Synthesis of LiâAl Layered Double Hydroxides (LDHs) for Efficient

Article pubs.acs.org/IECR

Synthesis of Li−Al Layered Double Hydroxides (LDHs) for Efficient Fluoride Removal Tao Zhang,†,‡ Qiurong Li,*,† Haiyan Xiao,† Hongxiao Lu,† and Yuming Zhou‡ †

College of Environmental and Chemical Engineering, Hebei Key Laboratory of Applied Chemistry, Yanshan University, Qinhuangdao 066004, China ‡ School of Chemistry and Chemical Engineering, Jiangsu Optoelectronic Functional Materials and Engineering Laboratory, Southeast University, Nanjing 210089, China ABSTRACT: A novel adsorbent of Li−Al layered double hydroxides (LDHs) was prepared through the precipitation of metal nitrates and further applied to remove excessive fluoride ions from water. The physical and chemical properties of synthesized materials were examined by powder transmission electron microscopy (TEM), X-ray diffraction (XRD) analysis, N2 adsorption/ desorption analysis, and thermogravimetric analysis (TGA). The TEM results indicated that the materials synthesized via coprecipitation present a preferential orientation of the nanoscale LDH platelets. The XRD analysis confirmed that the synthesized products have a highly crystalline nature and a well-ordered layer structure. The high specific surface areas (37.24− 51.27 m2/g) of calcined products were demonstrated to be beneficial for the adsorption of fluoride. For the adsorption experiment, the effects of the adsorption conditions including pH, coanions and adsorbent dose were investigated at initial fluoride concentration of 20 mg/L. The kinetics and isotherms of fluoride adsorption by calcined Li−Al LDHs were studied. The results indicated that the Li−Al LDHs can be effectively used to remove fluoride from water, where the maximum percentage removal (97.36%) could be reached and the adsorption equilibrium could be attained within 1 h. The kinetic data were well-fitted to the pseudo-second-order model, while the Freündlich isotherm model provided the better correlation of the equilibrium data. Based on FT-IR and kinetic analysis, the “memory effect” may play an important role in the early adsorption stage, while the ionexchange process may control the adsorption rate at the second adsorption stage. Since the Li−Al LDHs show excellent fluoride removal efficiency, they are expected to separate fluoride from water in pollution control. developing new materials for low-concentration fluoridated water treatment. Layered double hydroxides (LDHs), which are also known as lamellar compounds, have attracted considerable attention in the recent decade for their application in water pollution control.7−9 Because of the metal hydroxide layer having the positive surplus charges, they can adsorb some anions through electrostatic interactions.10 Hence, LDHs have also attracted attention as eco-friendly materials, because they are good adsorbents for the removal of various harmful anions.11,12 In recent years, several researchers have used LDHs as a controlled release drug delivery system.13−15 The intensity of van der Waals forces between host LDHs and intercalated drugs is much weaker than the chemical bond and coordination bond, which results in the release of drugs. Similarly in the adsorption system, the adsorbate may be released after adsorption. Hence, the stable fluoride adsorption capacity should be considered for LDHs materials in actual drinking water treatment. If the ionic radius of layer metal cations is very small, the strong intermolecular attraction of anions and layers may exist in the form of a chemical bond. The ionic radius of the Li+ (0.060 nm) is close to ionic radius of Mg2+ (0.065 nm), resulting in the similar nature of Mg2+ and Li+. Hence, Li+ and

1. INTRODUCTION Fluoride is one of the most toxic elements in drinking water when the concentrations for human consumption exceed the World Health Organisation (WHO) guideline of 1.5 mg/L.1,2 Fluoride contamination of groundwater has led to serious endemic of fluorosis in many parts of world, particularly in north and northeast China, India, Africa, and Mexico.2 It is estimated that 200 million people are drinking groundwater with fluoride concentrations above 1.5 mg/L.1 Several millions of people in northern China are suffering from fluorosis, so the removal of fluoride is extremely important for actual environmental pollution control. Several treatment processes such as precipitation, adsorption, ion exchange, and membrane techniques (reverse osmosis, nanofiltration, dialysis, and electrodialysis) are available for fluoride removal from aqueous solutions.3−6 Among the various presented technologies, adsorption is a widely used technique for environmental pollution management, because of its advantages, such as simple operation and low cost. The main disadvantage of adsorption has a very low removal efficiency in actual drinking water treatment, because the adsorption capacity decreased with the decrease of fluoride concentration for most adsorption systems. The question of the concentration of high-fluoride ground waters is normally below 20 mg/L,2 and the final fluoride concentration must below 1.5 mg/L after adsorption according to the WHO guideline. Considering adsorption capacity and removal efficiency, there is considerable interest in © 2012 American Chemical Society

Received: Revised: Accepted: Published: 11490

April 1, 2012 July 25, 2012 August 10, 2012 August 10, 2012 dx.doi.org/10.1021/ie300863x | Ind. Eng. Chem. Res. 2012, 51, 11490−11498

Industrial & Engineering Chemistry Research

Article

Figure 1. Transmission electron microscopy (TEM) micrographs of Li−Al LDHs: (A, B) CLDHs and (C, D) HPLDHs.

