Template-Free Synthesis of Magnetic La-Mn-Fe Tri-Metal Oxide Nanofibers for Efficient Fluoride Remediation: Kinetics, Isotherms, Thermodynamics and Reusability

The occurrence of fluoride contamination in drinking water has gained substantial concern owing to its serious threat to human health. Traditional adsorbents have shortcomings such as low adsorption capacity and poor selectivity, so it is urgent to develop new adsorbents with high adsorption capacity, renewable and no secondary pollution. In this work, magnetic electrospun La-Mn-Fe tri-metal oxide nanofibers (LMF NFs) for fluoride recovery were developed via electrospinning and heat treatment, and its defluoridation property was evaluated in batch trials. Modern analytical tools (SEM, BET, XRD, FTIR) were adopted to characterize the properties of the optimized adsorbent, i.e., LMF11 NFs with a La:Mn molar ratio of 1:1. The surface area calculated via BET method and pHpzc assessed using pH drift method of LMF11 NFs were 55.81 m2 g−1 and 6.47, respectively. The results indicated that the adsorption amount was highly dependent on the pH of the solution, and reached the highest value at pH = 3. The kinetic behavior of defluoridation on LMF11 NFs was dominated by the PSO model with the highest fitted determination coefficients of 0.9999. Compared with the other three isotherm models, the Langmuir model described defluoridation characteristics well with larger correlation coefficients of 0.9997, 0.9990, 0.9987 and 0.9976 at 15 °C, 25 °C, 35 °C and 45 °C, respectively. The optimized LMF11 NFs exhibited superior monolayer defluoridation capacities for 173.30–199.60 mg F−/g at pH 3 at 15–45 °C according to the Langmuir isotherm model. A thermodynamic study proved that the defluoridation by LMF11 NFs is a spontaneous, endothermic along with entropy increase process. In addition, the LMF11 NFs still showed high defluoridation performance after three reused cycles. These findings unveil that the synthesized LMF11 NFs adsorbent is a good adsorbent for fluoride remediation from wastewater owing to its low cost, high defluoridation performance and easy operation.


Introduction
Fluoride contamination in groundwater caused by the processes of natural geochemical and human production activities is a burning issue, which has attracted substantial attention since it is closely related to the safety and health issue for humans [1][2][3][4][5][6]. An appropriate amount of F − can enhance the hardness of teeth and the mineralization of hard tissues and prevent dental caries. Surplus fluoride in drinking water may lead to irremediable diseases in humans, namely fluorosis. The tolerance level of fluoride in drinking water recommended by the World Health Organization is no more than 1.5 mg/L [7,8]. However, endemic levels of fluorosis have been reported in 32 nations worldwide, and more than In this study, magnetically separable electrospun La-Mn-Fe tri-metal oxide nanofibers (LMF NFs) were prepared by electrospinning and heat treatment. The LMF NFs were used as a new type of adsorbent for fluoride remediation. The magnetic Fe 3 O 4 NPs are evenly dispersed along the nanofiber axis without obvious agglomeration. Moreover, the magnetic fibrous LMF NFs can effectively prevent the agglomeration of adsorbent during the defluoridation process and can be quickly separated from the solution using an external magnet after adsorption. The effects of the molar ratio of La/Mn, initial F − concentration, pH, LMF11 NFs dosage, shaking time, interfering anions, isotherm models and adsorption kinetics were investigated in detail.

Preparation of La-Mn-Fe Tri-Metal Oxide Nanofibers
La(NO 3 ) 3 ·6H 2 O and Mn(CH 3 COO) 2 ·4H 2 O were mixed in a beaker with La:Mn molar ratios of 2:1, 1:1 and 1:2, then completely dissolved in a small amount of DMF. Afterward, 1.8 g PAN powder, 10 mL DMF and 0.075 g Fe 3 O 4 nanoparticles (via a solvothermal method [44]) were injected into each sample to form precursor solutions namely (La(NO 3 ) 3 /Mn(CH 3 COO) 2 /Fe 3 O 4 /PAN/DMF). The electrospinning processes were conducted to produce precursor fibers [33,43,45,46]. Briefly, the precursor solution was added into a 5 mL syringe with a metal needle, then the electrospinning of the precursor solution was performed with an applied electric field of 150 kV m −1 and a flow rate of 1.2 mL h −1 at room temperature, to generate precursor nanofibers. After electrospinning, the resulting precursor nanofibers were dehydrated for 8 h and then put into a tube furnace for heat treatment at 500 • C for 2 h to prepare La-Mn-Fe tri-metal oxide nanofibers under an air atmosphere. La-Mn-Fe tri-metal oxide nanofibers with molar ratios of La/Mn being 2/1, 1/1 and 1 /2 were noted as LMF21 NFs, LMF11 NFs and LMF12 NFs, respectively. The graphic plan of the fabrication process of electrospun LMF NFs and the adsorption of fluoride is portrayed in Scheme 1.
high defluoridation efficiency, excellent recyclability and reusability.
In this study, magnetically separable electrospun La-Mn-Fe tri-metal oxide nanofibers (LMF NFs) were prepared by electrospinning and heat treatment. The LMF NFs were used as a new type of adsorbent for fluoride remediation. The magnetic Fe3O4 NPs are evenly dispersed along the nanofiber axis without obvious agglomeration. Moreover, the magnetic fibrous LMF NFs can effectively prevent the agglomeration of adsorbent during the defluoridation process and can be quickly separated from the solution using an external magnet after adsorption. The effects of the molar ratio of La/Mn, initial F − concentration, pH, LMF11 NFs dosage, shaking time, interfering anions, isotherm models and adsorption kinetics were investigated in detail.

