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Flexible Hierarchical ZrO2 Nanoparticle-Embedded SiO2 Nanofibrous Membrane as A Versatile Tool for Efficient Removal of Phosphate Xueqin Wang, Lvye Dou, Zhaoling Li, Liu Yang, Jianyong Yu, and Bin Ding ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11294 • Publication Date (Web): 18 Oct 2016 Downloaded from http://pubs.acs.org on October 26, 2016

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ACS Applied Materials & Interfaces

Flexible Hierarchical ZrO2 Nanoparticle-Embedded SiO2 Nanofibrous Membrane as A Versatile Tool for Efficient Removal of Phosphate Xueqin Wang,† Lvye Dou,‡ Zhaoling Li,‡ Liu Yang,‡ Jianyong Yu,§ and Bin Ding*,†,‡,§ †

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials,

College of Materials Science and Engineering, Donghua University, Shanghai 201620, China. ‡

Key Laboratory of Textile Science & Technology, Ministry of Education, College of

Textiles, Donghua University, Shanghai 201620, China. §

Nanofibers Research Center, Modern Textile Institute, Donghua University,

Shanghai 200051, China.

*Corresponding author: Prof. Bin Ding (Email: [email protected])

1 ACS Paragon Plus Environment


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ABSTRACT Functional nanoparticles modified silica nanofibrous materials with good flexibility, hierarchical mesoporous structure, and excellent durability would have broad applications in efficient removal of contaminant, yet have proven to be enormously challenging to construct. Herein, we reported a strategy for rational design and fabricating flexible, hierarchical mesoporous, and robust ZrO2 nanoparticle-embedded silica nanofibrous membranes (ZrO2/SiO2 NM) for phosphate removal by combining the chitosan dip-coating method with electrospinning technique. Our approach allows ZrO2 nanoparticles to be in situ firmly and uniformly anchored onto SiO2 nanofibers to drastically enlarge the specific surface area and porosity of membranes. Therefore, the resultant ZrO2/SiO2 NM exhibited prominent removal efficiency of 85% and excellent adsorption amount of 43.8 mg P g-1 membranes in 30 min towards phosphates. Furthermore, the removal performance towards different types of phosphates revealed the resultant membranes also could be used to remove phosphates in detergent and fertilizer water samples. More importantly, the membranes with good flexibility could directly be taken out from solution after use without any post-treatment. Such a simple and intriguing approach for fabricating nanofibrous membranes may provide a new platform for constructing membranes with superb phosphate removal performance. KEY WORDS: phosphates removal; electrospinning, nanofibrous membranes, flexible, ZrO2 nanoparticle-embedded SiO2 nanofibers


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1. INTRODUCTION Silica fibrous materials have attained great attention due to its good chemical stability, extraordinary low thermal conductivity, and corrosion resistance, which have been applied in various fields, including wastewater remediation, thermal insulation, catalysis, and high-temperature filtration.1-3 Compared with traditional silica fibrous materials, one dimensional silica nanofibers with obvious thinned fiber diameter, increased surface area, and porosity have become ideal alternatives of micro-scaled silica fibers to enhance application performance.4-6 Significantly, introducing nanoparticles onto the surface of silica nanofibers to construct hierarchical structure would further dramatically boost the performance of silica nanofibrous membranes, mainly encompass drastically enlarge surface area and high porosity.7-9 Nonetheless, the inherent brittleness of the currently fabricated silica-nanoparticles hierarchical nanofibers with small recoverable deformation have restricted their practical applications, thereby addressing this issue is paramount important. Up to now, various methods have been developed to improve the fragility of the silica nanofibrous membranes, from cooperation with polymer adhesives to growing nanoparticles on preformed polymers; whereas, the unique feature of the silica nanofibers and nanoparticles would partially be crippled ascribed to the introduction of polymeric component.10, 11 It is worthwhile to mention that maintaining flexibility is tremendously important to manufacture flexible ceramic devices (eg. membrane reactor, sensors, filters, etc.) for environmental remediation. Thus, it is highly desired to develop a versatile method to address such challenging but important problem.