F− may have as strong a intermolecular attraction as Mg2+. Mg−Al LDHs have a typical layer structure, which are the most widely used as adsorbents for removal harmful anions. Compared to Mg−Al LDHs, a unique compound in the LDHs that contains monovalent and trivalent matrix cations ([LiAl2(OH) 6]+A −·mH2O) has been prepared in many literature reports.16−18 However, there is no sufficient information about the application of Li−Al LDHs as environmental materials in the literature. Because of the strong intermolecular attraction between the fluoride ion and rareearth elements, rare-earth materials or rare-earth-modified materials have been widely used for defluoridation.19−21 Thus, the use of Li−Al LDHs for defluoridation seems very attractive from the point of view of strong intermolecular attraction. Although there have been many reports concerning the structure of Li−Al LDHs.16,18,22 We are aware that the applications of Li−Al LDHs for the removal of harmful anions are limited. Hence, the aim of this study was about the synthesis of Li−Al LDHs and their application for the adsorption of F− ions from water. The adsorption parameters, isotherms, and kinetics of fluoride were described.

dried sodium fluoride (NaF), and other fluoride test solutions were prepared via subsequent dilution of the stock solution. The total ionic strength adjustment buffer solution (TISAB) was prepared by NaNO3 and C6H5Na3O7·2H2O at pH ranging from 5 to 6. 2.2. Preparation of Li−Al LDHs. In current study, the coprecipitation method and homogeneous precipitation method were used to prepare Li−Al LDHs. The detailed synthesis procedures were carried out by following the reported method with a modification.10,16 (a) Coprecipitation: The Li−Al LDHs with molar ratio of 2 were synthesized according to the process described by Valente et al.10 In order to avoid the coexistence of CO32− ions, the ammonia solution (10 wt %) was used as the precipitant to prepare LDHs. The precipitation was performed at 90 °C under nitrogen atmosphere. The resulting precipitate was left 48 h at 90 °C for aging, after which the slurry was filtered and washed three times with distilled water. The final products prepared by coprecipitation method are denoted as CLDHs. (b) Homogenous Precipitation: The Li−Al LDHs were prepared by the urea precipitation method described by Britto et al.16 The initial mixture with molar ratio Al/Li/ urea = 2:1:10 was stirred for ∼20 min, and then was aged at 90 °C for 48 h. After the reaction, the resulting slurry was cooled to room temperature and washed thoroughly with distilled water. Samples that were synthesized via the homogeneous precipitation method are denoted as HPLDHs. 2.3. Characterization of Li−Al LDHs. The surface physical properties of the LDHs were analyzed by a surface-

2. MATERIALS AND METHODS 2.1. Reagents and Chemicals. The analytical-grade chemicals and distilled water were used to prepare all solutions used in this study. Starting materials Al(NO3)3·9H2O and LiNO3 used in the present study were obtained from Tianjin Guangfu Fine Chemical Research Institute. NaF, NaNO3, NaOH, HCl, NH3·H2O, and C6H5Na3O7·2H2O were purchased from Tianjin Windship Chemistry Technological Co., Ltd. A 200 mg/L F− ion stock solution was prepared using 11491

dx.doi.org/10.1021/ie300863x | Ind. Eng. Chem. Res. 2012, 51, 11490−11498


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distribution was controlled by the nucleation rate. The nucleation rate was determined by the rate of urea hydrolysis. Because of the lower nucleation rate, larger particles were formed during the aging process. This may explain the bigger size of HPLDHs, compared to CLDHs. As we know, adsorption is a typical surface interaction between an adsorbent and an adsorbate. The structures and surface charge of materials have a great impact on adsorption capacity. Hence, the LDHs with different surface morphology may have different fluoride removal efficiency. 3.1.2. XRD Analysis. The X-ray diffraction (XRD) patterns of as-synthesized LDHs and calcined products are shown in Figure 2. The reflections of synthesized products (see Figures

area analyzer (Model NOVA-4000e), the crystalline structure of the LDHs was characterized using an X-ray diffraction analyzer (Model D/MAX-2500/PC), the surface morphology of the samples was monitored with TEM (JEM-2010), the presence of functional groups in samples were determined from FT-IR (Nicolet is10) spectra before and after calcination, and the thermal decomposition of samples was investigated by thermogravimetry−differential thermal analysis (Model DTG60). 2.4. Sorption Experiments. In this study, the concentration of fluoride solution was measured by a fluoride ionselective electrode (PF-1). Batch adsorption studies were carried out as follows: 0.1 g of the calcined LDHs was added into 50 mL of sodium fluoride (NaF) at different initial concentrations. The effects of the adsorption conditions, including pH, coanions, and adsorbent dose, were investigated at an initial fluoride concentration of 20 mg/L. The time required for adsorption conditions was 24 h. The effect of initial pH on the adsorption was studied from pH 2 to 13. The effect of coanions was carried out in the presence of various competing anions. And the effect of adsorbent dose was investigated at fixed pH by varying the adsorbent dose between 0.01 g and 0.50 g. The amount of fluoride adsorbed at equilibrium (qe) was calculated by eq 1:

qe =

(Ce − C t)V m

(1)

where qe is the adsorption capacity (mg/g) at equilibrium, and Ce and Ct are the equilibrium concentration and concentration at time t (mg/L), respectively. V is the volume (mL) of solution, and m is the mass (g) of adsorbent used. The amount of fluoride adsorbed at time t (qt) was calculated by the following equation: qt =

(C 0 − C t )V m

Figure 2. X-ray diffraction (XRD) patterns of as-synthesized LDHs and calcined products ((a, b) LDHs derived from coprecipitation and homogeneous precipitation, respectively, and (c, d) calcined products of the LDHs derived from coprecipitation and homogeneous precipitation, respectively).

