Facile synthesis of magnetic La–Zr composite as high effective adsorbent for fluoride removal

Abstract

A novel magnetic composite of La–Zr was prepared by co-precipitation method, and its fluoride removal ability was investigated in batch studies. The sample was characterized by SEM, EDS and FTIR. Influence of various factors such as pH, presence of coexisting anions, contacting time and initial fluoride concentration were studied in detail by batch sorption experiments. The equilibrium data fitted to Langmuir, Freundlich and Lan–Fre isotherm model, and the maximum sorption capacity was calculated to be 88.5 mg g⁻¹, which is higher than lots of previous adsorbents. The kinetic data can be described well by the pseudo-second-order kinetic model. It was indicated that the overall rate of fluoride sorption is likely to be controlled by chemisorption process. Based on the results, the mechanism of adsorption was discussed in detail. There is no calcination in the preparing process, so this material is considered to be lower cost as compared with some metal elements-based oxides adsorbents, and is beneficial for the practical application.

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1. Introduction

Fluorine is quite a common element in the world. Due to the high reactivity, it usually does not naturally exist in its elemental form.¹ It can be either essential or harmful to humans and animals, which depends on the total amount ingested or the concentration in drinking water.² Drinking water is a major resource of fluoride intake. The optimum fluoride level in drinking water provided by World Health Organisation (WHO) is between 0.5 and 1.5 mg L⁻¹.³ If drinking water always contains fluoride higher than 1.5 mg L⁻¹, people will be easily caught by dental and skeletal fluorosis, which is a chronic disease manifested by mottling in mild cases, softening of bones, and neurological damage in severe cases.^4,5 Fluoride will enter public water systems either from natural and/or anthropogenic sources.^6,7 Some untreated wastewater contain high concentrations of fluoride (from tens to thousands of mg L⁻¹), which are far greater than the value of 1.5 mg L⁻¹ permitted by WTO.⁸ Nowadays, fluoride is still a big problem, and the WHO standard of fluoride limit is a worldwide challenge, especially in rural villages without proper treatment.⁹

Several treatment processes such as precipitation, adsorption, ion exchange, and membrane techniques (reverse osmosis, nanofiltration, dialysis, and electrodialysis) are available for fluoride removal.¹⁰ Adsorption is considered to be one of the more acceptable defluoridation techniques, because of its easy operation, high selectivity and low cost.¹¹ Because of small size and comparatively large surface area, nanosized sorbents are well known as useful tools in pollution treatments.^12,13 However, separation of nanosized adsorbents from matrices is always a problem in practical application. The residual nano particles will become new contaminations and cause the second pollution.¹⁴ Due to being easily enriched by magnetic field, magnetic nanosized sorbents get the opportunities to overcome this dilemma and appear promising.¹⁵

The components of composite adsorbents are important to improve properties. Recently, some rare earth elements and metal ions, such as La³⁺,¹⁶ Ce³⁺,¹⁷ Al³⁺,¹⁸ and Zr⁴⁺,¹⁹ have attracted increasing attention. Because of their high electrical affinities for fluoride ion, they have become important components in defluoriding composite adsorbents.^20,21 However, most of these adsorbents were prepared either by complicated methods, such as plasma treatment,¹⁷ or under severe conditions, such as hydrothermal process.²¹ The simple methods were limited and hence required.

In the present work, we reported a novel La–Zr magnetic composite with excellent defluoridation performance by a simple co-precipitation method, furthermore, no high energy consuming calcination operation in the preparation, which was used in most of rare earth and metal oxide based adsorbents. Its fluoride removing properties were investigated in this study. Based on the results, mechanism in the adsorption course was discussed in detail.

2. Experimental

2.1. Materials and methodology

Nano Fe₃O₄, lanthanum oxide (La₂O₃, 99.99%) and sodium fluoride (analytical reagent) were purchased from Aladdin Chemical Co., China. A stock solution of fluoride (1000 mg L⁻¹ F) was prepared by dissolving NaF (2.2105 g) in deionized water and fluoride-bearing solutions were prepared by diluting the stock solution to given concentrations with deionized water. Triblock polymer PEO–PPO–PEO (P123) was purchased from Aldrich Chemical Co. Ltm. The NaCl, Na₂SO₄, NaNO₃ were used as the anion sources competing with fluoride ion in the working solution for sorption experiment. Other reagents are all analytical reagents, commercially available, and used in this study without further purified.

