Al2O3 fibers [36], carbon fibers [37], etc. have been prepared successfully. The continuous fiber structure from electrospinning method endows the inorganic fiber membranes good flexibility instead of fragile property. The as-obtained electrospun inorganic fiber membranes show wide 5

applications in adsorption, filtration and catalysis. Among the reported flexible electrospun inorganic fibers, flexible electrospun SiO2 fibers have been investigated in detail by Ding’s group and were used as supporting or blended materials to be applied in multiple fields [34, 38, 39]. These studies give us an inspiration to consider the fabrication of flexible magnesium silicate fiber membrane. Herein, we report a novel flexible and robust magnesium silicate fiber membrane (MgSiFM) via hydrothermal reaction using electrospun SiO2 fiber as the flexible template. Different from the reported MgSi materials, our SiO2 precursor was a flexible fiber membrane. Under the hydrothermal process, SiO2 fibers were converted into MgSi fibers with the original flexibility retained. The physical and chemical characteristics of as-obtained fibers were evaluated by various characterized techniques. The removal performance toward cationic organic dye methylene blue (MB) by MgSiFM was investigated. Because of its large surface area, high magnesium silicate content, easy separation and good stability, MgSiFM exhibited excellent adsorption capacity and recyclability toward MB. More attractively, due to good membrane forming and mechanical properties, MgSiFM could act as the filter membrane for effective and fast filtration adsorption of MB, unlike other powdery magnesium silicate-based adsorbents. On the basis of obtained results, magnesium silicate fiber membrane showed its promising potential in the purification of cationic dye pollutants.

2. Experimental section 6

2.1. Materials Polyvinyl alcohol (PVA, Mw=75000-80000) was purchased from Beijing Chemical Factory. Tetraethyl orthosilicate (TEOS, 98%) was purchased from Aladdin. Methylene blue (MB) (C16H18ClN3S·3H2O) was purchased from Sinogharm Chemical Reagent Co., Ltd. Magnesium chloride (MgCl2) and ammonia chloride (NH4Cl) were obtained from Beijing Chemical Factory. Other inorganic reagents were also purchased from Beijing Chemical Factory. All chemicals were used as received without further purification. 2.2. Preparation of flexible SiO2 fiber membrane (SiO2FM) The SiO2 fiber membranes were fabricated in a typical synthesis (electrospinning and calcination) according to the previous reports [38, 39]. Firstly, a 12 wt% PVA solution (dissolving PVA powder in water at 80 °C) was added into a silica sol generated from the hydrolysis of TEOS with H3PO4 acted as a catalyst (the molar ratio of TEOS, H2O, and H3PO4 was 1:10:0.01). The mixture was stirred for 8 h with equal weight of the PVA solution and silica sol. Then, the above solution was loaded into a 5 mL syringe to conduct the electrospinning process using an electrospinning apparatus (WL-2C, Beijing Ion Beam Technology Co., Ltd., China). 15 kV was provided between the cathode and anode at a distance of 20 cm with a flow rate of 1.0 mL/h. The as-prepared PVA/silica fiber membranes were dried at 80 °C for 24 h. Finally, PVA/silica fiber membranes were placed into A vacuum tube furnace (OTF-1200X, Hefei Kejing Materials Technology Co., Ltd). Flexible SiO2 fiber membrane was obtained by the calcination of PVA/silica fiber membranes at 800 °C 7

in air with a heating rate of 5 °C/min to decompose the organic compound. 2.3. Preparation of magnesium silicate fiber membrane (MgSiFM) MgSiFM was fabricated from SiO2FM through a hydrothermal reaction [33]. Generally, the as-obtained SiO2FM was cut into 4 × 4 cm2 piece and was put into a hydrothermal synthesis reactor containing 40 mL solution containing magnesium chloride (72 mg), ammonia chloride (535 mg), and NH3·H2O (1 mL). After being immersed thoroughly, the mixed solution was transferred into the Teflon-lined stainless steel autoclave (50.0 mL) and heated to 140 °C for 10 h. After cooling down to room temperature, the as-prepared fiber membrane was washed by water and ethanol for several times, then dried in vacuum oven at 60 °C overnight to obtain MgSiFM. 2.4. Characterization The morphology and microstructure of the fibers were observed with field emission scanning electron microscopy (SEM Shimadzu SSX-550) and transmission electron microscopy (TEM; JEOL JEM-3010). Before SEM observation, the fiber samples on an aluminum stud were coated with platinum or gold using a sputter coater (Cressington 108Auto, UK). Energy-dispersive X-ray spectroscopy (EDX) of relevant samples were obtained from the scanning electron microscopy. The mean diameter of the fibers was calculated from measuring the different parts of the fibers at 100 different fibers from the SEM images using the commercial software package Image-Pro Plus. FT-IR spectra were purposed on a Bruker Vector-22 spectrometer (resolution: 1.5 cm-1, number of scans: 32) from 4000 to 400 cm-1 using 8

