Facile Fabrication of Multi-Structured SiO2@PVDF-HFP Nanofibrous Membranes for Enhanced Copper Ions Adsorption

The low-cost, heavy metal ion (Cu(II)) adsorptive multi-structured nanofibrous membranes of silicon oxide naonoparticles in-situ anchored polyvinylidene fluoride-hexafluoropropylene (SiO2@PVDF-HFP) fibers were fabricated by the facile electrospinning technique combined with sol–gel strategy. To explore the benefits of the structure-related Cu(II) adsorption capacity, the fiber diameters of SiO2@PVDF-HFP nanofibrous membranes were changed which also resulted in the change of their porosity. Taking advantage of the constructed multi-structures and efficient fiber morphology regulation which not only changed the PVDF-HFP nanofibrous membrane from hydrophobic to superhydrophilic but also increased the porosity of the membrane, the SiO2@PVDF-HFP nanofibrous membrane with a smaller diameter and a larger porosity exhibits higher Cu(II) adsorption capacity. The adsorption amount was approximate to 21.9 mg per gram of the membrane, which was higher than that of membranes with larger fiber diameter (smaller porosity) and the smooth one. Furthermore, the model isotherms of Freundlich and Langmuir, as well as the kinetic models of pseudo-first-order and pseudo-second-order were preferred to analyze the adsorption equilibrium data. The Freundlich model and the pseudo-first-order were well fitted to the adsorption experimental data. It not only uncovers the structure-related-property of multi-structured nanofibrous membranes, but also provides an efficient and facile way to design heavy metal ion adsorption materials.


Introduction
Heavy-metal ions are a severe environmental problem in that organic and toxic waste in water not only poses a major health risk to humankind, but also destroys the circulation of the ecological environment [1,2]. Fully cognizant of its hazards, researchers have developed a great deal of techniques to transfer and adsorb heavy metal ions in aqueous solution, such as chemical treatments [3][4][5][6][7][8], biochemical processes [9,10], physicochemical treatments [11,12], et al. Among these, adsorption technology was considered as the simple operation and much more efficient method for removal of heavy metal ions [13][14][15]. Accordingly, various adsorbent such as activated carbon, oxide minerals, and biosorbents were investigated [16,17]. However, what we cannot ignore is that some of these processes have major drawbacks due to high energy consumption and increased operational cost to overcome the additional waste to be disposed of. For example, there is an additional cost to recover the powders that had been used to remove heavy ions in water. angle value was obtained by measuring at five different positions of the same sample. The water adhesion forces were measured using the high-sensitivity micro-electro-mechanical balance system (Data-Physics DCAT11, Stuttgart, Germany). For the PVDF-HFP fibrous membrane, the water droplet (5 µL) was suspended with a metal cap and controlled to contact with the surface of the membrane with a constant speed of 0.005 mm·s −1 , and then controlled to leave. The force-distance curve was recorded at the real-time. For the SiO 2 @PVDF-HFP, the water droplet was controlled to contact the membrane surface and keep contacting until it was adsorbed. The force-time curve was recorded. The chemical compositions of fibrous membranes were characterized by Fourier Transform Infrared spectrometry (FTIR) on Nicolet 8700 FT-IR spectrometry (Madison, WI, USA) in the wave range from 400 to 4000 cm −1 . X-ray diffraction (XRD) (Bruker, Karlsruhe, Germany) were collected on a Rigaku-D/max 40,000 V X-ray diffractometer equipped with Cu Kα radiation (λ = 0.15418 nm) at a step width of 5 • /min. The concentrations of Cu(II) solutions were measured by inductively coupled plasma atomic emission spectrophotometer (ICP-AES) (CIROS EOP, Kleve, Germany) Cu(II) removal experiments: A series of adsorption experiments were performed to investigate the efficiency of the multi-structured SiO 2 @PVDF-HFP for the removal of Cu(II) from aqueous solutions. The Cu(II) solutions with different concentrations were prepared by dissolving CuSO 4 ·5H 2 O (Beijing Yili Fine Chemical Co., Beijing, China) in deionized water. (1) to determine the influences of pH and adsorption time on removing Cu (II), the pH values of CuSO 4 solutions were adjusted to 2, 3, 4, 5, and 6, respectively, by 1 mol/L NaOH (Beijing Yili Fine Chemical Co., Beijing, China). 30 mg of SiO 2 @PVDF-HFP fibrous membranes were added into 30 mL of CuSO 4 aqueous solution (100 mg/L) and then measured concentrations of the Cu solutions after 120 min. (2) in order to explore the equilibrium adsorption kinetics, 30 mg of as-prepared adsorbent was immersed into 100 mL of CuSO 4 aqueous solution (100 mg/L) at pH = 4.5, followed by sampling the solution after 5,10,20,30,50,80,120,200, 300 min for the analysis of Cu content in the solution. Thus, the adsorption kinetic model was determined. (3) 30 mg of multi-structured SiO 2 @PVDF-HFP fibrous membrane was immersed into 30 mL CuSO 4 aqueous solution (100 mg/L) with different initial Cu(II) concentrations (30,50,70, 100, 200 mg/L) at room temperature. (4) adsorption capacity of SiO 2 @PVDF-HFP with different fiber diameters was compared. When the adsorption reached the equilibrium (adjusted pH = 5 and 6), the Cu(II) content in the solution was measured. The adsorption capacity of the membrane for the Cu(II) removal was calculated as follows: where q e is the equilibrium adsorption quantity (mg/g), C 0 and C e represent the initial and equilibrium Cu(II) concentration (mg/L), respectively. V is the volume of the CuSO 4 aqueous solution (L), and m is the quality of membrane (g).