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Compared with the methods such as hydrothermal synthesis, laser ablation and template synthesis, electrospinnng has been considered as the most practical and effective technology for fabricating silica nanofibers with advantageous features, involving large surface area, torturous channel, lightweight, fine flexibility, robust mechanical strength, and excellent chemical stability.12-15 Meanwhile, researchers have attempted to anchor functional nanoparticles (eg. ZrO2, MnO2, MgO, etc.) on ceramic nanofibers, which would not only endow materials with multifunctionality, but also lead to drastic increment of specific surface area, thus dramatically enhanced the application performance of resultant membranes.16-18 However, the nanoparticles are tended to aggregate in fibrous structure especially at high concentration, thereby making the resultant membranes extremely brittle and the effective surface area decrease. In this sense, it is urgent to develop flexible nanoparticle-embedded silica nanofibrous membranes. Herein, we describe an intriguing method for fabricating flexible ZrO2 nanoparticle-embedded SiO2 nanofibrous membranes (ZrO2/SiO2 NM) by combining electrospinning technique with chitosan (CS) dip-coating method with superb adsorption performance towards phosphate, as illustrated in Figure 1. The flexible electrospun SiO2 nanofibers was selected as the network to uniformly growing ZrO2 nanoparticles (ZrO2 NPs), thus, leading to the formation of hierarchical structure with drastically enlarged surface area. In addition, the Zr(IV) could coordinate with phosphate ions to generate the ZrP compound, thereby the ZrO2 nanoparticles on ZrO2/SiO2 NM are capable of adsorbing phosphates ions in solution. Moreover, the


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membranes with large surface area, torturous porous structure, and good flexibility enable to adsorb phosphate from water with high removal efficiency, fast adsorption equilibrium, and superb adsorption capacity.

Figure 1. (a) Schematic illustrating the fabrication procedure of ZrO2/SiO2 NM. (b) Optical images showing the color change and flexibility of SiO2, CS/ZrAc4/SiO2, ZrO2/SiO2 NM. SEM images of (c) SiO2, (d) CS/ZrAc4/SiO2, and (e) ZrO2/SiO2 NM.

2. EXPERIMENTAL 2.1. Materials. Poly(vinyl alcohol) (PVA, Mw=88000), phosphoric acid (H3PO4, 85 wt%), zirconium acetate (Zr molar content of 15%~16%), chitosan (CS, Mw=210000, the deacetylation degree of 92%), acetic acid, tetraethyl otrhosilicate (TEOS), monosodium orthophosphate (NaH2PO4), disodium phosphate (Na2HPO4), sodium phosphate (Na3PO4), ascorbic acid, ammonium molybdate, potassium antimony tartrate, sodium hydroxide, and sulfuric acid were all purchased from Aladdin 5 ACS Paragon Plus Environment


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Industrial Co., China. All the dilutions were conducted by using Milli-Q water with resistivity higher than 18.2 MΩ. All reagents were used without any further refinement. 2.2. Preparing flexible SiO2 NM. Generally, the SiO2 NM were fabricated by calcining the elecrospun PVA/silica sol hybrid nanofibrous membranes.19 Firstly, the PVA powder was dropwise added into water at 80 °C under stirring for 4 h to obtain the 10 wt% PVA precursor solution. Subsequently, the silica sol was generated by hydrolyzing the TEOS aqueous solution in the presence of H3PO4 (as a caltalyst), with the molar ratio of TEOS:H3PO4:H2O=1:0.01:10. Following this, the resultant silica sol and PVA solution were mixed (with equal weight) under stirring for another 6 h. Subsequently, the as-prepared homogeneous spinning solution was transferred to a plastic syringe with metal needle, which was connected to a high voltage power supply with an applied voltage of 25 kV. The propulsion velocity of the syringe was kept at 1 mL h-1 with the tip-to-collector distance of 23 cm. During the electrospinning process, the relevant temperature and humidity were sustained at 23 ± 3 °C and 45 ± 3%. Finally, the obtained PVA/silica sol hybrid membranes were transferred to a muffle furnace at 800 °C in air to decompose the organic compound, and the SiO2 NM with good flexibility were obtained. 2.3. Fabrication of ZrO2/SiO2 NM. Figure 1a schematically showing the synthesis pathway of ZrO2/SiO2 NM. In a typical experiment, certain amount of CS powder was firstly dissolved in 1% acetic acid solution to obtain a 0.1 wt% CS aqueous solution. Then, zirconium acetate was dripped into CS solution to generate a homogeneous sol


