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Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Synthesis of Mg/Al Layered Double Hydroxides for Adsorptive Removal of Fluoride from Water: A Mechanistic and Kinetic Study Gautam Kumar Sarma and Md. Harunar Rashid* Department of Chemistry, Rajiv Gandhi University, Rono Hills, Doimukh, Arunachal Pradesh 791 112, India

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S Supporting Information *

ABSTRACT: Herein, we report the synthesis of Mg/Al-CO3 layered double hydroxides (LDHs) by the urea hydrolysis method under hydrothermal conditions at different aging times. The LDHs were characterized by different microscopic, spectroscopic, diffractometric, and thermal analysis techniques. The as-prepared LDHs were then used as adsorbents for the removal of fluoride ions from water. The adsorption process was systematically studied in terms of influences of pH, interaction time, temperature, adsorbent loading, fluoride ion concentration, and the effect of coexisting anions to understand the ideal conditions for adsorption. The adsorption process primarily followed pseudo-secondorder kinetics. The Langmuir monolayer adsorption capacities of different adsorbents are in the range of 15.13−27.03 mg g−1 at 303 K. The hydrothermal aging time has a negative impact on the adsorption capacities of the LDHs. The as-prepared LDHs are reusable up to five cycles of the fluoride ion adsorption−desorption process. developed and tested for the removal of fluoride from water. Although a handful of reports are available on the removal of fluoride by different adsorbents, defluoridation of water is still considered to be a very active field of research and hence needs the attention of the researchers to develop inexpensive but efficient adsorbents for fluoride removal. Layered double hydroxides (LDHs) are a versatile class of two-dimensional (2D) ionic lamellar compounds with a brucite structure. They belong to the anionic clay family with the general formula [M2+1−x M3+x(OH)2]x+ [Ax/n]n−· mH2O, where M2+ is a divalent cation, M3+ is a trivalent cation, and An− is the anion.17,18 The most widely used divalent and trivalent cations are Mg2+, Mn2+, Fe2+, Co2+, Cu2+, Ni2+, Zn2+ or Ca2+ and Al3+, Cr3+, Mn3+, Fe3+, Co3+, or La3+. The value of x generally falls in the range of 0.2−0.4, which is equal to the molar ratio, M2+/(M2+ + M3+).17,18 The wide variety of M2+, M3+, metal ions and equally diverse verity of An− anions make it a versatile class of material. LDHs have gained significant attention in the recent times for their potential applications in water treatment,7,19,20 drug carrier,21 photovoltaic cells,18 and catalysis,17,22 etc. Among different applications, the use of LDHs as adsorbents in water treatment has a great potential due to their low cost, high specific surface area, high tunable interior architecture, nontoxicity and exchangeable anionic features.23 Subsequently, over the years, many LDH-based adsorbents have been utilized as adsorbent for fluoride removal. Among them, Elhali et al. reported the defluoridation

1. INTRODUCTION Fluoride is known to have both a beneficial and harmful effect on human health. Fluoride concentration in between 0.4 and 1.0 mg L−1 has a beneficial effect, as it protects us from tooth decay by promoting calcification of dental enamel.1 However, fluoride concentration in excess of 1.5 mg L−1 leads to serious health problem such as fluorosis, osteoporosis, arthritis, brittle bones, cancer, infertility, brain damage, Alzheimer syndrome, and thyroid disorder.1,2 In recent times, fluoride contamination of drinking water has emerged as a serious environmental health hazard worldwide with countries such as USA, China, India, Kenya, etc. reporting very high concentrations of fluoride in drinking water.2,3 So, it is very important to remove excess fluoride from drinking water to maintain the optimal concentration of fluoride. Fluoride in water generally originates due to slow dissociation of fluoride based minerals such as fluorite [CaF2], fluorapatite [Ca5(PO4)3F], and cryolite [Na3AlF6], along with discharge of wastes from industries such as semiconductor manufacturing, coal power plants, electroplating, rubber and fertilizer production.2,4 Over the years, many treatment methods such as electrodialysis,5 ion exchange,6 adsorption,7−9 chemical precipitation, and coagulation10 have been employed to remove excess fluoride from water. Among the different treatment methods available for removal of the contaminant from water, adsorption has emerged as the most popular and promising technology because of the ease of operation, viability, simplicity of equipment, high efficiency, and low cost of the adsorbents.2,7,11 In recent years, various adsorbents, such as activated and organic acid-modified alumina,9,12 graphene,8 Fe3O4 nanocomposites,13−15 and Ce-doped bone char16 have been © XXXX American Chemical Society