Fabrication and Morphology of Multi-Structured SiO 2 @PVDF-HFP Nanofibrous Membranes
Polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) was used as the raw materials to fabricate the nanoparticle anchored on nanofiber multi-structured membranes. Figure 1 schematically illustrates the membrane fabrication process. An electrospinning technique was used to process (PVDF-HFP)/N,N -dimethylformamide (DMF) solution into a fibrous membrane. The obtained PVDF-HFP fibrous membrane was then immersed in the stirring alcohol, and TEOS was added dropwise at the same time. Since the electrospun PVDF-HFP nanofibrous membranes act as the skeleton, during the process of synthesis SiO 2 by Stöber sol-gel method [25], SiO 2 nanoparticles had been in-situ anchored on their surfaces.

Figure 1.
Schematic illustration of multi-structured SiO2@PVDF-HFP nanofibrous membrane preparation process. The PVDF-HPF nanofibrous membrane was obtained by electrospinning, and then was immersed in stirred alcohol, TEOS was successively dripped into the alcohol solution. After the minimum time of 6 h, the multi-structured SiO2@PVDF-HFP nanofibrous membrane was successfully fabricated.
In this work, the pore size of both the PVDF-HFP and SiO2-PVDF-HFP membranes were controlled by regulating the fiber diameters during the electrospinning process, mainly by adjusting the concentrations of electrospinng solution as well as the inner diameters of electrospun nozzles. As we know, the most straightforward means of increasing the pore size of electrospun scaffolds involves careful modification of system or process parameters during the fabrication process. For one thing, smaller pores can be achieved by decreasing the fiber diameters, a technique that is described by a number of groups as means of decreasing the pore size of scaffolds electrospun from both synthetic and natural polymers [26][27][28][29]. For the other thing, statistical modeling predicts a relationship between the fiber diameter and the pore size of electrospun scaffolds, where a smaller fiber diameter correlates with an increase in the pore size [24,30]. What's more, our previous works [31,32] also proved that the pore size of micro/nano fibrous membrane can be controlled by regulating the electrospinning parameters. Images in Figure 2 are the scanning electron microscopy (SEM) of asprepared samples. To obtain different fibers' diameter, different metal needles with the inner diameters of 0.6 mm (6G), 0.8 mm (8G), 1.0 mm (10G), and 1.2 mm (12G) were equipped during the fabrication process. Accordingly, Figure 2a-d are the typical SEM images of electrospun PVDF-HPF fibers with different fiber diameters, which are named as PVDF-HFP-6G, PVDF-HFP-8G, PVDF-HFP-10G, and PVDF-HFP-12G, respectively. The statistic average diameters are 150 nm, 201 nm, 214 nm, and 236 nm, respectively.
After 18 h of the sol-gel fabrication process, SEM images of SiO2 nanoparticle anchored samples are exhibiting in Figures 2e-h (named SiO2-PVDF-HFP-6G, SiO2-PVDF-HFP-8G, SiO2-PVDF-HFP-10G, and SiO2-PVDF-HFP-12G). It can be seen that the SiO2 nanoparticles are successfully fabricated and anchored on the PVDF-HFP fibrous membranes (orange dotted frames in Figure 2e-h). Meanwhile, for both the PVDF-HFP and SiO2@PVDF-HFP samples, the fibers are randomly oriented on the scaffold, and fibers' diameters are increased by increasing the nozzles' inner diameters. Schematic illustration of multi-structured SiO 2 @PVDF-HFP nanofibrous membrane preparation process. The PVDF-HPF nanofibrous membrane was obtained by electrospinning, and then was immersed in stirred alcohol, TEOS was successively dripped into the alcohol solution. After the minimum time of 6 h, the multi-structured SiO 2 @PVDF-HFP nanofibrous membrane was successfully fabricated.