(2)

where qt is the adsorption capacity (mg/g) at time t and C0 is the initial concentration.

2a and 2b) show sharp and symmetric peaks, even at a high angle range, indicating the highly crystalline nature of the samples. The XRD pattern of synthesized products agrees well with the peaks of standard LDHs (LiAl2(OH)7·xH2O, Joint Committee on Powder Diffraction Standards (JCPDS) File Card No. 31-0704), which confirms that the synthesized products were Li−Al LDHs. Meanwhile, the (2θ) angles at 11.68° and 23.59° correspond to the 003 and 006 crystal planes, which confirms that synthesized products have a typical and well-ordered layer structure. Compared with the peaks of CLDHs and HPLDHs, an asymmetrical peak (111) can be found at Figure 2b. Figure 2 also shows that the CLDHs display more sharp peaks appearing in relative angle. The patterns of calcined samples (Figures 2c and 2d) show that layered structure of the synthesized products is completely destroyed and indicate only aluminum oxide peaks. It can be seen that the peaks of calcined products became broader and the intensity of the peaks decreased in comparison with the uncalcined LDHs. This may be attributed to the formation of the amorphous mixed oxides. There are no phases of lithium-containing compounds on the patterns, which may be ascribed to the lithium being uniformly dispersed in the aluminum oxide. As we know, the Li−Al LDHs were derived from Al(OH)3 (bayerite),24 while the Mg−Al LDHs were derived from Mg(OH)2 (brucite).25 It is interesting to find that the phase of

3. RESULTS AND DISCUSSION 3.1. Characterization of the Li−Al LDHs. 3.1.1. TEM Analysis. Figure 1 shows the surface morphology of the Li−Al LDHs prepared by different methods. The representative micrographs of CLDHs and HPLDHs are presented in Figures 1A and 1B and Figures 1C and 1D, respectively. It can be seen from Figures 1A and 1B that the morphology of CLDHs was dominated by nanosheet structures, and the surfaces of the LDH platelets were smooth. We can clearly see the nanoscale LDH platelet with visible edges, where the layer structure was clear. But the diameters of the LDH platelets were not uniform; the majority of the particle size distribution of CLDHs was ∼30−60 nm. Since the LDHs layer has surplus positive charges, the layer structure might play an important role in fluoride adsorption. Figures 1C and 1D clearly show that the HPLDHs showed some aggregation, and their particle sizes were much larger than 100 nm. It also could be noticed that the layer structure is unclear. Larger particle sizes were obtained using the homogeneous precipitation method, relative to those obtained via other methods, because the hydrolysis of urea proceeds very slowly, which leads to a low degree of supersaturation during precipitation.23 The particle size 11492



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Table 1. Physical and Chemical Properties of Li−Al LDHs Prepared by Different Methods preparation method

dehydroxylation temperature (°C)

weight loss (%)

coprecipitation homogeneous precipitation

240−342 170−374

29.23 38.53

the calcined LDHs is dependent on the derived material. The XRD patterns of calcined Mg−Al LDHs show only a magnesium oxide phase, which has been previously reported by Zhu et al.26 3.1.3. Surface Area and Porosity Study. The calcination of LDHs often produces very reactive mixed oxides according to the literature.27,28 Moreover, thermal activation at moderate temperatures can produce a high surface area and a high degree of microporosity. From the thermal analysis, the dehydroxylation temperatures and weight loss of the two different types of LDHs are summarized in Table 1. The weight loss was attributed to the dehydroxylation of crystalline water and the interlayer anions. It can be seen that the HPLDHs have a wider range of dehydroxylation temperatures, compared to CLDHs. This can be attributed to the different content of interlayer water and anions. It is also noticed that the HPLDHs have higher dehydroxylation temperatures. This may be partly attributed to the carbonate was in the interlayer. The high temperature is needed to decompose the interlayer anions. During the calcination process, the Li−Al LDHs were decomposed into amorphous mixed oxides. The calcined LDHs will undergo a spontaneous rehydration (reconstruction) of the layered structure in an aqueous environment, which is called the “memory effect”.29 Through the memory effect, the F− ions can insert themselves into the crystalline structure more easily. The specific surface areas and pore size distribution of the sample were determined via the Brunauer−Emmett−Teller (BET) and Barrett−Joyner−Halenda (BJH) methods, respectively. The detailed results from the surface area and pore structure studies are also summarized in Table 1. The characteristics revealed that the calcined samples have large specific surface areas and pore structure, resulting in their excellent adsorption capacity for fluoride. The HPLDHs presents a surface morphology in the form of large size, possibly with a low specific surface area. However, the surface area and pore volume were larger than that of the CLDHs, as shown in Table 1. This phenomenon was due to the special pore structure of the samples. During the calcination process, the brucite-like layers will be destroyed through the breakup of the crystal structure, leading to the formation of the porous structure in the interlayer of HPLDHs. It will cause a considerable increase of the surface area. The total pore volumes of the two samples were calculated as 0.148 and 0.266 cc/g, respectively. The average pore diameter of CLDHs is 24.12 nm, whereas the average size of the HPLDHs is 24.18 nm (the so-called “mesoporous” size). 3.1.4. FT-IR Analysis. The FT-IR spectra are very helpful in the study of LDHs, especially for those containing interlayer anions. Figure 3 shows the FT-IR spectra of the pristine LDHs and calcined samples. As seen from the CLDHs in Figure 3a, the appearance of two sharp peaks at 3700−3500 cm−1 was attributed to the stretching N−H stretching for the as-prepared sample. This suggests that the obtained LDHs are doped by the NH4+ species. The broad adsorption band at 3436 cm−1 and small band at 1630 cm−1 could be ascribed to stretching vibrations and bending vibration of hydroxyl group. A sharp