The electron microscopic studies of samples were performed with a scanning electron microscope (FEI Quanta 200) under pressure < 5.0 × 10⁻³ Pa. The samples were coated with gold using a Polaron Sputter Coater. The test voltage was 20.00 kV. X-ray energy dispersive spectroscopy (EDS) was conducted on (FEI-Quanta 200, USA) electron microscope. Zeta-potential analysis was conducted on a Malvern Zeta sizer Nano ZS 90 instrument. X-ray powder diffractometer (XRD, Rigaku III/B max, Cu Ka) was used to analyze the samples. The Fourier transform infrared (FT-IR) spectra of the obtained samples were recorded with ALPHA-T FT-IR spectrometer (Bruker, Germany) in the range of 4000–600 cm⁻¹. The fluoride concentrations in solution were determined by the fluoride ion selective electrode. The pH was measured with a JENCO 6175 pH meter (Shanghai Renshi electronics Co. Ltd. USA).

2.2. Preparation of La–Zr magnetic composite

2.2.1 Preparation of Fe₃O₄@SiO₂. P123 (0.4 g) and boric acid (12.0 g) were dissolved in 160 mL deionized water. After Fe₃O₄ (0.6 g) was added, the mixture was dispersed by ultrasonic. Tetraethoxysilane (2 mL) was added to the suspension and stirred at 40 °C for 24 h. The solution was transferred to four 100 mL hydrothermal reaction kettles (Henghua, China; Teflon inner tank, steel shell), around 40 mL solution in each kettle, and reacted at 100 °C for 24 h, then cooled at room temperature. The as-obtained samples were washed with ethanol and deionized water in turn for three times. The products were collected by a magnetic field and dried at 40 °C under vacuum for 24 h to get Fe₃O₄@SiO₂.

2.2.2 Preparation of La–Zr magnetic composite. Fe₃O₄@SiO₂ (0.5191 g), 1 mol L⁻¹ La(NO₃)₃ (3.34 mL) and 0.2694 g ZrOCl₂·8H₂O were added in 160 mL deionized water and stirred at room temperature for 2 h. 0.8440 g H₂C₂O₄·2H₂O was dissolved in 5.72 mL deionized water and dripped into the above mixture with stirring. After that, 0.83 mL NH₃·H₂O was dripped into the mixture and reacted 2 h at 20 °C. The powder in the mixture was separated by magnetic field, and then washed by deionized water and ethanol in turn, until no Cl⁻ being detected by 0.5 wt% AgNO₃. After that, the powder was collected by magnetic field, and dried 13 h at 60 °C to get La–Zr magnetic composite.

2.3. Sorption experiments

Sorption experiments were carried out in batch conditions.

The adsorption isotherms were studied with an adsorbent dose of 1.00 g L⁻¹ at 25 °C. Typically, 0.02 g La–Zr magnetic composites were added to 20 mL fluoride solutions of several initial concentrations (20–300 mg L⁻¹). The solutions were shaken at 150 rpm for 24 h to achieve equilibrium, then separated by magnetic field. The residual fluoride concentration in solution was determined by the fluoride ion selective electrode PF-202-CF (Shanghai Leici Instrument Factory, China). The fluoride adsorption capacities at equilibrium of the adsorbents were calculated according to eqn (1):

where C₀ and C_e are the initial and equilibrium concentrations of fluoride ion (mg L⁻¹), respectively. V is the volume of the solution (mL) and m is the amount of adsorbent (g).

Kinetics experiments were conducted at 25 °C. 0.25 g La–Zr magnetic composites were immersed in 250 mL fluoride-bearing solutions with the initial concentrations of 50, 100, 300 mg L⁻¹, respectively. The mixtures were oscillated at 150 rpm and approximately 3 mL were taken from the suspension at predetermined times and quickly separated to measure the corresponding fluoride concentration, in order to calculate the time-dependent adsorption capacities.

The effects of different pH values on fluoride sorption were studied by adjusting the pH of solution using either 0.1 mol L⁻¹ NaOH or 0.1 mol L⁻¹ HCl solutions to the required pH range of 1–10. The effects of pH on fluoride adsorption were investigated using different initial fluoride concentrations of 10 mg L⁻¹, 25 mg L⁻¹ and 40 mg L⁻¹, respectively, with 20 mg adsorbent and 20.00 mL fluoride solutions. The effects of co-existing ions (Cl⁻, NO₃⁻ and SO₄²⁻) on fluoride adsorption were performed with an adsorbent dose of 1.0 g L⁻¹ and initial fluoride concentration of 10 mg L⁻¹ at 25 °C. The competing anions were set at six concentration levels of 0, 6, 12, 18, 24 and 30 mg L⁻¹, respectively. The mixed suspensions were oscillated at 150 rpm for 24 h at 25 °C.