In this work, the pore size of both the PVDF-HFP and SiO 2 -PVDF-HFP membranes were controlled by regulating the fiber diameters during the electrospinning process, mainly by adjusting the concentrations of electrospinng solution as well as the inner diameters of electrospun nozzles. As we know, the most straightforward means of increasing the pore size of electrospun scaffolds involves careful modification of system or process parameters during the fabrication process. For one thing, smaller pores can be achieved by decreasing the fiber diameters, a technique that is described by a number of groups as means of decreasing the pore size of scaffolds electrospun from both synthetic and natural polymers [26][27][28][29]. For the other thing, statistical modeling predicts a relationship between the fiber diameter and the pore size of electrospun scaffolds, where a smaller fiber diameter correlates with an increase in the pore size [24,30]. What's more, our previous works [31,32] also proved that the pore size of micro/nano fibrous membrane can be controlled by regulating the electrospinning parameters. Images in Figure 2 are the scanning electron microscopy (SEM) of as-prepared samples. To obtain different fibers' diameter, different metal needles with the inner diameters of 0.6 mm (6G), 0.8 mm (8G), 1.0 mm (10G), and 1.2 mm (12G) were equipped during the fabrication process. Accordingly, Figure 2a Figure 3a exhibits the relationship between electrospun nozzles type, solution concentrations and fiber diameter. With the increase of the inner diameter of electrospun needles as well as the electrospun solution concentration, the fiber diameter of the obtained PVDF-HFP fibers increased. The wettability of both PVDF-HFP and SiO2@PVDF-HFP fibrous membranes were carried out by measuring the water contact angles (WCA). A water droplet about 3 μL allowed to contact the surface of all as-prepared samples (Figure 3b). Water contact angles are 151.2° ± 0.6°, 147.7° ± 0.5°, 144.0° ± 0.7°, and 141.5° ± 0.9° corresponding to PVDF-HFP-6G, PVDF-HFP-8G, PVDF-HFP-10G, and PVDF-HFP-12G, respectively, indicating the PVDF-HFP nanofibrous membranes are hydrophobicity (blue line and inserted photos with dotted frames in Figure 3b). The PVDF-HFP natural property and porous membranes with high surface roughness are the main reasons for its hydrophobicity. Meanwhile, it also can be seen that the different diameter shows tiny influence to the wettability. With the increasing of fiber diameter, the WCA of PVDF-HFP fibrous membranes decreased slightly, attributing to that for a fibers-composed surface, the smaller fiber diameter usually brings about greater surface roughness, which can enhance or "enlarge" the wetting behavior [33,34]. In contrast, due to abundance of hydroxyl groups of hydrophilic SiO2 nanoparticles prepared via Stöber method, when 3 μL water droplet dropped on SiO2@PVDF-HFP membrane surfaces, it immediately spreads on the membrane with the contact angle near to 0°, exhibiting superhydrophilic property (rose line and inserted photos with dotted frames in Figure 3b).  Figure 2e-h). Meanwhile, for both the PVDF-HFP and SiO 2 @PVDF-HFP samples, the fibers are randomly oriented on the scaffold, and fibers' diameters are increased by increasing the nozzles' inner diameters. Compared with PVDF-HFP fibers, the fiber diameters of SiO 2 @PVDF-HFP increased, which can result from the coating of sol during the SiO 2 fabrication process via the Stöber method. Figure 3a exhibits the relationship between electrospun nozzles type, solution concentrations and fiber diameter. With the increase of the inner diameter of electrospun needles as well as the electrospun solution concentration, the fiber diameter of the obtained PVDF-HFP fibers increased. The wettability of both PVDF-HFP and SiO 2 @PVDF-HFP fibrous membranes were carried out by measuring the water contact angles (WCA). A water droplet about 3 µL allowed to contact the surface of all as-prepared samples ( Figure 3b). Water contact angles are 151.2 • ± 0.6 • , 147.7 • ± 0.5 • , 144.0 • ± 0.7 • , and 141.5 • ± 0.9 • corresponding to PVDF-HFP-6G, PVDF-HFP-8G, PVDF-HFP-10G, and PVDF-HFP-12G, respectively, indicating the PVDF-HFP nanofibrous membranes are hydrophobicity (blue line and inserted photos with dotted frames in Figure 3b). The PVDF-HFP natural property and porous membranes with high surface roughness are the main reasons for its hydrophobicity. Meanwhile, it also can be seen that the different diameter shows tiny influence to the wettability. With the increasing of fiber diameter, the WCA of PVDF-HFP fibrous membranes decreased slightly, attributing to that for a fibers-composed surface, the smaller fiber diameter usually brings about greater surface roughness, which can enhance or "enlarge" the wetting behavior [33,34]. In contrast, due to abundance of hydroxyl groups of hydrophilic SiO 2 nanoparticles prepared via Stöber method, when 3 µL water droplet dropped on SiO 2 @PVDF-HFP membrane surfaces, it immediately spreads on the membrane with the contact angle near to 0 • , exhibiting superhydrophilic property (rose line and inserted photos with dotted frames in Figure 3b).