BET surface area (m2/g) pore volume (cc/g) 37.24 51.27

0.148 0.266

average pore diameter (nm) 24.12 24.18

Figure 3. FT-IR spectra of Li−Al LDHs before and after calcination ((a, b) LDHs derived from coprecipitation and homogeneous precipitation, respectively, and (c, d) calcined products of LDHs derived from coprecipitation and homogeneous precipitation, respectively).

band at ∼1385 cm−1 was assigned to the stretching vibration of the interlayer NO3−. In the low-frequency region, the bands at 977, 772, and 532 cm−1 are ascribed to the lattice vibration modes attributed to M−O and O−M−O vibrations. The spectrum of the HPLDHs (Figure 3b) was very similar to the spectrum of CLDHs, except for the bands at 3700−3400 cm−1. However, it is important to mention that the band at 1385 cm−1 is slightly broader than that of the CLDHs. This may be attributed to the peak overlapping of the interlayer anions. The CO32−, as interlayer anions, may be produced by hydrolysis of urea during preparation. The FT-IR spectra of the calcined samples are presented in Figures 3c and 3d. The interlayer anions of CLDHs were completely decomposed by calcination, which can be proved by the disappearance of band at 1385 cm−1. However, two small peaks were observed at 1450−1350 cm−1 for the calcined HPLDHs. It indicated that the interlayer anions were not completely decomposed. It should be noted that the small band at ∼1440 cm−1 was observed in the FT-IR spectrum of the calcined HPLDHs, which indicates that the CO32− was indeed intercalated into the gallery of HPLDHs. It is known that CO32− ions have a very strong affinity to LDHs. Hence, the adsorption sites of LDHs are occupied by those interlayer anions. It can be speculated that the presence of interlayer anions may have significant influence on the fluoride removal efficiency. 3.2. Evaluation of Batch Adsorption Parameters. 3.2.1. Effect of Solution pH. The solution initial pH influences the surface charge of Li−Al LDHs via the protonation and deprotonation of host layer cations,12 thus affecting the percentage removal of fluoride in actual water treatment. The effect of initial pH on the adsorption of fluoride was studied from pH 2 to 13 at an ambient temperature of 25 °C and a constant initial fluoride concentration of 20 mg/L. Figure 4 shows the effect of initial solution pH on fluoride removal on 11493



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Figure 4. Removal of fluoride as a function of solution pH. (Conditions: initial concentration = 20 mg/L, adsorption time = 24 h, adsorption temperature = 25 °C, and dose = 2 g/L.)

Figure 5. Removal of fluoride as a function of adsorbent dose. (Conditions: initial concentration = 20 mg/L, adsorption time = 24 h, adsorption temperature = 25 °C, and pH 6−7.)

LDHs, and it can be seen that pH values had a significant effect on fluoride adsorption. The results revealed that the percent removal decreases with increasing pH. The maximum percentage of fluoride removal by the LDHs was observed under acidic conditions. At low pH, the surfaces of LDHs were exposed and carried positive charges, which results in the electrostatic interactions between fluoride ions and the positively charged surface. The percent removal of fluoride was found to be very low at higher pH values, perhaps because of competition between OH− and F− ions on the adsorption sites. Figure 4 also illustrates the adsorption capacity of LDHs prepared via different methods, and it can be seen that the percent removal of CLDHs was higher than HPLDHs, mainly due to the adsorption sites of HPLDHs being occupied by incomplete decomposed interlayer anions. On the other hand, the surface properties of adsorbents, such as surface density, surface energy, surface area, and pore structure may have some influence on fluoride removal efficiency. The different morphologies of two materials should result in different surface properties. Considering the practical application of water treatment, further studies were carried out at neutral pH. 3.2.2. Effect of Adsorbent Dose. In order to study the effect of the adsorbent mass for fluoride removal, the experiments were carried out with an initial fluoride concentration of 20 mg/L by varying the adsorbent dose between 0.01 g and 0.50 g at an adsorption temperature of 25 °C, and the results are shown in Figure 5. It can be seen that the fluoride removal increased from 11.8% to 94.4% (equilibrium concentration was ∼1.0 mg/L) when the mass of the CLDHs increased from 0.01 g to 0.50 g. It can be clearly seen from Figure 5 that the calcined HPLDHs have a lower removal efficiency, compared to calcined CLDHs. It is also noticed that the percent removal increases with an increasing adsorbent dose and, at a certain adsorbent dose, reaches a constant value, where no more fluoride is removed from the solution. The enhancement of removal efficiency is ascribed to an increase in adsorption surface area of LDHs and the high number of unsaturated adsorption sites. However, it is difficult for fluoride adsorption with a percentage removal above 98%, which is due to the adsorption equilibrium between free fluoride and LDHs. Considering the cost and efficiency of water treatment, an