powder-pressed KBr pellets (mass ratio of KBr and sample: 100:1) at room temperature. X-ray diffraction (XRD) measurement was implemented out using a Rigaku D/Max 2500 diffractometer with Cu Kα radiation (λ = 1.54 Å) at a generator voltage of 40 kV and a generator current of 40 mA. The mechanical properties of the fiber membranes were performed by assembling the membranes (dimensions: length = 40 mm, width = 20 mm) between two stainless steel clamps with a tensile speed of 10 mm·min−1 on a mechanical strength microtest device (410R250, Test Resources, Shakopee, MN, USA). The specific surface area of the fiber sample was derived from N2 adsorption-desorption isotherms, which were obtained at -196 °C on a Micromeritics 2420 sorptometer. The zeta potential of MgSiFM was measured by ZEN3600 Zetasizer (Malvern Co., Ltd. UK) at 20 °C. All the measurements were performed at a dynamic pH of the electrolyte (KCl, 0.01 mol/L, pH 2-12). The MgSiFM samples were rinsed and soaked in the electrolyte with different pH values (10 mg MgSiFM in 30 mL electrolyte). The zeta potential values were determined after the 48-hour agitation. 2.5. Bath adsorption experiments toward MB Batch adsorption experiments were performed on a model BETS-M1 shaker (Kylin-Bell Lab Instruments Co., Ltd., China) with a shaking speed of 120 rpm. The initial pH values of the MB solution were adjusted in the range of 2−10 by dropwise adding 0.01 mol/L NaOH or 0.01 mol/L HCl solutions to study the effects of initial solution pH. The pH values of the adsorption solutions were measured using a pH meter (Starter 2100, Ohaus Instruments Co., Ltd.). Kinetic experiments were 9

performed by mixing 9 mg of adsorbent into 100 mL of MB solution with the initial concentration of 20 mg/L. The adsorption isotherms for MB were investigated by initial concentrations ranging from 20 to 600 mg/L with the adsorbent dosage of 0.1 g/L. For the desorption-readsorption experiment, 5 mg adsorbent was added into 40 mL dye solution (20 mg/L) for 48 h. The MB-adsorbed adsorbent was regenerated by the calcination at 450 °C in muffle furnace (KSI-1100X, Hefei Kejing Materials Technology Co., Ltd) in air for 2 h to remove adsorbed MB molecules. The adsorbent was reused in adsorption experiments directly without any treatments and the process was repeated five times. All the adsorption experiments were performed at 20±2 °C. The concentration of MB in solution was determined by using a Shimadzu UV-2501 UV−vis spectrophotometer based on the standard curve. The adsorption capacity (q) of MB onto each adsorbent was calculated on the basis of the following equation: (1) where C0 and Ce (mg/L) are the initial and the equilibrium concentration of MB in the aqueous solution, respectively. V (L) is the volume of the solution, and W (g) is the mass of the dry adsorbent. 2.6. Filtration adsorption experiments toward MB Filtration adsorption experiments were conducted with a laboratory-made dead-end filtration device. Round filter membrane with an effective filtration area of 0.8 cm2 (~20 mg) was placed in the filtration cell (Fig. S1). MB solution (initial concentration: 10 mg/L, pH value: 7) was forced to pass through the filtration cell using a syringe pump (Longer Precision Pump Co., Ltd, Hebei, China) at constant flow rates (2-8 10

mL/min). Note that, the membrane filtration system was stabilized with deionized water prior to the experiment. For reusability evaluation of MgSiFM filter membrane, MB solution (initial concentration: 10 mg/L, pH value: 7) was firstly passed through the membrane at a flow rate of 2 mL/min. After the filtration experiment, the used MgSiFM was regenerated by the calcination at 450 °C in muffle furnace (KSI-1100X, Hefei Kejing Materials Technology Co., Ltd) in air for 2 h. The regenerated MgSiFM filter membrane was reused in filtration experiment and the above cycle was repeated five times.

3. Results and discussion

Fig. 1. Schematic illustration of the preparation of flexible magnesium silicate fiber membrane (MgSiFM).

3.1. MgSiFM fabrication and characterization The synthetic route to flexible magnesium silicate fiber membrane (MgSiFM) is 11

schematically illustrated in Fig. 1. First, flexible SiO2 fiber membrane (SiO2FM) was prepared by the calcination of electrospun PVA/silica composite fiber membrane, which has been reported in the previous work [38, 39]. Then, SiO2FM was acted as structure director for the fabrication of magnesium silicate fiber membrane through hydrothermal reaction, with its original flexibility retained. During the hydrothermal process, MgSiMF was obtained from SiO2FM, which is a chemical reaction. Surface silica on SiO2FM was gradually dissolved to release silicate ions in the alkaline solution. At the same time, the silicate ions reacted with magnesium ions to form magnesium silicate nanosheets, which assemble with each other to form “flower-like” morphology onto the surface of undissolved SiO2 fibers [23, 24]. Thus, MgSiFM can be fabricated using common and simple techniques, which makes it suitable for large-scale preparation. In addition, this synthetic route may also be applied to prepare other flexible metal silicate fiber membranes, such as manganese silicate, nickel silicate, copper silicate, cobalt silicate, and so on.