FTIR and XRD Analysis
The Fourier Transform Infrared Spectroscopy (FTIR) was used to characterize the chemical compositions of as-prepared PVDF-HFP, SiO2@PVDF-HFP electrospun nanofibrous membrane, and the synthetic inorganic silica nanoparticles (Figure 4a). It was carried out in the range of 400 cm −1 -4000 cm −1 to analysis and confirm the presence of functional groups [35]. Compared with the spectra of PVDF-HFP porous membrane, peaks in the range of 3639-2730 cm −1 appeared, which can be attributed to the anti-symmetric stretching vibration peaks of the silanol (Si-OH) group for the SiO2@PVDF-HFP membrane [36][37][38]. The peak around 1630 cm −1 is ascribed to the bending vibration of O-H groups [37]. The presence of SiO2 is verified by absorption bands at 1135 to 990 cm −1 [38,39], Si-O-Si anti-symmetric stretching vibrations, and a very weak Si-OH bending vibration absorption peak near 944 cm −1 [39,40]. Since Si-OH is susceptible to thermal dehydration to form Si-O-Si, almost no visible Si-OH absorption peak can be detected in the spectrum. In addition, peaks in the range of 852-769 cm −1 and 510-418 cm −1 are represented the Si-O-Si asymmetric stretching vibration and The heavy metal ion removal process was in an aqueous solution system. Such a reaction process can be deemed as the interfacial reaction in which the interaction between the aqueous solution and the membrane plays important role. Accordingly, the drawing force of the porous membranes towards the water droplet was measured by using a micro-force balance system. Two testing models were employed: in Model 1, a water droplet (5 µL) was brought to contact the membrane and then pulled off, which was used to probe the force between the water droplet and the hydrophobic PVDF-HFP porous membrane (schematic illustration on top of Figure 3c). While, in Model 2, the water droplet was brought to contact the membrane without pulling off, from which water spreading or adsorbing time by the superhydrophilic SiO 2 @PVDF-HFP porous membrane can be measured, and the real-time force change was recorded (schematic illustration on top of Figure 3d).
As shown in Figure 3c, when water droplets contacted the hydrophobic PVDF-HFP porous surface and then moved downward, before being fully separated from fibrous surface, the force gradually increased with increasing the "moving downward" distance (D). At the time when the water droplet separated from the membrane, the force reached a maximum value (F) after which it then decreased abruptly. The recorded corresponding force for the porous membrane PVDF-HFP-6G with smallest fiber diameter was 54.88 µN, which was higher than that of PVDF-HFP-8G (49.98 µN), PVDF-HFP-10G (32.34 µN), and PVDF-HFP-12G (31.36 µN). It indicates that the nanofibrous membrane PVDF-HFP-6G exhibits higher adhesive force to water which is in favor to adsorb solution in further sol-gel SiO 2 fabrication process. Figure 3d shows the testing results of superhydrophilic SiO 2 @PVDF-HFP porous membranes. It should be pointed out that for the superhydrophilic porous surface, once the water droplet contacted on its surface, it can be adsorbed immediately. The maximum force value appears when the water droplet contacted on the membrane and drops rapidly in a short time, regarded as a shorter time to reach the peak suggesting faster water spreading or adsorbing. The recorded time was 13.60 s, 14.88 s, 18.60 s, and 18.35 s for SiO 2 @PVDF-HFP-6G, SiO 2 @PVDF-HFP-8G, SiO 2 @PVDF-HFP-10G, and SiO 2 @PVDF-HFP-12G, respectively. That is, the multi-structured SiO 2 @PVDF-HFP-6G nanofibrous membrane shows stronger and faster absorbed affinity to aqueous solution, which contributes to the interaction between the ions in aqueous solution and the porous membrane surface.

FTIR and XRD Analysis
The Fourier Transform Infrared Spectroscopy (FTIR) was used to characterize the chemical compositions of as-prepared PVDF-HFP, SiO 2 @PVDF-HFP electrospun nanofibrous membrane, and the synthetic inorganic silica nanoparticles (Figure 4a). It was carried out in the range of 400 cm −1 -4000 cm −1 to analysis and confirm the presence of functional groups [35]. Compared with the spectra of PVDF-HFP porous membrane, peaks in the range of 3639-2730 cm −1 appeared, which can be attributed to the anti-symmetric stretching vibration peaks of the silanol (Si-OH) group for the SiO 2 @PVDF-HFP membrane [36][37][38]. The peak around 1630 cm −1 is ascribed to the bending vibration of O-H groups [37]. The presence of SiO 2 is verified by absorption bands at 1135 to 990 cm −1 [38,39], Si-O-Si anti-symmetric stretching vibrations, and a very weak Si-OH bending vibration absorption peak near 944 cm −1 [39,40]. Since Si-OH is susceptible to thermal dehydration to form Si-O-Si, almost no visible Si-OH absorption peak can be detected in the spectrum. In addition, peaks in the range of 852-769 cm −1 and 510-418 cm −1 are represented the Si-O-Si asymmetric stretching vibration and bending vibration, respectively [39,41]. Such characteristic peaks of SiO 2 nanoparticles appeared in the SiO 2 @PVDF-HFP spectrum proving that silica nanoparticles successfully loaded on the fibers' surface.