Received: March 26, 2018 Accepted: June 6, 2018

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DOI: 10.1021/acs.jced.8b00242 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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of groundwater by calcined Mg/Al LDHs.24 Kameda et al. reported the synthesis of recyclable Mg/Al LDHs for removal of fluoride from water.7 Kim et al. reported the use of Mg/Al LDH for removal of fluoride from high fluoride concentration.19 They further investigated the fluoride removal activity of calcined LDHs. Noorjahan et al. reported the preparation of novel Co modified Mg/Al LDHs for removal of fluoride.25 Calcined Mg/Al LDHs were reported for removal of fluoride by Cai et al.26 Lv et al. further investigated the influence of different factors on the removal of fluoride by calcined Mg/Al LDHs.27 So, it is evident that most of the Mg/ Al LDH based studies focused on the use of a calcined form for the removal of fluoride without giving proper attention to asprepared LDHs. Herein we report the synthesis of Mg/Al-CO3 LDH by the urea hydrolysis method under hydrothermal conditions. We optimized the reaction conditions to synthesize LDH for better adsoption capacities. The synthesized LDHs were then used as an adsorbent for removal of fluoride from water. The isotherm and kinetics of the adsorption process was thoroughly investigated along with its possible mechanism of fluoride uptake.

characterization techniques are provided in the Supporting Information (SI). 2.2. Study of Adsorption of Fluoride Ions. The fluoride adsorption experiments were carried out in a 100 mL plastic conical flask by mixing a fixed amount of adsorbent with 10 mL of aqueous fluoride solution. Polypropylene conical flasks were used instead of glass vessels to avoid possible interaction of fluoride with glass as reported earlier.28 The mixture was agitated in a thermostatic water bath shaker (NSW, Mumbai, India) at a constant speed of 200 rpm for a predetermined time interval. The adsorbents were then separated from the mixture by centrifugation at 10 000 rpm for 15 min, and the amount of fluoride remaining unadsorbed in the supernatant solution was determined spectrophotometrically (Agilent Cary60 spectrophotometer) using the SPADNS method.29,30 The amount of fluoride adsorbed per unit mass of the adsorbent (qe, mg g−1) and the extent of adsorption (%) were computed using the following equations:

qe =

2. EXPERIMENTAL SECTION 2.1. Synthesis of Mg/Al-CO3 Layered Double Hydroxides. A conventional urea hydrolysis method under hydrothermal condition was adopted to synthesize Mg/Al-CO3 LDHs (Scheme 1). In a typical synthesis, MgCl2·6H2O

C0 − Ct m

(1)

%adsorption =

C0 − Ct × 100 C0

−1

(2) −1

where, C0 (mg L ) and Ct (mg L ) are the initial fluoride concentration and that after adsorption on “m” g of the adsorbent in 1 L solution at time, t min, respectively. To monitor if there is any adsorption of fluoride on the walls of the container, a blank experiment was carried out by agitating the fluoride solution until the equilibrium time without adding the adsorbent and calculating its fluoride concentration. The batch adsorption studies were carried out under different experimental conditions and the details are provided in Table S1 in the SI.

Scheme 1. Schematic Representation of the Preparation of Mg/Al LDH

3. RESULTS AND DISCUSSION 3.1. X-ray Diffraction Analysis (XRD). To study the crystalline properties and to identify the crystal structure of the products, the powder XRD patterns of all the as-prepared LDHs were recorded. Figure 1 shows the XRD patterns of the as-prepared LDHs prepared at different hydrothermal aging time. The sharp peaks observed at 2θ = 11.65, 23.32, 34.9, 39.42, 46.86, 60.65, and 62.0° are typical for Mg/Al-CO3 LDH (JCPDF Card No. 14-0191) which correspond to (003),

(Merck, India), AlCl3 (Merck, India), and urea (SRL, India) with a molar ratio of 2:1:15 were dissolved in 100 mL of methanol (Merck, India) solution (methanol:water = 90:10) at room temperature (303 K). After being stirred for 30 min in a magnetic stirrer, the clear solution was transferred into a 250 mL Teflon lined autoclave and aged at 423 K for 6 h. After that, the reactor containing the product was allowed to cool down to room temperature naturally. The solid precipitate was collected by centrifugation at 10 000 rpm for 15 min (Remi CPR 24 plus centrifuge). The isolated solid product was purified from any traces of excess reactants by repeated washing with water and then with ethanol, and finally the solid product was isolated by centrifugation. The isolated solid product was dried in vacuum at 333 K for 12 h and the dried products were stored in a desiccator. The product was designated as sample Mg_Al-6 where the numerical value represents the hydrothermal aging time. A similar set of reactions was carried out at a hydrothermal aging time of 12 and 24 h, and the respective samples were designated as Mg_Al-12 and Mg_Al-24 in order to ascertain the role of aging time on the size and morphology of the LDHs. All the asprepared LDHs were characterized by different spectroscopic, microscopic, and diffractometric techniques. The details of the

