Startseite Electrospun nanofibers membranes of La(OH)3/PAN as a versatile adsorbent for fluoride remediation: Performance and mechanisms
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Electrospun nanofibers membranes of La(OH)3/PAN as a versatile adsorbent for fluoride remediation: Performance and mechanisms

  • Shaoju Jian , Jinlong Wu , Li Ran , Weisen Yang , Gaigai Duan , Haoqi Yang EMAIL logo , Fengshuo Shi , Yuhuang Chen , Jiapeng Hu EMAIL logo und Shaohua Jiang EMAIL logo
Veröffentlicht/Copyright: 10. November 2024
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e-Polymers
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Abstract

Excessive existence of fluoride in water resources can lead to harmful impacts on ecosystems and organisms. Electrospun polyacrylonitrile (PAN) nanofiber membranes loaded with La(OH)3 nanorods composites (La(OH)3/PAN electrospun nanofiber membranes [ENFMs]) are fabricated and used as an efficient fluoride scavenger. Adsorbent fabricate protocols, pH, initial F− concentration, adsorbent dosage, and adsorption time, in addition to coexisting anions, were systematically evaluated. The investigation unveils that a pH of 3.0 is optimal for F− remediation. The adsorption kinetics and isotherm of La(OH)3/PAN ENFMs are well described by the pseudo-second-order model (R 2 > 0.997) with characteristics of chemisorption and Langmuir isotherm (R 2 > 0.999) with the feature of single-layer coverage. The existence of Cl−, SO4 2−, NO3 −, and CO3 2− does not significantly hinder fluoride removal by La(OH)3/PAN ENFMs with the exception of PO4 3−. Calculations of ΔH, ΔG, and ΔS reveal that the nature of F− adsorption onto La(OH)3/PAN ENFMs is endothermic and favorable at a higher temperature.

Keywords: electrospun nanofiber membrane; La(OH)3 ; defluoridation; kinetics; water treatment

1 Introduction

Wastewater is a global issue and has attracted much attention from all over the world including heavy metal ions, dyes, fluoride, phosphate, and so on (1,2,3,4,5,6). With the increase in agricultural activities as well as industrial production like electroplating, semiconductor manufacturing, fertilizer production, and aluminum production facilities, the emission of fluoride into water sources continues to increase (7). Massive disposal of fluoride seriously threatens the ecosystem of aquatic life and public health. Excess intake of fluoride can pose severe illness including the fluorosis of dental and skeletal, disruption of endocrine glands, and Alzheimer’s disease in humans (8). Hitherto, considering it a global crisis, regulating the fluoride concentration of drinking water is essential to prevent these health issues. Fluoride-contaminated water resources can be treated by diverse water management methods including coagulation, ion exchange, membrane technology, electrodialysis, chemical precipitation, and adsorption. The foremost bottleneck of the above strategies is that the ability of water defluoridation is unsatisfactory and cannot meet the acceptable level.

Adsorption has emerged as a mainstream and advanced strategy for fluoride removal. Indeed, the key to water defluoridation via adsorption is designing efficient functional adsorbents. The ideal defluoridation materials have the characteristics of operational flexibility, low economic costs, high removal efficiency, excellent selectivity and reusable performance, broad applicability, etc. Currently, numerous materials involving metal oxides/hydroxides (9), activated carbon (10,11,12), metal-organic frameworks (MOFs) (13), and biopolymer adsorbents (14) have been designed for fluoride abatement. Adsorption adsorbents can capture fluoride via hydrogen bonding, electrostatic attraction, as well as ligand exchange according to the adsorption mechanisms (15). In fact, defluoridation adsorbents that act mainly through ligand exchange are preferred materials because they exhibit excellent selectivity towards fluoride when other anions coexist. Hence, metal oxides/hydroxides with various −OH groups on the surface which can be replaced by F− via ligand exchange are ideal defluoridation materials. Up to date, ZrO2 (16), La-Mn bimetal oxides (9), Ce-AlOOH, etc. (17), have been explored for the remediation of fluoride from wastewater.

Lanthanum (La), as an eco-friendly and highly electropositive rare earth metal element with abundant sources, exists in Earth’s crust and owns good fluoride capture performance owing to its Lewis acid properties. In particular, nano-La-based oxides or hydroxides show superior affinity and selectivity toward fluoride, since a covalent chemical bond can be generated between the La3+ and F−. For instance, Na and Park reported that the maximum fluoride uptake of La(OH)3 powders was up to 242.2 mg·g−1 (18). However, nano-structured La(OH)3 materials usually encounter nanoparticle agglomeration during the adsorption process. Besides, nanoparticles are difficult to effectively control in the application process, which may generate the ecological risk of secondary pollution. Moreover, another obstacle of nanoparticles is the difficulty of solid–liquid separation. In light of this, incorporating La(OH)3 into host materials such as polymer membrane (19,20), biomass (21), hydrogel (22), and MOFs (23) can evaluate the adsorption and solid-liquid separation properties. Vences-Alvarez et al. (24) proposed carbon fiber as the matrix for immobilization of conventional La(OH)3 and found that the maximum uptake of fluoride was only 9.98 mg·g−1. Obviously, the fluoride removal performance was insufficient. This is mainly due to the difficulty in forming small-size nanoparticles inside the inert supporting material. In addition, the pore channels of supporting material further hinder the accessibility and reactivity of fluoride ions, so the properties of the matrix play a crucial role in the design of efficient defluoridation nanocomposites. Continuous electrospun nanofiber membranes (ENFMs) are prospective host membranes for fabricating composites considering their high porosity, good mechanical properties, simple preparation, and easy recovery (25). Therefore, nano-La-based oxides or hydroxides nanoparticles impregnated into ENFMs via an in situ precipitation process can be utilized directly as well as simply separated from a solid–liquid system after adsorption. Currently, some studies have developed some La-based oxides/hydroxides immobilized in ENFMs and utilized them to remove arsenate, phosphate, and humic acid (26). For example, Li et al. (27) loaded La(OH)3 nanorods on PVA/PEI crosslinked nanofiber membrane for phosphate recovery with a maximum uptake of 165.9 mg P·g−1 La. In addition, the ENFM composites had superior separability as well as reusability. Nevertheless, few researchers were performed to scavenge fluoride.