To analysis the crystal property of as-prepared samples, Figure 4b exhibits the X-ray diffraction (XRD) spectrum. For SiO 2 nanoparticles, XRD peaks appeared in the low diffraction angle region near 2θ = 23 • , indicating that the obtained SiO 2 nanoparticles are amorphous [38,42]. Once 2θ over 23 • , the diffraction intensity gradually decreased with slightly fluctuations, and finally tended to become smooth. Making a comparison with PVDF-HFP and SiO 2 nanoparticles, the diffraction spectrum of the SiO 2 @PVDF-HFP sample shown a typical SiO 2 amorphous diffraction peak near 2θ = 20 • , which can also be seen in the two-dimensional diffraction patterns (insertions in Figure 3b), indicating that the amorphous SiO 2 nanoparticles anchored on PVDF-HFP nanofibrous membrane [39,43,44]. diffraction intensity gradually decreased with slightly fluctuations, and finally tended to become smooth. Making a comparison with PVDF-HFP and SiO2 nanoparticles, the diffraction spectrum of the SiO2@PVDF-HFP sample shown a typical SiO2 amorphous diffraction peak near 2θ = 20°, which can also be seen in the two-dimensional diffraction patterns (insertions in Figure 3b), indicating that the amorphous SiO2 nanoparticles anchored on PVDF-HFP nanofibrous membrane [39,43,44].

Copper Ion (Cu(II)) Adsorption in Aqueous Solution
In addition, batch adsorption experiments were carried out to study the adsorption and removal capacity of multi-structured SiO2@PVDF-HFP electrospun nanofibrous membranes. Copper ions (Cu(II)) in aqueous solution acts as a "target" degradation for adsorption. On account of the substantial verified electrostatic/adsorption properties of an adsorbent with the solution pH value, it is known that the initial pH value exhibits a significant effect on adsorption capacity of metal ions [45,46]. To explore the optimized pH value, the effect of solution pH values (from pH = 1.0 to 6.0, since the precipitations of copper hydroxide would form at the solution pH values above 7) [47] on the adsorptions of Cu(II) ion were investigated (Figure 5a). When the pH value is in the range of 1 to 5, the Cu(II) simply adsorbed. Once the pH changed to 6, the adsorption amounts were increased abruptly. It can be attributed to that the surface of the adsorbent had positive charges at low pH values (1)(2)(3)(4)(5). When the pH values were raised over 5, the -OH groups on the SiO2 nanoparticle surfaces suggested making the adsorbent membrane surface more negatively charged [48]. Meanwhile, the incorporation of negative functional groups including siloxane, silanol, and amine groups into the SiO2@PVDF-HFP fibrous membrane which enhanced the available active sites for adsorption process [48,49]. For one thing, the Cu(II) metal ions with positive charges can easily interact with the surface of electrospun membrane. Cu(II) would hydrolyze to Cu(OH)2, Cu(OH) + , and Cu2(OH)2 2+ when the pH over 5 which will promote the adsorption [47,50]. Whereas, further increase in the pH level once at high pH, i.e., pH > 7 (alkaline solutions), the metal cations start to react with hydroxide ions to form metal hydroxide and get precipitated.