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containing different contents of zirconium acetate (0.5, 1, 2, 4, and 8 mol%). Subsequently, the SiO2 NM were cut into 3 × 3 cm2 pieces, impregnated into the as-prepared sol for 5 min, and dried in an oven at 50 °C for 20 min to fix the CS layer containing zirconium acetate on SiO2 nanofibers. Following this, the SiO2/CS NM were calcined in a vacuum tube furnace at 800 °C for 30 min with a heating rate of 2

°C min-1 in N2 flow to generate the black ZrO2/SiO2 NM with different contents of ZrO2. The ZrO2/SiO2 NM with x wt% of zirconium acetate were signified as ZrO2/SiO2-x. 2.4. Phosphate adsorption measurement. To evaluate the phosphate removal performance of ZrO2/SiO2 NM, a typical phosphate (NaH2PO4) was selected as a model because it is one of the mainly phosphate sources that can cause eutrophication. Generally, 10 mg of ZrO2/SiO2 NM was immersed into 50 mL of NaH2PO4 solution (initial concentration ranged from 0 to 20 mg L-1, pH value of 5) to oscillate for a designated time (0~60 min) at different temperature ranged from 25 to 60 °C, then the concentration of the NaH2PO4 was assayed by using a molybdenum blue spectrophotometry method.20, 21 Prior to titrate the concentration of phosphate solution, a molybdate solution was prepared by mixing 100 mL of 13 g mL-1 ammonium molybdate, 100 ml of 0.35 g mL-1 potassium antimony tartrate, and 300 ml of 50% sulfuric acid (with the volume ratio of sulfuric acid and water 1:1) together, consequently, the as-prepared solution could be stored at 2~8 °C for 6 months. During titration, 1 mL 100 g L-1 ascorbic acid were added to phosphate solution and stirring for 30 s; following this, 2 mL as-prepared molybdate solution was added and



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stirred for 10 min, then the color of solution gradually changed from colorless to blue and became stable due to the formation of phosphorus molybdenum blue. Subsequently, the concentration of the phosphate was quantitatively analyzed according to the UV-vis spectra of the solution at the wavelength of 710 nm at room temperature based on the based on the Beer–Lambert's law. After obtaining the concentration of phosphate solution, the adsorption capacities of the ZrO2/SiO2 NM towards phosphates could be calculated based on equation of qt = (C0-Ct)V0/m0, where qt is the adsorption capacity at designated time, C0 and Ct is the concentration of phosphate at initial and designated time, V0 is the phosphate solution’s volume, and m0 is the weight of ZrO2/SiO2 NM. To evaluate the practicability of membranes, the adsorption performance of ZrO2/SiO2 NM towards different types of phosphates and the recycling performance are systematically investigated. We take five kinds of phosphates for examples to test the adsorption capacity of ZrO2/SiO2 NM towards different kinds of phosphates at room temperature, including Na2HPO4, Na3PO4, NaH2PO4, and two water samples (one is the detergent solution, denoted as sample 1; another is fertilizer solution, denoted as sample 2). The adsorption process and calculation method were the same as mentioned above. To investigate the recycling performance, the membranes with adsorbed phosphates were immersed into 0.1 mol/L NaOH solution to dissolve phosphate for several times, then the UV-vis spectra of the leachates were tested until the adsorption peak at 710 nm disappear; therefor, the regenerated ZrO2/SiO2 NM were used to measure phosphate adsorption performance for another cycle.


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2.5. Apparatus. A scanning electron microscope (SEM, Tescan Inc., USA) was employed to character the morphology and energy dispersive X-ray spectroscopy (EDX) of relevant samples. A fourier transform infrared spectroscopy (FT-IR) spectra (Nicolet 8700, Thermo Nicolet Co., USA) was used to examine the chemical structure change of relevant samples during entire process. The phase structure of the membranes were characterized with X-ray diffraction (XRD, D/Max-2550 PC Rigaku Co., Japan). N2 adsorption-desorption isotherms and BET surface areas were examined by an ASAP 2020 analyzer (Micromeritics Co., USA). A fiber tensile tester (XQ-1C, Shanghai Lipu Instrument Research Center of Co., Ltd., China) was used to measure mechanical behavior of relevant membranes. A softness tester (RRY-1000, Hangzhou Qingtong & Boke Automation Technology Co., Ltd., China) was utilized to measure bending rigidity of ceramic nanofibrous membranes. A UV–vis fiber-optic spectrometer (PG 2000pro, Ideaoptics Technology Ltd., China) was applied to record the spectra of various solutions.