Herein, La(OH)3 nanorods are embedded in electrospun polyacrylonitrile (PAN) nanofiber membranes via an in situ method to form La(OH)3/PAN ENFMs for recovering fluoride. The well-dispersed La(OH)3 nanorods fixed within the 1D PAN nanofiber matrix efficiently prevent the aggregation of binding sites. Simultaneously, the systematic adsorbent performance, such as the impact of pH, selectivity, adsorption isotherms, and kinetics are explored to quantify the applicability of the La(OH)3/PAN ENFMs in real wastewater treatment applications.

2 Experiment

2.1 Materials

PAN (M n = 150,000), N,N-dimethylformamide (DMF), La(NO3)3·6H2O, NaOH, HCl, Na3PO4, NaF, NaNO3, NaCl, Na2SO4, and Na2CO3 are purchased from Sinopharm chemical reagent Co., Ltd. (Shanghai, China). All solvents and reagents are utilized without further treatment in this study.

2.2 Preparation of La(OH)3/PAN ENFMs

In this work, La(OH)3/PAN ENFMs are prepared by electrospinning technology (28) and in situ precipitation method. First, 15 wt% PAN solution is obtained by dissolution of 1.5 g PAN powder in 8.5 g DMF solvent. Then, the approximate amount of La(NO3)3·6H2O powder is added to the viscous 15 wt% PAN solution and further stirred at room temperature for another 2 h. The mass ratios of La(NO3)3·6H2O to PAN in different samples are set to be 2:1, 1.5:1, 1:1, 1:2, and 1:3, respectively. The La(NO3)3/PAN/DMF spinning solution is acquired and transferred into an electrospinning machine. High voltage electrospinning (15–20 kV; fade rate: 1.0 mL·h−1; receiving distance: 20 cm), followed by in situ precipitation with NaOH solution for 12 h, rinsed with deionized water many times until the pH becomes neutral and drying nanofibers (under a vacuum oven for 12 h at 45°C) are performed to prepare the final La(OH)3/PAN ENFMs for fluoride abatement. In the meantime, neat PAN ENFMs are fabricated through the same method as a control.

2.3 Fluoride adsorption procedure on La(OH)3/PAN ENFMs

Bath defluoridation investigations are done to quantify the fluoride adsorption capacity of La(OH)3/PAN ENFMs using 50 mL NaF solution with an adsorbent dosage of 0.3 g·L−1 at 25°C. The influences of various parameters viz., pH (2–9), adsorbent dosage (0.2–0.6 g·L−1), initial fluoride concentration (C 0: 10–40 mg·L−1), shaking time (0–480 min), and co-existing anions (PO4 3−, Cl−, SO4 2−, CO3 2−, and NO3 −), are systematically scrutinized for optimization of defluoridation conditions. The adsorption isotherm trials are carried out with 15 mg La(OH)3/PAN ENFMs, and 50 mL of F− solution with varying C 0 (10–40 mg·L−1) at 25°C, 35°C, and 45°C. The kinetic study is researched at pH = 3 and 25°C by stirring 150 mg La(OH)3/PAN ENFMs in 500 mL of 10 and 20 mg·L−1 F− solution. The required supernatant is picked up at the desired point to dilute and monitor the residual fluoride concentration using the F−-equipped ion meter.

3 Results and discussion

3.1 Adsorbent characterization

The morphology of La(NO3)3/PAN ENFMs with La(NO3)3·6H2O to PAN in a 1:1 ratio by mass and the corresponding La(OH)3/PAN ENFMs is analyzed by SEM, which is an effective technology to investigate the surfaces of materials while its element mapping can be used to analyze the element composition of materials (29,30,31). As shown in Figure 1, the nanofibers with regular cylinder structures are continuously and randomly arranged. The average diameters of La(NO3)3/PAN and La(OH)3/PAN ENFMs are approximately 388 ± 77 nm (Figure 1a) and 358 ± 48 nm (Figure 1b), respectively. What is more, a large number of nanoparticles are uniformly distributed on La(OH)3/PAN ENFMs surface, indicating the successful formation of La(OH)3 after alkali treatment. The chemical constituents of La(OH)3/PAN ENFMs are ascertained using SEM elemental mappings, as depicted in Figure 1c–g. Clearly, the main constituent elements of La(OH)3/PAN ENFMs are La, O, C, and N, and all the elements are well deposited on the PAN nanofibers. Moreover, the observation is validated by the result of TEM, which can be applied to characterize the porous structure of materials (32,33,34). Apparently, La(OH)3 particles are evenly embedded in the nanofibers as well as loaded around the PAN nanofibers, which illustrates that this approach can effectively decrease the agglomeration of La(OH)3 particles (Figure 1h).

Figure 1 SEM images of (a) La(NO3)3/PAN ENFMs, (b) La(OH)3/PAN ENFMs, (c–g) mapping spectra, and (h) TEM image of La(OH)3/PAN ENFMs. (i) XRD, (j) N2 adsorption–desorption curves, and (k) corresponding aperture distribution curve of La(OH)3/PAN ENFMs.
Figure 1

SEM images of (a) La(NO3)3/PAN ENFMs, (b) La(OH)3/PAN ENFMs, (c–g) mapping spectra, and (h) TEM image of La(OH)3/PAN ENFMs. (i) XRD, (j) N2 adsorption–desorption curves, and (k) corresponding aperture distribution curve of La(OH)3/PAN ENFMs.

