Outstanding fluoride removal from aqueous solution by a La-based adsorbent

A La-based adsorbent was prepared with La(NO3)3·6H2O, 2-methylimidazole and DMF via amide-hydrolysis and used for fluoride decontamination from aqueous water. The obtained adsorbent was lanthanum methanoate (La(COOH)3). The effects of pH value, initial F− concentration and interfering ions on defluoridation properties of as-prepared La(COOH)3 were assessed through batch adsorption tests. The adsorption kinetics, isotherm models and thermodynamics were employed to verify the order, nature and feasibility of La(COOH)3 towards fluoride removal. The results imply that La(COOH)3 is preferable for defluoridation over a wide pH range of 2 to 9 without interference. Simultaneously, the defluoridation process of La(HCOO)3 accords to the pseudo-second order model and Langmuir isotherm, revealing chemical adsorption is the main control step. The maximum fluoride capture capacities of La(COOH)3 at 30, 40 and 50 °C are 245.02, 260.40 and 268.99 mg g−1, respectively. The mechanism for defluoridation by La(COOH)3 was revealed by PXRD and XPS. To summarize, the as-synthesized La based adsorbent could serve as a promising adsorbent for defluoridation from complex fluoride-rich water.


Introduction
The uoride pollution in aquatic ecosystems has drawn global concern in recent years. Excessive intake of uoride from water leads to thyroid disease, osteoporosis, skeletal and dental uorosis, and even brain damage. 1 Groundwater with high uoride is exposed in more than 25 countries such as India, American, Pakistan, Africa, Sri Lanka, Mexico, China and so on. 2,3 Accordingly, exploring an efficient and feasible uoride remediation technology is imperative. Up to now, various remediation options for deuoridation such as anion exchange, 4 membrane technology, 5 adsorption, 6 precipitationcoagulation, 7 reverse osmosis (RO), 8 and electrodialysis 9,10 were conventionally established for uoride retention. Among these water purication techniques, adsorption technology has been indicated to be an eye-catching and economical option owing to its comprehensive benets of simplicity of design, operational exibility, inexpensive expenditure, and high efficiency. A great variety of adsorbents like nano sized metal-oxide adsorbents, 11 carbon-based adsorbents, 12 glass bers, 13 montmorillonite, 2 Mxenes, 14,15 polymers, [16][17][18] clay, 19,20 and metal-organic frameworks (MOFs) [21][22][23][24] have been designed to recover uoride ions and metal ions in water. However, most traditional deuoridation adsorbents have the limitations of low removal efficiency, are highly pH-dependent over a narrow pH range or have poor selectivity.
Currently, adsorbents based on rare-earth metal element have been recognized as effective adsorbent materials. Zr MOFs based on ZrCl 4 and tetrauoroterephthalic acid exhibited good deuoridation performance over a wide pH range of 3 to 10 with the maximum uptake of 204.08 mg g −1 computed by Langmuir model. 25 Various La-based materials have been wildly utilized for remediation of phosphate, metal ions, arsenic and uoride because of the strong affinity and high selectivity of rare earth metal lanthanum (La), the biocompatible, the low cost, and environmentally friendly. [26][27][28] The La-MOF@50%PANI, which was fabricated with terephthalic acid (1,4-BDC) ligand through a one-pot technique possessed superior removal efficiency toward Pb 2+ with maximum capture uptake of 185.19 mg g −1 . 29 Yin et al. explored ve La-MOFs viz. La-BTC, La-BDC, La-BPDC, La-PMA, and La-BHTA for immobilization of uoride, the maximum uptake of uoride reached 105.2, 171.7, 125.9, 158.9 and 145.5 mg g −1 at 25°C, respectively. 3 Fe-Mg-La tri-metal nanocomposite 30 prepared by co-precipitation without calcination exhibit efficient deuoridation performance with maximum capacity of 47.2 mg g −1 . Prabhu S M et al. 31 developed La(HCOO) 3 through an acid catalyst amide-hydrolysis mechanism using lanthanum nitrate hexahydrate, benzoic acid (BA) and DMF as materials. The maximum adsorption of AsO 4 3− by La(COOH) 3 was found to be 2.623 mmol g −1 . Until now, La(COOH) 3 prepared with La(NO 3 ) 3 $6H 2 O, 2-methylimidazole and DMF via basic amide-hydrolysis mechanism and used as an adsorbent for deuoridation has never been reported. Herein, La-based adsorbent (lanthanum methanoate) was fabricated via amide-hydrolysis mechanism for abating excess F − from aqueous solution. Characterization of the as-prepared adsorbent was thoroughly evaluated by SEM, PXRD and XPS. The deuoridation property of La(COOH) 3 was quantied in detail. To elucidate the mechanism of uoride decontamination by La(COOH) 3 , the kinetics models and adsorption isotherms were systematically studied via adsorption.