3. Results and discussions 3.1. Morphologies of ZrO2/SiO2 NM. To prepare nanoparticle immobilized nanofibrous membranes with superb adsorption performance for phosphate removal, we rationally designed the ZrO2/SiO2 NM based on following three definite criteria: (1) the SiO2 nanofibers should be assembled into a flexible network with robust mechanical properties. (2) The ZrO2 NPs must be non-aggregated and tightly anchored onto the surface of SiO2 nanofibers at high contents.22, 23 (3) The constructed hierarchical structure need to provide abundant active sites available for targeted



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compounds to immobilize.24,

25

The first criterion was satisfied by combining the

sol-gel method and electrospinning technique. In order to meet the other two requirements, a general and easy-access CS dip-coating method was used, in which the CS layer was acted as a carrier to fix zirconium acetate on the surface of fiber and consequently result in formation of the carbon layer to embed ZrO2 NPs on SiO2 fibers and prevent the nanoparticles falling off. The representative SEM images of SiO2 NM displayed in Figure 1b revealed the nanofibers were randomly deposited as a nonwoven fabrics with an average fiber diameter of 246 nm. After dipping into CS solution, the fibers obviously adhered with adjacent fibers and the fiber diameter increased to 290 nm, demonstrating the CS layer has successfully been loaded on SiO2 fibers surface with a thickness of approximately 40 nm (Figure 1c). Significantly, the amino and hydroxyl groups on CS molecules could associate with zirconium ions, thereby the CS layer could act as the adhesive to fix the zirconium acetate on SiO2 nanofiber homogeneously.26 During the calcination in N2 flow, the CS layer gradually decomposed and carbonized, meanwhile the zirconium acetate would be converted to ZrO2 NPs via the in situ nucleation/growth. As the CS layer transferred to carbon layer, the ZrO2 NPs would be inlayed into carbon layer, thereby prompted the nanoparticles firmly and non-aggregated anchored onto SiO2 nanofibers surface (Figure 2a).27 The FT-IR spectra also clearly verified the loading and decomposition of CS layer. It is clear that the characteristic peaks around 1453 and 1570 were belonged to the bending and stretching vibration of acylamide bond, and the peak located at 3403 cm-1 described


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the stretching vibration of hydroxyl groups in CS molecules, respectively;28 after calcination, all characteristic peaks of CS disappeared, revealing the completely decomposition of CS layer on fibers. Significantly, the membranes exhibited excellent flexibility and could be bent 180° without obvious damage during the entire processes (Figure 1e).

Figure 2. (a) FT-IR spectra and (b) XRD patterns of SiO2, CS/ZrAc4/SiO2, and ZrO2/SiO2 NM. (c) EDX spectrum of ZrO2/SiO2 NM. (d) The distribution of nanoparticle size on SiO2 nanofibers. To further estimate the contents of ZrO2 NPs in ZrO2/SiO2 NM (taken zirconium acetate of 4 wt% as an example), the EDX spectral analysis were used. As illustrated in Figure 2b, the contents of Si, Zr and C elements were of 38.5%, 6.35%, and 0.96%, respectively, demonstrating the presence of a high content of ZrO2 NPs in membrane. XRD analysis shown in Figure 2c revealed that a new peak at 2θ values of 10.4° appeared after impregnated SiO2 NM into CS solution, which was induced by the 11 ACS Paragon Plus Environment