12

Fig. 2. SEM images of PVA/silica composite fibers (a), SiO2FM (b) and MgSiFM (c, d). TEM images of SiO2FM (e) and MgSiFM (f).

The morphology of the obtained fiber samples was characterized by electron microscopy. As shown in the representative SEM images (Fig. 2a~2d), all the fibers displayed long and continuous fibrous morphology; and the fibers were randomly stacked as nonwoven fabrics. However, fiber diameters and surface structures were different. Due to the complete decomposition of organic compounds, the average diameter of SiO2 fibers decreased to 400 nm from the 431 nm of PVA/silica 13

composite fibers (Fig. S2). For these two samples, the fibers’ surfaces were both smooth. After the hydrothermal reaction, large numbers of nanosheets were observed on the surface of MgSi fibers (Fig. 2c, 2d). The change of surface structure was also studied by TEM observation. SiO2 fibers were homogeneous and the surface was smooth (Fig. 2e). For MgSi fibers, their surfaces were course and the twisted nanosheets were connected with each other to form “flower-like” surface morphology (Fig. 2f). Similar “flower-like” surface morphologies were also found in MgSi nanotubes, spheres and short fibers [22, 24, 25]. Accompanying the change of surface morphology, the average diameter increased significantly to 572 nm (Fig. S2), attributing to the transformation of SiO2 to MgSi. Through the analysis of fiber morphology, we can initially confirm the formation of MgSi fibers. Another purpose of this work is to fabricate flexible and robust fiber membrane. Thus, the mechanical properties, including tensile strength and flexibility were studied. Stress-strain curves of SiO2FM and MgSiFM are shown in Fig. S3a. The tensile strength of SiO2FM was 5.10±0.77 MPa and this value decreased to 5.52±0.49 MPa for MgSiFM. This phenomenon can be explained by the reason that the generated MgSi structures make MgSiFM less dense than SiO2FM. However, MgSiFM could still raise a weight of 100 g (Fig. S3b). The flexibility of MgSiFM was further proved by multiple-bending/folding tests, in which MgSiFM remained intact without any cracks after bending or folding (Fig. S3c~S3d). The mechanical property results of MgSiFM could guarantee its usability in practical adsorption or filtration applications.

14

Fig. 3. EDS spectra of obtained fiber membranes (a) and SEM elemental mapping for single MgSi fiber (b).

The chemical composition and other physical properties of obtained fiber samples were also characterized. EDX spectra from corresponding SEM images were used to analyze the elementary composition (Fig. 3a). For PVA/silica composite fiber membrane, C, O and Si elements were observed. After the calcinations, SiO2FM displayed only O and Si elements without C, indicating that the polymer template has 15

been decomposed completely. In the EDX spectrum of MgSiFM, except for O and Si peaks, an obvious new peak for Mg element appeared corresponding to elemental composition of magnesium silicate. Furthermore, all the three samples showed Pt element at 2.06 keV due to the coating platinum before SEM observation. As shown in SEM elemental mapping images of Si, Mg, and O on a single fiber of MgSiFM (Fig. 3b), three elements (Si, Mg, and O) uniformly distributed on MgSi fiber, indicating that the chemical composition of MgSiFM is homogeneous. FT-IR spectra of SiO2FM and MgSiFM were used to analyze the chemical structures of fiber samples (Fig. 4a). In the spectrum of SiO2FM, the bands at 1233, 1074, 802 and 457 cm-1 were attributed to the stretching and deformation vibrations of SiO2 [40]. The bands at 3426 and 1626 cm-1 were ascribed to O–H vibrations from adsorbed water [41]. No other bands were observed, suggesting that PVA polymer template has been removed and pure SiO2 fibers are prepared successfully. After the hydrothermal reaction, the spectrum of MgSiFM showed two new peaks at 3679 and 661 cm-1, which are associated to the characteristic absorption bands for Mg–OH and Si–O–Si, respectively [40, 42]. Moreover, the band at 1074 cm-1 shifted to 1016 cm-1, which belongs to Si–O–Si stretching vibration in tetrahedral sheets [41]. The FT-IR results demonstrate the transformation process from SiO2 to MgSi during the hydrothermal reaction. To further confirm the formation of MgSi, XRD patterns of SiO2FM and MgSiFM were recorded and displayed in Fig. 4b. SiO2FM exhibited only one broad peak at 22° due to its composition of amorphous silica [43]. However, MgSiFM displayed well-defined diffraction peaks at 19.5°, 34.5°, 53.7°, 60.9° and 71.4°, which 16