Preparation of La-Mn-Fe Tri-Metal Oxide Nanofibers
La(NO3)3·6H2O and Mn(CH3COO)2·4H2O were mixed in a beaker with La:Mn molar ratios of 2:1, 1:1 and 1:2, then completely dissolved in a small amount of DMF. Afterward, 1.8 g PAN powder, 10 mL DMF and 0.075 g Fe3O4 nanoparticles (via a solvothermal method [44]) were injected into each sample to form precursor solutions namely (La(NO3)3/Mn(CH3COO)2/Fe3O4/PAN/DMF). The electrospinning processes were conducted to produce precursor fibers [33,43,45,46]. Briefly, the precursor solution was added into a 5 mL syringe with a metal needle, then the electrospinning of the precursor solution was performed with an applied electric field of 150 kV m −1 and a flow rate of 1.2 mL h −1 at room temperature, to generate precursor nanofibers. After electrospinning, the resulting precursor nanofibers were dehydrated for 8 h and then put into a tube furnace for heat treatment at 500 °C for 2 h to prepare La-Mn-Fe tri-metal oxide nanofibers und.er an air atmosphere. La-Mn-Fe tri-metal oxide nanofibers with molar ratios of La/Mn being 2/1, 1/1 and ½ were noted as LMF21 NFs, LMF11 NFs and LMF12 NFs, respectively. The graphic plan of the fabrication process of electrospun LMF NFs and the adsorption of fluoride is portrayed in Scheme 1.

Batch Adsorption Studies
The defluoridation performance of LMF NFs was evaluated using batch experiments (Scheme 1). Adsorption kinetics were performed in the time range from 1 to 60 min under Adsorption isotherm studies were carried out at varying original F − concentrations of 10-60 mg F − /L at a temperature range of 15-45 • C with LMF11 NFs dosage of 0.2 g/L in 50 mL of respective solution. Various concentrations (10,30,50,80 or 100 mg/L) of five commonly occurring anions, including nitrate (NO 3 − ), sulfate (SO 4 2− ), chloride (Cl − ), carbonate (CO 3 2− ), phosphate (PO 4 3− ) were studied under initial F − concentration of 20 mg F − /L with an adsorbent dosage of 0.2 g/L at 25 • C and pH 3 for 12 h. The effect of pH (2-9) along with LMF11 NFs dosage (0.2-0.6 g/L) on adsorption property was assessed under 50 mL initial F − concentration of 20 mg F − /L at 25 • C for 12 h.

Reusability Test
The reusability of the F − adsorbed magnetic LMF11 NFs was determined to quantify the cost-effectiveness of the adsorbent for F − remediation in the successive adsorptiondesorption cycles. The original concentration of F − was 20 mg F − /L at pH 3 and 25 • C for 8 h for each regeneration cycle. The F − adsorbed magnetic LMF11 NFs were recovered by an external magnet, and then eluted with 100 mL of 0.1 M NaOH to regeneration [43,47,48], vacuum-dried and used for a new run.

Characterization of Adsorbent
The crystal structures of LMF11 NFs before and after adsorption were established by X-ray diffraction (XRD, D8 ADVANCE X, Bruker, Saarbrucken, Germany), The BET specific surface area of the LMF11 NFs adsorbent was examined using a Micrometrics ASAP 2020 analyzer (Micromeritics, Norcross, GA, USA). The TEM and SEM images of the magnetic electrospun LMF11 NFs were acquired from TEM JEM 2100 (JEOL, Tokyo, Japan) and SEM 300U (VEGA, Tescan, Brno, Czech Republic). The point of zero charge of LMF11 NFs was assessed via a pH drift method [49].