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hydrogen bonds between hydroxyl groups and amino groups in CS molecules; the results also verified the successful fixing of CS layer on fibers, which match well with the aforementioned FT-IR spectral analysis.29 After calcination, the relevant peaks were well positioned at 2θ values of 30.12° (111), 34.96° (200), 50.22° (220), and 59.74° (311), which matched well with standard pattern of tetragonal ZrO2 phase (JCPDS: 50-1089).30 Furthermore, the crystallite size of ZrO2 NPs were calculated to be 34 nm based on the well-known Scherrer’s formula, which was in consistent with the nanoparticle size counted from SEM observation (Figure 2d). The introduction of ZrO2 NPs is critical to construct a functional surface with hierarchical structure on fiber surface, thus, we systematically investigated the influence of ZrO2 NPs concentration on morphologies and structure of nanofibrous membranes. As expected, the ZrO2 NPs were well anchored onto the SiO2 nanofiber surface and the amount of ZrO2 NPs located on fibers dramatically increased with increasing zirconium acetate from 0.5 to 8 wt% (Figure 3). It is worthwhile to note that barely ZrO2 NPs were presented among the gap of nanofibers when the zirconium acetate concentration lower than 4 wt%, suggesting the effective construction of hierarchical structure with remarkably increased specific surface area. Nonetheless, large amount of ZrO2 NPs were observed to aggregate in the voids of fibers when the zirconium acetate contents further increased to 8 wt%, which might be ascribed to the exorbitant viscosity of CS sol that leading to the inhomogeneous dispersion of ZrO2 NPs.


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Figure 3. SEM images of (a) ZrO2/SiO2-0.5, (b) ZrO2/SiO2-1, (c) ZrO2/SiO2-4 NM, and (d) ZrO2/SiO2-8 NM. In sharp contrast to the brittle nature of ceramic nanofibrous materials, the resultant ZrO2/SiO2 NM could bear the bending of nearly 180° without obvious damage (Figure 1e). More importantly, the pristine SiO2 NM exhibited a nonlinear elastic behavior with a tensile stress of 6.2 MPa, Young’s modulus of nearly 310 MPa, and the bending rigidity of 12.2 mN, as shown in Figure 4. Such exceptional flexibility might be ascribed to the entanglement network and high aspect ratio (>1000) of SiO2 nanofibers.31,32 After the ZrO2 NPs introduced onto nanofibrous membranes, the tensile stress of ZrO2/SiO2 NM gradually decreased from 5.8 to 3.6 MPa with increasing zirconium acetate contents from 0.5 to 4 wt %. It is reasonable considering the fact that the synthesized ZrO2 NPs on fiber surface would restrict the slippage of nanofibers during tensile process, which was also confirmed by the drastic decrease of tensile strain. Even so, the ZrO2/SiO2-4 NM still exhibited good flexibility, which endows the resultant membranes with great promise to be used in practical 13 ACS Paragon Plus Environment


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applications. Whereas, the strength of the membrane was dramatically decreased as the zirconium acetate contents reached 8 wt% due to the inhomogeneous dispersion of ZrO2 NPs and the filled gaps between nanofibers restricted the slippage of nanofibers.

Figure 4. The tensile stress, strain, and bending rigidity of various ZrO2/SiO2 NM. 3.2. Quantitatively porous structure analysis. The introduction of ZrO2 NPs would construct a hierarchical structure on SiO2 nanofibers, thereby endowing the nanofibrous membranes with dramatically increased specific surface area and torturous channels. Therefore, the N2 adsorption-desorption isotherms were tested to quantitatively analysis the specific surface area and pore distribution of relevant ZrO2/SiO2 NM. The isotherms of all the samples (Figure 5a) illustrated versatile adsorption behaviors involving monolayer adsorption, multilayer adsorption, and capillary condensation, confirming the existence of mesoporous structure in resultant nanofibrous membranes.33, 34 In addition, the isotherms exhibited narrow hysteresis loop over the region of 0.4 < P/P0 < 1, revealing existence of the opened mesopores and confirming the capillary evaporation and condensation of N2 were persistently occurred during the adsorption process.35, 36 Significantly, the BET specific surface 14 ACS Paragon Plus Environment

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area of ZrO2/SiO2 NM gradually increased from 5.26 to 17.35 m2 g-1 with increasing zirconium acetate contents, indicating the significance role of ZrO2 NPs played in enlarging effective surface area. Moreover, the Frenkel–Halsey–Hill (FHH) model was used to quantitatively analysis the roughness surface structure and porous structure, shown in Figure 5b and Supplementary method.37 The obvious differential slopes in high coverage region demonstrated the fractal dimension of ZrO2/SiO2 NM were 2.71, 2.72, 2.74, 2.78, and 2.80, respectively, with increasing the contents of zirconium salts. The gradually increased fractal dimension value suggested that the surface roughness feature of the membranes, which would consequently endows the membranes with enhanced surface area. Moreover, the Barrett-Joyner-Halenda (BJH) porous structure analysis verified the polydispersity of pore width in resultant ZrO2/SiO2 NM with a well-defined peak was observed at 32 nm, which was consistent with the aforementioned results about SEM image and XRD pattern. Furthermore, the BJH pore volume of ZrO2/SiO2-8 NM (0.064 cm3 g-1, shown in Table 1) was 4-folder higher than that of pristine SiO2 NM (0.014 cm3 g-1), such hierarchical porous structure would potentially dramatically improve the phosphate removal performance of resultant membranes.