X-ray diffraction (XRD) analysis as an efficient technique can be applied to analyze the phase composition, crystal structure, and orientation of materials (35,36,37). Figure 1i depicts the XRD patterns of La(OH)3/PAN ENFMs with mass ratios of PAN/La(NO3)3·6H2O being 3/1 and 1/1. Diffraction peaks located at 2θ = 17.3° (100), 28.3° (101), 31.1° (200), 39.8° (201), 48.9° (211), and 55.8° (220) are well indexed to the lattice planes of La(OH)3 (JCPDS No. 83–2034) (38), and no more peaks in crystalline form were detected, suggesting that La(NO3)3 is successfully converted to well-crystalline La(OH)3 after alkali treatment. In the meantime, the diffraction peak at 2θ = 16.8° (100) belonging to amorphous PAN (39) is overlapped. The results unveil the successful fabrication of La(OH)3/PAN ENFMs.

Specific surface area and pore size are important for the analysis of the effects of pore structure on the performance of materials (40,41,42). As represented in Figure 1j, the N2 adsorption–desorption isotherm of La(OH)3/PAN ENFMs corresponds to a type H3 isotherm. BET analysis denotes that the specific surface area La(OH)3/PAN ENFMs determined through the BET method is 18.666 m2·g−1, and the total pore volume is 0.115 cm3·g−1. It can be observed from the aperture distribution curve (Figure 1k) that La(OH)3/PAN ENFMs own abundant mesopores in the region of 10–50 nm, displaying a mesopore structure. The contact surface between La(OH)3/PAN ENFMs and adsorbate (F−) can enhance the adequate internal mesoporous structure, leading to sufficient available binding sites for capturing fluoride.

3.2 Impact of adsorbent fabricate protocols on defluoridation performance

The effect of the ratio of La(NO3)3·6H2O/PAN by mass in the spinning solution on the defluoridation performance by La(OH)3/PAN ENFMs is presented in Figure 2a. As is evident, the defluoridation property of neat PAN ENFMs shows a negligible fluoride uptake of only 0.5 mg·g−1. Meanwhile, the fluoride uptake of La(OH)3/PAN ENFMs ascends rapidly as the increasing of mass ratio of La(NO3)3·6H2O/PAN from 1/3 to 1/1. It is probably ascribed to the higher lanthanum content and well-dispersed La(OH)3 nanoparticles in the composite (20). Additionally, the defluoridation property of La(OH)3/PAN ENFMs is not significantly improved by further increasing the mass ratio of La(NO3)3·6H2O/PAN from 1.5/1 to 2/1. Since more La(OH)3 nanoparticles generate and tend to agglomerate along the one-dimensional direction (Figure S1), this may reduce available binding sites for fluoride, thereby affecting fluoride removal efficiency. Thus, the ratio of La(NO3)3·6H2O/PAN by mass being 1:1 is selected to fabricate the subsequent La(OH)3/PAN ENFMs for scavenging F−.

Figure 2 The impact of (a) the mass ratio of La(NO3)3·6H2O/PAN and (b) the concentration of NaOH used in the adsorbent preparation process on the defluoridation performance by La(OH)3/PAN ENFMs.
Figure 2

The impact of (a) the mass ratio of La(NO3)3·6H2O/PAN and (b) the concentration of NaOH used in the adsorbent preparation process on the defluoridation performance by La(OH)3/PAN ENFMs.

The fluoride adsorption amount of adsorbent can be affected by alkali treatment through the conversion of La(NO3)3 to La(OH)3. The defluoridation of La(OH)3/PAN ENFMs obtained under various NaOH concentrations is assessed at a pH of 3, a dosage of 0.3 g·L−1, a C 0 of 20 mg·L−1, and a contact time of 480 min. The plot of fluoride uptake versus NaOH concentration (C NaOH) is portrayed in Figure 2b. Evidently, the fluoride uptake first ascends and then goes down slightly, reaching a maximum fluoride uptake of 59.98 mg·g−1 at C NaOH of 1.0 mol·L−1. The optimal defluoridation property proves that C NaOH of 1.0 mol·L−1 is suitable for the transformation of La(NO3)3 to La(OH)3. Hence, C NaOH in the subsequent preparation of the adsorbent is chosen as 1.0 mol·L−1.

3.3 Effect of operational factors on defluoridation performance

Figure 3a demonstrates the variation in fluoride uptake by La(OH)3/PAN ENFMs with pH value. Clearly, La(OH)3/PAN ENFMs exhibit the highest defluoridation property at pH = 3, with an adsorption amount of 30.5 mg·g−1. While pH drops below 3, the uptake of La(OH)3/PAN ENFMs for fluoride is only 11.09 mg·g−1. It may be ascribed to the appearance of HF under a strongly acidic environment (43), which hinders the adsorption of F− onto the active sites. At higher pH values, the dramatic reduction in fluoride uptake occurs owing to the enhanced electrostatic repulsion as well as a decrease in the exchange probability of OH− on La(OH)3 with negatively charged ions of F− which alleviates the F− removal. Furthermore, the isoelectric point (pHpzc) of La(OH)3/PAN ENFMs assessed via a pH drift method is 6.06 (Figure 3b). At pH values below 6.06, the surface of La(OH)3/PAN ENFMs acquires a positive charge. As a result, more H+ is involved, thereby adsorption through electrostatic attraction between electronegative F− and La(OH)3/PAN ENFMs facilitates the fluoride adsorption process. Conversely, in the scenario of pH > 6.06, the electrostatic repulsion is unfavorable for the defluoridation process. Therefore, a pH of 3 is chosen for subsequent defluoridation research.