Figure 1. Powder XRD patterns of the as-prepared LDHs. B



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like morphologies. Further, we have also carried out the transmission electron microscopy (TEM) analysis of the asprepared LDHs to examine the structural property. A representative TEM image of LDH (sample Mg_Al-6) is shown in Figure 2d. The image clearly shows the presence of ball-like assembled structures which also support the SEM results. The energy dispersive X-ray analyses on the samples show the presence of peaks of elemental Mg, Al, and O in the samples (Figure S1 in the SI). This confirms that the asprepared LDH is composed of Mg, Al, and O. Further to confirm the results, we have carried out an X-ray photoelectron spectroscopy (XPS) study on sample Mg_Al-6. The survey scan spectrum (Figure S2 in the SI) exhibits the peaks in the region of O 1s and Al 2s, Al 2p, Mg 2s and Mg 2p indicating the presence of Al and Mg in the (III) and (II) oxidation states. However, to determine the exact composition of the LDH, we carried out an atomic absorption spectroscopy (AAS) study on sample Al_Mg-6 which confirmed that the sample consists of 37.6% Al and 51.7% Mg which gives the Al:Mg ratio of 1:1.4. 3.3. N2 Gas Adsorption−Desorption Study. It is known that the specific surface area of LDHs plays the important role in deciding their effectiveness as adsorbents or catalysts. To measure the specific surface area of the as-prepared LDHs, N2 gas adsorption−desorption isotherms were recorded on the samples. The BET isotherms of different samples are shown in Figure 3. All the LDH samples exhibit type IV isotherms with a type H3 hysteresis loop which is quite common for nonporous or macroporous material with aggregates of plate-like particles with slit shaped pores.32 In all the three LDH samples, the monolayer formation starts at very low value of P/Po and continues up to P/Po = 0.7 after which the adsorption− desorption curves become steep. This is a clear indication that after the monolayer formation, gradual filling of the pores starts, and subsequently the amount of gas adsorbed per g of the adsorbent increases sharply. Beyond P/Po = 0.7, the amount of N2 gas adsorbed increases sharply from ∼29.2 to ∼66.3 cm3/g at P/Po = 1.0 in samples Mg_Al-6 and Mg_Al-24. However, the value is close to 90 cm3/g for sample Mg_Al-12. This sharp increase in adsorption in all the samples might be attributed to the rapid filling of the pores at high relative pressures. Also, it is clear that sample Mg_Al-12 exhibits the maximum adsorption capacity at higher relative pressure, whereas the adsorption capacity of the remaining two samples is the same. From these results it can be inferred that the asprepared LDHs do not follow any particular trend in N2 gas

(006), (009), (015), (018), (110), and (113) planes. These crystal planes suggests the presence of hexagonal lattice with rhombohedral 3R symmetry as reported earlier.31 The XRD studies further indicate that the hydrothermal aging time for the preparation of LDH by the urea hydrolysis route has no influence on the crystalline properties of the products. However, as the aging time was increased from 6 to 12 h to 24 h new peaks (marked by an asterisk (∗) in Figure 1) corresponding to (002) and (011) planes for Al(OH)3 were observed at 2θ = 18.5 and 20.2°, respectively (JCPDF Card No. 18-0031). 3.2. Electron Microscopy Study. To study the morphological evolution and tomography of the as-prepared LDHs, the electron microscopy study was carried on all the samples. Figure 2 panels a, b, and c show the field emission scanning

Figure 2. FE-SEM images of the as-prepared LDHs: (a) sample Mg_Al-6, (b) Mg_Al-12, and (c) Mg_Al-24. (d) TEM image of asprepared LDH recorded from sample Mg_Al-6. Inset in Panel d shows the magnified TEM image of individual LDHs.

electron microscopy (FESEM) images of samples Mg_Al-6, Mg_Al-12, and Mg_Al-24 prepared by aging for 6, 12, and 24 h, respectively. All the micrographs show the presence of flake-

Figure 3. N2 adsorption−desorption isotherms of different LDH samples: (a) Mg_Al-6, (b) Mg_Al-12, and (c) Mg_Al-24. C



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adsorption. However, from the BET isotherms we can infer that there are changes in the pore structures due to variations in hydrothermal aging time, which ultimately results in the variation in adsorption capability of the LDHs. Besides, there are slight variations in the hysteresis loops also for different samples. That is, the samples prepared by aging for 6 and 12 h show the start of a hysteresis loop closure around the relative pressure of P/Po = 0.3. But the sample prepared at the aging time of 24 h (sample Mg_Al-24) exhibits the start of hysteresis loop closure at relative pressure of 0.6. The results indicate that as the hydrothermal aging time was increased from 6 to 12 h to 24 h, the BET specific surface area decreased significantly (Table 1). We assumed that this decrease might be due to Table 1. Specific Surface Area and Pore Characteristics of the As-Prepared LDHs adsorbent

specific surface area (m2/g)

pore volume (cm3/g)

pore radius (Å)

Mg_Al-6 Mg−Al-12 Mg_Al-24

63.35 49.24 26.73

0.068 0.111 0.078

18.31 33.3 15.68

Figure 4. TGA thermograms of as-prepared LDHs.