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Figure 5. (a) N2 adsorption–desorption isotherms of various ZrO2/SiO2 NM with different contents of zirconium acetate. (b) The plot of ln(V/Vmono) against ln(ln(P0/P)) reconstructed from the N2 desorption isotherm on the basis of FHH model. Table 1. The BET surface area and BJH pore volume of relevant ZrO2/SiO2 NM. Sample

BET surface area (m2 g-1)

BJH pore volume (cm3 g-1)

SiO2

5.26

0.014

ZrO2/SiO2-0.5

7.12

0.023

ZrO2/SiO2-1

9.35

0.033

ZrO2/SiO2-2

14.57

0.041

ZrO2/SiO2-4

16.79

0.053

ZrO2/SiO2-8

17.35

0.064


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3.3. Effect of ZrO2 NPs contents on phosphate adsorption performance. The embedded ZrO2 NPs could coordinate with phosphate, therefore, it can be exploited to remove phosphate in water with plenty of other advantageous features, for instance, high corrosion resistance, excellent thermal and chemical stability.38 Consequently, we further demonstrated that the ZrO2/SiO2 NM with enhanced mesoporous structure and specific surface area could be used to efficiently remove phosphate to prevent the water bodies from eutrophication. The results dedicated that the adsorption capacity and removal efficiency of pristine SiO2 NM was only 2.4 mg P g-1 membrane and 6% (Figure S1); meanwhile, the after modified with ZrO2 NPs, the resultant membranes could achieve a higher adsorption capacity and a maximum adsorption capacity of 45.8 mg P g-1 membrane was achieved as the zirconium acetate concentration increased 4 wt% (Figure 6a). Further increasing the salts contents to 8 wt%, the phosphate adsorption capacity would be not changed drastically and achieve a plateau due to the barely changed specific surface area, which were consistent with the aforementioned results.



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Figure 6. (a) Adsorption performance of various ZrO2/SiO2 NM with different contents of zirconium acetate. (b) The effect of phosphate concentration on adsorption performance and the relevant fitting curves of Langmuir adsorption and Freundlich adsorption. (c) The kinetic adsorption performance of ZrO2/SiO2 NM towards phosphate as a function of time and the relevant fitting curves based on pseudo-first-order and pseudo-second-order. (d) Adsorption performance of ZrO2/SiO2-4 NM towards different types of phosphate aqueous solution. 3.4. Optimization the adsorption conditions of phosphate. The initial concentration of phosphates is of paramount importance on adsorption capacity. As can be seen from Figure 6b, the adsorption capacity drastically increased as the initial concentration increased from 0 to 10 mg L-1 and became flattened after reached the maximum adsorption capacity of approximately 45 mg P g-1 membrane. More importantly, we used Langmuir (equation 1) and Freundlich models (equation 2) to


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further quantitatively analysis the adsorption process.39

qe =

qmax Kα Ce Kα Ce + 1

(1)

qe = K F Ce n

(2)

Where qe is the equilibrium adsorption amount, qmax is the maximum adsorption capacity, Ce is the concentration of phosphate at equilibrium state, Kα and KF are the corresponding adsorption constants, and n is the equilibrium constants. The results shown and Table 2 demonstrated that the Langmuir model is preferable to describe the process of phosphates adsorption as its correlation coefficient (0.97) is higher than that of Freundlich model, and the theoretical maximum adsorption capacity is calculated to be 57.38 mg g-1, thus the phosphate removal performance could be further enhanced in actually. The resulted indicated that the phosphates ions was monolayer adsorbed on the surface of nanofiber, and the interaction between phosphates ions was respectively weak. Table 2. Corresponding parameters calculated from the Langmuir and Freundlich adsorption model Langmuir adsorption -1

Freundlich adsorption

qmax (mg g )

Kα (L mg )