Figure 3 (a) Impact of pH on defluoridation performance by La(OH)3/PAN ENFMs (C 0 = 10 mg·L−1, T = 25°C, dosage = 0.3 g·L−1, t = 480 min), (b) pHpzc obtained by pH drift method, influence of (c) dosage and (d) coexisting anions on defluoridation performance by La(OH)3/PAN ENFMs.
Figure 3

(a) Impact of pH on defluoridation performance by La(OH)3/PAN ENFMs (C 0 = 10 mg·L−1, T = 25°C, dosage = 0.3 g·L−1, t = 480 min), (b) pHpzc obtained by pH drift method, influence of (c) dosage and (d) coexisting anions on defluoridation performance by La(OH)3/PAN ENFMs.

Several studies were done using various dosages (0.2–0.6 g·L−1) of La(OH)3/PAN ENFMs at a C 0 of 10 mg·L−1, a pH of 3, and a duration of 480 min. As displayed in Figure 3c, the fluoride uptake upsurges quickly when the dosage of La(OH)3/PAN ENFMs is raised from 0.2 to 0.3 g·L−1 on account of the greater accessible binding sites (44). The optimal defluoridation performance is attained at a La(OH)3/PAN ENFMs dosage of 0.3 g·L−1, showing a maximum fluoride uptake of 30.95 mg·g−1, whereas further ascending the dosage of La(OH)3/PAN ENFMs leads to an excess of adsorption sites at a fixed initial F− concentration, thereby reducing the fluoride capture capacity of La(OH)3/PAN ENFMs.

In general, F− coexists with multiple competing anions with different valence states including PO4 3−, CO3 2−, SO4 2−, NO3 −, and Cl− in both actual industrial wastewaters and natural water. The water defluoridation performance of adsorbents may be hindered by these anions. Hence, the defluoridation efficiency of La(OH)3/PAN ENFMs in the existence of these anions at five concentration gradients (10–50 mg·L−1) is assessed (Figure 3d). It can be observed from Figure 3d that representative Cl−, SO4 2−, NO3 −, and CO3 2− have negligible influence on fluoride capture regardless of them are at low or high concentration except of PO4 3−, substantiating the excellent selectivity. The fluoride density dramatically decreases by 37.38% and 70.53% in the PO4 3− concentration of 10 and 50 mg·L−1, respectively. Clearly, PO4 3− has a noticeable negative impact on fluoride removal by La(OH)3/PAN ENFMs. It is probably ascribed to the higher charge/size ratio (45) as well as the solubility of LaPO4 (pK sp = 33.4) (15).

3.4 Impact of adsorption time and kinetics of adsorption

The equilibrium time for fluoride adsorption onto La(OH)3/PAN ENFMs is observed from the plots of time-series concentration, as exhibited in Figure 4a. Evidently, the fluoride uptake increases substantially from 0 to 40 min for all concentrations due to the ample binding sites and adsorbate. Exceeding 80% of fluoride is removed in the initial 60 min and does not proceed further after 150 min regardless of C 0 on account of the saturation of binding sites on La(OH)3/PAN ENFMs. Moreover, the defluoridation rate in this work is faster than previously reported La-based adsorbents like Zr-La/PP (500 min) (46), La-ZIF-8 (270 min) (15), La-doped Mg/Al LDHs (1,440 min) (47), La-Al-HP (540 min) (48), and PC@La composites (600 min) (49).

Figure 4 (a) The curves of the amount of F− adsorbed on La(OH)3/PAN ENFMs against time, (b) PFO model, (c) PSO model, and (d) IPD model (C 0 = 10 and 20 mg·L−1).
Figure 4

(a) The curves of the amount of F− adsorbed on La(OH)3/PAN ENFMs against time, (b) PFO model, (c) PSO model, and (d) IPD model (C 0 = 10 and 20 mg·L−1).

To better insight into the defluoridation behavior of La(OH)3/PAN ENFMs, three typical adsorption kinetics including pseudo-first-order (PFO), pseudo-second-order (PSO), and intraparticle diffusion (IPD) models are used to describe the defluoridation rate and mechanism. As portrayed in Figure 4b and c and Table 1, the linear correlation coefficients (R 2) of PFO are 0.5773 and 0.8532, respectively, which are lower than those of PSO (0.9999 and 0.9972), respectively. The observation implies that the defluoridation behavior by La(OH)3/PAN ENFMs is more consistent with the PSO. What is more, the values of Q e,cal for the PSO model exhibit quite agreement with those of Q e,exp, confirming that the defluoridation mechanism by La(OH)3/PAN ENFMs is primarily related to chemisorption encompassing ligand exchange and ionic interaction.