Characterization of Adsorbent
The morphological details of La(NO 3 ) 3 /Mn(CH 3 COO) 2 /Fe 3 O 4 /PAN and LMF11 NFs were determined by SEM, as portrayed in Figure 1a,b. Obviously, the surfaces of La(NO 3 ) 3 /Mn(CH 3 COO) 2 /Fe 3 O 4 /PAN fibers are continuous and smooth with an average diameter of 1.28 ± 0.10 µm. Fe 3 O 4 NPs are evenly distributed on the surface of the La(NO 3 ) 3 /Mn(CH 3 COO) 2 /Fe 3 O 4 /PAN and LMF11 NFs. After heat treatment, the volume and diameter of fibers decline due to the decomposition of Mn(CH 3 COO) 2 , La(NO 3 ) 3 and PAN. The continuous LMF11 NFs are hollow and rougher compared to that before calcination, which possess an average diameter of 626 ± 57 nm. Figure 1d displays the TEM image of LMF11 NFs, it is obvious that the spherical Fe 3 O 4 NPs are uniformly dispersed along the fiber axis without obvious agglomeration. The chemical composition of LMF11 NFs is verified by EDS analysis, as shown in Figure 1c. EDS illustrates that the LMF11 NFs consist of La (14.02%), Mn (13.22%), Fe (4.45%) and O (68.31%) elements. Apparently, the detected La/Mn molar ratio in the sample is quite consistent with the designed value. The BET surface area and BJH pore distribution of LMF11 NFs are analyzed by N 2 adsorptiondesorption isotherm (Figure 1e). The total pore volume and surface area of LMF11 NFs are 0.276 cm 3 g −1 and 55.81 m 2 g −1 . The inset of Figure 1e states that the main pore sizes of LMF11 NFs locate at 3.06 nm and 17.39 nm, indicative of numerous mesopores in the adsorbent [29]. The isoelectric point (pH pzc ) of LMF11 NFs calculated according to the literature [50] is 6.47 ( Figure 1f). This signifies that the surface charge of LMF11 NFs is positive and protonated at pH < 6.47 while negative at pH > 6.47. Thus, negative ions such as F − can be could easily absorbed into the positively charged LMF11 NFs owing to their columbic attractions at a pH range below pH pzc [51].

Preparation Optimization
The capture capacities of F − on as-prepared adsorbents involving La NFs, LMF21 NFs, LMF11 NFs, LMF12 NFs, Mn NFs and Fe 3 O 4 NPs were determined. Then, 0.2 g/L of adsorbents were placed into 50 mL fluoride solutions with a concentration of 20 mg F − /L for the adsorption test, the temperature of 25 • C, pH of 3 and shaking time for 12 h were adopted, respectively. The fluoride binding amounts were determined, as portrayed in Figure 2. The maximum fluoride uptakes of La NFs, Mn NFs and Fe 3 O 4 NPs are 104.12, 30.71 and 15.71 mg F − /g, respectively. While the maximum uptakes of F − on LMF21 NFs, LMF11 NFs, LMF12 NFs are 147.68, 182.03 and 131.67 mg F − /g, respectively. Evidently, the LMF NFs possess higher adsorption capacity than those of single metal oxide due to the synergistic effect of La, Mn and Fe [33,34]. The fluoride uptake of LMF11 adsorbent with La/Mn molar ratio being 1/1 is the highest. Hence, LMF11 NFs is designed as fluoride scavenger in the subsequent experiments.
negative ions such as F − can be could easily absorbed into the positively charged LM NFs owing to their columbic attractions at a pH range below pHpzc [51].

Preparation Optimization
The capture capacities of F − on as-prepared adsorbents involving La NFs, LM NFs, LMF11 NFs, LMF12 NFs, Mn NFs and Fe3O4 NPs were determined. Then, 0.2 g/ adsorbents were placed into 50 mL fluoride solutions with a concentration of 20 mg for the adsorption test, the temperature of 25 °C , pH of 3 and shaking time for 12 h w adopted, respectively. The fluoride binding amounts were determined, as portraye Eviden the LMF NFs possess higher adsorption capacity than those of single metal oxide du the synergistic effect of La, Mn and Fe [33,34]. The fluoride uptake of LMF11 adsor with La/Mn molar ratio being 1/1 is the highest. Hence, LMF11 NFs is designed as f ride scavenger in the subsequent experiments.

The Effect of pH
The effect of pH on the defluoridation performance of LMF11 NFs was evaluated at various pH values from 2 to 9 ( Figure 3). The maximum binding amount (47.79 mg F − /g) and removal efficiency (95.58%) for LMF11 NFs occurs at a pH of 3 owing to the columbic attractions (Equation (1)). LMF11 NFs show a dramatic decrease in defluoridation property at pH > 3 and pH < 3. Most fluoride ions exist in the form of electrically neutral HF (pKa HF = 2.95) [52] when pH < 3, which can reduce the columbic forces between LMF11 NFs and adsorbate [42]. Meanwhile, the diminished binding amount at higher pH conditions is attributed to electrostatic repulsion between F − and negatively charged LMF11 NFs along with the competition between F − and OH − for active sites [41]. Additionally, the equilibrium pH of the F − solution changed from 2 to 2.18, 3−5.49, 4-6.59, 5-6.75, 6-6.59, 7-6.34, 8-6.83, 9-7.14 after adsorption, respectively. The improvement of pH after adsorption in an initial pH range of 2-6 suggests the existence of an ion exchange of hydroxyl groups bonded on the surface of the adsorbent with fluoride ions (Equations (2) and (3)). While the decline of pH after adsorption in an initial pH range from 7 to 9 (>pH pzc ) is mainly ascribed to the competitive adsorption between hydroxyl groups and adsorbate (F − ) (Equation (4)). A pH of 3 was chosen to utilize for subsequent studies.

≡M-OH
where ≡M indicative of La, Fe and Mn metal ions.

