Ultralight and Superelastic Nanofiber Aerogels with In-Situ Loaded Polypyrrole for High-Efficient Cr(VI) Adsorption

Constructing 3-D nanomaterial-based adsorbents with good mechanical properties has attracted increasing attention. This paper reports a simple strategy to prepare ultralight and superelastic 3-D aerogels as efficient adsorbents for heavy metal Cr(VI) in wastewater by in-situ loading of polypyrrole on cellulose acetate nanofibers followed by freeze-drying method. The obtained aerogel possesses obvious 3-D porous and cross-linked structure, in which polypyrrole is uniformly coated on the nanofiber surface. Besides, the aerogel shows good mechanical properties with high compressive strength of 14.49 kPa, and can be processed into any desired shape. Especially, due to the presence of large specific surface area and high porosity, the obtained nanofiber aerogel shows significant Cr(VI) adsorption with maximum adsorption capacity of 244.65 mg/g. The present work proposes a high-quality nanofiber aerogel for efficiently adsorbing Cr(VI).


Introduction
With the acceleration of global industrialization process, water pollution is becoming more and more serious. Heavy metal pollution is a common and harmful source of water pollution. Among the types of heavy metal pollution, chromium pollution is the second-largest source of pollution after lead. The main forms of chromium in nature are Cr(III) and Cr(VI) [1][2][3][4]. Among them, Cr(VI) has the characteristics of contact sensitization, inhalation carcinogenicity, possible genetic defects, and high mobility, which is harmful to human health and the ecological environment [3][4][5][6][7]. The traditional treatment methods for Cr(VI) in water include chemical precipitation, electrochemical, membrane separation, etc. [8][9][10] In contrast, adsorption is a relatively efficient and environmentally friendly treatment method [11]. To efficiently treat high-toxic Cr(VI) in water, various adsorption materials have been designed and prepared, such as activated carbon, agricultural waste, metal oxide nanoparticles, mineral clay, etc. [12][13][14][15] However, these materials often show low adsorption efficiency, and complex secondary separation is required after adsorption, which not only significantly increases the working difficulty but also may cause secondary pollution. In recent years, the merits of outstanding adsorption ability, high adsorption efficiency, good environmental stability, and easy processing make polypyrrole (PPy) a rising star in the field of adsorption [16][17][18]. However, pure PPy tends to form clusters after polymerization, and its surface area is very small, inevitably leading to poor adsorption efficiency.
How to prepare PPy-based composites with large specific surface area and good recyclability has become a research hotspot. Some substances with larger specific surface areas, such as graphene oxide (GO), multi-walled carbon nanotubes (MWTNTs), SiO 2 nanoparticles, Fe 3 O 4 nanoparticles, etc., have been chosen as substrate materials for pyrrole polymerization [19][20][21][22]. Compared with pure polypyrrole, these composites generally show large surface area and can efficiently solve the recovery problem of adsorbents, but their 2-D layered stacking structure greatly limits their shape designability. In addition, these materials have reduced liquid permeability due to their small pore size. Moreover, because of their weak structure, it is difficult to assemble them onto carrier media for stable use, which significantly limits their future large-scale applications.
Recently, aerogels, as size-controllable lightweight soft materials, have shown obvious development prospects in the field of adsorption [23][24][25]. Several functional aerogels, such as silica colloidal aerogels, graphene composite aerogels, and polymer sponges, have recently been used in the field of adsorption [26][27][28]. Although traditional aerogels have the advantages of low density, high porosity, and high specific surface area, they also have inherent limitations of low strength and poor toughness. To address this issues, nanofibrous aerogels have been developed and prepared in combination with electrospinning technology [29][30][31]. On the basis of retaining the advantages of traditional aerogels, nanofibrous aerogels can also greatly improve the mechanical properties of aerogels and become an excellent candidate for adsorbents [32][33][34][35][36][37]. Therefore, it is of great significance to explore an economical, environmentally friendly and shape-designable polypyrrole composite aerogel material for Cr(VI) adsorption.
In view of the above contents, the present study reports our success in constructing ultralight and superelastic 3-D nanofiber composite aerogels (PNFA) as efficient adsorbents for Cr(VI) in wastewater by in-situ polymerization of polypyrrole on cellulose acetate (CA) nanofibers followed by freeze-drying method. The obtained PNFA shows high porosity, large specific surface area and environmental stability, and its maximum adsorption capacity of Cr(VI) can reach up to 322.58 mg/g. The PNFA also exhibits good mechanical properties with high compressive strength and can be processed into any desired shape. This work opens a new avenue for the large-scale production of assembled adsorption devices for water pollution treatment.