3. Results and discussion

3.1. Physicochemical characterization

The composite adsorbent can be easily enriched by magnetic field due to its magnetism, as shown in ESI Fig. S1.† Fig. S1† indicates behavior of the composite in the absence and presence of external magnetic field to show its magnetic property. The magnetic property apparently facilitates practical application. Fig. 1 shows SEM images of Fe₃O₄@SiO₂ (Fig. 1(a)), La₂(C₂O₄)₃·10H₂O (Fig. 1(b)) and La–Zr magnetic composite (Fig. 1(c)). One can see from Fig. 1(c) that three significantly different kinds of morphologies are existent in the composite. They are spherical, prismatic and amorphous, respectively. As compared with Fig. 1(a) of Fe₃O₄@SiO₂ SEM, the spherical ones are attributed to nano Fe₃O₄@SiO₂ particles, and their diameter are around 300 nm according to the scale bar; as compared with Fig. 1(b) of La₂(C₂O₄)₃, the prismatic ones are attributed to La₂(C₂O₄)₃·10H₂O particles. The particles are around 1 μm to 10 μm long according to Fig. 1(b). EDS spectrum of La–Zr magnetic composite (Fig. S2(a)†) indicates the ratio of atom% of La, Zr, C, O, Fe, Si is 3.31 : 2.49 : 18.93 : 66.56 : 4.12 : 4.59. EDS spectrum of La–Zr magnetic composite after adsorbed F⁻ (Fig. S2(b)†) indicates the ratio of atom% of La, Zr, C, O, Fe, Si, F is 2.33 : 2.00 : 14.69 : 37.45 : 8.37 : 24.58 : 10.58. It confirms the adsorption of fluoride.

Fig. 1 SEM of (a) nano Fe₃O₄@SiO₂, (b) La₂(C₂O₄)₃·10H₂O, (c) ZrO(OH)₂ and (d) La–Zr magnetic composite.

Samples of La–Zr magnetic composite before and after fluoride adsorption were characterized by FTIR, as shown in Fig. 2. In the spectrum before adsorption, the peak at 1317 cm⁻¹ can be assigned to the bending vibration of Zr–OH groups.²² After adsorption, this peak is significantly reduced. This confirms that fluoride has replaced a substantial faction of surface hydroxyl groups bound to zirconium. A new peak at 1448 cm⁻¹ appears in the FTIR spectrum after adsorption, that may be due to the formation of M–F bonds after fluoride adsorption (M = Zr or La).²⁰ The significant blue shift of the carbonyl group peak from 1615 cm⁻¹ to 1639 cm⁻¹ might also be related to the possible bonding of F⁻ to the La–Zr oxalate. The results suggested that during the adsorbed fluoride ions process, lanthanum combined with fluoride ions.

Fig. 2 FTIR spectra of La–Zr magnetic composite before and after F⁻ (300 mg L⁻¹) adsorption.

The XRD pattern of composite (Fig. 3) has typical characteristic peaks of La₂(C₂O₄)₃·10H₂O (JCPDS card no. 49-1255) and Fe₃O₄ (JCPDS card no. 75-0033), however, there are no obvious peaks of zirconium. It suggests that zirconium exists in the composite as an amorphous form, because it was prepared without high-temperature calcination. Similar results were observed and reported elsewhere.^31,32

Fig. 3 XRD patterns of La–Zr magnetic composite and Fe₃O₄.

3.2. Effect of pH on defluoridation

The effects of pH on fluoride removal by La–Zr magnetic composite with various initial concentrations were investigated. The pH of solution was controlled to be 2–10 by adding HCl and NaOH solution. As shown in Fig. 4, the adsorption capacities are increased with rise of pH from 2 to 3. After around pH = 3, the adsorption capacities don't increase any more, but keep stable or decrease a little at first and then keep stable, depending on the initial fluoride concentration. The phenomenon is easily understood based on the results of zeta-potential charge analysis. It can be seen in Fig. 5, the pH_zp is 3. It means at pH < 3, surface charge of the adsorbent is positive and the adsorption of anionic fluoride is favorable. At pH > 3, the surface becomes negative and the adsorption of anionic fluoride becomes unfavorable according to the electrostatic attraction. Nonetheless, there is still appreciable amount of fluoride adsorbed on the composite under the condition of pH > 3. The results suggest that there are other factors (such as ion diffusion etc.) affecting the behavior of adsorbate at the vicinity of the adsorbent. In addition, the adsorption capacities decrease with the decrease of pH from 3.0 to 2.0. This might be due to the formation of HF (aqueous solution):

H⁺ + F⁻ = HF (aq)

Fig. 4 Effects of pH on the removal of fluoride by La–Zr magnetic composite with initial concentrations of 10 mg L⁻¹, 25 mg L⁻¹ and 40 mg L⁻¹, respectively.