can be indexed to magnesium silicate (Mg3Si4O9(OH)4, JCPDS No. 03-0174). This further verified its composition of silicon-oxygen tetrahedron frameworks [25, 43]. The surface chemical composition of the obtained fiber membranes was also determined by XPS analysis. As shown in Fig. 4c, only the peaks of O and Si could be observed in the full-scanning XPS spectrum of SiO2FM. After hydrothermal reaction, new peaks at 307.5 eV and 50.1 eV appeared with high intensity, corresponding to Mg element in MgSiFM. Based on the integrated peak areas from XPS spectrum of MgSiFM, the Mg atomic fraction and Si atomic fraction were quantitatively estimated and found to be 12.56 at. % and 16.81 at. %, respectively. Thus, the atomic fraction ratio of Mg to Si was 0.747 (12.56/16.81) which is close to the atomic fraction ratio in Mg3Si4O9(OH)4 (3/4=0.75), suggesting that the surface chemical composition of MgSiFM is magnesium silicate. Moreover, the high resolution O1s spectra were fitted and analyzed (Fig. 4d). For SiO2FM, the O1s spectrum could be subdivided into two Gaussian-Lorenz peaks. The peaks at 532.7 eV was assigned to Si–O–Si and the peak at 533.9 eV was assigned to adsorbed H2O or –OH [42, 44]. For MgSiFM, the O1s spectrum consisted of three fitting peaks. The new peak at 533.0 eV corresponded to Si–O–Mg moieties of MgSi [44]. The above results and analysis confirmed the successful preparation of flexible magnesium silicate fiber membrane.

17

Fig. 4. FT-IR spectra (a), XRD patterns (b), XPS full-scan spectra (c), high resolution peaks for O 1s (d) and N2 adsorption/desorption isotherms (e) of SiO2FM and MgSiFM. The pore size distribution of MgSiFM (f).

Specific surface area and pore size distribution have strong influences on the adsorption

performance,

adsorption–desorption

which

has

measurement,

been as 18

investigated indicated

in

by

nitrogen

Fig.

4e.

(N2) The

Brunauer–Emmett–Teller (BET) surface area of SiO2FM was only 11.2 m2/g and is consistent with previous reports [38, 39]. Apparently, the N2 adsorption-desorption isotherms of MgSiFM showed typical type IV curve with H3 hysteresis loop, exhibiting the characteristics of a porous material. The calculated BET surface area was 463.4 m2/g, which is significantly larger than the value of SiO2FM, due to the “flower-like” nanosheet structure and intrinsic property of MgSi. The average pore diameter and pore volume for MgSiFM were 3.93 nm and 0.34 cm3/g, respectively (Fig. 4f). With high surface area and pore volume, MgSiFM can provide large number of active sites for capturing pollutants from wastewater, making it favorable for adsorption.

.

19

Fig. 5. (a) The effect of pH on adsorption of MB by MgSiFM (inset is molecular structure of MB), (b) zeta potential of MgSiFM different pH value, (c) removal efficiency toward MB by different adsorbents, (c) adsorption kinetics curve of MB onto MgSiFM, (d) the pseudo-second-order kinetic fitting plot from (c), and (f) Weber and Morris model graphs.

3.2. MB adsorption experiments 20

Through the above-mentioned characterization and discussion, we have demonstrated the successful preparation of flexible and robust magnesium silicate fiber membrane, which is beneficial for the practical adsorption application. The adsorption performance of MgSiFM toward cationic dye methylene blue (MB, inset in Fig. 5a) was also investigated and shall be discussed in the following section. 3.2.1. Effect of initial solution pH The pH value of the adsorption solution is a significant factor during the adsorption process, which influences the surface charge and property of both adsorbent and adsorbate. The pH effect study will give the guidance in the water treatment process. The initial pH values effect on the MB adsorption by MgSiFM were conducted by varying solution pH from 2.0 to 10.0. As the results shown in Fig. 5a, the initial pH values indeed influenced the adsorption capacity significantly. It is shown that the adsorption capacity toward MB increased as initial pH increases in the range of 2-7, and then became constant after pH value of 7. To explain this variation tendency, the surface charges of MgSiFM were determined by the zeta-potential over a range of pH values from 2 to 12. As displayed in Fig. 5b, the isoelectric point (IEP, zeta potential = 0 V) occurred at pH = 4.3. The material is positively charged in the solution when its pH is below IEP and it has a negative charge at solution pH > IEP [45]. The pKa value of MB is equal to 3.8 [46]. Thus MB will exist in molecular forms at pH<3.8, which is not conducive to be adsorbed by charged MgSiFM. Therefore, MgSiFM showed low adsorption capacity toward MB at pH<3.8. However, in the pH range of 3.8–4.3, abundant H+ ions occupied the surface of MgSiFM via the competition with 21