LDHs was found to be 18, 13, and 11%, respectively, for samples Mg_Al-6, Mg_Al-12, and Mg_Al-24. The second decomposition occurring at the temperature range of 190−285 °C represents a mass loss of about 3% in all the samples due to the removal of CO2 and interlayer H2O molecules. The mass loss in the final step of thermal decomposition beyond 285 °C was 31, 24, and 25%, respectively, for samples Mg_Al-6, Mg_Al-12, and Mg_Al-24 and is associated with dehydroxylation and decarbonation processes as reported earlier.20 This step leads to the collapse of the layered structures thereby forming mixed metal oxide. 3.6. Kinetics of Adsorption. To investigate whether the as-synthesized LDHs can be used as an adsorbent for water treatment, we have chosen the removal of toxic ions such as fluoride from water. The extent of adsorption of fluoride on the LDHs was studied by recording the time-dependent UV− vis spectra of the mixture of aqueous suspensions of LDH and fluoride solution of known concentration. We observed that fluoride and LDH interactions attained equilibrium within 180 min (Figure 5a). The adsorption of fluoride on LDH occurred in two phases: first a rapid adsorption of fluoride on LDH took place up to ∼30 min followed by comparatively slow adsorption resulting in establishing equilibrium within 180 min. Initially, the bare surface available on the LDH allowed the faster adsorption of fluoride. But as the fluoride ions gradually cover the surfaces of LDH, the rate of adsorption decreases. Similar trends were observed in all the as-prepared LDHs. To understand the possible mechanism, the adsorption processes were tested with two well-known kinetic models, namely, the Lagergren and pseudo-second-order kinetics model. The pseudo-first-order Lagergren model37 is one of the most widely used models which is given by the following equation:

aggregation of the particles at higher aging time. Similar observations were also reported earlier by other researchers.33,34 Further to get an insight into these observations, we have calculated pore volume and pore diameter following the Barrett−Joyner−Halenda (BJH) method, and the data are presented in Table 1. It is observed that the pore size and pore volume exhibited by sample Mg_Al-12 is higher compared to that of other as-prepared LDHs (Table 1). From the above BET isotherms and the measured values of specific surface areas, pore sizes, and pore volumes, it is clear that the LDH samples prepared at a hydrothermal aging time of 12 h exhibits the highest pore size and volume. Also the amount of N2 gas adsorbed by this particular LDH is a maximum due to their larger pore volumes. 3.4. FTIR Analysis. The FTIR spectra of all the as-prepared LDH samples are shown in Figure S3a in SI which shows the presence of prominent peaks at 3442, 1628, and 1353 cm−1. The broad peak at 3442 cm−1 is assigned to −OH stretching in the brucite-like structure. The relatively sharp peaks appearing at 1628 and 1353 cm−1 are associated with the H−O−H bending mode of interlayer water molecules35 and the presence of CO32− in the interlayer region.36 The presence of CO32− in the LDH interlayer is due to a release of CO2 during urea hydrolysis. Besides, a number of weakly intense peaks were observed in the region from 400 to 800 cm−1 (Figure S3b in SI) which can be assigned to M−O- and M−O−M-type lattice vibrations in the LDH as reported earlier.25 The FTIR spectra further revealed that the aging time during hydrothermal synthesis has no adverse effect on the lattice structures of the LDHs as no significant changes were observed in the spectra of samples Mg_Al-6, Mg_Al-12, and Mg_Al-24. 3.5. Thermal Analysis. To investigate the thermal effect on the mass of the LDHs, thermogravimetric analysis (TGA) was carried on the samples. Figure 4 shows the TGA thermograms of all the as-prepared LDH samples which show three decomposition temperatures. The first decomposition took place at a temperature range of room temperature to about 190 °C which is attributed to the removal of the adsorbed water molecule in the LDHs. The amount of interlayer water molecules present in different

ln(qe − qt ) = ln qe − k1t

(3)

where qt is the amount adsorbed per unit mass at time t, qe is the amount adsorbed per unit mass at equilibrium, and k1 is the first order rate coefficient. Fluoride adsorption on LDHs yielded linear curves between ln (qe − qt) and t (r: −0.88 to −0.98) with k1 values ranging from 2.11 × 10−2 to 2.37 × 10−2 min−1 at 303 K. The first order mechanism however becomes untenable since qe obtained from the linear fitted plots does not match the experimental values and showed large deviations ranging from 11.95% to +50.51% (Table 2). On application of D