Table 1

Kinetic constants of La(OH)3/PAN ENFMs on the removal of F−

Models Parameters C 0 (mg·L−1)
10 20
PFO ln(Q e – Q t) = lnQ e – k 1 t k 1 × 103 (min−1) 4.20 11.62
Q e (mg·g−1) 3.85 23.64
R 2 0.5773 0.8532
PSO t Q t = 1 k 2 Q e 2 + t Q e k 2 × 102 (g·mg−1·min−1) 1.0581 0.7391
Q e,exp (mg·g−1) 31.96 64.66
Q e,cal (mg·g−1) 32.24 68.59
R 2 0.9999 0.9972
IPD Q t = k P t 0.5 + C k p1 (mg·g−1·min−0.5) 2.5580 10.4425
C 15.13 −17.37
R 2 0.9190 0.9895
k p2 (mg·g−1·min−0.5) 0.1690 2.5055
C 29.70 35.93
R 2 0.9425 0.9214
k p3 (mg·g−1·min−0.5) 0.0204 0.1527
C 31.56 61.58
R 2 0.6358 0.8698

The diffusion behavior of fluoride onto La(OH)3/PAN ENFMs is evaluated by the IPD model. As portrayed in Figure 4d, multi-linearity variations in defluoridation capacity with t 0.5 in the IPD model are found, unveiling the operational three steps in the defluoridation process. During the first step, an instant adsorption takes place within an initial 40 min assigned to the fast external mass transfer of fluoride onto La(OH)3/PAN ENFMs. The defluoridation rate (k p1) in the first step ascends from 2.5580 to 10.4425 mg·g−1·min−0.5 as the C 0 rises from 10 to 20 mg·L−1 (Table 1). This is reasonable that F− diffuses faster as the initial concentration ascends. The second step is between 50 and 120 min. The k p2 decrease to 0.1690 and 2.5055 mg·g−1·min−0.5 for initial 10 and 20 mg·L−1 concentration tests, respectively, indicating a relatively slow adsorption stage, and the diffusion happens in the interior of La(OH)3/PAN ENFMs. Eventually, the stable third step illustrates the final saturation status. None of the intercepts (C) shown in Table 1 for all linear segments are close to zero. The results indicate that the defluoridation by La(OH)3/PAN ENFMs is dominated by IPD and liquid film diffusion (50).

3.5 Effect of C 0 and isotherm investigations

The equilibrium fluoride uptakes of F− on the La(OH)3/PAN ENFMs with respect to C 0 are represented in Figure 5a. Obviously, the fluoride densities consecutively rise as C 0 rises from 10 to 25 mg·L−1 at all given experimental temperatures and a pH of 3. The reason is that more driving force provided by high concentration is beneficial to conquer the resistance of mass transfer between solids and aqueous phases as well as sufficient binding sites towards fluoride. When C 0 is above 25 mg·L−1, most of the binding sites on La(OH)3/PAN ENFMs are occupied and adsorption equilibrium is reached. The fluoride adsorption capacity of La(OH)3/PAN ENFMs slightly rises as the reaction temperature evaluates from 25°C to 45°C. The diffusion rate of F− in solution can be accelerated with the rising of temperature; thus, F− migrates more rapidly to the surface and pore of the La(OH)3/PAN ENFMs (14), thereby elevating adsorption capacity. It is speculated that the defluoridation process of La(OH)3/PAN ENFMs occurs endothermically. Besides, the retaining fluoride concentration after adsorption can be declined to less than 1.0 mg·L−1 when C 0 is below 15 mg·L−1, which is lower than the WHO’s issued fluoride limit of 1.5 mg·L−1.

Figure 5 (a) Adsorption isotherms of F− on La(OH)3/PAN ENFMs, (b) Langmuir model, (c) Freundlich model, and (d) relationship between lnK D and 103/T.
Figure 5

(a) Adsorption isotherms of F− on La(OH)3/PAN ENFMs, (b) Langmuir model, (c) Freundlich model, and (d) relationship between lnK D and 103/T.

Two typical isotherm models viz., Langmuir and Freundlich (51), are employed to fit the equilibrium data of La(OH)3/PAN ENFMs, and the corresponding parameters are tabulated in Table 2. Apparently, the linear correlation coefficients (R 2) derived from Freundlich models (0.7599–0.8158) are lower than those of Langmuir models (0.9994–0.9996) at 25–45°C (Figure 5b and c). This implies that the latter is superior for delineating the defluoridation process by La(OH)3/PAN ENFMs, reflecting monolayer adsorption (13). Besides, the maximum fluoride uptakes of La(OH)3/PAN ENFMs are found to be 71.48, 71.62, and 73.05 mg·g−1 at 25–45°C according to Langmuir isotherm, respectively, which are noticeably larger than most similar adsorbents (Table 3). Moreover, the mass ratio of La(NO3)3·6H2O/PAN is 1/1 in the spinning solution; thus, the theoretical loading amount percentage of La(OH)3 in the adsorbent is only approximately 44%, which is less than half of the total dosage of La(OH)3/PAN ENFMs used in the defluoridation investigations, the fluoride uptake could potentially be double if calculated based solely on the amount of La(OH)3 in the composite.

Table 2

Isotherm constants of La(OH)3/PAN ENFMs on the removal of F−

T (°C) Langmuir Freundlich
Q e (mg·g−1) Q m (mg·g−1) b R 2 K F 1/n R 2
25 69.26 71.48 1.7867 0.9995 43.05 0.1946 0.7599
35 70.98 71.63 2.9829 0.9997 42.28 0.1667 0.8158
45 71.49 73.05 2.7217 0.9997 47.45 0.1731 0.7900
Table 3

Defluoridation performance comparison against other similar adsorbents

Adsorbents Conditions (F− contents; pH; dosage; t) Q m (mg·g−1) Ref.
La3+-impregnated cross-linked gelatin −; 5–7; 0.4 g·L−1; 40 min 21.28 (52)
Zirconium-lanthanum complexed polyvinyl alcohol films 2–10 mg·L−1; 7; −; 360 min 8.35 (53)
La(iii)- and Y(iii)-impregnated alumina 2 mmol·L−1; 6–8; 2.5 g·L−1; − 6.36 (54)
Zr-impregnated cellulose biopolymer adsorbent −; 5; 10 g·L−1; 60 min 4.95 (55)
Electrospun alumina nanofibers 10–100 mg·L−1; 7; 5 g·L−1; 180 min 1.2 (56)
UiO-66-NH2 composite nanofiber membranes 20 mg·L−1; 4–10; 0.2 g·L−1; 20 min 95 (57)
GAC–La 1–80 mg·L−1; 7; 3.33 g·L−1; – 9.98 (24)
biomass cellulose−CeO2 nanocomposite membrane 20–100 mg·L−1; 3; –; – 48.0 (44)
CS/PVA-La 5–200 mg·L−1; 7; 1.2 g·L−1; 15 min 28.72 (58)
La-Al-Scoria adsorbent 0.21–20.82 mg·L−1; 7.2; 20 g·L−1; 300 min 23.91 (59)
La(OH)3/PAN ENFMs 10–40 mg·L −1 ; 3; 0.3 g·L −1 ; 150 min 73.05 This work

The bold values is represented the result obtianed in this work.