The Effect of pH
The effect of pH on the defluoridation performance of LMF11 NFs was evaluate various pH values from 2 to 9 ( Figure 3). The maximum binding amount (47.79 mg and removal efficiency (95.58%) for LMF11 NFs occurs at a pH of 3 owing to the col bic attractions (Equation (1)). LMF11 NFs show a dramatic decrease in defluorida property at pH > 3 and pH < 3. Most fluoride ions exist in the form of electrically neu HF (pKa HF = 2.95) [52] when pH < 3, which can reduce the columbic forces betw LMF11 NFs and adsorbate [42]. Meanwhile, the diminished binding amount at hi pH conditions is attributed to electrostatic repulsion between F − and negatively cha LMF11 NFs along with the competition between F − and OH − for active sites [41]. A tionally, the equilibrium pH of the F − solution changed from 2 to 2.18, 3−5.49, 4-6.5 6.75, 6-6.59, 7-6.34, 8-6.83, 9-7.14 after adsorption, respectively. The improvement o after adsorption in an initial pH range of 2-6 suggests the existence of an ion excha of hydroxyl groups bonded on the surface of the adsorbent with fluoride ions (Equat (2) and (3)). While the decline of pH after adsorption in an initial pH range from 7 (>pHpzc) is mainly ascribed to the competitive adsorption between hydroxyl groups adsorbate (F − ) (Equation (4)). A pH of 3 was chosen to utilize for subsequent studies.
where M indicative of La, Fe and Mn metal ions.  However, the initial concentration of F − keeps constant and fails to saturate the binding sites on LMF11 NFs. It means that the binding sites cannot be fully used. In addition, the binding sites may be agglomerated together, leading to partial binding sites covering each other, and then the unit adsorption capacity cuts down [41]. The adsorbent dosage is selected as 0.2 g/L in the following defluoridation experiments in light of the economy and practicability of the adsorbent.
Polymers 2022, 14, x FOR PEER REVIEW 7 of fluoride removal percentage. However, the initial concentration of F − keeps constant an fails to saturate the binding sites on LMF11 NFs. It means that the binding sites cann be fully used. In addition, the binding sites may be agglomerated together, leading partial binding sites covering each other, and then the unit adsorption capacity cu down [41]. The adsorbent dosage is selected as 0.2 g/L in the following defluoridatio experiments in light of the economy and practicability of the adsorbent.

The Effect of Initial F − Concentration (C0)
The fluoride uptake significantly depends on the initial concentration of F − . A shown in Figure 5, the fluoride uptake of LMF11 NFs elevates gradually with an i crease in C0 (10 mg F − /L to 45 mg F − /L). The reason is that with the increase in F − conce tration, the number of fluoride ions near the surface of the LMF11 NFs increases appa ently, and the binding sites on the surface of LMF11 NFs are more fully surrounded b fluoride ions, thus, more fluoride ions are adsorbed by the adsorbent, resulting in the i creasing of fluoride binding amount [53]. Then it approaches saturation at higher C0 du to the saturation of active sites. While the fluoride removal efficiency declines with th rise of fluoride concentration. It is because the LMF11 NFs adsorbent have a limited a sorption uptake. When LMF11 NFs adsorbent achieve adsorption saturation, the remo al percentage will decline with the rise of the initial fluoride concentration. In additio at C0 = 60 mg F − /L, the binding amount leaps from 175.82 mg F − /g at 15 °C to 193.53 m F − /g at 45 °C . The higher the temperature of the adsorption system, the larger the bin ing amount, validating the endothermic nature of the defluoridation process [29].  The fluoride uptake significantly depends on the initial concentration of F − . As shown in Figure 5, the fluoride uptake of LMF11 NFs elevates gradually with an increase in C 0 (10 mg F − /L to 45 mg F − /L). The reason is that with the increase in F − concentration, the number of fluoride ions near the surface of the LMF11 NFs increases apparently, and the binding sites on the surface of LMF11 NFs are more fully surrounded by fluoride ions, thus, more fluoride ions are adsorbed by the adsorbent, resulting in the increasing of fluoride binding amount [53]. Then it approaches saturation at higher C 0 due to the saturation of active sites. While the fluoride removal efficiency declines with the rise of fluoride concentration. It is because the LMF11 NFs adsorbent have a limited adsorption uptake. When LMF11 NFs adsorbent achieve adsorption saturation, the removal percentage will decline with the rise of the initial fluoride concentration. In addition, at C 0 = 60 mg F − /L, the binding amount leaps from 175.82 mg F − /g at 15 • C to 193.53 mg F − /g at 45 • C. The higher the temperature of the adsorption system, the larger the binding amount, validating the endothermic nature of the defluoridation process [29].
Polymers 2022, 14, x FOR PEER REVIEW 7 of 2 fluoride removal percentage. However, the initial concentration of F − keeps constant and fails to saturate the binding sites on LMF11 NFs. It means that the binding sites canno be fully used. In addition, the binding sites may be agglomerated together, leading to partial binding sites covering each other, and then the unit adsorption capacity cut down [41]. The adsorbent dosage is selected as 0.2 g/L in the following defluoridation experiments in light of the economy and practicability of the adsorbent. The fluoride uptake significantly depends on the initial concentration of F − . A shown in Figure 5, the fluoride uptake of LMF11 NFs elevates gradually with an in crease in C0 (10 mg F − /L to 45 mg F − /L). The reason is that with the increase in F − concen tration, the number of fluoride ions near the surface of the LMF11 NFs increases appar ently, and the binding sites on the surface of LMF11 NFs are more fully surrounded by fluoride ions, thus, more fluoride ions are adsorbed by the adsorbent, resulting in the in creasing of fluoride binding amount [53]. Then it approaches saturation at higher C0 due to the saturation of active sites. While the fluoride removal efficiency declines with the rise of fluoride concentration. It is because the LMF11 NFs adsorbent have a limited ad sorption uptake. When LMF11 NFs adsorbent achieve adsorption saturation, the remov al percentage will decline with the rise of the initial fluoride concentration. In addition at C0 = 60 mg F − /L, the binding amount leaps from 175.82 mg F − /g at 15 °C to 193.53 mg F − /g at 45 °C . The higher the temperature of the adsorption system, the larger the bind ing amount, validating the endothermic nature of the defluoridation process [29].