the cationic dyes, making the surface −Si–OH groups on MgSiFM exist as the form of –Si–OH2+. The electrostatic repulsion between –Si–OH2+ and cationic MB could also lead to a low adsorption capacity at pH value of 4. As increasing the pH value, the surface −Si–OH groups tend to transform as −Si–O- groups and the MgSiFM becomes negatively charged, which is beneficial for the adsorption of cationic MB via electrostatic attraction. Moreover, due to the electrostatic attraction, more MB molecules have the chance to enter into the silicon–oxygen tetrahedral frameworks through ion-exchange. Therefore, a gradual increased adsorption capacity was observed in the pH range of 5–7. After pH value of 7, the surface −Si–OH groups have transformed to −Si–O- groups. The electrostatic effect reaches saturation. Thus, the adsorption capacity showed little change in alkaline condition (pH: 8-10), suggesting that the OH- ions have almost no influence on the MB adsorption by MgSiFM. The results are consistent with previous reports [43, 47]. The detailed adsorption mechanism is elucidated by XPS and FT-IR investigation below. Considering the goal of effective treatment of dye polluted wastewater, a pH value of 7 is fixed in subsequent adsorption study.

3.2.2. Comparative adsorption To explore the change of adsorption capacity after the formation of MgSi, a comparative adsorption test of related adsorbents was conducted (MB solution: 100 mL of 20 mg/L, adsorbent: ~ 9 mg, pH value: 7). Except for precursor SiO2 fiber membrane, magnesium silicate coated electrospun polyacrylonitrile fiber membrane 22

(MgSi/PAN, the preparation process is shown in our previous study [33]) was also compared. As shown in Fig. 5c, the experimental group treated by MgSiFM showed faster and higher removal of MB. After 48 h adsorption, the removal efficiencies for MgSiFM, MgSi/PAN and SiO2FM were 94.82%, 61.50% and 8.95%, respectively. The results revealed that the hydrothermal reaction to generate magnesium silicate could improve the adsorption capacity of SiO2 fibers toward MB effectively. In comparison with coating magnesium silicate onto electrospun polymer fibers, the flexible magnesium silicate fibers in this work showed better adsorption performance attributing to the higher content of MgSi and larger BET.

3.2.3. Adsorption kinetics To better understand the adsorption rate and rate control step, adsorption kinetic curve of MB adsorption by MgSiFM was measured and displayed in Fig. 5d. A rapid adsorption was observed in the first 8 h due to a large number of available adsorption sites on MgSiFM surfaces. After these sites are covered, MB molecules have to come into the fibers leading to a slow adsorption rate until the adsorption equilibrium. Two widely-used kinetic models (the pseudo-first-order kinetic model and the pseudo-second-order kinetic model) were applied to analyze kinetic data. Their linear equations are expressed as follows [39]: (2) (3) where qt and qe (mg•g-1) are the adsorption capacity at time t and equilibrium time, 23

respectively. k1 (h-1) and k2 (g•h-1•mg-1) are the pseudo-first order model rate constant and the pseudo-second order model rate constant, respectively. Their linear fitting curves are displayed in Fig. S4 and Fig. 5d. The calculated kinetic parameters are listed in Table 1. The parameters obtained from the pseudo-second-order kinetic model showed higher correlation coefficient (R2=0.9983) and the calculated qe was much closer to experimental value qexp, revealing that the MB adsorption by MgSiFM is better fitter with the pseudo-second-order kinetic model. Table 1 Kinetic parameters of MB adsorption by MgSiFM. Pseudo-first-order model Experimental

qe

R2

k1

qexp (mg/g)

(mg/g)

(h )

147.70

117.49

0.22

-1

0.9896

Pseudo-second-order model qe

k2

(mg/g)

(g/mg•h)

154.56

3.63×10-3

R2

0.9983

The kinetic data were further studied with intraparticle diffusion model (Weber-Morris model) to analyze rate controlling steps which affect the adsorption process [48]. Its linear form is shown as follows: (4) where kd is the intra-particle diffusion rate constant and L is the thickness of boundary layer. As shown in Fig. 5f, the plot of qt versus t0.5 gave three stages. The first step showed a sharp adsorption of MB, due to the diffusion of MB molecules through the solution to the exterior surface of magnesium silicate fibers. The second stage was the gradual sorption step and intra-particle diffusion is the rate-control step. The third part was attributed to the final equilibrium step where very low dye concentration of MB 24