Synthesis of La-based adsorbent
1.300 g of La(NO 3 ) 3 $6H 2 O and 0.825 g of 2-methylimidazole were dissolved thoroughly in 40 mL of DMF under stirring for 10 min, respectively. Then the above solutions were mixed under vigorous stirring for 30 min, and reacted at 150°C for 2 h in a 150 mL solvothermal autoclave. Subsequently, in order to eliminate the unreacted agents from pores of lanthanum methanoate, the precipitation was washed with reaction solvent and methanol several times and dried at 80°C. The preparation scheme of La-based adsorbent was displayed in Scheme 1.

Adsorption tests
The 100 mg L −1 F − standard stock solution was prepared by pouring an appropriate amount of NaF into 1 L deionized (DI) water. For batch deuoridation experiments, 0.01 g lanthanum methanoate was immersed in to 50 mL of F − solution at 30°C for 12 h in triplicate. Adsorption kinetics examinations were performed with 1 L of F − solution (10 and 20 mg L −1 calculated by the mass of uoride anions). To investigate the adsorption isotherms, the deuoridation experiments were conducted by varying the initial F − concentration (10-80 mg L −1 ) at 30, 40 and 50°C, respectively. The effect of pH adjusted by 0.1 M HCl or NaOH solution was examined in the range of 2-9 with 50 mL of 20 mg L −1 F − solution. 10-50 mg L −1 of Ca 2+ , Mg 2+ , NO 3 − , Cl − , CO 3 2− , PO 4 3− and SO 4 2− were selected as interfering ions in adsorption condition (C 0 = 10 mg L −1 , pH = 8, V = 50 mL) to identify the selectivity of lanthanum methanoate. The residual F − concentrations aer adsorption were monitored by F −selective electrode using the standard method, 2,32 and the capture capacity of lanthanum methanoate was computed according to our previous work. 11

Material characterization
The surface morphology and size of lanthanum methanoate were determined by scanning electron microscopy (VEGA300U, Tescan). The pristine and used lanthanum methanoate were veried by X-ray diffraction (D8 ADVANCE, Bruker) equipped with Cu Ka radiation in the 2q range of 10-80°at 2°min −1 . XPS measurements were examined using a Thermo Scientic K-Alpha spectrometer (Thermo Fisher, USA). A Nicolet 330 FTIR spectrometer (Thermo Fisher Scientic Ltd, USA) was used to record the Fourier transform infrared spectra (FTIR).