R

KF

n

R2

57.58

0.2771

0.97

16.056

0.387

0.89

-1

2

The kinetic removal performance was then studied by monitoring the concentration of phosphate at designed time intervals, as illustrated in Figure 6c. As expected, the removal efficiency steeply increased and 85% of NaH2PO4 could be removed by resultant membranes within 30 min, and a completely removal could be achieved in 19 ACS Paragon Plus Environment


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60 min. Notably, the adsorption capacity gradually increased and reached the maximum adsorption capacity of 45.8 mg P g-1 membranes in 60 min. Significantly, the pseudo-first-order (equation 3) and pseudo-second-order models (equation 4) were used to further quantitatively analyze the adsorption kinetic.40

qt = qe −

qt =

qe ek1t

(3)

k2 qe 2t 1 + k2 qet

(4)

Where qt is the adsorption amount at a design intervals, qe is the equilibrium adsorption amount, t is the adsorption time, k1 and k2 are the rate constants. The calculated results displayed in Table 3 revealed that the pseudo-first-order model is more suitable for depicting the adsorption kinetic with relatively higher correlation coefficient (R2 of 0.99). The results confirmed that the adsorption of phosphate is physisorption ascribed to the coordination between zirconium and phosphates ions, and also demonstrated a faster adsorption kinetic rate (0.575) compared with adsorbents reported in the literatures. Table 3. Estimated kinetic parameters of the adsorption isotherms. Kinetic model

qe (mg g-1)

k

R2

Pseudo-first-order

48.71

0.575

0.99

Pseudo-second-order

64.27

0.0008

0.97

Considering the temperature is also a factor affect the adsorption kinetic, thereby we investigated the phosphate removal performance at different temperatures (ranged from 25 to 60 °C, taken ZrO2/SiO2-4 NM as an example). As expected, the adsorption 20 ACS Paragon Plus Environment

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capacity towards phosphate gradually increased from 43.2 to 47.7 mg P g-1 membrane with increasing adsorption temperature, as illustrated in Figure. S2. This phenomenon is reasonable considering the activity of phosphate molecules enhanced as the temperature increased, thus the physical adsorption towards phosphate increased; meanwhile, the ionization and hydrolysis of phosphate enhanced, which would lead to the increment of the coordination interaction between ZrO2 and phosphate. Considering the adsorption performance of the resultant membranes was insignificantly affected by the temperature, all the experiment was conducted at room temperature.

3.5. Adsorption performance towards different species of phosphates. Interestingly, the membranes exhibited different adsorption performance towards different types of phosphonate (at the same concentration of 10 mg L-1 and pH value of 5, as shown in Figure 6d), involving Na2HPO4, Na3PO4, and NaH2PO4, with the adsorption capacity of 29.4, 43.8, and 44.2 mg P g-1 membranes, respectively. The results would be reasonable considering the fact that the strength of Lewis acid–base interactions between different phosphate ions and ZrO2 are different (the pKa value of H2PO4- and HPO42- were 7.2 and 12.4), revealing the ionization of H2PO4- is stronger than HPO42-; thus, the adsorption of H2PO4- is enhanced due to the additional electrostatic forces and ion-exchange.8,

21, 41

With respect to Na3PO4 solution, the

PO43- would be transferred to HPO42- via hydrolysis, therefore, the adsorption capacity of ZrO2/SiO2 NM towards Na3PO4 is approximately equal to that of NaH2PO4.42 21 ACS Paragon Plus Environment


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More importantly, the practical applicability of ZrO2/SiO2 NM was tested by measuring the adsorption performance towards two water samples (one is detergent aqueous solution, another is fertilizer aqueous solution). The adsorption amount towards two samples reached 13.2 and 25.8 mg P g-1 membranes within 30 min; meanwhile, the ZrO2/SiO2 NM exhibited unexpected flexibility (Figure 1e), thus, the membranes with adsorbed phosphate would be taken out from solution to prevent the redissolving of phosphate. Such adsorption performance is very promising in contrast to the traditional remediation materials such as aluminium slats, ferric salts, and copper sulfate, because those materials might generate secondary contamination or dissolved phosphates under the low dissolved oxygen condition.6, 43 Furthermore, the bounded phosphates on membranes would be released from membranes by adjusting the pH value of solution based on its amphoteric property, thereby the as-prepared membranes could potentially be reused in practical applications. 3.6. Recycling adsorption performance study. Recycling performance is of paramount importance to practical application, herein we systematically investigated the reusability of resultant ZrO2/SiO2 NM, as shown in Figure 7. After adsorption, the membranes was immersed in 0.1 mol L-1 NaOH solution to dissolve the adsorbed phosphates for regeneration of ZrO2/SiO2 NM, then reused for another cycle. The results depicted that the desorption ration of ZrO2/SiO2 NM could up to 95% without no change on structure (see SEM image shown in Figure 7b), therefore, the adsorption capacity of the regenerated membrane is nearly the same with freshly fabricated ones. The good recycling performance of the membranes are ascribable to tow merits: the 22 ACS Paragon Plus Environment