Characterization
The chemical composition of the nanofibrous aerogels was analyzed by Fourier infrared spectroscopy (Perkin Elmer Frontier), X-ray photoelectron spectroscopy (Thermo Scientific K-Alpha, USA), and Raman spectroscopy (Horiba Scientific LabRAM HR Evolution). The aerogel surface morphology was observed by scanning electron microscope (Hitachi S-4800, Japan). The thermal stability of the nanofibrous aerogels was evaluated by thermogravimetric analysis (TA SDT650). The concentration of Cr(VI) was detected by the UV-Vis spectrophotometer (JEM2100Plus, Japan). The compressive stress-strain curves of the aerogels were evaluated using the universal tensile machine (Jinjian UTM-1422, China) with a testing rate of 10 mm/min.

Preparation of Short CA Nanofibers
CA nanofibers were prepared by using a TEADFS-100 electrospinning machine (Beijing Technova Technology Co., Ltd., China). First, CA powder (2.8 g) was dissolved in a mixed solution containing 5 mL DMAC and 10 mL acetone (volume ratio of 1:2), and then magnetically stirred at room temperature to obtain a 20 wt% homogeneous CA electrospinning solution. The spinning solution was placed in a 10 mL syringe, the inner diameter of the spinneret was 0.51 mm, a high voltage of 20 kV was applied, the spinning distance was adjusted to 15 cm, the spinning solution advancing speed was 0.015 mL/min, and the temperature was 35 °C to collect nanofibers on aluminum foil. Finally, short CA nanofibers can be obtained by breaking the collected long CA nanofibers.

Preparation of PNFA
The short CA nanofibers were uniformly dispersed in 3.4 wt% APS solution and kept at 5 °C for 2 h. Then, pyrrole monomer was added to the mixture, and cross-linking polymerization was carried out at 5 °C for 6 h. Here, pyrrole not only exists as a crosslinking agent but also acts as a nanofiber surface modifier to reduce and adsorb Cr(VI). Keeping the polymerized homogeneous system in a -20 °C environment for 12 h, and then putting it into the LGJ-12 freeze dryer (Beijing Songyuanhuaxing Technology Co., Ltd., China) for 24 h to obtain PNFA. The adsorption ability of Cr(VI) was adjusted by changing the mass ratio of the pyrrole monomer and CA nanofibers (0.5:1; 1:1; 2:1; 3:1; 5:1) in the whole system. The shape of the aerogel was regulated by dispersing CA nanofibers and polymerizing pyrrole into different shaped molds.
To study adsorption kinetics, PNFA was placed in different concentrations (100 ppm; 150 ppm; 200 ppm) of Cr(VI) solutions at 15 °C under a pH of 2, and samples with the same volume were taken at different times. To determine the adsorption isotherm of Cr(VI), the temperature was adjusted to 25 °C, 35 °C, and 45 °C respectively, and the pH of the system was adjusted to 2 with 0.2 M HCl. Then, PNFA was put into Cr(VI) solution with different concentrations (100 ppm-350 ppm) for obtaining absorption equilibrium.
The amount of adsorption was calculated by the following formula: where q t (mg/g) is the instantaneous adsorption amount of Cr(VI) ions by the adsorbent, C 0 (mg/L) is the initial concentration of Cr(VI) solution, C t (mg/L) is the concentration of Cr(VI) solution at different times, m (g) is the mass of the adsorbent, and V (L) is the volume of the Cr(VI) solution. (1)