remained in the solution. Furthermore, none of the lines passed through the origin, suggesting that intra-particle diffusion is not the only rate-controlling step in the whole adsorption process. 3.2.4. Adsorption isotherm The maximum adsorption capacity, another important parameter to evaluate the adsorbents, was investigated by adsorption isotherms. Moreover, the adsorption property and the interaction could also be obtained. The equilibrium isotherm data (Fig. 6a) were analyzed by two classical isotherm models, namely Langmuir and Freundlich. Their linear equations are as follows [42]: Langmuir isotherm (homogeneous and monolayer adsorption): (5) Freundlich isotherm (heterogeneous and multilayer adsorption): (6) where qe is the equilibrium adsorption capacity (mg/g), Ce is the equilibrium concentration (mg/L), and qm and b are Langmuir constants related to maximum adsorption capacity and binding energy, respectively; KF and n are empirical constants that indicate the Freundlich constant and heterogeneity factor, respectively.

Table 2 Langmuir and Freundlich constants of MB adsorption by MgSiFM. Langmuir isotherm qmax

b

(mg/g)

(L/mg)

609.75

0.0821

Freundlich isotherm 2

R

KF

0.9991

25

152.48

n

R2

4.19

0.9598

Fig. 6. (a) Adsorption isotherm of MB onto MgSiFM, (b) Langmuir linear plot from (a), and (c) comparison of the adsorption capacity of MgSiFM toward MB with other MgSi-based adsorbents.

According to the fitted curves (Fig. 6b, Fig. S5) and correlation coefficients (Table 2), Langmuir model could better explain the adsorption of MB onto MgSiFM, assuming that all the adsorption sites on MgSiFM are homogeneous and on interaction occurs between adsorbed species. The maximum adsorption capacity from the Langmuir fitting was 609.75 mg/g and this value was compared with the related magnesium silicate-based adsorbents. As shown in Fig. 6c, the maximum adsorption capacity toward MB by MgSiFM was higher than most of previously reported MgSi-based MB adsorbents. This is because that the reported powdery and nanoscale 26

MgSi adsorbents are easy to aggregate, which can decrease the adsorption capacity. For MgSiFM, the long and continuous fibrous morphology could prevent the aggregation of MgSi nanostructure, thus being beneficial for the adsorption. Moreover, it is observed that the BET surface area of MgSiFM was similar to that of other MgSi-based adsorbents, suggesting that the flexible MgSiFM prepared in this study also has a moderate surface area. If the adsorption capacity is evaluated based on the surface area, MgSiFM also exhibited higher adsorption capacity per unit surface area than most of the adsorbents (above the dotted line in Fig. 6c). To show the good treatment effect of MgSiFM on cationic pollutants, the adsorption performance toward cationic dye (Rhodamine B, RhB) and cationic herbicide (diquat) by MgSiFM were also studied. The adsorption isotherm curves of RhB and diquat by MgSiFM are displayed in Fig. S6. Fitted parameters are listed in Table S1. According to Langmuir fitting, the maximum adsorption capacities toward RhB and diquat were 549.14 mg/g and 405.45 mg/g, which are also very high. The results indicate that MgSiFM also has the ability to remove other cationic pollutants.

27

Fig. 7. Coexisting ions (a) and ionic strength (b) effect on MB adsorption by MgSiFM. Adsorption–desorption cycles (c) and XRD patterns of MgSiFM after MB adsorption and calcination regeneration (d). In each case, 2.5 mg of MgSiFM was added into 20 mL MB solution (20 mg/L) at pH 7.

3.2.5. Effect of co-existing ions and reusability Usually, there are various inorganic ions in the real water media, which could influence the organic matters adsorption. To study the competitive adsorption by inorganic ions, the adsorption toward MB co-existing with cations (Na+, Mg2+, Ca2+) and anions (SO42-, Cl-, NO3-) was conducted. As shown in Fig. 7a, co-existing ions showed almost slight interferences on the MB adsorption by MgSiFM. In addition, the effect of ionic strength on MB adsorption onto MgSiFM was also explored by varying 28

concentrations of NaCl solutions (Fig. 7b). As increasing the NaCl concentration to 0.1 M, the adsorption capacity slightly decreased. The results indicate that MgSiFM has the ability to resist interference of co-existing ions for MB adsorption, making it possible for MgSiFM to be used to remove MB from saline waste-water. To reduce the economic cost during the water treatment process, reusability is a vital factor in practical applications. Previous reported magnesium silicate-based adsorbents had to be separated by centrifugation and magnetic field assistance [23, 27], which are troublesome and generate additional cost. In this study, due to MgSiFM’s good membrane-forming property, MgSiFM could be easily separated from the solution by tweezers (Fig. S7) thus is more suitable for wastewater treatment. The saturation MB adsorbed-MgSiFM was regenerated by calcining the composite membrane at 450 oC for 2 h. As the digital photos shown in Fig. S8, blue MB had been removed completely by the calcination due to its pyrolysis. Moreover, its flexibility was still maintained well. The regenerated MgSiFM was reused in the adsorption experiments (Dosage of MgSiFM: 5 mg, initial concentration/volume of MB: 20.0 mg·L-1/40 mL) and the removal efficiency remained above 98% after five cycles

(Fig.