amount of 0.1 g CLDHs may be a good selection for the rest of the batch experiments. 3.2.3. Effect of Co-anions. Some competitive anions exist in the actual groundwater; therefore, the adsorption experiments were carried out in the presence of various competing anions. The effects of PO43−, HPO42−, CO32−, NO3−, Cl−, HCO3−, and SO42− on the sorption of fluoride on the adsorbent were studied by taking equal ionic strengths of the competing anions, relative to that of the fluoride solution (1.053 mmol/L). The removal efficiencies obtained from the interference of the anions are shown in Figure 6. Evidently, the presence of competitive anions affected the removal efficiency to some extent, and the percent removal of fluoride in the presence of anions decreased in the following order:

Figure 6. Removal of fluoride as a function of the co-anions. (Conditions: initial concentration = 20 mg/L, dose = 2 g/L, adsorption time = 24 h, adsorption temperature = 25 °C, and pH 6−7.) 11494



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this fact was further supported by the calculated rate constants as shown in Table 2. The maximum percent removal could reach 97.36%, and the equilibrium concentration was 0.99), although the adsorption process was divided into two stages. This suggests that the present adsorption system can be defined by the pseudo-second-order kinetic model in the adsorption step. The rate-limiting step in the adsorption process is the “surface reaction”. It is suggested that the adsorption process might be considered as chemisorption. From the pseudo-second-order rate constants, it should be noticed that the adsorption rate was higher in the first stage (K21) than in the second stage (K22) for fluoride adsorption. Also note that the adsorption rate decreased as the concentrations increased; this fact was supported by Figure 7. Based on the kinetic analysis, the possible mechanisms of adsorption can be described as follows. The mechanism of the rapid step is controlled by the “memory effect”. The calcination of LDHs at moderate temperatures removes the interlayer water, interlayer anions, and the hydroxyl groups. The result is the formation of the amorphous aluminum oxide with dispersed Li+ ions as a solid solution. The calcined products are able to regenerate the layered structure when they are exposed to fluoride solution. During the process, water is absorbed to reform the hydroxyl layers, and F− ions are incorporated into the interlayer galleries as interlayer anions. This process was governed by electrostatic interaction between F− ions and the positive charge of the LDH host layers. Hence, the fluoride adsorption of this process was very fast. The pseudo-second-order model is based on the assumption that the rate-limiting step may be chemisorption. During the second adsorption step, the adsorption reaction may be involved in F− ions and hydroxide groups. Because of the strong interactions between F− ions and metal cations of the layers, ion exchange between F− ions and the hydroxide groups may occur at the surface of the layers:

NO3− ≈ Cl− < SO4 2 − ≈ HCO3− < CO32 − < HPO4 2 − < PO4 3 −

The divalent and trivalent anions have a greater effect than monovalent anions on removal efficiency; similar results have been observed in the literature for fluoride adsorption by Mg− Al LDH.30 This may be due to a high negative charge density of ions that were easily adsorbed by the layer positive charge. The competitive anions were introduced on the surface of LDHs, which can provide less adsorption sites for fluoride and reduce the adsorption capacity. Meanwhile, it can be seen that monovalent anions had no effect on the removal of fluoride. Considering the practical applicability of adsorbents for defluoridation under actual field conditions, real water (such as tap water, groundwater, and lake water) was used to prepare the fluoride solution (20 mg/L). The results on the defluoridation efficiency from the real water samples are shown in Figure 6. It was observed that the real water only slightly affected the percent removal of fluoride. The final fluoride concentrations for all of the real water samples could reach the drinking water standard. It is evident from the results that the calcined Li−Al LDHs can be effectively employed for the removal of fluoride from real water. 3.3. Kinetic Studies of Fluoride Adsorption. The effect of adsorption time at the adsorption temperature of 25 °C is presented in Figure 7. It can be seen that the adsorption of

Figure 7. Removal of fluoride, as a function of adsorption time. (Conditions: dose = 2 g/L, adsorption temperature = 25 °C, and pH 6−7.)

fluoride increases as the agitation time increases and attains equilibrium earlier for solutions with lower initial concentration. The adsorption was very fast in the first 10 min and became almost asymptotic after 40 min. It should be noticed that the time required for reaching adsorption equilibrium increased from ∼20 min at 20 mg/L to >50 min at 60 mg/L; Table 2. Kinetic Model Parameters for Adsorption of Fluoride pseudo-first-order

pseudo-second-order

intraparticle diffusion

conc (mg/L)

R12

K11

R2 2

K12

R1 2

K21

R2 2

K22

R1 2

Kd1

R2

Kd2

20 40 60

0.8540 0.9942 0.9314

0.2982 0.1907 0.0627

0.9988 0.9443 0.6706

0.0972 0.0518 0.1745

0.9999 0.9959 0.9988

0.1235 0.0884 0.0482

0.9998 0.9963 0.9969

0.0936 0.0860 0.0664

0.7260 0.9780 0.9597

4.0640 2.6752 1.4710

0.8500 0.9469 0.9989

0.1271 0.2921 1.0060

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Table 3. Kinetic Models and Their Linearized Expressionsa expression

plot

refs

eq

pseudo-first-order

kinetic model

ln(qe − qt) = ln qe − k1t

ln(qe − qt) vs t

31−33

(3)

pseudo-second-order

t qt

t/qt vs t

33−35

(4)

intraparticle diffusion

qt = k pt 1/2 + C

qt vs t1/2

35, 36

(5)

=

1 k 2qe2

+

t qe

a

Note: K1, K2, and Kp are rate constants of the pseudo-first-order kinetic reaction, the pseudo-second-order kinetic reaction, and intraparticle diffusion, respectively.