In Figure 5b, the effect of initial concentration of Cu(II) ions on the adsorption capacity was also depicted. The relationship between initial Cu(II) concentration and the removal rate by multistructured SiO2@PVDF-HFP nanofibrous membrane can be obtained according to the slope in the plot of adsorption amount (Q (mg/g)) versus initial Cu(II) concentration (C (mg/L)). The overall trend exhibits that the adsorption capacity of SiO2@PVDF-HFP electrospun membrane increased with the increasing of initial Cu(II) concentrations. However, it can be seen that the slope of the Q~C plot was larger when the initial Cu(II) concentration was below 50 mg/L than once it was over 50 mg/L, exhibiting that the multi-structured nanofibers exhibit a higher removal rate at relatively lower initial

Copper Ion (Cu(II)) Adsorption in Aqueous Solution
In addition, batch adsorption experiments were carried out to study the adsorption and removal capacity of multi-structured SiO 2 @PVDF-HFP electrospun nanofibrous membranes. Copper ions (Cu(II)) in aqueous solution acts as a "target" degradation for adsorption. On account of the substantial verified electrostatic/adsorption properties of an adsorbent with the solution pH value, it is known that the initial pH value exhibits a significant effect on adsorption capacity of metal ions [45,46]. To explore the optimized pH value, the effect of solution pH values (from pH = 1.0 to 6.0, since the precipitations of copper hydroxide would form at the solution pH values above 7) [47] on the adsorptions of Cu(II) ion were investigated (Figure 5a). When the pH value is in the range of 1 to 5, the Cu(II) simply adsorbed. Once the pH changed to 6, the adsorption amounts were increased abruptly. It can be attributed to that the surface of the adsorbent had positive charges at low pH values (1)(2)(3)(4)(5). When the pH values were raised over 5, the -OH groups on the SiO 2 nanoparticle surfaces suggested making the adsorbent membrane surface more negatively charged [48]. Meanwhile, the incorporation of negative functional groups including siloxane, silanol, and amine groups into the SiO 2 @PVDF-HFP fibrous membrane which enhanced the available active sites for adsorption process [48,49]. For one thing, the Cu(II) metal ions with positive charges can easily interact with the surface of electrospun membrane. Cu(II) would hydrolyze to Cu(OH) 2 , Cu(OH) + , and Cu 2 (OH) 2 2+ when the pH over 5 which will promote the adsorption [47,50]. Whereas, further increase in the pH level once at high pH, i.e., pH > 7 (alkaline solutions), the metal cations start to react with hydroxide ions to form metal hydroxide and get precipitated. In Figure 5b, the effect of initial concentration of Cu(II) ions on the adsorption capacity was also depicted. The relationship between initial Cu(II) concentration and the removal rate by multi-structured SiO 2 @PVDF-HFP nanofibrous membrane can be obtained according to the slope in the plot of adsorption amount (Q (mg/g)) versus initial Cu(II) concentration (C (mg/L)). The overall trend exhibits that the adsorption capacity of SiO 2 @PVDF-HFP electrospun membrane increased with the increasing of initial Cu(II) concentrations. However, it can be seen that the slope of the Q~C plot was larger when the initial Cu(II) concentration was below 50 mg/L than once it was over 50 mg/L, exhibiting that the multi-structured nanofibers exhibit a higher removal rate at relatively lower initial concentrations. At first, plenty of Cu(II) were adsorbed on the surface of the membrane at any initial concentrations by the coaction of both the active sites of fibers and the roughness of the multi-structured membrane. With the increase in initial Cu(II) ion concentration, the ion coordination sites on the surface of the nanofiber membrane were rapidly occupied [45]. Taking advantages of the roughness of the obtained multi-structured fibrous surface, plenty of Cu(II) ions can only be adsorbed on the multi-structured nanofiber surface [51]. The adsorption of Cu(II) ions can only occupy on the rough areas of the membrane, which cause the Cu(II) adsorption rate to decrease.

Adsorption Kinetics and Isotherms Simulation
To better understand the adsorption ability of the multi-structured SiO2@PVDF-HFP nanofibrous membranes toward Cu(II) metal ions, the adsorption experimental data are normally used to evaluated by the kinetic models. The kinetic models usually consist of pseudo first-order adsorption model and pseudo second-order adsorption model [52][53][54][55], both of which can be expressed by kinetic equations: Pseudo first-order adsorption model: where qt is the amount of Cu(II) adsorbed (mg/g) at time t, qe is the amount of Cu(II) adsorbed (mg/g) at equilibrium time, respectively, k1 is the rate constant (1/min) of pseudo first-order adsorption. Pseudo second-order adsorption model: where k2 (g/mg/min) represents the rate constant of pseudo-second-order reaction. The value of qe and k2 can be determined by the slope and intercept of the straight line of the plot of "t/ qt versus t".
The first-order kinetic equation means that the adsorption rate is linear with a reactant concentration, while the second-order kinetics means adsorption rate is linear with two reactants. In this work, the applicability of suitable kinetic model can be evaluated through the magnitude of the so-called correlation coefficients, i.e., R 2 . Generally, the higher R 2 represents better applicability of the corresponding kinetic model [52,54]. Both the pseudo-first-order adsorption model and the pseudosecond-order adsorption model were applied to fitted the adsorption behavior. As shown in Figure  6a and Figure 6b, the pseudo-second-order kinetic model (R 2 = 0.99735) was better described the kinetic data of Cu(II) removal process in this work. More details can be seen in Table 1. The qe values (qe(cal) = 10.05025 mg/g) calculated from the pseudo-second-order model for Cu(II) metal ions adsorption are closer to the experimental data (qe(exp) =9.9818 mg/g), indicated the adsorption process of the multi-structured electrospun SiO2@PVDF-HFP fibrous membrane on Cu(II) ions is more Furthermore, the adsorption time is also one of the most important factors to evaluate the removal performance of heavy metal ions [45], and its influence on adsorption capacity of multi-structured SiO 2 @PVDF-HFP-6G fibrous membrane in Cu(II) aqueous solution are shown in Figure 5c. It exhibited a relatively high adsorption rate during the early stage of the adsorption process, then reduced sharply with the increasing of adsorption time, and finally reached the equilibrium time of 120 min. During the whole process, the concentration of Cu(II) in aqueous solution decreased from 110 mg/L to 99.02 mg/L, and keep constant after 120 min.