3.6 Adsorption thermodynamics

The spontaneity and the energy changes of the defluoridation process by La(OH)3/PAN ENFMs can be confirmed by adsorption thermodynamics. The obtained thermodynamic factors evaluated from Figure 5d are tabulated in Table 4. ΔG declines from −7.33 to −9.02 kJ·mol−1 as the temperature rises from 25 to 45°C. The negative ΔG values illustrate a spontaneous defluoridation behavior. Increasing the temperature is presumably conducive to the adsorption reaction. Meanwhile, ΔH (8.84 kJ·mol−1) as well as ΔS (56.34 J·mol−1·K−1) are more than zero, respectively, meaning an endothermic as well as increased randomness at the La(OH)3/PAN ENFMs adsorbent–solution interface.

Table 4

Thermodynamic factors of PAN/La(OH)3 ENFMs on the removal of F−

T (°C) ΔG 0 (kJ·mol−1) lnK D ΔS 0 (J·mol−1·K−1) ΔH 0 (kJ·mol−1)
15 −7.33 3.05 56.34 8.84
25 −8.03 3.24
35 −8.56 3.34
45 −9.02 3.41

3.7 Mechanism of fluoride adsorption on La(OH)3/PAN ENFMs

Multiple technologies involving FTIR, XRD, SEM-EDS, and TEM are adopted to reveal the mechanism of defluoridation on La(OH)3/PAN ENFMs. As portrayed in Figure 6a, the distinct bands appearing at 2,923 and 2,243 cm−1 are corroborated −CH2 and C≡N stretching frequencies of PAN respectively (60). The intensities of the two characteristic bands at approximately 673 and 540 cm−1 represent the vibrations of the La–OH bond (61) in La(OH)3. This indicates the successful conversion of La(NO3)3/PAN to La(OH)3/PAN. After adsorption, the intensities of bands belonging to La–OH bond dramatically reduce, indicating that a ligand exchange reaction takes place between −OH and F−, probably yielding a complex. The XRD patterns of bare and used La(OH)3/PAN ENFMs are illustrated in Figure 6b. It is clear that the intensities of bands indexing to the hexagonal phase of La(OH)3 (JCPDS No. 83-2034) (38) significantly declined or vanished, and nine diffraction peaks representing LaF3 crystals are observed after fluoride adsorption. Therefore, it can be inferred that F− and La(OH)3 in the adsorbent undergo ligand exchange to generate a complex namely LaF3, which contributes to the fluoride removal from solutions. The SEM image of the F−-loaded adsorbent is displayed in Figure 6c. Obviously, the adsorbent maintains a good fibrous morphology, meanwhile the granular-shaped La(OH)3 nanoparticles on the surface of the adsorbent have been transformed into sharped edged LaF3 crystals with flat-nanostructures after fluoride adsorption. EDS mapping of the used La(OH)3/PAN ENFMs (Figure 6d) indicates a uniform distribution of F, La, C, O, and N elements on the surface, illustrating the successful immobilization of fluoride on the adsorbent. More evidence for the crystals in the adsorbent after defluoridation is further confirmed by TEM (Figure 6e). Meanwhile, based on the result of pH (Figure 3a) and pHpzc (Figure 3b), the possible defluoridation of La(OH)3/PAN ENFMs includes electrostatic attraction and ligand exchange.

Figure 6 (a) FTIR spectra and (b) XRD patterns of neat and used La(OH)3/PAN ENFMs, (c) SEM image, (d) EDS mapping, and (e) TEM image of used La(OH)3/PAN ENFMs.
Figure 6

(a) FTIR spectra and (b) XRD patterns of neat and used La(OH)3/PAN ENFMs, (c) SEM image, (d) EDS mapping, and (e) TEM image of used La(OH)3/PAN ENFMs.

4 Conclusions

La(OH)3/PAN ENFMs are synthesized by direct electrospinning technology and an in situ method and then utilized to eliminate F− from the contaminated water. The defluoridation performance of the La(OH)3/PAN ENFMs is dramatically enhanced by incorporating La(OH)3. The optimal defluoridation condition is a dosage of 0.3 g·L−1 and pH = 3. The Langmuir isotherm described the adsorption process of F− by La(OH)3/PAN ENFMs, corresponding to maximum fluoride uptakes of 71.48–73.05 mg·g−1 at 25–45°C. Adsorption of F− onto La(OH)3/PAN ENFMs is best explained using the PSO model. The defluoridation mechanism of La(OH)3/PAN ENFMs is controlled by intra-particle diffusion as well as liquid film diffusion. The defluoridation process on La(OH)3/PAN ENFMs is spontaneous as well as endothermic. The observation enables us to confirm the light prospect for La(OH)3/PAN ENFMs to treat fluoride wastewater.