Preparation and Digital Photos of PNFA
The fabrication process of the nanofibrous aerogel for efficient Cr(VI) adsorption described in this study is shown in Fig. 1a. Here, the initial CA nanofibers prepared by electrospinning were too long and seriously entangled, which is not conducive to the subsequent uniform dispersion in solution. Therefore, the CA nanofibers were sheared and crushed to form short nanofibers with good dispersity to prepare the following PNFA. As shown in Fig. S1, the diameter of the nanofibers obtained by electrospinning is about 600 nm, and the length of the short CA nanofibers after shearing is about 50-200 μm. The short CA nanofibers were uniformly dispersed in the aqueous solution, and oxidant APS was added to form a uniform oxidation system for the subsequent uniform polymerization of pyrrole. Then the brown-yellow pyrrole monomer was added, the color of the system gradually transited from the initial light yellow to dark green under low-temperature conditions. After the polymerization completed, the system appears a black color. The cause of this phenomenon is due to the black appearance nature of PPy. Specifically, the polymerization of pyrrole was mainly carried out on the surface of nanofiber, so with the gradual polymerization of pyrrole, the color of the white fiber surface gradually deepened, and eventually stabilized to black. Finally, the polymerized mixture was freeze-dried to obtain PNFA. Since PNFA is mainly composed of overlapping nanofibers, it shows remarkably porous and ultra-light properties and can be stably placed on the stamen without obvious shape change of the stamen (Fig. 1b). The aerogels were formed due to the synergistic effect of pyrrole cross-linking and hydrogen bonding on the fiber surface. Besides, the obtained PNFA can quickly recover to the original state after releasing bending pressure, and the shape of PNFA can be easily designed according to actual needs (Fig. 1c, d). Based on the above results and discussions, it can be concluded that PNFA aerogel with good mechanical properties is successfully prepared.

Characterization of PNFA
Figure 2a-c show the SEM images of the as-prepared ultralight and superelastic composite aerogels. The aerogel takes CA nanofibers as the skeleton and is formed by cross-linking and overlapping of PPy. It can be clearly seen that the prepared 3D composite aerogel has an obvious porous structure, and the pore size distribution is relatively uniform, which effectively increases the specific surface area of the material and provides more contact sites for the subsequent efficient adsorption of Cr(VI). The polymerization of pyrrole allows the nanofibers in the aerogel to be cross-linked together, which can endow the aerogel with excellent mechanical properties while maintaining the aerogel morphology. The effect of pyrrole amount on the pore structure of the aerogel and the coating of the fiber surface was studied by fixing The obtained results show that when the mole ratio of pyrrole: APS was 2:1, the pore structure is obvious and evenly distributed, and the fiber surface can be uniformly coated. When the amount of pyrrole was too small or too much, the pore structure is not obvious and the shape is irregular, and the fiber surface also shows incomplete coating or bulk agglomeration. Figure S4 displays the FTIR spectrum of PNFA. For comparison, the FTIR spectra of CA and PPy were also recorded. According to the results, it can be found that the stretching vibration bands at 3450 cm −1 (N-H/O-H This suggests that PNFA is consisted of CA and PPy. In addition, Fig. S4b presents the XRD pattern of PNFA, in which broad diffraction peaks can be clearly observed, suggesting that the degree of structural order in PNFA is low. EDS mapping results of single pyrrole-coated nanofibers in Fig. 2d show that C/N elements from PPy and N/S/O elements from APS are uniformly distributed on the fiber surface, leading to the core-shell structure of nanofiber. The typical core-shell structure was also confirmed by the TEM image (Fig. 2d). Figure 2e-f shows the XPS spectra of PNFA. It can be seen from the XPS spectra that with the increase of pyrrole amount during the preparation process, the N1s peak intensity increased continuously, which was due to the increase of PPy coated on the fiber surface. The N 1s core-level spectrum of PNFA can be fitted into four peak components with binding energies of about 401.98, 400.98, 399.88, and 399.38 eV, respectively. These four peak components can be attributed to the −N + =, −N + −, −N=, and −NH− species, respectively. This is consistent with the APS structure and the structure of polypyrrole prepared at a low APS/pyrrole ratio. The above results powerfully demonstrate the successful coating of pyrrole on the nanofiber's surface.
As shown in Fig. 2g, when the additive amount of pyrrole is different, the obtained PNFA exhibits an obvious difference in density. Within the range of variables set, the bulk density of the prepared PNFA ranges from 20.14 to 21.78 mg/cm 3 . Therefore, the dosage of pyrrole should be reasonably controlled. Fig. S5a shows the Raman spectra of CA nanofibers, PPy, and PNFA in characteristic D and G bands. The D band represents the disordered carbon and defects in the carbonaceous material, the G band comes from the ordered sp 2 -bonded carbon atoms, and the intensity ratio of the D band to the G band can be used to evaluate the disorder degree of the carbonaceous material. The intensities of the D and G bands of PPy are basically equal, indicating that there are a lot of defects in its structure. However, the intensity ratio of the D and G bands of PNFA decreases significantly, suggesting that the structural defects in PNFA were reduced and the ordering degree was greatly increased. The cause of this phenomenon is due to the uniform coating of PPy on the surface of the CA nanofibers. Fig. S5b presents the thermal stability of CA nanofibers and PNFA under a nitrogen atmosphere. It can be observed that the overall thermal stability of the material decreases due to the complexation of pyrrole, which is mainly due to the thermal degradation of polypyrrole under high temperature over 200 ℃. Besides, as shown in Fig. 2h, when the deformation of the PNFA prepared in this study reaches 80%, the compressive strength can reach up to 14.49 kPa, proving that PNFA has good soft elasticity.