M−OH + F− ⇌ M−F + OH−

(6)

the other hand, the change of charge density and pore structure may also require energy during the adsorption process. The adsorption experimental data were fitted with Langmuir and Freündlich isotherm models.38−40 The models are represented mathematically as follows:

where M denotes the metal cations of the LDHs layer. A process similar to that described by reaction 6 would occur in the presence of aluminum oxide.37 3.4. Equilibrium Adsorption Isotherm. Effect of temperature was studied in the range of 10−40 °C at initial fluoride concentrations of 20−200 mg/L. The adsorption isotherms are shown in Figure 8. The increase in the initial concentration led

Ce C 1 = + e qe KLqm qm

ln qe = ln KF +

(7)

⎛1⎞ ⎜ ⎟ln C e ⎝n⎠

(8)

where KL is the Langmuir isotherm constant (mL/mg), KF the Freündlich isotherm constant, and the term (1/n) the adsorption intensity. A comparison of the experimental adsorption data with the linearized plot of the Langmuir and Freündlich models is shown in Table 4. It can be seen that the experimental data were better fitted by the Freündlich isotherm than the Langmuir isotherm. These results may indicate that some heterogeneity on the surface or pore structure of the calcined Li−Al LDHs will play an important role in fluoride adsorption. Furthermore, the multilayer adsorption of fluoride has been proposed by the Freündlich isotherm model. The Freündlich constant (n) is determiend to be >1, which indicates that the adsorption of fluoride was favorable under the studied conditions.

Figure 8. Adsorption isotherms of fluoride ion adsorption onto CLDHs. (Conditions: dose = 2 g/L, adsorption time = 24 h, and pH 6−7.)

4. CONCLUSION The Li−Al LDHs with high sorption capacity for fluoride have been successfully synthesized by the coprecipitation and homogeneous precipitation methods. The X-ray diffraction (XRD) patterns of synthesized products exhibit a single crystalline phase with the characteristic reflections of LDHs. The morphology of CLDHs only has nanosheet structures, and the surfaces of the LDH platelets were smooth. But the samples of HPLDHs present a surface morphology in the form of a large size. To remove fluoride from water, the calcined Li−Al LDHs were employed as adsorbents and some adsorption conditions were carefully discussed. It was found that the LDHs was suitable for fluoride adsorption in an acidic solution, and the coexisting anions have little effect on the fluoride adsorption, except for HPO42− and PO43−, the optimal

to an increase in fluoride adsorption capacity. The increase in the adsorption temperature led to an increase in adsorption capacity. The maximum adsorption capacity increased from 34.77 mg/g to 42.43 mg/g as the temperature increased from 10 °C to 40 °C, which indicated that the adsorption process of LDHs was endothermic. The increase in fluoride adsorption capacity with increasing temperature might be attributed to the mechanism of adsorption. Based on the kinetic analysis, the ion exchange process of the ions may occur in the second adsorption step. Hence, the mechanism of adsorption may involve chemisorption; energy is needed for chemisorption. On

Table 4. Isotherm Model Constants and Correlation Coefficients for Adsorption of Fluoride onto CLDHs at Different Temperatures Langmuir

Freündlich

temperature (°C)

R2

KL(L/mg)

qmax (mg/g)

R2

n

KF (mg/g (L/mg)1/n)

10 25 40

0.9619 0.9794 0.9814

4.1707 × 10−2 4.3911 × 10−2 6.0701 × 10−2

37.61 46.53 47.24

0.9933 0.9951 0.9799

2.789 2.442 3.004

5.747 5.916 8.869

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adsorbent dose was 0.1 g for 50-mL fluoride solutions. The adsorption kinetics of fluoride adsorption was well-represented by the pseudo-second-order kinetic model, and the adsorption isotherms of adsorption were best fitted by the Freündlich isotherm model. Based on Fourier transform infrared (FT-IR) analysis, the interlayer anions of CLDHs were decomposed by calcination, suggesting that the adsorption mechanism was controlled by the “memory effect” in the early adsorption stages. From the kinetic analysis, the chemisorption may be involved in the adsorption process; it can be inferred that the ion-exchange process was involved in the second adsorption stage.