The unique characteristics of electrospun fibrous material are the very high surface area to volume ratios, high porosity that results in high surface cohesion, making it an excellent candidate for adsorption. By adjusting the fiber diameter, the morphology of fibrous membrane can be easily regulated, which influences the porosity of the fibrous membrane. As showing in Figure 5d, the Cu(II) adsorption capacity of multi-structured SiO 2 @PVDF-HFP electrospun nanofibrous membranes with different fiber diameters as well as the SiO 2 anchored smooth PVDF-HFP (prepared via sip-coating, named SM) were measured under the conditions of a pH value of 6, and an initial Cu(II) ion concentration of 50 mg·L −1 . The adsorption efficiency gradually decreased corresponding to the membranes 6G, 8G, 10G, and 12G. The SiO 2 @PVDF-HFP-6G exhibits the highest Cu(II) adsorption capacity (21.9 mg per gram of membrane). Meanwhile, the adsorption capacity of the SM membrane was much lower than that of the other multi-structured electrospun SiO 2 @PVDF-HFP membranes which, mainly due to that the rough fibrous membranes, have larger surface areas providing more available active sites which facilitated the adsorption of Cu(II) metal ions [39,45,51]. In addition, the relation between the average fiber diameter and the porosity were measured via anhydrous ethanol uptake, and the results were shown in Figure 5e. For both the SiO 2 @PVDF-HFP and PVDF-HFP membranes, the porosity decreased with the decreasing of the average fiber diameter. The porosity of SiO 2 @PVDF-HFP-6G, SiO 2 @PVDF-HFP-8G, SiO 2 @PVDF-HFP-10G, and SiO 2 @PVDF-HFP-12G were 89.74%, 83.32%, 80.54%, and 74.69%, respectively, which are higher than that those of PVDF-HFP membranes. Thus, it can be seen that for one thing, the constructed multi-structures offer larger porosity. For the other thing, the average fiber diameter of multi-structured nanofibrous membranes plays an important role in determining the porosity as well as affects the adsorption of Cu(II), i.e., for the membranes with similar morphology, the smaller fiber diameter caused larger porosity, which enhanced the Cu(II) adsorption in aqueous solution.
The recyclable property of multi-structured SiO 2 @PVDF-HFP-6G nanofibrous membrane was studied by repeating the Cu(II) removal experimental process five times by using the same nanofibrous membrane. The regeneration of Cu 2+ metal ions by the membrane was treated with 50 mL hydrochloric acid solution at pH = 1. After 2 h stirring at 25 • C, these membranes were rinsed with diluted alcohol and deionized water repeatedly. As shown in Figure 5f, the adsorption amount of obtained nanofibrous membrane after five times recycle usage was 18.4 mg/g, which exhibits only slight decrease from that of adsorption amount when used for the first time (21.9 mg/g), showing a superior potential for recycle application.

Adsorption Kinetics and Isotherms Simulation
To better understand the adsorption ability of the multi-structured SiO 2 @PVDF-HFP nanofibrous membranes toward Cu(II) metal ions, the adsorption experimental data are normally used to evaluated by the kinetic models. The kinetic models usually consist of pseudo first-order adsorption model and pseudo second-order adsorption model [52][53][54][55], both of which can be expressed by kinetic equations: Pseudo first-order adsorption model: where q t is the amount of Cu(II) adsorbed (mg/g) at time t, q e is the amount of Cu(II) adsorbed (mg/g) at equilibrium time, respectively, k 1 is the rate constant (1/min) of pseudo first-order adsorption. Pseudo second-order adsorption model: where k 2 (g/mg/min) represents the rate constant of pseudo-second-order reaction. The value of q e and k 2 can be determined by the slope and intercept of the straight line of the plot of "t/q t versus t".