Effect of PNFA Preparation Conditions on Cr(VI) Adsorption
In order to accurately measure the Cr(VI) solution concentration before and after adsorption, 10 sets of different Cr(VI) solution concentrations were prepared. Figure 3a, b display the corresponding UV-Vis absorption curves and the resulting standard curve. The center of the UV spectral peak of Cr(VI) in this experiment was at 351.5 nm. As shown in Fig. S6, the linear relationship between concentration and absorbance (Y = 0.01098X − 0.03921) is obtained by processing the UV absorption curve, in which the linear correlation coefficient (R 2 ) can reaches up to 0.999. It shows that the obtained standard curve has high reliability and can be used for the accurate measurement of Cr(VI) solution concentration. Here, 3D aerogel materials have the advantage of a large specific surface area, and polypyrrole possesses reduction-adsorption ability. Therefore, the PNFA prepared by combining the two is expected to show effective adsorption of Cr(VI) in wastewater. The effect of the mole ratio of pyrrole and oxidant on the adsorption performance of Cr(VI) was further studied. As shown in Fig. 3c, for Cr(VI) solution with an initial concentration of 100 mg/L at a temperature of 15 °C without adjusting the pH of the solution, the adsorption capacity of aerogels is about 27 mg/g when the mole ratio of pyrrole and oxidant reaches 1:1. Especially, when the mole ratio of pyrrole: APS is 2:1, the adsorption amount reaches 28.34 mg/g. Therefore, the mole ratio of pyrrole and oxidant in the aerogels is kept at 2:1 in the subsequent adsorption experiments. Figure 3c illustrates the adsorption capacities of pyrrole for Cr(VI) ion under different pH conditions. The results show that the adsorption performance of aerogels for Cr(VI) decreases significantly with the increase in pH. At pH = 2, the aerogel has the high-est adsorption capacity for Cr(VI), reaching 31.39 mg/g. This is because the treatment of Cr(VI) by aerogel mainly contains two processes: (1) Cr(VI) was reduced to Cr(III) and then adsorbed; . When the pH gradually increases, the increasing OH − in the system would compete with CrO 4 2− for the same adsorption site. Therefore, subsequent experiments were carried out under the condition of pH = 2. Besides, as shown in Fig. S7, the color of the solution changes from bright orange to pale yellow after adsorption, further confirming the good adsorption ability of PNFA. Figure 4a shows the effect of contact time and the initial concentration of the solution on the adsorption capacity of Cr(VI). The results show that with the prolongation of adsorption time, the amount of Cr(VI) adsorbed by aerogel gradually increases until it reaches the adsorption equilibrium. The formula for calculating the equilibrium adsorption capacity at adsorption equilibrium is:

Adsorption Kinetics
where q e (mg/g) is the equilibrium adsorption capacity, C e (mg/L) is the concentration of the solution at adsorption equilibrium.
Besides, according to the results in Fig. 4a, it can be found that the larger the initial solution concentration, the larger the adsorption capacity. In this study, the ambient temperature was 15 °C, and the initial pH was 2. When the initial solution concentrations were 100, 150, and 200 mg/L, the equilibrium adsorption capacities could reach up to 31.76 mg/g, 44.46 mg/g, and 48.05 mg/g, respectively. It can be seen that the composite aerogel materials prepared in this study has high adsorption capacity towards Cr(VI) ions.
In order to clarify the relationship between the rate of the whole process and various physical factors (such as adsorption time, initial temperature of the solution, etc.), we carried out a kinetic analysis on the reduction-adsorption behavior of PNFA for Cr(VI) ions. The commonly used models for adsorption kinetic analysis include the pseudo-first-order kinetic model and the pseudo-secondorder kinetic model, whose mathematical expressions are: where K 1 (mg/g) and K 2 (min/g/mg) are the pseudo-firstorder and pseudo-second-order kinetic adsorption rate constants, respectively. The pseudo-first-order kinetic model believes that the adsorption rate of the adsorbent to the adsorbate in solution is determined by the adsorption capacity of the adsorbent. It can be seen from Fig. 4b that the fitting effect is poor. The fitting parameters of the pseudo-first-order kinetics are summarized in Table 1. The calculation results show that not only the correlation coefficient is low, but also the calculated value (q e,exp ) of the equilibrium adsorption capacity is far from the experimental value (q e,cal ). Therefore, the pseudo-first-order kinetic model is not suitable to describe the kinetic process of Cr(VI) removal by PNFA. As can be seen from Fig. 4c, the fitting effect of the pseudo-secondorder kinetic model is good. The fitting parameters of the pseudo-second-order kinetics in Table 1 also show that the correlation coefficient (R 2 ) is maintained above 0.999 stably, and the q e,exp of the equilibrium adsorption capacity is basically the same as the experimental values (q e,cal ). This indicates that the pseudo-second-order kinetic model can effectively describe the reduction-adsorption process of PNFA towards Cr(VI).

Adsorption Isotherm and Thermodynamic Study
Under the condition of fixed adsorption temperature (25 °C, 35 °C, 45 °C), the relationship between the equilibrium concentration (C e ) of Cr(VI) and the equilibrium adsorption capacity (q e ) was studied, as presented in Fig. 5a. On the one hand, with the increase of the initial concentration of the solution, the equilibrium adsorption capacity increases gradually. On the other hand, higher adsorption temperature leads to higher adsorption capacity, and the increased rate of adsorption capacity at a higher temperature is significantly faster than that at a lower temperature. This may be because the higher temperature of the solution provides thermal energy to the adsorbate, resulting in a faster absorption rate and higher adsorption capacity. In order to understand the migration pattern of the adsorbate to the adsorbent, two classical adsorption models were used to fit the adsorption data, namely the Langmuir adsorption isotherm model and the Freundlich adsorption isotherm model. The Langmuir adsorption isotherm model believes that the adsorption of adsorbent to adsorbate is monolayer adsorption, and the adsorption effect of each adsorption site on adsorbate is the same, and there is no mutual interference between adsorbates. The Freundlich adsorption isotherm model holds that the adsorbate is adsorbed by a multi-molecular layer on the adsorbent. According to the different distances, the adsorption effect of the adsorbent on the adsorbate is different, and there is mutual interference between the adsorbates. Its mathematical expressions are: where q m (mg/g) is the saturated adsorption capacity of the adsorbent for Cr(VI), b (L/mg) is the Langmuir adsorption constant, K f (mg/g) and n are the Freundlich adsorption constants.
The fitting results are shown in Fig. 5b, c. It can be seen intuitively that the linear fitting effect of the Langmuir model is better. Table 2 lists the fitting parameters of the two models, and the linear correlation coefficient (R 2 ) was used to indicate the fitting effect. The higher correlation coefficient further proves that the Langmuir model can describe the isotherm adsorption data better than the Freundlich model. This indicates that the adsorption of the adsorbate on the adsorbate is more inclined to the monolayer adsorption. According to the Langmuir model, when the adsorption temperature increases from 25 to 45 °C, the saturated adsorption capacity can increase from 182.82g to 322.58 mg/g. In addition, Table S1 (5) C e q e = 1 q m b + C e q m (6) lnq e = lnK f + lnC e n , According to the equilibrium concentration data of Cr(VI) solution after adsorption at different temperatures, various thermodynamic parameters of the reductionadsorption process can also be determined. The mathematical relationship between the known thermodynamic parameters ΔG 0 , ΔH 0 , and ΔS 0 is: where m (g) is the adsorbent dose, R (J/mol/K) is the gas constant and T (K) is the absolute solution temperature. Figure 5d is obtained by plotting 1/T against lnK d , and the values of ΔH 0 and ΔS 0 can be calculated according to  the slope and intercept ( Table 3). The positive value of ΔH 0 indicates that the reduction-adsorption of Cr(VI) is an endothermic process. This is consistent with the aforementioned results that when the adsorption temperature increases, the adsorption rate and the adsorption capacity increase. A positive value of ΔS 0 indicates that the degree of disorder of the system at the adsorption interface increases. The negative value of ΔG 0 indicates that the reduction-adsorption process is autogenic.