The Effect of Contact Time
The defluoridation experiments were kinetically conducted towards F − removal on LMF11 NFs by varying the time intervals in the range of 0-60 min using an adsorbent dosage of 0.2 g/L with given concentrations (C 0 = 20 and 50 mg F − /L) at 25 • C and pH = 3. As depicted in Figure 6, exceeding 70% of F − could be removed within the first 20 min at C 0 = 20 and 50 mg F − /L, showing an excellent defluoridation property. In this stage, numerous binding sites in LMF11 NFs and the high concentration of fluoride ions in the solution decide the rapid adsorption rate. Hence the larger adsorption rate in the incipient step is ascribed to the strengthening of the diffusion rate of F − provided by concentration gradient along with the existence of numerous available binding sites on the LMF11 NFs [54]. The adsorption tends to dynamic equilibrium after 10 min in 20 mg F − /L solution and 30 min in 50 mg F − /L solution. Clearly, at higher C 0 , it takes longer shaking time to attain equilibrium. Based on the aforementioned analysis, the effect of pH (A), initial concentration of F − (B) and LMF11 NFs dosage (C) (Supplementary Materials, Tables S1 and S2) was determined using response surface methodology (RSM) to optimize the adsorption parameters [55,56]. It is clear that the experimental results of the single factor variable are very close to the RSM results (Supplementary Materials, Table S3, Figures S1 and S2).

The Effect of Contact Time
The defluoridation experiments were kinetically conducted towards F − remova LMF11 NFs by varying the time intervals in the range of 0-60 min using an adsor dosage of 0.2 g/L with given concentrations (C0 = 20 and 50 mg F − /L) at 25 °C and pH As depicted in Figure 6, exceeding 70% of F − could be removed within the first 20 m C0 = 20 and 50 mg F − /L, showing an excellent defluoridation property. In this stage, merous binding sites in LMF11 NFs and the high concentration of fluoride ions in th lution decide the rapid adsorption rate. Hence the larger adsorption rate in the incip step is ascribed to the strengthening of the diffusion rate of F − provided by concentra gradient along with the existence of numerous available binding sites on the LMF11 [54]. The adsorption tends to dynamic equilibrium after 10 min in 20 mg F − /L solu and 30 min in 50 mg F − /L solution. Clearly, at higher C0, it takes longer shaking tim attain equilibrium. Based on the aforementioned analysis, the effect of pH (A), in concentration of F − (B) and LMF11 NFs dosage (C) (Supplementary Materials, Table and S2) was determined using response surface methodology (RSM) to optimize the sorption parameters [55,56]. It is clear that the experimental results of the single fa variable are very close to the RSM results (Supplementary Materials, Table S3, Fig  S1 and S2). One of the major barriers that limit the widespread application of adsorption t nology in practical water treatment is the selectivity of adsorbent. The effects of commonly occurring anions including nitrate (NO3 − ), sulfate (SO4 2− ), chloride (Cl − ), bonate (CO3 2− ) and phosphate (PO4 3− ) on defluoridation by LMF11 NFs were invest ed. The competitive experiments were conducted by weighing 0.01 g LMF11 NFs in binary system that contained 50 mL 10 mg F − /L F − paired with different concentrat (10,30,50,80 or 100 mg/L) of the interfering anions, respectively. The result is depi in Figure 7. Obviously, NO3 − , Cl − , SO4 2− and CO3 2− anions at all concentrations of 10 mg/L almost do not prevent the F − removal, illustrating the excellent selectivity and bility of LMF11 NFs adsorbent. Whereas the binding capacity drops quickly from 9 to 52.16 mg F − /g with the rise of PO4 3− concentration from 0 to 100 mg/L, manifesti remarkable competitive effect of PO4 3− on defluoridation. This may be explained by Ksp of LaPO4 (3.7 × 10 −23 ) [54], which favors PO4 3− to displace the adsorbed F − on LM NFs compared with that of LaF3, thereby deteriorating the fluoride binding capacity. same result has also been reported by several studies using rare earth element-based terials for fluoride remediation [27,57].