Adsorbent characterization
The surface morphology of La-based adsorbent is depicted in Fig. 1a. Clearly, it exhibits a strong polyhedron solid structure and aggregates in block with several small debris on the surface. The particle size of La-based adsorbent is micron with strong solid structure. Fig. 1b displays the EDX mapping of La-based adsorbent, the EDS spectrum indicates that the presence of C, O and La in the particles (Fig. 1c). The structure of the La-based adsorbent was veried by PXRD pattern and FTIR spectra ( Fig. 1d (Fig. 1d), suggesting that the La-based adsorbent is pure La(HCOO) 3 . In the spectrum of La-based adsorbent (Fig. 1e), the band occurred at 3429 cm −1 corresponds to the -OH stretching vibration. The signicant peaks appeared at 1600 cm −1 and 1426 cm −1 are indicative of C-O asymmetric and symmetric vibrations, revealing the carboxylic groups in the methanoate. 33 The bands at 2914 cm −1 and 1354 cm −1 ascribed to the -CH stretching and bending mode illustrates the presence of methanoate, 31 manifesting that the La 3+ ions are successfully coordinated with the groups of -OOCH to generate the La(COOH) 3 . The peak located at 423 cm −1 represents the La-O stretching vibration in La(COOH) 3 . 33,34 The La(COOH) 3 may be generated via basic amide-hydrolysis mechanism and the possible pathway is illustrated in Scheme S1 † in ESI. 35,36 3.2. Effects of parameters on deuoridation performance 3.2.1. Impact of the initial pH values on deuoridation. The change of pH value can not only affect the charge on the La(COOH) 3 surface but also affects the existence species of uoride in the solution. Fig. 2a illustrates the inuence of pH on deuoridation of La(COOH) 3 . It is clear that La(COOH) 3 exhibits an outstanding deuoridation efficiency over a very broad pH range of 2 to 9, the decontamination performance is more than 85 mg g −1 with original F − concentration of 20 mg L −1 in this pH range. The highest uptake of uoride is 98.2 mg g −1 at pH of 8. Even at pH of 9, the capture capacity of uoride is 96.4 mg g −1 . The removal efficiency (85.2 mg g −1 ) is relatively low at pH of 2, since part of F − exist as HF at pH of 2. 1,37 The isoelectric point (pH pzc ) of La(COOH) 3 assessed using pH dri method 38 is noted as 5.6 ( Fig. 2b), implying that the supercies of La(COOH) 3 is positively charged when pH < 5.6 which is favourable for deuoridation, and negatively charged when pH > 5.6. Hence, the uoride capture capacity maintains at a relative high level in the pH ranging from 3 to 6 because of the electrostatic attraction between the negative charged F − and the positively charged La(COOH) 3 . La(COOH) 3 is deprotonated when pH > pH pzc . However, with further increase of pH (7-9), the uoride capture capacity is higher than 96 mg g −1 and reaches the highest value at pH 8, reecting that the deuoridation of La(COOH) 3 takes place not only by electrostatic attraction, but also through ligand exchange mechanism between the F − and the -OOCH group of La(COOH) 3 adsorbent as well as non-specic electrostatic attraction 3,37 at alkaline medium. Hence, a pH of 8 is selected as the optimum pH for subsequent experiments.
3.2.2. Impact of original F − concentration. Fig. 3 presents the impact of original F − concentration in the range of 10 to 80 mg L −1 keeping adsorbent dosage xed at 0.01 g on the deuoridation of La(COOH) 3 at altered temperature for 12 h. Notably, the F − capture capacity enlarges gradually with the rise of original F − concentration range of 10 to 60 mg L −1 owing to the acceleration of the diffusion rate of F − caused by concentration gradient, and then no signicant variation in binding capacity is observed due to the equilibration of binding sites of the La(COOH) 3 at higher-uoride solutions under a constant dosage condition. Additionally, at a high uoride concentration system, the deuoridation process of La(COOH) 3 is favourable at higher temperature. Taking all the above-mentioned factors into account, it can be concluded that La(COOH) 3 prepared in this study has Fig. 2 The impact of pH on defluoridation (a) and pH pzc (b) of the adsorbent.  a signicant anti-interference ability and the inhibitory effect of PO 4 3− and CO 3 2− should be particularly concerned in practical complex engineering application. Fig. 5a depicts the time-dependent deuoridation on La(COOH) 3 under the xed adsorbent dose of 0.1 g and 1 L of F − solution with various original F − concentrations (10 and 20 mg L −1 ) at pH of 8, respectively. As seen from Fig. 5a, the binding capacity rises rapidly as time increases within 30 minutes, and then attains equilibrium at a contact time of 100 minutes. To interpret the rate-limiting step and determine the deuoridation behaviour of La(COOH) 3 , three commonly used reaction-based models 41 were conducted to t the experimentally measured data. The tting patterns and dependable factors are recorded in Fig. 5b-d and Table 1. Notably, the pseudo-second-order model (PSO) with higher tted determination coefficients (i.e., 0.9999 at 10 mg L −1 and 0.9999 at 20 mg L −1 ) provide a better description for the kinetics dates than the pseudo-rst-order model (PFO), revealing the strong chemical interaction. Meanwhile, the Q e (i.e., 95.15 and 201.21 mg g −1 at 10 and 20 mg L −1 ) of La(COOH) 3 estimated using PSO are accorded with experimental ones (94.96 and 197.10 mg g −1 at 10 and 20 mg L −1 , respectively). Additionally, the diffusion route of F − in the adsorption system is determined by Weber-Morris diffusion model. As portrayed in Fig. 5d, twoplatform stages observed in the intra-particle diffusion model mean that the deuoridation on La(COOH) 3 is consist of multiple diffusion mechanisms. The initial fast stage takes place within 30 min, the maximum adsorption rates (k id1 ) at 10 and 20 mg L −1 F − solutions are 3.4649 and 6.3507 mg g −1 min −0.5 during this period, while the adsorption rates (k id2 ) in the subordinate stage occurred between 30 and 100 minutes decline to 0.0964 and 1.3654 mg g −1 min −0.5 , respectively. Notably, none of the two-stage tting line segments pass through the origin point, which illustrates that the deuoridation process by La(COOH) 3 is governed by intra-particle diffusion as well as affected by diverse diffusion mechanisms involving surface adsorption, intra-pore diffusion, and external mass transfer.