Adsorption Mechanism
The adsorption mechanism of Cr(VI) was further investigated by XPS analysis on the surface of PNFA after adsorption (Fig. 6). The S 2p peak in the XPS spectrum of PNFA before adsorption comes from the oxidant APS, and the weakening of the peak intensity after adsorption may be because the residual APS was dissolved in the adsorption solution and was desorbed from PNFA. Figure 6a shows that the N 1s peak intensity is significantly weakened after adsorption, which is because the -NH + -and -N + = in polypyrrole are consumed for reducing Cr(VI) when the pH is lower than 6. Compared with Fig. 2f, it can be seen from Fig. S8 that the peak intensities of -NH + -and -N + = have a significant decrease, which further confirms that polypyrrole is involved in the adsorption process. Compared with the XPS spectrum of PNFA before adsorption, the spectrum after adsorption exhibits intense two new energy bands centered at 577.58 eV and 587.38 eV, which attribute to the Cr 2p3/2 and Cr 2p1/2 respectively, demonstrating the existence of both Cr(III) and Cr(VI) on the surface of PNFA. The existence of Cr(III) proves that the adsorption process involved the reduction of Cr(VI) to Cr(III). Considering that -NH + -and -N + = from pyrrole possess reducing properties when the pH is less than 6, it is believed that Cr(VI) is reduced to Cr(III) by the action of pyrrole, which can also explain by the obvious intensity decrease of the N peak in the XPS spectrum. The reduction of Cr(VI) can be expressed by the following equation: Through the previous analysis, we believed that the adsorption of Cr(VI) by PNFA mainly contains two parts: (1) Cr(VI) is reduced to Cr(III) by the -NH + -and -N + = units from PPy; (2) Cr(VI) and Cr(III) are adsorbed by PNFA.

Conclusion
In summary, we successfully prepared nanofibrous composite aerogels (PNFA) with high porosity and large specific surface area by in situ polymerization of pyrrole on the three-dimensional CA nanofibrous framework. The obtained composite aerogel has high adsorption efficiency for Cr(VI) due to more adsorption sites. When pH was 2, the maximum adsorption capacity can reach up to 322.58 mg/g with an initial solution concentration of 200 ppm. The adsorption process of Cr(VI) by PNFA follows the pseudo-second-order  Fig. 6 a XPS spectra of PNFA before and after Cr(VI) adsorption, and b Cr2p spectra of PNFA after Cr(VI) adsorption 1 3 kinetic model and the Langmuir isotherm adsorption model. The prepared PNFA shows good mechanical property with high compressive strength of 14.49 kPa. Moreover, the PNFA has a large space for shape design and is easy to handle after adsorption, which exhibits broad development prospects in the field of assembled adsorption devices.