Layered Double Hydroxides (LDH), anionic clays, is a family of compounds which have attracted considerable attention in recent years^[1-4] such as catalysis^[5], adsorption^[6], ion exchange^[7], biomedical application^[8] and Defluoridation Capacity (DC)^[9].

  The structure of most of these materials corresponds to that of hydrotalcite^[10]. LDHs are characterized by the general formula [M^II_1-xM^III_x(OH)₂]^x+[X_n/x^n-m H₂O]^x-, symbolized by [M^IIM^III-X^n-], where M^II and M^III are the di- and trivalent cations and X^n- is an exchangeable interlayer anion, notably (carbonate, nitrate, halides, complexes anions, oxo-anion, etc.)^[11,12].

  A large number of LDH are a combination with divalent cations, for instance Mg²⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺ and trivalent cations, namely Al³⁺, Cr³⁺, Mn³⁺, Fe³⁺, Co³⁺, La^3+[13,14]. The charge density on the layers of the LDH will clearly depend upon the M^II / M^III ratio which usually lies between 2 and 4^[15].

  For synthesizing LDH, several methods are described, such as the hydrothermal route^[16], anionic exchange method^[17], and coprecipitation method^[18]. Amongst these methods, the co precipitation in aqueous solution is the most common method for LDH synthesis^[19]. The benefit of this method is that several synthesis parameters can be controlled independently during the precipitation process^[19,20]. Recently, hydrotalcite composite received attention in the domain of water treatment^[21]. Those Hydrotalcite-like compound, also known as Layered Double Hydroxides (LDH), constitute an important class of inorganic materials with desirable properties to remove anionic pollutants from water^[22,23]. The presence of sizable interlayer spaces and large quantity of exchangeable anions, make LDH compound and their calcined products excellent adsorbents for the removal of toxic anions from contaminated waters^[24-26].

  Fluoride contamination in drinking water, due to natural and anthropogenic activities, has been recognized as one of the major problems worldwide imposing a serious threat to Human health^[27]. Drinking water is the single major source of fluoride intake which, when consumed in excess, may lead to various diseases, especially osteoporosis, arthritis, cancer, brain damage, Alzheimer syndrome and thyroid disorder^[28,29]. Among several treatment technologies applied for Fluoride removal, there are various methods have been suggested to reduce the fluoride concentration in water, for instance, precipitation, ion exchange, filtration adsorption by LDH product^[30-34]. However, to our knowledge, the performance of as-synthesized LDH prepared under ultrasound irradiation in the defluoridation of water has not yet been reported. In fact the present work deals with the preparation of carbonate – intercalated, Mg-Al, LDHs by co precipitation method at constant p H and examines the effect of ultrasound treatment on the structural, textural and sorption behavior of fluoride anion from contaminated water by the LDH products. This study also investigates the thermal decomposition of Mg-Al LDHS1 prepared without ultrasound treatment.

Materials and Methods

Experimental
Preparation of [Mg-Al-CO₃]
  Mg–Al–CO₃–LDHs1; with Mg/Al ratio 3.00 was synthesized by coprecipitation method at constant pH. A solution of MgCl₂6H₂O (0.75 M) and Al (Cl)₃9H₂O (0.25 M) in 80 ml of distilled water was dropwise added at a constant rate (about 1 ml/min) at room temperature to 100 ml of aqueous solution containing 0.35 mol of NaOH and 0.09 mol of Na₂CO₃. The pH was maintained constant (pH = 10) by the drop wise addition of aqueous solution of NaOH (1 mol/l). Once the addition was completed, the solution was maintained at 75°C for 48h. The suspension was filtered, washed several times with distilled water, and then dried at room temperature and at 105°C for 18h.

  The conventional synthesis of Mg–Al–CO₃–LDH_s2 under ultrasound treatment was carried out following the same procedure as mentioned above without ultrasound treatment: Mg–Al–CO₃–LDH_s1 (at constant pH and composition ratio of Mg and Al was also the same as described above), then the suspension was conducted for 3 hours by ultrasound instruments.

The amplitude of the ultrasonic waves and pulses were set as follows:
- Amplitude of ultrasonic waves: 50%
- Pulse: 2 sec on; 1 sec off.
  The suspension was filtered, washed several times with distilled water, and then dried at room temperature and at 105°C for 18h.

Adsorption of fluoride
  The experimental procedure is carried out as follows: 0.1 g of the compound LDH prepared under ultrasound treatment (LDH_s2), calcined layered double hydroxides CLDH and the sample, prepared without ultrasound treatment (LDH_s1), are suspended each in a solution of NaF(10 mg/l). The pH of the solution is adjusted at 7 using 0.01 M HCl and/or NaOH. The solution is stirred at room temperature (25°C). Afterwards; the concentration of fluoride is determined by a specific probe (wtw ino Lab series Ph / ion745) at times t defined. We can determine the concentration of fluoride removed by mass of the compound. The amount of fluoride (Q_e) and removal efficiency (E) are calculated using the Following equations:
Q_e = ((C₀- C_e)/m) × V
E = Qe ×100
Where Q_e is the amount of fluoride adsorbed per unit, the mass of the adsorbent at equilibrium (mg / g), C₀ is the initial concentration of fluoride ion in the solution (mg.L^-1) and C_e is the equilibrium concentration of fluoride ion in the solution (mg.L^-1). V is the volume of the solution in contact with the adsorbents (L), m is the weight of the adsorbent used in the experiment (LDH_s1, CLDH or LDH_s2) (g) and E is the removal efficiency (%).