The Effect of Interfering Anions
One of the major barriers that limit the widespread application of adsorption technology in practical water treatment is the selectivity of adsorbent. The effects of five commonly occurring anions including nitrate (NO 3 − ), sulfate (SO 4 2− ), chloride (Cl − ), carbonate (CO 3 2− ) and phosphate (PO 4 3− ) on defluoridation by LMF11 NFs were investigated. The competitive experiments were conducted by weighing 0.01 g LMF11 NFs into a binary system that contained 50 mL 10 mg F − /L F − paired with different concentrations (10,30,50,80 or 100 mg/L) of the interfering anions, respectively. The result is depicted in Figure 7. Obviously, NO 3 − , Cl − , SO 4 2− and CO 3 2− anions at all concentrations of 10-100 mg/L almost do not prevent the F − removal, illustrating the excellent selectivity and stability of LMF11 NFs adsorbent. Whereas the binding capacity drops quickly from 98.20 to 52.16 mg F − /g with the rise of PO 4 3− concentration from 0 to 100 mg/L, manifesting a remarkable competitive effect of PO 4 3− on defluoridation. This may be explained by the K sp of LaPO 4 (3.7 × 10 −23 ) [54], which favors PO 4 3− to displace the adsorbed F − on LMF11 NFs compared with that of LaF 3 , thereby deteriorating the fluoride binding capacity. The same result has also been reported by several studies using rare earth element-based materials for fluoride remediation [27,57].
The values of 1/n (0.1961-0.2543) established from the Freundlich model lie in range of 0 < 1/n < 1 at 15 to 45 °C , signifying beneficial adsorption by LMF11 NFs un studied conditions [52]. The E value derived from the D-R model represents free energ values in ranges of 1-8 kJ mol −1 , 8-16 kJ mol −1 and greater than 16 kJ mol −1 are indicativ electrostatic physical adsorption, electrostatic interaction or synergy and chemical ads tion, respectively [65]. The E value calculated from the D-R model gradually elevates f 14.06 to 17.96 kJ/mol with ascending the temperature from 15 to 45 °C , which denotes the defluoridation on LMF11 NFs proceeds from ion exchange to chemisorption [66].
The values of 1/n (0.1961-0.2543) established from the Freundlich model lie in the range of 0 < 1/n < 1 at 15 to 45 • C, signifying beneficial adsorption by LMF11 NFs under studied conditions [52]. The E value derived from the D-R model represents free energy. E values in ranges of 1-8 kJ mol −1 , 8-16 kJ mol −1 and greater than 16 kJ mol −1 are indicative of electrostatic physical adsorption, electrostatic interaction or synergy and chemical adsorption, respectively [65]. The E value calculated from the D-R model gradually elevates from 14.06 to 17.96 kJ/mol with ascending the temperature from 15 to 45 • C, which denotes that the defluoridation on LMF11 NFs proceeds from ion exchange to chemisorption [66].      respectively. This indicates that LMF11 NFs exhibit a higher fluoride removal rate at a lower F − concentration, resulting in a shorter time needed to attain the adsorption equilibrium. Similar results were observed in other La-modified adsorbents [54]. In addition, on the basis of the three-segment linear fitting Weber and Morris model, the intercepts (C) are not zero (Figure 9d), reflecting that the defluoridation mechanism of LMF11 NFs is complex and composed of surface adsorption, intraparticle diffusion along with external liquid film diffusion [48]. The order of k int is k int1 > k int2 > k int3 at C 0 = 20 and 50 mg F − /L, revealing that the surface or film diffusion is a rate-controlling step. The fluoride ions adsorb quickly on the external surface of LMF11 NFs at beginning. Pore diffusion or intraparticle is a rate-limiting step at the second stage and the diffusion into mesopores/micropores is dominant and then achieves equilibrium on the exterior surface at the third stage, where intraparticle diffusion decreases owing to the extremely low F − concentration in the adsorption system [43,48].

T (°C )
G 0 (kJ mol − 1 ) lnKD H 0 (kJ mol −1 ) S 0 (J mol − For an ideal adsorbent, it is vital to possess high reusability without a signifi loss of removal efficiency. Figure 11 shows the renewability of LMF11 NFs. Notably F − uptakes of LMF11 NFs are 97.50, 96.46, 95.73 and 94.76 mg F − /g at the original, second and third cycles, respectively. It can be concluded that the defluoridation formance is still effective for up to three consecutive reuse-regeneration cycles. Th sult states that LMF11 adsorbent exhibits sufficient chemical stability and reusab thereby it is suitable for F − removal from wastewater.

Reusability
For an ideal adsorbent, it is vital to possess high reusability without a significant loss of removal efficiency. Figure 11 shows the renewability of LMF11 NFs. Notably, the F − uptakes of LMF11 NFs are 97.50, 96.46, 95.73 and 94.76 mg F − /g at the original, first, second and third cycles, respectively. It can be concluded that the defluoridation performance is still effective for up to three consecutive reuse-regeneration cycles. The result states that LMF11 adsorbent exhibits sufficient chemical stability and reusability, thereby it is suitable for F − removal from wastewater.