Fig. 5 The pH-zeta potential curve of the composite adsorbent.

It results in lower available fluoride concentration than the actual concentration adsorption on La–Zr composite in very low pH conditions. The similar phenomenon was observed by other workers as well.²² Although the adsorbent at pH ≈ 3 has shown the maximum adsorption capacities, it is not normal in most natural conditions and destructive to fluoride ion selective electrode.²³ So, the following works are performed at nearly neutral condition without adjusting pH.

3.3. Effects of competing anions

One of the major problems which limit wide application of sorption method to eliminate pollutants is ion selectivity. Fig. 6 shows the effects of competing anions such as Cl⁻, NO₃⁻ and SO₄²⁻ onto La–Zr magnetic composite. The experiments were conducted under a binary system, where contained 10 mg L⁻¹ fluoride ions paired with 0, 6, 12, 18, 24, 30 mg L⁻¹ of each competing anion. As shown in Fig. 6, it indicates that the defluoridation capacity of the composite shows decrease in uptake of fluoride ion as the concentration of competing anions increases. The presence of competitive anions will affect the adsorption capacity in different extent, depending on the species. In which, Cl⁻ has the least effects. The composite remains ∼87% defluoridation capacity with the highest Cl⁻ coexisting. SO₄²⁻ has the maximum effects. The composite remains ∼55% defluoridation capacity with the highest SO₄²⁻ coexisting.

Fig. 6 Effect of competing anions on F⁻ adsorption by La–Zr magnetic composite.

3.4. Adsorption isotherms

To understand the adsorption performance and adsorption mechanism, adsorption isotherm of fluoride adsorption on La–Zr magnetic composite was investigated. 0.02 g La–Zr magnetic composite was contacted with 20.00 mL fluoride solution whose concentrations in the range of 20–300 mg L⁻¹. Each sample was allowed to equilibrate for 24 h at 298 K, with constant shaking at 150 rpm, prior to analysis for the final F⁻ concentration.

Langmuir, Freundlich and Lan–Fre isotherm models shown as below:^24–28

Q_e = K_FC_eⁿ (3)

where C_e is the equilibrium concentration (mg L⁻¹), Q_e is the amount sorbed at equilibrium (mg g⁻¹), Q_m is the calculated maximum sorption capacity (mg g⁻¹), K_L is the Langmuir constant related to the energy of adsorption (L mg⁻¹), K_F and n are the Freundlich constant related to the sorption capacity of the adsorbent (mg g⁻¹) (L mg⁻¹)^1/n and the adsorption intensity, respectively. K_FL (L mg⁻¹)ⁿ is Langmuir–Freundlich constants related to the sorption capacity of the adsorbent.

Langmuir isotherm is a valid monolayer sorption on a surface containing a finite number of binding sites, and assumes the energy of adsorption is constant. The Freundlich isotherm describes the multilayer adsorption of heterogeneous systems and assumes that different sites have several adsorption energies involved. Langmuir–Freundlich isotherm is also known as Sip's equation. The model is valid for localized adsorption without adsorbate–adsorbate interactions. The data fitting to Langmuir, Freundlich and Lan–Fre isotherm models are shown in Fig. 7. The calculated isotherm parameters along with regression coefficients are given in Table 1. It indicates that the maximum sorption capacity (Q_m) of La–Zr magnetic composite for fluoride is found to be 126.03 and 88.50 mg g⁻¹ by Langmuir and Lan–Fre isotherm models, respectively. The comparison of the Q_m values with those of other reported adsorbents shows that La–Zr magnetic composite is effective for removal of F⁻, as shown in Table 2. It is observed that the data fitted Lan–Fre isotherm model the best, according to its highest regression coefficient (R²) obtained.

Fig. 7 Comparison between the measured and modeled isotherm profiles of La–Zr composite adsorption F⁻.