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Figure 5. (a) Variation in adsorption of fluoride ion on LDHs with time at 303 K and (b) pseudo-second-order plots for the adsorption of fluoride on different LDHs. ([fluoride] = 10 mg L−1; [LDHs] = 1.0 g L−1, pH = 6.1 and equilibrium time = 180 min).

where kid is the intraparticles diffusion rate constant. The plots of all the samples at 303 K are quite linear (r ≈ +0.91 to +0.99; Table 2) (Figure S4 in the SI); however, these are not the ratedetermining steps as they do not fulfill the necessary condition of zero intercepts. But, as the intercepts are very close to zero (intercepts: +0.58 to +0.64) it is possible that they have some influence in the overall rate process, even though they are not the rate-determining step.41 There is also the possibility that fluoride ion may diffuse very slowly across a liquid layer over the LDHs before they can successfully interact with the active sites and this step might be the rate-determining step. Keeping this in mind, we plotted the curves −ln (1 − F) versus t to validate the liquid film diffusion model represented by the following equation,40,42

Table 2. Rate Coefficient and Deviation of Experimental and Computed qe Values for Adsorption of Fluoridea layered double hydroxides (LDHs) order pseudo-second

Lagregreen first

intraparticle diffusion

liquid film diffusion

rate coefficient k2 × 103 r qe (expt) qe (plots) deviation (%) k1 × 102 r qe (expt) qe (plots) deviation (%) ki intercept r kfd intercept r

Mg_Al-6 Mg_Al-12 6.16 0.99 4.31 4.76 −9.45 2.11 −0.98 4.31 3.70 11.95 0.307 0.58 0.99 0.021 0.05 0.98

12.20 0.99 3.33 3.62 −8.01 2.37 −0.88 3.33 2.95 12.88 0.226 0.63 0.95 0.024 0.05 0.94

Mg_Al-24 16.40 0.99 2.98 3.25 −8.31 2.11 −0.98 2.98 1.98 50.51 0.208 0.64 0.91 0.021 0.18 0.98

ln(1 − F ) = −k fdt

where F is the fractional attainment of equilibrium = qt/qe and kfd is the diffusion rate coefficient. Here again despite the plots being linear (r ≈ +0.94 to +0.98; Table 2) (Figure S5 in the SI), they do not meet the condition of zero intercept although the values of the intercepts are very close to zero (+0.05 to +0.18). From the study of all these kinetic models, we can conclude that the adsorption of fluoride on LDHs is not an easy process, and hence a single model cannot explain the kinetics data satisfactorily. This implies that at different stages of adsorption, that is, an initial fast adsorption up to ∼30 min and then relatively slow adsorption until equilibrium; different fundamentals govern the adsorption processes. Overall, we can say that the adsorption process followed pseudo-second-order kinetics as that model showed the best fit to the experimental data, but liquid film diffusion and intraparticle diffusion might play some role at different stages of adsorption. 3.7. Adsorption Isotherm. A variety of isotherm models have been proposed over the years some of which were with firm theoretical foundation and others being of an empirical nature. A few of the most common isotherms having the widest applications were used in this study to explain the adsorption equilibrium data. The adsorption data conformed to both Freundlich43 and Langmuir isotherms models.44 The linear isotherms are 1 ln qe = ln Ce + ln k f (7) n

Fluoride (10 mg L−1) on different as-prepared LDHs (1.0 g L−1) at 303 K. (k1 in min−1, k2 in g mg−1 min−1 and qe in mg g−1).

a

the pseudo-second-order kinetics as per the well-known rate equation38 (eq 4), t 1 1 = + t 2 qt qe k 2qe (4) linear plots (Figure 5b) are obtained for t/qt vs t (r: +0.99) and the second order rate coefficient at 303 K was found to be 6.16 × 10−3 to 16.40 × 10−3 g mg−1 min−1 (Table 2). The relatively close agreement between the experimental qe values and those obtained from the linear fitted plots (deviation −9.45 to −8.01%) strongly support a pseudo-second order mechanism for fluoride take-up by the LDHs. Since the prepared LDH materials have a layered structure and the fluoride ions are very small, it is possible for the fluoride ion to enter into the pores of LDH molecules through intraparticle diffusion. This fact was further examined by plotting qt versus t0.5 in accordance with the following equation,39,40 qt = k idt 0.5

(6)

(5)