Impact of contact time and adsorption kinetics.
3.2.5. Adsorption isotherm. Four classical adsorption isotherms including D-R, Langmuir, Temkin, and Freundlich 42-44 were applied to identify the interactions between the F − equilibrium concentration and binding uptake by La(COOH) 3 at altered temperature. As displayed in Fig. 6a- Table 2. The free energy (E) is computed using the D-R model, as depicted in Fig. 6c. From the calculations, the E values of La(COOH) 3 for Table 1 The fitted factors of kinetics models for defluoridation on La(COOH) 3

Model
Parameter   uoride adsorption are 13.3492, 13.8554 and 14.6897 kJ mol −1 at 30-50°C, respectively. It is evident that the values of E from 8 to 16 kJ mol −1 manifest that both ligand exchange and electrostatic attraction are the mainly mechanism. 3,43,[45][46][47] To portray the affinity between the F − and the La(COOH) 3 , the dimensionless constant separation factor (R L ) values calculated according to the literature 21,37 decline quickly with the increasing of initial F − concentrations in the range of 10-80 mg L −1 and approach to zero, manifesting that the adsorbing system is encouraged at even higher F − concentrations (Fig. S1 †). In addition, the values of R L drop as temperature rises and are less than 1. Hence, the deuoridation process of La(COOH) 3 occurs under favorable condition. 3.2.6. Thermodynamics studies. To explore the effect of temperature on the deuoridation of La(COOH) 3 , the thermodynamic investigation is utilized to study the deuoridation process ( Fig. S2 † and Table 3). The calculated DH 0 (41.41 kJ mol −1 ) and DS 0 (166.28 J mol −1 K −1 ) indicate that the uoride removal onto La(COOH) 3 is an endothermic nature and random state, revealing that the increase of temperature promotes the capture of uoride. DG 0 values are −10.26, −12.24 and −13.66 kJ mol −1 at 30-50°C, respectively, demonstrating that the deuoridation process on La(COOH) 3 is highly feasible and spontaneous. In addition, the deuoridation mechanism can be further elucidated by thermodynamic factors. The calculated DH 0 (41.41 kJ mol −1 ) is larger than physical adsorption (2.1-20.9 kJ mol −1 ), but less than predicted chemical adsorption (80-400 kJ mol −1 ), implying that the deuoridation on La(COOH) 3 is composed of chemisorption and physisorption process.