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hierarchical structure of the membrane prevents the falling off of ZrO2 NPs from SiO2 fiber, thus endowing the membrane with stable adsorption performance; the exceptional flexibility guarantee the integrity of the membranes during the entire regeneration/reuse process.

Figure 7. (a) The cyclic adsorption performance of ZrO2/SiO2 NM towards phosphates and (b) SEM image of ZrO2/SiO2 NM after using for 5 cycles. 3.7. Adsorption performance comparison between different adsorbents. On the basis of the aforementioned results, we compared the phosphates adsorption



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performance of resultant ZrO2/SiO2 NM with adsorbents reported in literatures, as shown in Figure 8. Significantly, the resulting ZrO2/SiO2 NM obviously achieved the higher removal efficiency (~90%) and adsorption capacity (~45 mg g-1) compared with other ceramic-based phosphate adsorbents (Table S1), such as GO/ZrO2 nanosheets, SBA-15/ZrO2, Fe3O4/ZrO2 nanoparticles, and MgO/carbon microbeads.18, 36, 44, 45

We attributed those high removal efficiency and adsorption capacity to two

contributors: (1) the resultant membranes have quantities of active sites for phosphates binding, (2) the hierarchical mesoporous structure of the membranes facilitate the mass transfer in the torturous channels of nanofibrous membranes, thereby improved the accessibility to binding with phosphate.46, 47 More importantly, the as-prepared ZrO2/SiO2 NM exhibited good flexibility in dramatic contrast to the brittle nature of ceramic nanofibrous membranes, thus, such membranes with unexpected flexibility could facilely be taken out from solution after use without any tedious or time-consuming post-processing. Therefore, such membranes with excellent phosphate removal performance would be good alternatives to traditional adsorbents for phosphate remediation.

Figure 8. The adsorption capacity and removal efficiency comparison between 24 ACS Paragon Plus Environment

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different phosphates adsorbents.

4. CONCLUSION In summary, we present an intriguing approach for preparing flexible and hierarchical mesoporous ZrO2/SiO2 NM by combining CS impregnating method with ceramic nanofibrous membranes. The CS layer enables the ZrO2 NPs to non-aggregated grow on the SiO2 nanofiber surface with good homogeneous and high contents. Taking advantages of large specific surface area and hierarchical mesoporous structure, ZrO2/SiO2 NM exhibited excellent phosphate removal performance with removal efficiency of 85% and adsorption capacity of 43.8 mg P g-1 membranes within 30 min. Moreover, the membranes possess good flexibility and mechanical properties, thereby the membranes enable to be picked out after utilization without any post-processing. Furthermore, the membranes could be used to adsorb phosphate in detergent and fertilizer aqueous solution, thus, endows the ZrO2/SiO2 NM with great potential to remediate eutrophication water. Such exceptional ZrO2/SiO2 NM might also provide a new platform for constructing membranes with superb phosphate removal performance.

ASSOCIATED CONTENT

Supporting Information Available: Calculation method for fractal dimension based on FHH model, the UV-vis spectra showing the phosphate removal performance of SiO2 and ZrO2/SiO2-4 NM, the adsorption performance towards phosphates at different temperature, and the phosphate removal performance of various adsorbents. This material is available free of charge via the Internet at http://pubs.acs.org. 25 ACS Paragon Plus Environment


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AUTHOR INFORMATION Corresponding Author *Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT This work is supported by the National Natural Science Foundation of China (51322304), the Fundamental Research Funds for the Central Universities, and the “DHU Distinguished Young Professor Program”.


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