  1. Funding information: This work is financially supported by Nanping City resource chemical industry science and technology innovation joint funding project (N2023Z009); Fujian Province College Natural Science Foundation of Fujian Province (2024J01921, 2024J01923); The Open Fund of Fujian Provincial Key Laboratory of Eco-Industrial Green Technology (WYKF-EIGT2022-1); Fujian Province College Students’ Innovation and entrepreneurship training program (S202410397038); National College Students’ Innovation and entrepreneurship training program (202310397014).

  2. Author contributions: Shaoju Jian: investigation, conceptualization, writing – original draft. Jinlong Wu: investigation, conceptualization. Li Ran: investigation, data curation. Weisen Yang: investigation, data curation. Gaigai Duan: conceptualization. Haoqi Yang: writing – review and editing. Fengshuo Shi: data curation. Yuhuang Chen: methodology, data curation. Jiapeng Hu: writing – review and editing. Shaohua Jiang: writing – review and editing.

  3. Conflict of interest: The authors declare that they have no known personal relationships or competing financial interests that could have appeared to influence the work reported in this paper.

  4. Data availability statement: Data are contained within the article.

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Received: 2024-08-18
Revised: 2024-09-15
Accepted: 2024-09-19
Published Online: 2024-11-10

© 2024 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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  62. Large-scale production of highly responsive, stretchable, and conductive wrapped yarns for wearable strain sensors
  63. Preparation of natural raw rubber and silica/NR composites with low generation heat through aqueous silane flocculation
  64. Molecular dynamics simulation of the interaction between polybutylene terephthalate and A3 during thermal-oxidative aging
  65. Crashworthiness of GFRP/aluminum hybrid square tubes under quasi-static compression and single/repeated impact
  66. Review Articles
  67. Recent advancements in multinuclear early transition metal catalysts for olefin polymerization through cooperative effects
  68. Impact of ionic liquids on the thermal properties of polymer composites
  69. Recent progress in properties and application of antibacterial food packaging materials based on polyvinyl alcohol
  70. Additive manufacturing (3D printing) technologies for fiber-reinforced polymer composite materials: A review on fabrication methods and process parameters
  71. Rapid Communication
  72. Design, synthesis, characterization, and adsorption capacities of novel superabsorbent polymers derived from poly (potato starch xanthate-graft-acrylamide)
  73. Special Issue: Biodegradable and bio-based polymers: Green approaches (Guest Editors: Kumaran Subramanian, A. Wilson Santhosh Kumar, and Venkatajothi Ramarao)
  74. Development of smart core–shell nanoparticles-based sensors for diagnostics of salivary alpha-amylase in biomedical and forensics
  75. Thermoplastic-polymer matrix composite of banana/betel nut husk fiber reinforcement: Physico-mechanical properties evaluation
  76. Special Issue: Electrospun Functional Materials
  77. Electrospun polyacrylonitrile/regenerated cellulose/citral nanofibers as active food packagings
https://doi.org/10.1515/epoly-2024-0083
Schlagwörter für diesen Artikel
electrospun nanofiber membrane; La(OH)3; defluoridation; kinetics; water treatment
Creative Commons
BY 4.0