Linear Langmuir isotherm: E



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Ce 1 1 = + Ce qe bqm qm

complex, which is, however, not likely to be a stable complex as indicated by comparatively low values for “b”. It was also noted that a higher hydrothermal aging time has a negative impact on the adsorption. When the monolayer adsorption capacities of some other reported adsorbents for fluoride are compared (Table 4), it is observed that the as-prepared LDHs have a

(8)

where Ce is the equilibrium concentration of the adsorbate (mg L−1); qe is the amount of the adsorbate adsorbed per unit mass (mg g−1), kf (mg1−1/n L1/n g−1) and n are Freundlich coefficients related to adsorption capacity and adsorption intensity, respectively. It is generally accepted that the reciprocal Freundlich intensity (1/n) is < 1.0 for favorable adsorption. qm (mg g−1) and b (L mg−1) are Langmuir coefficients related to adsorption efficiency and adsorption equilibrium constant, respectively. At 303 K, the Freundlich isotherm plots are linear (r = +0.99; Table 3) with (1/n) < 1

Table 4. Comparison of the Adsorption Capacities of the As-Prepared LDHs with Some Reported Adsorbents adsorbents Zn−Al LDH Fe3O4 nanocomposite hydrous ferric oxide graphene cerium-doped bone char core−shell Fe3O4@alginate-La particles La3+ impregnated chitosan/βcyclodextrin organic acid modified mesoporous alumina ethanol treated iron oxide hydroxyapatite/multiwalled carbon nanotubes calcined Li/Al LDH magnetic Fe3O4@Fe−Ti composites Mg_Al LDH

Table 3. Isotherm Parameters for Adsorption of Fluoride Ions from Aqueous Solution on LDHs at 303 Ka parameters

Mg_Al-6

Freundlich Isotherm 0.86 kf 1/n 0.82 r 0.99 Langmuir Isotherm qm 27.03 b 0.029 r 0.97 Temkin Isotherm kT 0.761 B 3.23 r 0.96

Mg_Al-12

Mg_Al-24

0.70 0.85 0.99

0.57 0.80 0.99

24.10 0.027 0.98

15.13 0.033 0.99

0.720 2.87 0.96

0.695 2.14 0.96

Langmuir monolayer adsorption capacity (mg/g)

ref

4.16 9.43 6.71 17.65 13.6 45.23

30 13 1 8 16 14

8.14

45

47.2−62.5

12

60.8 39.22

15 46

158.7 41.8

47 48

27.03

this work

reasonably high adsorption capacity for fluoride compared to some of the reported metal oxides and other adsorbents. However, the adsorption capacity is lower than some of the reported surface functionalized adsorbents and calcined Mg/Al LDH. But those adsorbents required mostly multistep tedious synthesis procedures, whereas calcined LDHs possess very high surface area due to calcinations of LDHs at very high temperature that results in increased fluoride adsorption capacity in the calcined LDH. The Temkin isotherm was formulated by taking into consideration the effects of adsorbate−adsorbate interactions. The Temkin isotherm has the form30

[Fluoride, 2−30 mg L−1; LDH amount, 1.0 g L−1] kf in mg1‑1/n L1/n g−1, qm and b are in mg g−1 and L mg−1, kT in L mg−1. a

(Figure S6 in the SI) indicating favorable adsorption. It was observed that hydrothermal aging time in the formation of LDHs has a significance effect on the adsorption capacity. The adsorption results showed a gradual decrease with increase in aging time in the order: Mg_Al-6 > Mg_Al-12 > Mg_Al-24. Langmuir plots of Ce/qe versus Ce (Figure 6) are also linear (r ≈ +0.97 to +0.99, Table 3) at 303 K. The trend in Langmuir monolayer adsorption capacity, Mg_Al-6 > Mg_Al-12 > Mg_Al-24, is similar to that of the Freundlich adsorption capacity. The Langmuir equilibrium coefficient b (Table 3) suggests a favorable formation of a fluoride−LDH adsorption

i RT yz zz ln(k TCe) qe = jjj (9) k b { This isotherm is based on the assumption that (a) the heat of adsorption of all the molecules in the layer decreases linearly with coverage due to adsorbate−adsorbate interactions and (b) the adsorption is characterized by a uniform distribution of binding energies, up to some maximum binding energy. The linear form of the equation is qe = B ln k T + B ln Ce

(10) −1

where, B = (RT/b) with kT (L mg ) being the equilibrium binding constant corresponding to maximum binding energy and b (J mol−1) and B are the Temkin coefficients. The dimensionless constant B is assumed to be related to heat of adsorption of the process. At 303 K, the Temkin isotherm plots (qe versus ln Ce) for fluoride on LDHs are linear (r ≈ +0.96) (Figure S7 in the SI) indicating that both adsorbate− adsorbate and adsorbate−adsorbent interactions are likely to play key role in uptake of fluoride by the LDHs. The Temkin coefficients are presented in Table 3. The B values for all the