Adsorption mechanism via PXRD and XPS characterization
To gain deeper insight into the mechanism of deuoridation by La(COOH) 3 , PXRD and XPS analysis is recorded to verify the changes in structure of pristine and used adsorbent. Aer uoride uptake, the characteristic peaks belonged to La(COOH) 3 disappear and a series of new reections corresponded to the hexagonal phase of LaF 3 (PDF No. 32-0483) emerge in the PXRD pattern 52,54 (Fig. 7a), signifying that precipitation and ligand exchange is identied as the dominating deuoridation mechanism.
Furthermore, XPS analysis is conducted to further elucidate the structural composition of the fresh and used adsorbent. As depicted in Fig. 7b, obvious signals of C and La can be found in the survey spectrum of fresh La(COOH) 3 . As for La 3d, two peaks appeared at 835.3 and 838.4 eV are corresponded to La 3d 5/2  spin state, while peaks located at 851.9 and 855.0 eV are ascribed to La 3d 3/2 orbital. 31,55 Aer deuoridation, a new F 1s peak centred at 686.5 eV with a slight shi (∼1.8 eV) to high binding energy compared with the F 1s standard spectrum of NaF (684.7 eV) emerges in the spectrum, 56,57 revealing that uoride binding to the La(COOH) 3 (Fig. 7c). Meanwhile, the La 3d 5/2 peaks detected at 838.7 and 842.4 eV, and La 3d 3/2 peaks appeared at 855.5 and 859.0 eV shi to high binding energy direction (3.4-4.0 eV), is attributed to the bond of La-F formed via ion exchange as well as precipitation 3 (Fig. 7d). Hence, according to the isoelectric point (pH pzc ) of La(COOH) 3 , PXRD and XPS analysis, the deuoridation process onto La(COOH) 3 is signicantly controlled by precipitation, ligand exchange between La and uoride along with the electrostatic interaction, which is in agreement with the previous results obtained from kinetic and isotherm analysis. The possible deuoridation mechanism is shown in Scheme 2.

Conclusions
Lanthanum methanoate with particle size in micrometre was successfully fabricated for abating excess F − from aqueous solution. La(COOH) 3 functioned excellently over a wide pH variety from 2 to 9, with the largest decontamination performance at pH of 8. Fluoride adsorption behaviour on La(COOH) 3 conform with the PSO model and Langmuir isotherm well, reecting that single-layer chemisorption. The maximum uptakes of La(COOH) 3 for uoride achieve 245.02-268.99 mg g −1 at the temperatures of 30-50°C, respectively, which are better than those of most adsorbents based on rare earth metal elements recorded in the literature. The deuoridation mechanism of La(COOH) 3 is presided by precipitation, ligand exchange as well as electrostatic attraction. The results illustrate that the synthesized La-based adsorbent can be developed to immobilize uoride-rich water.

Author contributions
Conceptualization, investigation, writingoriginal dra preparation, Weisen Yang, Fengshuo Shi and Shaoju Jian; analysis, Wenlong Jiang; data curation, Yuhuang Chen; methodology, Kaiyin Zhang; writingreview & editing, Shaohua Jiang and Chunmei Zhang; project administration and supervision, Jiapeng Hu. All authors have read and agreed to the published version of the manuscript.

Conflicts of interest
There are no conicts to declare.