Thermodynamic Study
The thermodynamic factors (namely G 0 , H 0 and S 0 ) derived from the plot of lnKD versus 10 3 /T ( Figure 10) are tabulated in Table 4. The G 0 values at 15-45 °C are assessed at −5.89, −6.69, −7.08 and −7.64 kJ mol −1 , respectively. The values of G 0 are negative, signifying the defluoridation process by LMF11 NFs is feasible and spontaneous [75]. The positive H 0 (10.45 kJ mol −1 ) and S 0 (69.12 J mol −1 k −1 ) confirm the defluoridation by LMF11 NFs is endothermic along with an entropy increase process [76].  For an ideal adsorbent, it is vital to possess high reusability without a significant loss of removal efficiency. Figure 11 shows the renewability of LMF11 NFs. Notably, the F − uptakes of LMF11 NFs are 97.50, 96.46, 95.73 and 94.76 mg F − /g at the original, first, second and third cycles, respectively. It can be concluded that the defluoridation performance is still effective for up to three consecutive reuse-regeneration cycles. The result states that LMF11 adsorbent exhibits sufficient chemical stability and reusability, thereby it is suitable for F − removal from wastewater.

Adsorption Mechanism
The defluoridation mechanism of LMF11 NFs was analyzed by PXRD and FTIR. Figure 12a shows the PXRD spectra of the LMF11 NFs and F − adsorbed LMF11 NFs. Seven characteristic peaks appearing at 2θ = 23.0 • , 32.8 • , 40.5 • , 47.1 • , 58.2 • , 68.6 • and 77.8 • , assigned to peak indices of (100), (110), (111), (200), (211), (220) and (310), respectively, portray the cubic perovskite phase of LaMnO 3 (JCPDS NO. 74-0440) [33,77]. The detected characteristic peaks occur at 2θ = 30.7 • (220), 35.8 • (311) and 62.9 • (440), signifying that the cubic phase of Fe 3 O 4 has been generated [44]. Meanwhile, the wide peak located at 2θ = 29.5 • (220) [78] assigns to La 2 O 3 . In summary, the LMF11 NFs are composed of La 2 O 3 , LaMnO 3 and Fe 3 O 4 . After defluoridation, the disappearance of the diffraction peak ascribed to La 2 O 3 and the presence of new reflections belonged to the hexagonal structure of LaF 3 (PDF No.32-0483) [43] state that the ion exchange between -OH on the surface of LMF11 NFs and F − is the dominating defluoridation mechanism of hydrous La 2 O 3 in adsorption system. Moreover, the reflection intensity of Fe 3 O 4 and LaMnO 3 turns slightly weak after fluoride uptake owing to the decrease in crystallinity caused by the entry of F − into the crystal lattice. ed characteristic peaks occur at 2θ = 30.7° (220), 35.8° (311) and 62.9° (440), signifying that the cubic phase of Fe3O4 has been generated [44]. Meanwhile, the wide peak located at 2θ = 29.5° (220) [78] assigns to La2O3. In summary, the LMF11 NFs are composed of La2O3, LaMnO3 and Fe3O4. After defluoridation, the disappearance of the diffraction peak ascribed to La2O3 and the presence of new reflections belonged to the hexagonal structure of LaF3 (PDF No.32-0483) [43] state that the ion exchange between -OH on the surface of LMF11 NFs and F − is the dominating defluoridation mechanism of hydrous La2O3 in adsorption system. Moreover, the reflection intensity of Fe3O4 and LaMnO3 turns slightly weak after fluoride uptake owing to the decrease in crystallinity caused by the entry of F − into the crystal lattice.
As seen from the FTIR spectrum of the neat LMF11 NFs (Figure 12b), the bands at 3434, 1630, 1485, 851 and 590 cm − 1 assign to the surface hydroxyl groups stretching mode, chemisorbed water bending mode, CO3 2 − stretching mode, La-O stretching mode, and Mn-O along with Fe-O stretching mode [44,77,79]. Meanwhile, the two peaks ascribed to the metal hydroxyl bond (M-OH) [67,80]    As seen from the FTIR spectrum of the neat LMF11 NFs (Figure 12b), the bands at 3434, 1630, 1485, 851 and 590 cm −1 assign to the surface hydroxyl groups stretching mode, chemisorbed water bending mode, CO 3 2− stretching mode, La-O stretching mode, and Mn-O along with Fe-O stretching mode [44,77,79]. Meanwhile, the two peaks ascribed to the metal hydroxyl bond (M-OH) [67,80] at 1121 and 1067 cm −1 are detected. After fluoride uptake, the bands at 3430 and 1632 cm −1 turn obviously weak and two peaks at 1127 and 1067 cm −1 are not detected, validating that the bonding of fluoride through electrostatic interaction as well as the replacement of -OH by F − via ion exchange [81].  [82].
F − loaded LMF11 adsorbent was further verified by SEM and EDS (Figure 12c,d). The used LMF11 NFs have preserved fibrous morphology. However, the morphological surface of the used LMF11 NFs acquires modified, since there are lots of nanoparticles on the surface of F − loaded LMF11 NFs after fluoride adsorption, indicating that fluoride ions have covered the LMF11 NFs surface and LaF 3 nanoparticles have been yielded via ion exchange during the defluoridation process, which is consistent with PXRD analysis. Apparently, the EDS spectrum of F − loaded LMF11 NFs portrayed in Figure 12d suggests the presence of La (13.32%), Mn (12.11%), F (15.05%), Fe (3.74%) and O (55.78%) in the used LCF11 NFs.
The pH and pH pzc investigation of LMF11 NFs on F − removal reveal that it is more prone to higher removal efficiency in the acidic condition, owing to opposite charges on LMF11 NFs surface and F − which is responsible for columbic attractions. The values of E DR calculated by D-R isotherm are 14.06, 14.91, 16.58 and 17.96 kJ mol −1 , respectively, at 15, 25, 35 and 45 • C, confirming that the F − adsorption is owing to both ion exchange and chemisorption [29,83]. In a word, the defluoridation process onto LMF11 NFs is significantly governed by hydrogen bond, columbic interaction, as well as ion exchange. The plausible defluoridation mechanism by LMF11 NFs is depicted in Figure 13.  (Figure 12c, The used LMF11 NFs have preserved fibrous morphology. However, the morphologi surface of the used LMF11 NFs acquires modified, since there are lots of nanopartic on the surface of F − loaded LMF11 NFs after fluoride adsorption, indicating that fluori ions have covered the LMF11 NFs surface and LaF3 nanoparticles have been yielded v ion exchange during the defluoridation process, which is consistent with PXRD analys Apparently, the EDS spectrum of F − loaded LMF11 NFs portrayed in Figure 12d su gests the presence of La (13.32%), Mn (12.11%), F (15.15%), Fe (3.74%) and O (55.78%) the used LCF11 NFs.
The pH and pHpzc investigation of LMF11 NFs on F − removal reveal that it is mo prone to higher removal efficiency in the acidic condition, owing to opposite charges LMF11 NFs surface and F − which is responsible for columbic attractions. The values EDR calculated by D-R isotherm are 14.06, 14.91, 16.58 and 17.96 kJ mol −1 , respectively, 15, 25, 35 and 45 °C , confirming that the F − adsorption is owing to both ion exchange a chemisorption [29,83]. In a word, the defluoridation process onto LMF11 NFs is sign cantly governed by hydrogen bond, columbic interaction, as well as ion exchange. T plausible defluoridation mechanism by LMF11 NFs is depicted in Figure 13.