Figure 6. Langmuir plots for fluoride adsorption (2−30 mg L−1) on different LDHs (1.0 g L−1) at 303 K. F



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the adsorption of fluoride, we carried out adsorption at different temperatures ranging from 303−333 K. It was observed that when the temperature was increased from 303 to 323 K the adsorption of fluoride increases from 41.2% to 55.6% (Figure S11 in the SI). A further increase in the temperature has little effect on the adsorption of fluoride as only 1% adsorption was observed when the temperature was increased from 323 to 333 K. This may occur because at higher temperature the adsorbed fluoride would have higher thermal energy causing it to desorb from the LDH.51 3.9. Reusability of the Prepared Adsorbents. The reusability of the adsorbents was examined for five cycles of fluoride adsorption−desorption. Each time after adsorption, the solid adsorbent was isolated by centrifugation and was dispersed in distilled water by spinning vortex for 5 min followed by ultrasonic vibration for 15 min to remove the adsorbed fluoride from the surface of the adsorbent. The suspension was then again centrifuged to isolate the solid adsorbent. The process was repeated for three times and finally the isolated solid was used for another batch of adsorption. The reusability of the LDH was tested for five times. Realistically, the adsorbent could be used for a maximum three cycles; after that a quite significant decrease in adsorption capacity was observed. For the first three cycles, the adsorption capacity of Mg_Al-6 was almost steady with a minimal decrease of 12.16%. However, after the third cycle the adsorption capacity dropped down to 45.63% (Figure 7).

LDH adsorbents showed a constant decreasing trend with increase in hydrothermal aging time of preparation of LDHs. This signifies that heat of adsorption also gradually decreases. Significantly, the equilibrium binding constant (kT) is also greater in Mg_Al-6 samples compared to that of the rest. This suggests that this particular LDH (sample Mg_Al-6) forms a stronger bond compared to the rest of the adsorbents. 3.8. Effect of Different Adsorption Parameters on the Removal of Fluoride. 3.8.1. Influence of pH. It has been reported that the pH of the medium plays an important role in adsorption. Thus, to optimize the pH of the medium for better adsorption, we studied the fluoride adsorption on LDHs at different initial pH values ranging from pH 4.0 to pH 10.0. Figure S8 in the SI shows the effect of initial pH on the adsorption of fluoride by the as-prepared LDH (sample Mg_Al-6). It is observed that maximum fluoride adsorption occurred at pH 6.0 and both above and below this pH adsorption decreases. The decrease in the adsorption capacity in acidic pH might be attributed to the possible protonation of the fluoride ions and the dissolution of the layered materials.30 Moreover, the decrease in adsorption capacity in alkaline pH might be due to the increased competition between fluoride ions and OH− ions for the interlayer spaces in LDH.30 3.8.2. Influence of Adsorbent Loading. It is interesting to study the effect of adsorbent loading on the removal of fluoride ions. So in the current study, we have varied the LDHs loading from 1.0 to 5.0 g L−1 by keeping the initial concentration of fluoride the same. It was observed that the adsorption of fluoride ions increased from 38.88 to 83.81% for Mg_Al-6 as the adsorbent loading was increased from 1.0 to 5.0 g L−1 (Figure S9 in the SI). This increase might be the result of an increase of available adsorption sites. However, a decreasing trend was observed for the amount of fluoride adsorbed per unit mass (qe) with an increase in adsorbent loading (Mg_Al6:3.89 to 1.68 mg g−1) (Figure S9 in the SI). Hence, it is possible that some of the adsorption sites were not available for the fluoride ion at higher adsorbent loading due to coagulation of the adsorbent, which will hinder the approach of fluoride ions to the adsorption site and ultimately results in decreased adsorption per unit mass even though there is a net increase in percentage adsorption. 3.8.3. Influence of Coexisting Ion. Ground water generally contains various anions. Therefore, these anions can compete with fluoride for the adsorption sites. Thus, it is very important to study the influence of coexisting anions on fluoride adsorption. From the literature it is clear that the most common anions such as SO42−, PO43−, NO3−, Cl−, CO32−, etc. are present in water in much higher concentrations than fluoride.49,50 Therefore, in this study, we used coexisting anions in 10-fold concentration (100 mg L−1) than fluoride concentration (10 mg L−1) to mimic groundwater sample. It was observed that the presence of coexisting ion has a significant effect on fluoride removal. That is the adsorption of fluoride decreased from 15.88% to 45.28% for sample Mg_Al-6 in the presence of different mono- and bivalent anions which follow the order Cl− < NO3− < HPO42− < CO32− < SO42−. This trend clearly shows that the adsorbent has more affinity toward divalent anions compared to monovalent anions. The observation is represented in the form a bar diagram (Figure S10 in the SI). This observation is contrary to the results reported for Co−Mg−Al LDH.25 3.8.4. Effect of Temperature. Temperature generally plays a significant role during adsorption. To investigate its effect on

Figure 7. Bar diagram showing the variation of % adsorption of fluoride ion (10 mg L−1) in successive batches with sample Mg_Al-6 (1.0 g L−1).