7c).

The

chemical

compositions

of

MB

adsorbed-MgSiFM

(MB-MgSiFM) and regenerated MgSiFM after five cycles were measured by XRD. As shown in Fig. 7d, all of the peaks for both materials could also be indexed to JCPDS No. 03-0174, indicating that MB adsorption and calcination regeneration couldn’t destroy the silicate structure. Moreover, the fiber morphology of MgSiFM after five cycles is displayed in Fig. S9. Except for a few broken fibers, most of the 29

fibers still kept their long and continuous morphology. The results indicate that MgSiFM has good stability and reusability. Due to its simple preparation, high adsorption capacity, easy separation and excellent cyclic stability, our prepared magnesium silicate fiber membrane can be a good MB adsorbent candidate in practical water treatment.

30

Fig. 8. FT-IR (a) and XPS (b) spectra of MB-MgSiFM. High resolution of Si 2p peaks for MgSiFM before and after MB adsorption.

3.2.6. Adsorption mechanism 31

The mechanism of MB adsorption by MgSiFM was investigated by FT-IR and XPS analysis. The FT-IR spectrum of MgSiFM after MB adsorption is displayed in Fig. 8a. Compared with the spectrum of MgSiFM, the presence of new bands appearing at 1603, 1490, 1389, and 1334 cm-1, which correspond to the characteristic peaks of MB [51, 52], confirmed the successful adsorption of MB onto MgSiFM. In comparison with the XPS full-scan spectrum of MgSiFM (Fig. 4c), MB-MgSiFM’s XPS full-scan spectrum (Fig. 8b) showed the new C 1s binding energy peak at 284.5 eV, which can also confirm the MB adsorption. In the Effect of initial solution pH section (Section 3.2.1), it has been demonstrated that MB adsorption by MgSiFM is attributed to the electrostatic attraction between −Si–OH groups and MB molecules [47]. Moreover, the previous studies have reported that the high adsorption capacity of MgSi also owns to an ion exchange process between Mg2+ in MgSi and cationic pollutants [21, 25]. According to XPS spectrum of MB-MgSiFM, the atomic fraction ratio of Mg to Si was calculated to be 0.545 (4.93/9.04), which is much lower than that of MgSiFM (0.747, Fig. 4c), suggesting that the content of Mg element in MgSiFM decreases after MB adsorption. Moreover, according to Fig. 7d, MgSiFM still kept its silicate structure after MB adsorption. These results indicate that MB adsorption by MgSiFM is also attributed to the ion exchange process between Mg2+ and MB. To further analyze the electrostatic attraction and ion exchange processes, high resolution XPS spectra of Si 2p before and after MB adsorption were studied (Fig. 8c). Before MB adsorption, the Si 2p peak was fitted by three Gaussian-Lorenz peaks at 104.1, 103.3, and 102.6 eV, assigning to Si–O, Si–O–Mg, and Si–OH, respectively 32

[42, 44]. Their contents, calculated from the peak area ratio, are also displayed in Fig. 8c. After MB adsorption, the Si 2p peak could be divided into three peaks as well. However, the area ratio of Si-OH and Si–O–Mg both decreased. The decreased content of Si–OH (from 27.3% to 25.8%) is ascribed to the electrostatic interaction and the decreased content of Si–O–Mg (from 47.7% to 24.9%) is ascribed to the ion exchange process [23, 33]. As one can see from the difference values, ion exchange process played a major role in the adsorption mechanism. On the basis of the above results, the adsorption mechanism toward MB by MgSiFM can be summarized as follows: firstly, −Si–OH groups become deprotonated and cationic MB molecules are adsorbed onto MgSiFM through electrostatic attraction. Subsequently, MB molecules have the opportunity to be further adsorbed through ion exchange, which is the primary adsorption process.

3.3. Filtration adsorption experiments

33

Fig. 9. (a) Photo of the laboratory-made dead-end filtration device, (b) optical image of the detachable filtration cell, (c) break-through curves for passage of MB solutions through MgSiFM at various flow rates, and (d) breakthrough curves for filtration adsorption

of

MB

by

fresh

MgSiFM

and

filter

membrane

after

five

filtration-regeneration cycles.