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Sorption−Desorption Kinetic and Equilibrium Studies. Ind. Eng. Chem. Res. 2004, 43, 2209. (12) Vreysen, S.; Maes, A. Adsorption Mechanism of Humic and Fulvic Acid onto Mg/Al Layered Double Hydroxides. Appl. Clay Sci. 2008, 38, 237. (13) Xia, S. J.; Ni, Z. M.; Xu, Q.; Hu, B. X.; Hu, J. Layered Double Hydroxides as Supports for Intercalation and Sustained Release of Antihypertensive Drugs. J. Solid State Chem. 2008, 181, 2610. (14) Pan, D.; Zhang, H.; Zhang, T.; Duan, X. A Novel OrganicInorganic Microhybrids Containing Anticancer Agent Doxifluridine and Layered Double Hydroxides: Structure and Controlled Release Properties. Chem. Eng. Sci. 2010, 65, 3762. (15) Hussein, M. Z. b.; Zainal, Z.; Yahaya, A. H.; Foo, D. W. V. Controlled Release of a Plant Growth Regulator, A-Naphthaleneacetate from the Lamella of Zn-Al-Layered Double Hydroxide Nanocomposite. J. Controlled Release 2002, 82, 417. (16) Britto, S.; Kamath, P. V. Polytypism in the Lithium−Aluminum Layered Double Hydroxides: The [LiAl2(OH)6]+ Layer as a Structural Synthon. Inorg. Chem. 2011, 50, 5619. (17) Poeppelmeier, K. R.; Hwu, S. J. Synthesis of Lithium Dialuminate by Salt Imbibition. Inorg. Chem. 1987, 26, 3297. (18) Britto, S.; Kamath, P. V. Structure of Bayerite-Based Lithium− Aluminum Layered Double Hydroxides (LDHs): Observation of Monoclinic Symmetry. Inorg. Chem. 2009, 48, 11646. (19) Raichur, A. M.; Jyoti Basu, M. Adsorption of Fluoride onto Mixed Rare Earth Oxides. Sep. Purif. Technol. 2001, 24, 121. (20) Viswanathan, N.; Meenakshi, S. Enhanced Fluoride Sorption Using La(III) Incorporated Carboxylated Chitosan Beads. J. Colloid Interface Sci. 2008, 322, 375. (21) Zhang, T.; Li, Q. R.; Liu, Y.; Duan, Y. L.; Zhang, W. Y. Equilibrium and Kinetics Studies of Fluoride Ions Adsorption on CeO2/Al2O3 Composites Pretreated with Non-Thermal Plasma. Chem. Eng. J. 2011, 168, 665. (22) Thiel, J. P.; Chiang, C. K.; Poeppelmeier, K. R. Structure of Lithium Aluminum Hydroxide Dihydrate (LiAl2(OH)7·2H2O). Chem. Mater. 1993, 5, 297. (23) Duan, X.; Evans, D. G. Structure and Bonding. In Layered Double Hydroxides; Mingos, D.M. P., Ed.; Springer−Verlag: New York, 2005. (24) Besserguenev, A. V.; Fogg, A. M.; Francis, R. J.; Price, S. J.; O’Hare, D.; Isupov, V. P.; Tolochko, B. P. Synthesis and Structure of the Gibbsite Intercalation Compounds [LiAl2(OH)6]X {X = Cl, Br, NO3} and [LiAl2(OH)6]Cl·H2O Using Synchrotron X-ray and Neutron Powder Diffraction. Chem. Mater. 1997, 9, 241. (25) León, M.; Díaz, E.; Bennici, S.; Vega, A.; Ordóñez, S.; Auroux, A. Adsorption of CO2 on Hydrotalcite-Derived Mixed Oxides: Sorption Mechanisms and Consequences for Adsorption Irreversibility. Ind. Eng. Chem. Res. 2010, 49, 3663. (26) Zhu, M. X.; Li, Y. P.; Xie, M.; Xin, H. Z. Sorption of an Anionic Dye by Uncalcined and Calcined Layered Double Hydroxides: A Case Study. J. Hazard. Mater. 2005, 120, 163. (27) Guo, Y.; Li, D.; Hu, C.; Wang, E.; Zou, Y.; Ding, H.; Feng, S. Preparation and Photocatalytic Behavior of Zn/Al/W(Mn) Mixed Oxides Via Polyoxometalates Intercalated Layered Double Hydroxides. Microporous Mesoporous Mater. 2002, 56, 153. (28) Velu, S.; Suzuki, K.; Okazaki, M.; Osaki, T.; Tomura, S.; Ohashi, F. Synthesis of New Sn-Incorporated Layered Double Hydroxides and Their Thermal Evolution to Mixed Oxides. Chem. Mater. 1999, 11, 2163. (29) Cheng, X.; Huang, X.; Wang, X.; Sun, D. Influence of Calcination on the Adsorptive Removal of Phosphate by Zn-Al Layered Double Hydroxides from Excess Sludge Liquor. J. Hazard. Mater. 2010, 177, 516. (30) Lv, L.; He, J.; Wei, M.; Evans, D. G.; Duan, X. Factors Influencing the Removal of Fluoride from Aqueous Solution by Calcined Mg-Al-CO3 Layered Double Hydroxides. J. Hazard. Mater. 2006, 133, 119.

AUTHOR INFORMATION

Corresponding Author

*Tel.:+86335-8387744. Fax: +86335-8061569. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Jenna Shorten and anonymous reviewers for their suggestions, which have significantly improved the quality of this manuscript. We are very thankful to the Natural Science Foundation of China (No. 51077013) for the financial support.