The first-order kinetic equation means that the adsorption rate is linear with a reactant concentration, while the second-order kinetics means adsorption rate is linear with two reactants. In this work, the applicability of suitable kinetic model can be evaluated through the magnitude of the so-called correlation coefficients, i.e., R 2 . Generally, the higher R 2 represents better applicability of the corresponding kinetic model [52,54]. Both the pseudo-first-order adsorption model and the pseudo-second-order adsorption model were applied to fitted the adsorption behavior. As shown in Figure 6a,b, the pseudo-second-order kinetic model (R 2 = 0.99735) was better described the kinetic data of Cu(II) removal process in this work. More details can be seen in Table 1. The q e values (q e(cal) = 10.05025 mg/g) calculated from the pseudo-second-order model for Cu(II) metal ions adsorption are closer to the experimental data (q e(exp) = 9.9818 mg/g), indicated the adsorption process of the multi-structured electrospun SiO 2 @PVDF-HFP fibrous membrane on Cu(II) ions is more consistent with the pseudo-second-order kinetics [56].  Isothermal adsorption models are also used to evaluated the adsorption simulation [54], which consist of Langmuir adsorption model and Freundlich adsorption model. Based on the relationship between the initial Cu(II) concentrations with the adsorption capacity of multi-structured electrospun SiO2@PVDF-HFP-6G membrane shown in Figure 5b, the Freundlich and Langmuir isotherm models were used to fit the equilibrium. The isotherm equations are expressed as follows [54,57,58].
Langmuir isotherm model: where qe is the amount of the metal ions adsorbed (mg/g), qm is the maximum adsorption capacity (mg/g), Ce is the equilibrium concentration of the metal ions (mg/L), and KL is the Langmuir constant related to the free energy of adsorption (L/mg). The values of qm and KL can be derived from the slope and intercept of Ce/qe versus Ce. Freundlich isotherm model: Where qe is the equilibrium adsorption capacity (mg/g), and KF and n are Freundlich constants related to the adsorption capacity and adsorption intensity, respectively. If 1/n is smaller than unity, the  Isothermal adsorption models are also used to evaluated the adsorption simulation [54], which consist of Langmuir adsorption model and Freundlich adsorption model. Based on the relationship between the initial Cu(II) concentrations with the adsorption capacity of multi-structured electrospun SiO 2 @PVDF-HFP-6G membrane shown in Figure 5b, the Freundlich and Langmuir isotherm models were used to fit the equilibrium. The isotherm equations are expressed as follows [54,57,58].
Langmuir isotherm model: where q e is the amount of the metal ions adsorbed (mg/g), q m is the maximum adsorption capacity (mg/g), C e is the equilibrium concentration of the metal ions (mg/L), and K L is the Langmuir constant related to the free energy of adsorption (L/mg). The values of q m and K L can be derived from the slope and intercept of C e /q e versus C e . Freundlich isotherm model: ln q e = 1 n ln C e + ln K F where q e is the equilibrium adsorption capacity (mg/g), and K F and n are Freundlich constants related to the adsorption capacity and adsorption intensity, respectively. If 1/n is smaller than unity, the adsorption process is favorable. The values of K F and n can be obtained from the slope and intercept evaluated from the plot of lnq e versus lnC e . The Freundlich isotherm model is suitable for the adsorption on heterogeneous surfaces which can describe adsorption data over a restricted range. The Langmuir adsorption model is mainly used for the adsorption process with strong interaction with the adsorption membrane surface, and the adsorption process is single layer adsorption. The isotherm simulation results are shown in Figure 6c,d, and much more details are presented in Table 2. Making comparison of the correlation coefficients, it was found that the equilibrium data was best described by Freundlich isotherm model (R 2 = 0.97377) than that from Langmuir isotherm model (R 2 = 0.92495) [49]. It was caused by the multilayer reaction adsorption of Cu(II) ions onto the heterogeneous surface of multi-structured SiO 2 @PVDF-HFP fibrous membranes, i.e., the stronger binding sites are occupied firstly by the membrane, which is consistent with the mechanism of Freundlich isotherm adsorption [53]. And the Freundlich constants (1/n) was smaller than 1 [56], indicating the adsorption of Cu(II) metal ions onto the multi-structured SiO 2 @PVDF-HFP fibrous membranes are the favorable processes.

Conclusions
In summary, SiO 2 nanoparticles were successfully anchored on electrospun PVDF-HFP nanofibrous membranes, forming superhydrophilic multi-structured SiO 2 @PVDF-HFP nanofibrous membranes. The benefit of the structure-related Cu(II) adsorption capacity is that the multi-structured SiO 2 @PVDF-HFP nanofibrous membrane with smaller fiber diameter, larger porosity exhibits greater affinity to aqueous solutions and higher Cu(II) adsorption capacity. Taking advantages of constructed SiO 2 nanoparticles and fiber morphology regulation, not only was the Cu(II) adsorption capacity enhanced, but also the adsorption simulation was fitted well with the kinetic model of pseudo-first-order and the isotherm model of Freundlich. This work uncovers the structure-related-property of multi-structured nanofibrous materials, and provides an efficient and facile way to design the heavy metal ion adsorption materials.