This might occur because adsorbed fluoride was not completely removed during washing which might block some of the active sites on the LDH. This also indicated that just washing with distilled water might not be enough for complete removal of the adsorbed fluoride from the surface of LDHs. To investigate this, we recorded the zeta potential of sample Mg_Al-6 before and after fluoride adsorption. The results show a decrease in the zeta potential value from +7.2 mV to −10.2 mV (Figure S12 in the SI). This decrease in the zeta potential value supports the claim of adsorption of fluoride ions on the surface of LDHs. 3.10. Mechanism of Fluoride Adsorption. To investigate the fate of LDHs during the adsorption of fluoride, we carried out XRD, FTIR, SEM, and zeta potential studies on isolated LDHs after adsorption. No significant changes in XRD G



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patterns of LDH (Mg_Al-6) due to fluoride adsorption were observed (Figure S13 in the SI). This suggests that the interlayer carbonate ions remain mostly intact after adsorption which leads us to believe that the adsorption occurred on the surface of the LDH rather than ion exchange. This suggestion is further backed by the zeta potential data in which a decrease in the zeta potential value from +7.2 mV to −10.2 mV was observed for LDH before and after adsorption. The FTIR study also showed some changes in the peak position in the region of 400−800 cm−1 in LDH (sample Mg_Al-6) (Figure S14 in the SI) due to adsorption suggesting the involvement of the M−O or M−O−M bonds in the adsorptions. On the basis of the results of pre and postadsorption analysis of LDH, we conclude that the surfaces of the as-prepared LDHs contain surplus positive charges which act as the driving force for removal of the negatively charged fluoride ions. As a result the zeta potential value becomes negative. SEM images of LDH samples after adsorption also did not show any significant changes in the morphology thereby showing the stability of the adsorbent (Figure S15 in SI) during the process of adsorption and desorption.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Md. Harunar Rashid: 0000-0001-7470-8384 Funding

G.K.S. acknowledges the Science and Engineering Research Board, India, for providing the fellowship (N-PDF File No. PDF/2016/000110). Thanks are also due to partial support from Science and Engineering Research Board, India (EMR/ 2015/001912). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank SAIF-NEHU, NEIST-Jorhat, and Department of Chemistry, Gauhati University, for help with TEM, FESEM, zeta potential measurements, XRD, and TGA.

4. CONCLUSION We report the optimized reaction condition for the preparation of Mg/Al-CO3 LDHs by the urea hydrolysis method under hydrothermal condition. The LDHs are highly crystalline possessing flake-like structures even at the aging time of 6 h at 150 °C. The morphology and crystallinity of the prepared LDHs are well maintained even after 12 and 24 h of aging. However, a new phase such as Al(OH)3 can be observed with increased aging time. Increase in hydrothermal aging time has a profound effect on the specific surface area, and it was observed that the specific surface area gradually decreased on samples prepared at higher hydrothermal aging time. The assynthesized LDHs were successfully used as adsorbent for removal of fluoride from water. The adsorption process primarily followed pseudo-second-order kinetics and Freundlich isotherm for all the adsorbents. Langmuir adsorption capacity decreases as the aging time increases. The adsorption of fluoride is favorable at slightly acidic pH and the best adsorption was observed at pH 6.0. This is beneficial as in most of the cases natural drinking water has a pH in the ranges of 6.0 to 8.5. The presence of coexisting ions affects the fluoride adsorption, and the percent adsorption on LDHs (sample Mg_Al-6) decreases from 15.88% to 45.28% in the presence of coexisting ions. The LDHs are highly stable as no changes in morphology and crystallinity were observed due to fluoride adsorption. The LDHs exhibits better adsorption capacity compared to some of the reported adsorbents and are reusable up to five adsorption−desorption cycle. Thus, Mg/AlCO3 LDHs are suitable and effective for removal of fluoride ions from water.

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adsorption with dose of adsorbent, effect of anions and temperature on adsorption, zeta potential curves, XRD patterns and SEM images of the recovered samples (PDF)

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b00242. Table: characterization processes. Figures: EDX spectra, XPS survey scan spectra, FTIR spectra (before and after adsorption), plots based on intraparticle and liquid film diffusion model, Freundlich and Temkin isotherms, effect of pH on fluoride adsorption, variation of H



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