Table 1 Isotherm parameters for the removal of F⁻ by La–Zr magnetic composite at 298 K

Isotherm model	Parameter
Langmuir	Q0 (mg g^−1)	126.03
	KL (L mg^−1)	0.0225
	R^2	0.7792
Freundlich	KF [(mg g^−1) (L mg^−1)^1/n]	12.524
	1/n	0.409
	R^2	0.6643
Lan–Fre	Qm (mg g^−1)	88.50
	KL (L mg^−1)	5.67 × 10^−8
	n	5.144
	R^2	0.9794

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Table 2 Comparison of maximum adsorption capacity (Q_m) of different adsorbents towards fluoride ion

Adsorbent	Qm (mg g^−1)	References
Zr–Mn composite	3.05	1
MgAl–CO3 LDHs	141.64	2
Polypyrrole/alumina (Ppy–AlO)	8.00	3
Fe–Al–Ce hydroxide nano-adsorbent	4.46	5
Alginate bioencapsulated nano-hydroxyapatite	4.97	7
Zr(IV) loaded alginate	18.05	8
Titanium and lanthanum oxides impregnated on granular activated carbon (TLAC)	27.8	9
Li–Al layered double hydroxides (LDHs)	47.24	10
Alginate entrapped Fe(III)–Zr(IV) binary mixed oxide	0.98	11
Fe3O4@Al(OH)3	88.48	20
CeO2–ZrO2 nanocages	175	21
Fe–Al-impregnated granular ceramic adsorbent	3.56	23
Biocarbon–Mg/Al layered double hydroxides	83.30	26
Ce(IV) + Zr(V) mixed oxide	19.50	27
Cellulose@hydroxyapatite nanocomposites	4.20	28
Alumina supported carbon composite	37	29
Nano zirconium chitosan composite	96.58	30
Zirconium-ion oxide	9.80	31
Zirconium impregnated cellulose	4.95	32
Cocus nucifera midribs	18.50	33
La–Zr magnetic composite	88.50	Present study

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3.5. Adsorption kinetics

Fig. 8(a) shows the plots of fluoride adsorption kinetic data obtained at three different initial concentrations and at 293 K on La–Zr magnetic composite. It shows that the adsorption almost reach equilibrium at 60 min.

Fig. 8 (a) Adsorption capacity of fluoride vs. time with initial concentration of 50 mg L⁻¹, 100 mg L⁻¹ and 300 mg L⁻¹, (b) pseudo-first order kinetic, (c) pseudo-second order kinetic for the removal of fluoride ion by La–Zr magnetic composite.

In order to explain the experimental data, the time-dependent adsorption data have been analyzed using the linear form of the pseudo-first-order kinetic equation and pseudo-second-order kinetic equation.

The pseudo-first-order kinetic model of Lagergren is given as:²⁷

ln(Q_e − Q_t) = ln Q_e − k₁t (5)

where k₁ is the pseudo-first-order rate constant of adsorption (1/min), Q_e and Q_t are the amounts of fluoride adsorbed at equilibrium (mg g⁻¹) and at time t (min), respectively. A linear form plot (Fig. 8(b)) of contacting time t versus ln(Q_e − Q_t) gives the slope of k₁. The values of k₁ and Q_e, and the regression coefficients evaluated from the pseudo-first-order linear portions are presented in Table 3. As shown in Table 3, the regression coefficients for the first-order-kinetic model obtained at all the studied concentrations are relatively high (R² > 0.9). However, the calculated Q_e values are not reasonable, because they are too small as compared with those of experimental Q_e.

Table 3 Kinetics data of La–Zr magnetic composite adsorbed fluoride ion

Initial F^− concentration (mg L^−1)	Qe exp (mg g^−1)	Pseudo-first order			Pseudo-second order
		Qe cal (mg g^−1)	k1 (1/min)	R^2	Qe cal (mg g^−1)	k2 (g mg^−1) (1/min)	R^2
50	26.80	12.03	0.01431	0.9476	26.25	4.61 × 10^−4	0.9949
100	66.44	24.53	0.01346	0.9377	64.94	2.28 × 10^−5	0.9980
300	100.02	10.08	0.01023	0.6753	98.04	1.19 × 10^−6	0.9998