Conclusions
In this study, magnetic electrospun La-Mn-Fe tri-metal oxide nanofibers (LMF NF were developed through electrospinning and heat treatment and used for fluoride reco ery. The prepared LMF11 NFs with a La:Mn molar ratio of 1:1 gained the highest fluori adsorption amount. The results of the investigation were summarized as follows: (1) The LMF11 NFs presented fiber morphology with an average diameter of 626 ± nm. The results showed that the spherical Fe3O4 NPs were uniformly dispers along the fiber axis without obvious agglomeration, and the total pore volume a surface area of LMF11 NFs were 0.276 cm 3 g −1 and 55.81 m 2 g −1 , respectively. T isoelectric point (pHpzc) of LMF11 NFs was 6.47.

Conclusions
In this study, magnetic electrospun La-Mn-Fe tri-metal oxide nanofibers (LMF NFs) were developed through electrospinning and heat treatment and used for fluoride recovery. The prepared LMF11 NFs with a La:Mn molar ratio of 1:1 gained the highest fluoride adsorption amount. The results of the investigation were summarized as follows: (1) The LMF11 NFs presented fiber morphology with an average diameter of 626 ± 57 nm.
The results showed that the spherical Fe 3 O 4 NPs were uniformly dispersed along the fiber axis without obvious agglomeration, and the total pore volume and surface area of LMF11 NFs were 0.276 cm 3 g −1 and 55.81 m 2 g −1 , respectively. The isoelectric point (pH pzc ) of LMF11 NFs was 6.47. (3) The Langmuir isotherm and PSO model were favored divulging that the F − adsorption on LMF11 NFs was seen as a single-layer chemisorption. The maximum binding amount calculated from the Langmuir isotherm model was as high as 173. .60 mg F − /g at pH = 3 at 15-45 • C. A thermodynamic study proved that the defluoridation by LMF11 NFs is a spontaneous, endothermic as well as entropy increase process. The regeneration of the F − loaded LMF11 NFs exhibited a high defluoridation efficiency of 94.76 mg F − /g at C 0 = 20 mg F − /L up to three repetitions. (4) On the basis of the isoelectric point, FTIR and PXRD analysis, it was confirmed that LMF11 NFs worked with the defluoridation mechanisms including hydrogen bonding, electrostatic attraction and ion exchange.
In summary, the fabricated LMF11 NFs possessed good defluoridation performance and high selective adsorption ability, indicating that the material has great potential as a promising adsorbent for fluoride decontamination. Further study will be conducted on the removal of phosphate, arsenate and other pollutants in the future.