The above-mentioned batch adsorption experiments have revealed that magnesium silicate fiber membrane shows good adsorption performance toward MB. Compared with the previous reported powdery magnesium silicate-based adsorbents, the flexible and robust MgSi fiber membrane has its specific advantages to be used as a filter membrane for continuous and rapid removal of MB. The laboratory-made dead-end filtration device (Fig. 9a) was used to explore the filtration adsorption performance of MgSiFM. The MgSiFM was cut into a circle with a diameter of 1.2 cm and put into a 34

detachable filtration cell (Fig. 9b). MB solution with a concentration of 10 mg/L was selected as the feeding solution to elevate the filtration adsorption performance of MgSiFM. The flow rate of MB solution was controlled by a syringe pump. Three different flow rates of 2, 4 and 8 mL/min were used. Fig. 9c shows the breakthrough curves obtained by plotting the ratio between the outlet (Ct) and feed (C0) concentration of MB as a function of treated capacity (kg wastewater/kg MgSiFM) for different flow rates. At the flow rate of 2 mL/min, MgSiFM could remove 99% of MB with the treated capacity of ~2000 kg wastewater/kg MgSiFM (about 40 mL MB solution). The filtration performance was compared with reported MB filter membranes. As shown in Table S2, our performance is close or superior to many other reports. As demonstrated in Movie S1, the MB solution could be effectively decolored through this filtration process. In this study, c/c0 = 0.1 is defined as the breakthrough point, and c/c0 = 0.8 as the saturation point. As depicted in Fig. 9c, the breakthrough point occurred faster at higher flow rate, and the saturation point also appeared faster with increasing the flow rate. The breakthrough point decreased from 3961 to 571 kg wastewater/kg MgSiFM when the flow rate changed from 2 to 8 mL/min. The saturation point also decreased from 13380 to 4570 kg wastewater/kg MgSiFM. This can be explained that an increased flow rate results in decreasing in the contact time of the adsorbent and adsorbate according to mass transfer concepts. Thus, MB molecules do not have enough time to penetrate and diffuse deeply into the magnesium silicate fibers. In addition, N6MM (commercial nylon-6 microfiltration membrane, pore diameter 35

= 1.2 m, from Shanghai XinYa purification Equipment Co., Ltd) and MgSi/PAN fiber membrane were further tested for comparison at the flow rate of 2 mL/min. Nylon-6 microfiltration membrane (N6MM) can be obtained easily and is often employed as filtration usage. Thus, N6MM is chosen as control group in this study. SEM image of N6MM is shown in Fig. S10 and the detailed information of MgSi/PAN fiber membrane can be obtained in our previous study [33]. As the results shown in Fig. S11 and Table S3, the breakthrough point for MgSiFM was ca. 60 times larger than that of N6MM and 3 times larger than that of MgSi/PAN fiber membrane. The high filtration performance of MgSiFM in comparison with MgSi/PAN fiber membrane and N6MM is attributed to the high content of magnesium silicate and fibrous structure. Furthermore, the recyclability of MgSiFM filter membrane was also investigated. The filtered MgSiFM was also regenerated by calcination at 450 oC for 2 h. The filtration-regeneration cycles were conducted at a flow rate of 2 mL/min. After five cycles, the saturation point showed a slight decrease and breakthrough curves in the Fig. 9d indicated that the filtration performance could also be successfully maintained. The filtration results suggest that MgSiFM can also act as a practical filter membrane for effective and fast removal of MB, that powdery magnesium silicate-based adsorbents don’t possess.

4. Conclusions In summary, we have demonstrated the preparation of flexible magnesium silicate fiber membrane (MgSiFM) by hydrothermal reaction using electrospun flexible SiO2 36

fiber membrane as silicon template for the first time. MgSiFM exhibited good mechanical stability, that could improve its operability and practicability. The BET surface area and pore volume were 463.4 m2/g and 0.34 cm3/g, respectively. The adsorption performance of MgSiFM toward cationic dye methylene blue (MB) was studied. The adsorption processes could be fitted well with pseudo-second-order model and Langmuir isotherm model. The maximum adsorption capacity was 609.75 mg/g, which is higher than most of the reported MgSi-based adsorbents. In addition, MgSiFM could be easily recycled by calcination. Different from powdery MgSi-based adsorbents, MgSiFM could remove MB through a fast and efficient membrane filtration adsorption process. The obtained results indicate that MgSiFM is a promising candidate for the removal of MB during practical wastewater treatment process. Furthermore, we anticipate that this work may also provide a universal strategy for the preparation of other flexible metal silicate fiber membranes in a broad range of applications, such as adsorption, catalysts, separation, electrode materials, and so on.

Acknowledgements This work is supported by the research grants from the National Natural Science Foundation of China (No. 51773082 and 21474043) and Jilin Provincial Industrial Innovation Program (No. 2016C024).

References 37

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45

Graphical Abstract

46

Highlights ★ Flexible and robust magnesium silicate fiber membrane (MgSiFM) is prepared.

★ MgSiFM shows good mechanical stability with a high tensile strength of 5.10

MPa. ★ MgSiFM has the excellent adsorption capacity of 609.75 mg/g for methylene blue

(MB). ★ MgSiFM could also act as a filter membrane for continuous and fast MB removal.

47