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The pseudo-second-order kinetic equation, which is more likely to predict the behavior over entire sorption period when chemical sorption being the rate-controlling step, is given as:³⁴

where k₂ is the pseudo-second-order rate constant of adsorption (g mg⁻¹ min⁻¹), Q_e and Q_t are the amounts of fluoride adsorbed at equilibrium (mg g⁻¹) and at time t (min), respectively. Fig. 8(c) shows the linear plots of the pseudo-second-order equation. The plots are found to be linear over all the time with good regression coefficients (R² > 0.99), confirming the applicability of the pseudo-second-order kinetic model. The pseudo-second-order rate constant (k₂) and equilibrium capacity (Q_e) determined from the slopes and intercepts of the plots (Fig. 8(c)) are listed in Table 3. The calculated Q_e values are very close to the experimental Q_e values (Table 3). It indicates that the adsorption of fluoride on La–Zr magnetic composite is more appropriately described by the pseudo-second-order kinetic model. The rate of adsorption decreases with increasing solute concentrations. As shown in Table 3, with the increase of initial fluoride concentration, k₂ values decrease but Q_e values increase.

3.6. Mechanism of fluoride adsorption

Revealing the adsorption mechanism plays an important role in understanding the material characteristics and designing new adsorbent. To understand the mechanism, the equilibrium data are fitted to Dubinin Radushkevich model, which is expressed as follows:

ln Q_e = ln Q_m − Kε² (7)

where Q_e is the equilibrium amount of solute adsorbed (mg g⁻¹); Q_m is the adsorption capacity; K is the constant related to adsorption energy (mol² J⁻²); T is 293 K; C_e is the equilibrium concentration of fluoride ions in solution (mg L⁻¹); R is the gas constant (8.314 J mol⁻¹ K⁻¹); ε can be calculated from eqn (8).³⁰ The mean free energy of adsorption (E), defined as the free energy change when one mole of ion is transferred to the surface of the solid from infinite in solution,³⁰ can be calculated from the K value using the eqn (9) as

E = (2K)^−0.5 (9)

It is considered that sorption process with values of E in the range of 8–16 kJ mol⁻¹ is governed by ion exchange. In the case of E < 8 kJ mol⁻¹, sorption is mainly affected by physical forces. In the case of E > 16 kJ mol⁻¹, sorption process is dominated by particle diffusion course.³⁴ If E is in the range of 40–400 kJ mol⁻¹, sorption process is mainly chemical sorption.³⁵

A plot of ln Q_e against ε² (Fig. 9) gives a straight line with slope (−K = −1.36801 × 10⁻⁴ mol² J⁻²), intensity (ln Q_m = 4.70) and R² = 0.9314. The E value in the present system is found to be 60.46 kJ mol⁻¹. It confirms that in the sorption process, concentrations of both adsorbate and adsorbent are involved in rate-determining step, and the adsorption may be a chemical sorption or chemisorption.^33,36

Fig. 9 Dubinin–Radushkevich (D–R) equation plot at 25 °C.

Lack of calcination, magnetic La–Zr composite is obviously not a mixed oxide of ZrO₂ and La₂O₃. It is supposed to be a mixture of ZrO(OH)₂,³⁷ Fe₃O₄ and La₂(C₂O₄)₃·10H₂O. Being calcinated or not is the important difference between this composite and some rare earth and metal elements-based oxides, which were reported as fluoride adsorbents, such as Fe–Al–Ce trimetal oxide,³⁸ mixed rare earth oxides,³⁹ Ce(IV)–Al(III) binary oxide,¹⁷ La–Al–scoria,¹⁶ Al(III)–Zr(IV) binary oxide.⁴⁰ Calcination is an operation consuming time and high energy. Our study provides a simple method to get high performance adsorbent without calcination, that will effectively reduce the cost, and be beneficial to the practical application.

4. Conclusions

This study demonstrates that La–Zr magnetic composite shows excellent fluoride removal performance. The adsorption isotherm can be described very well by Lan–Fre isotherm model, and the maximum capacity is calculated to be 88.50 mg g⁻¹. Although the saturated adsorbent would be hard to be defluorinated and regenerated due to its adsorption mechanism being considered as chemosorption, the high adsorption ability and easy operation still put it in the list of promising material to deal with fluoride polluted waters in large scale.

Acknowledgements

Authors are thankful to National Natural Science Foundation of China (51368044, 51568051, 51238002), the Science and Technology Supporting Program of Jiangxi Province (20151BBG70018), the Project Program of Shanghai Academy of Spaceflight Technology (SAST201459), the Open Project Program of Advanced Laboratory of POPs Cyclic Utilization and Control (ST201422015), the Open Project Program of State Key Laboratory of Food Science and Technology, Nanchang University (No. SKLF-KF-201417).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra27929a

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