Artikel in diesem Heft

  1. Research Articles
  2. Flame-retardant thermoelectric responsive coating based on poly(3,4-ethylenedioxythiphene) modified metal–organic frameworks
  3. Highly stretchable, durable, and reversibly thermochromic wrapped yarns induced by Joule heating: With an emphasis on parametric study of elastane drafts
  4. Molecular dynamics simulation and experimental study on the mechanical properties of PET nanocomposites filled with CaCO3, SiO2, and POE-g-GMA
  5. Multifunctional hydrogel based on silk fibroin/thermosensitive polymers supporting implant biomaterials in osteomyelitis
  6. Marine antifouling coating based on fluorescent-modified poly(ethylene-co-tetrafluoroethylene) resin
  7. Preparation and application of profiled luminescent polyester fiber with reversible photochromism materials
  8. Determination of pesticide residue in soil samples by molecularly imprinted solid-phase extraction method
  9. The die swell eliminating mechanism of hot air assisted 3D printing of GF/PP and its influence on the product performance
  10. Rheological behavior of particle-filled polymer suspensions and its influence on surface structure of the coated electrodes
  11. The effects of property variation on the dripping behaviour of polymers during UL94 test simulated by particle finite element method
  12. Experimental evaluation on compression-after-impact behavior of perforated sandwich panel comprised of foam core and glass fiber reinforced epoxy hybrid facesheets
  13. Synthesis, characterization and evaluation of a pH-responsive molecular imprinted polymer for Matrine as an intelligent drug delivery system
  14. Twist-related parametric optimization of Joule heating-triggered highly stretchable thermochromic wrapped yarns using technique for order preference by similarity to ideal solution
  15. Comparative analysis of flow factors and crystallinity in conventional extrusion and gas-assisted extrusion
  16. Simulation approach to study kinetic heterogeneity of gadolinium catalytic system in the 1,4-cis-polyisoprene production
  17. Properties of kenaf fiber-reinforced polyamide 6 composites
  18. Cellulose acetate filter rods tuned by surface engineering modification for typical smoke components adsorption
  19. A blue fluorescent waterborne polyurethane-based Zn(ii) complex with antibacterial activity
  20. Experimental investigation on damage mechanism of GFRP laminates embedded with/without steel wire mesh under low-velocity-impact and post-impact tensile loading
  21. Preparation and application research of composites with low vacuum outgassing and excellent electromagnetic sealing performance
  22. Assessing the recycling potential of thermosetting polymer waste in high-density polyethylene composites for safety helmet applications
  23. Mesoscale mechanics investigation of multi-component solid propellant systems
  24. Preparation of HTV silicone rubber with hydrophobic–uvioresistant composite coating and the aging research
  25. Experimental investigation on tensile behavior of CFRP bolted joints subjected to hydrothermal aging
  26. Structure and transition behavior of crosslinked poly(2-(2-methoxyethoxy) ethylmethacrylate-co-(ethyleneglycol) methacrylate) gel film on cellulosic-based flat substrate
  27. Mechanical properties and thermal stability of high-temperature (cooking temperature)-resistant PP/HDPE/POE composites
  28. Preparation of itaconic acid-modified epoxy resins and comparative study on the properties of it and epoxy acrylates
  29. Synthesis and properties of novel degradable polyglycolide-based polyurethanes
  30. Fatigue life prediction method of carbon fiber-reinforced composites
  31. Thermal, morphological, and structural characterization of starch-based bio-polymers for melt spinnability
  32. Robust biaxially stretchable polylactic acid films based on the highly oriented chain network and “nano-walls” containing zinc phenylphosphonate and calcium sulfate whisker: Superior mechanical, barrier, and optical properties
  33. ARGET ATRP of styrene with low catalyst usage in bio-based solvent γ-valerolactone
  34. New PMMA-InP/ZnS nanohybrid coatings for improving the performance of c-Si photovoltaic cells
  35. Impacts of the calcinated clay on structure and gamma-ray shielding capacity of epoxy-based composites
  36. Preparation of cardanol-based curing agent for underwater drainage pipeline repairs
  37. Preparation of lightweight PBS foams with high ductility and impact toughness by foam injection molding
  38. Gamma-ray shielding investigation of nano- and microstructures of SnO on polyester resin composites: Experimental and theoretical study
  39. Experimental study on impact and flexural behaviors of CFRP/aluminum-honeycomb sandwich panel
  40. Normal-hexane treatment on PET-based waste fiber depolymerization process
  41. Effect of tannic acid chelating treatment on thermo-oxidative aging property of natural rubber
  42. Design, synthesis, and characterization of novel copolymer gel particles for water-plugging applications
  43. Influence of 1,1′-Azobis(cyclohexanezonitrile) on the thermo-oxidative aging performance of diolefin elastomers
  44. Characteristics of cellulose nanofibril films prepared by liquid- and gas-phase esterification processes
  45. Investigation on the biaxial stretching deformation mechanism of PA6 film based on finite element method
  46. Simultaneous effects of temperature and backbone length on static and dynamic properties of high-density polyethylene-1-butene copolymer melt: Equilibrium molecular dynamics approach
  47. Research on microscopic structure–activity relationship of AP particle–matrix interface in HTPB propellant
  48. Three-layered films enable efficient passive radiation cooling of buildings
  49. Electrospun nanofibers membranes of La(OH)3/PAN as a versatile adsorbent for fluoride remediation: Performance and mechanisms
  50. Preparation and characterization of biodegradable polyester fibers enhanced with antibacterial and antiviral organic composites
  51. Preparation of hydrophobic silicone rubber composite insulators and the research of anti-aging performance
  52. Surface modification of sepiolite and its application in one-component silicone potting adhesive
  53. Study on hydrophobicity and aging characteristics of epoxy resin modified with nano-MgO
  54. Optimization of baffle’s height in an asymmetric twin-screw extruder using the response surface model
  55. Effect of surface treatment of nickel-coated graphite on conductive rubber
  56. Experimental investigation on low-velocity impact and compression after impact behaviors of GFRP laminates with steel mesh reinforced
  57. Development and characterization of acetylated and acetylated surface-modified tapioca starches as a carrier material for linalool
  58. Investigation of the compaction density of electromagnetic moulding of poly(ether-ketone-ketone) polymer powder
  59. Experimental investigation on low-velocity-impact and post-impact-tension behaviors of GFRP T-joints after hydrothermal aging
  60. The repeated low-velocity impact response and damage accumulation of shape memory alloy hybrid composite laminates
  61. Exploring a new method for high-performance TPSiV preparation through innovative Si–H/Pt curing system in VSR/TPU blends
  62. Large-scale production of highly responsive, stretchable, and conductive wrapped yarns for wearable strain sensors
  63. Preparation of natural raw rubber and silica/NR composites with low generation heat through aqueous silane flocculation
  64. Molecular dynamics simulation of the interaction between polybutylene terephthalate and A3 during thermal-oxidative aging
  65. Crashworthiness of GFRP/aluminum hybrid square tubes under quasi-static compression and single/repeated impact
  66. Review Articles
  67. Recent advancements in multinuclear early transition metal catalysts for olefin polymerization through cooperative effects
  68. Impact of ionic liquids on the thermal properties of polymer composites
  69. Recent progress in properties and application of antibacterial food packaging materials based on polyvinyl alcohol
  70. Additive manufacturing (3D printing) technologies for fiber-reinforced polymer composite materials: A review on fabrication methods and process parameters
  71. Rapid Communication
  72. Design, synthesis, characterization, and adsorption capacities of novel superabsorbent polymers derived from poly (potato starch xanthate-graft-acrylamide)
  73. Special Issue: Biodegradable and bio-based polymers: Green approaches (Guest Editors: Kumaran Subramanian, A. Wilson Santhosh Kumar, and Venkatajothi Ramarao)
  74. Development of smart core–shell nanoparticles-based sensors for diagnostics of salivary alpha-amylase in biomedical and forensics
  75. Thermoplastic-polymer matrix composite of banana/betel nut husk fiber reinforcement: Physico-mechanical properties evaluation
  76. Special Issue: Electrospun Functional Materials
  77. Electrospun polyacrylonitrile/regenerated cellulose/citral nanofibers as active food packagings
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Heruntergeladen am 7.9.2025 von https://www.degruyterbrill.com/document/doi/10.1515/epoly-2024-0083/html
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