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REFERENCES

(1) Bhatnagar, A.; Kumar, E.; Sillanpäa,̈ M. Fluoride Removal from Water by AdsorptionA Review. Chem. Eng. J. 2011, 171, 811. (2) Jagtap, S.; Yenkie, M. K.; Labhsetwar, N.; Rayalu, S. Fluoride in Drinking Water and Defluoridation of Water. Chem. Rev. 2012, 112, 2454. (3) Viswanathan, N.; Meenakshi, S. Selective Fluoride Adsorption by a Hydrotalcite/Chitosan Composite. Appl. Clay Sci. 2010, 48, 607. (4) Batistella, L.; Venquiaruto, L. D.; Di Luccio, M.; Oliveira, J. V.; Pergher, S. B. C.; Mazutti, M. A.; de Oliveira, D. b.; Mossi, A. J.; Treichel, H.; Dallago, R. r. Evaluation of Acid Activation under the Adsorption Capacity of Double Layered Hydroxides of Mg−Al−CO3 Type for Fluoride Removal from Aqueous Medium. Ind. Eng. Chem. Res. 2011, 50, 6871. (5) Pontié, M.; Dach, H.; Leparc, J.; Hafsi, M.; Lhassani, A. Novel Approach Combining Physico-Chemical Characterizations and Mass Transfer Modelling of Nanofiltration and Low Pressure Reverse Osmosis Membranes for Brackish Water Desalination Intensification. Desalination 2008, 221, 174. (6) Mohapatra, M.; Anand, S.; Mishra, B. K.; Giles, D. E.; Singh, P. Review of Fluoride Removal from Drinking Water. J. Environ. Manage. 2009, 91, 67. (7) He, S.; Zhao, Y.; Wei, M.; Evans, D. G.; Duan, X. Fabrication of Hierarchical Layered Double Hydroxide Framework on Aluminum Foam as a Structured Adsorbent for Water Treatment. Ind. Eng. Chem. Res. 2011, 51, 285. (8) Zhao, Q.; Chang, Z.; Lei, X.; Sun, X. Adsorption Behavior of Thiophene from Aqueous Solution on Carbonate- and DodecylsulfateIntercalated ZnAl Layered Double Hydroxides. Ind. Eng. Chem. Res. 2011, 50, 10253. (9) Dadwhal, M.; Sahimi, M.; Tsotsis, T. T. Adsorption Isotherms of Arsenic on Conditioned Layered Double Hydroxides in the Presence of Various Competing Ions. Ind. Eng. Chem. Res. 2011, 50, 2220. (10) Valente, J. S.; Hernandez-Cortez, J.; Cantu, M. S.; Ferrat, G.; López-Salinas, E. Calcined Layered Double Hydroxides Mg-Me-Al (Me: Cu, Fe, Ni, Zn) as Bifunctional Catalysts. Catal. Today 2010, 150, 340. (11) Lazaridis, N. K.; Pandi, T. A.; Matis, K. A. Chromium(VI) Removal from Aqueous Solutions by Mg-Al-CO3 Hydrotalcite: 11497



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(31) Ahmaruzzaman, M.; Laxmi Gayatri, S. Batch Adsorption of 4Nitrophenol by Acid Activated Jute Stick Char: Equilibrium, Kinetic and Thermodynamic Studies. Chem. Eng. J. 2010, 158, 173. (32) Chabani, M.; Amrane, A.; Bensmaili, A. Kinetic Modelling of the Adsorption of Nitrates by Ion Exchange Resin. Chem. Eng. J. 2006, 125, 111. (33) Hameed, B. H.; Tan, I. A. W.; Ahmad, A. L. Adsorption Isotherm, Kinetic Modeling and Mechanism of 2,4,6-Trichlorophenol on Coconut Husk-Based Activated Carbon. Chem. Eng. J. 2008, 144, 235. (34) Liu, Q. S.; Zheng, T.; Wang, P.; Jiang, J. P.; Li, N. Adsorption Isotherm, Kinetic and Mechanism Studies of Some Substituted Phenols on Activated Carbon Fibers. Chem. Eng. J. 2010, 157, 348. (35) Sarı, A.; Şahinoğlu, G.; Tüzen, M. Antimony(III) Adsorption from Aqueous Solution Using Raw Perlite and Mn-Modified Perlite: Equilibrium, Thermodynamic, and Kinetic Studies. Ind. Eng. Chem. Res. 2012, 51, 6877. (36) Xue, Y.; Hou, H.; Zhu, S. Adsorption Removal of Reactive Dyes from Aqueous Solution by Modified Basic Oxygen Furnace Slag: Isotherm and Kinetic Study. Chem. Eng. J. 2009, 147, 272. (37) López Valdivieso, A.; Reyes Bahena, J. L.; Song, S.; Herrera Urbina, R. Temperature Effect on the Zeta Potential and Fluoride Adsorption at the α-Al2O3/Aqueous Solution Interface. J. Colloid Interface Sci. 2006, 298, 1. (38) Chen, S.; Yue, Q.; Gao, B.; Xu, X. Equilibrium and Kinetic Adsorption Study of the Adsorptive Removal of Cr(VI) Using Modified Wheat Residue. J. Colloid Interface Sci. 2010, 349, 256. (39) Vimonses, V.; Lei, S.; Jin, B.; Chow, C. W. K.; Saint, C. Kinetic Study and Equilibrium Isotherm Analysis of Congo Red Adsorption by Clay Materials. Chem. Eng. J. 2009, 148, 354. (40) Liu, T.; Yang, M.; Wang, T.; Yuan, Q. Prediction Strategy of Adsorption Equilibrium Time Based on Equilibrium and Kinetic Results To Isolate Taxifolin. Ind. Eng. Chem. Res. 2011, 51, 454.

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