Structural characterisation techniques
X-Ray diffraction
  Powder X-Ray Diffraction (PXRD) patterns are determined on a P analytical X’ Pert PRO MPD powder X-ray diffractometer equipped with an X’cellerator detector operating with a secondary monochrometer and using a CuKα radiation source (K_α1 = 1.5406 Å and K_α2 = 1.5444 Å). The diffraction pattern was recorded under ambient atmosphere over an angular range of 5 - 120° (2θ), with a step length of 0.017° (2θ) and a counting time of 27.5.s step^-1. Each of the powder diffraction patterns confirmed that the synthetic compound was formed by rhombohedra symmetry, hydrotalcite -type phase (space group ).

Infrared spectroscopy
  The Infrared (IR) absorption spectrum was carried out on a pellet sample prepared by mixing 1.0 mg sample for a total weight (samples + KBr) of 200 mg. The spectrum was obtained on a Perkin-Elmer FT-IR system PC spectrophotometer (at the University of Sfax) in the 4000 – 400 cm^-1 range (30 scans) with 2 cm^-1 spectral resolution. A 200 mg KBr pellet was used as reference to correct the background.

DTA-TGA analysis
  In order to estimate the interlayer water content, thermo gravimetric (TGA) and Differential Thermal Analysis (DTA) were performed using a thermal analyzer (SDT Q600 V8.1 Build 99, TA instrument). Curves were recorded at room temperature up to 800°C at a rate of 10°C/min and using 13.66 mg of the sample, which was initially in equilibrium with this air flow.

Heating stage Raman spectroscopy
  The Raman spectra were recorded at room temperature in the backscattering configuration on a T64000 Jobin- Yvon-Horiba spectrometer equipped with the diffraction grating 600 lines/ mm under a microscope (Olympus Bx41) with a 100× objective focusing on the 514 nm line from an argon−krypton ion laser (coherent, Innova). The spot size of the laser was estimated at 0.8 μm and the spectral resolution at 2 cm⁻¹. The samples were isothermally annealed at different temperatures (50,100, 150, 230, 280, 320, 360, 400, 440, 500, 550 and 600°C).

Results

Characterization of [Mg-Al-CO₃^2-]
Structural characteristics
Effect of ultrasound treatment
  The XRD patterns for the resulting solids are shown in Figure 1. For comparison, the XRD patterns for the LDH synthesized in a conventional constant- pH process are also illustrated. In each case, The XRD patterns exhibit the characteristic reflections corresponding to the (003), (006), (009) and (110) planes for Mg-Al-CO₃-LDHS, respectively, indicating the formation of mostly well-crystallized hydrotalcite-like phase indexed to typical carbonate hydrotalcite structure with the rhombohedra system with the R3¯m space group (JCPDS 22-700).

Figure 1: The XRD patterns for the LDH synthesized in a conventional constant- pH process.
(a): Without Ultrasound treatment (Mg-Al-CO₃ LDH_S1)
(b): Under Ultrasound treatment (Mg-Al-CO₃ LDH_S2)

  For LDH synthesized under ultrasound treatment (Mg–Al–CO₃–LDH_s2), the reflection intensity decreases while the half width increases relative to that prepared by a constant pH without sonication, indicating smaller LDH crystallites or inhibited crystallization in the former process. The broadening of the (110) reflection around 60° indicated some disorder in the structure, which was directly related to a reduced aggregation of the particles during sonication. The XRD pattern of each prepared sample was analyzed by the Rietveld method, using the pattern matching routine of the Fullprof program^[35] integrated in Winploter software^[36]. The detailed structural parameters for the Mg–Al–CO₃– LDHs under different conditions are listed in Table1. The crystallite size of Mg-Al–CO₃ LDHS like- phase prepared by different methods can be calculated using Scherer equation^[37]:
  D = Bλ /β½ cosθ

  where D is the average crystallite size of the phase under investigation, B is the Scherer constant (0.89), λ is the wavelength of the X-ray beam used (1.54056 Å), β½ is the full width at half maximum (FWHM) of diffraction peak and θ is the diffraction angle. The calculation of the crystallite size by the Scherer formula was created and based on the first two peaks with (003) and (006) plane that characterize the hydrotalcite compound.

  The crystallite size in both a and c directions decreased due to the presence of ultrasound treatment compared to the crystallite size for Mg-Al LDH_S1 prepared at constant -pH without sonication.

Table 1: Structural parameters of LDH crystallines prepared using different methods.

^aa = 2d₁₁₀
^bc = 3d₀₀₃
^cValue calculated from the Scherrer equation

  The lattice parameters a = 2d₀₀₆ (cation–cation distance in the brucite-like layer) and c = 3dS₀₀₃ (thickness of one brucite-like layer and one interlayer) were calculated (Table 1). The parameter a (a = 3.06 Å) was the same and essentially independent of the synthesis method of Mg-Al LDH_s. As for the lattice parameter c, it corresponds to the 3 layers rhombohedra polytype 3R^[38]. The resulting interlayer distance d₀₀₃ = c/3 depends on the layer charge density, the nature of the interlayer anion (CO₃²⁻) and the number of water molecules in the interlayer space.

  The basal spacing, which corresponding to the d value of the 003 diffraction peak in the pattern, for the sample prepared under ultrasound treatment is c/3 = 8.766 Å, very much larger than the value reported for Mg-Al hydrotalcite prepared by conventional method without ultrasound treatment with what is expected for the intercalated carbonates^[38] setting in D^3h orientation with the 3-fold axis being parallel to the c-direction (d₀₀₃ = 7.65 Å at constant pH). This increase of the basal spacing related to the c parameter can be explicated by a different orientation of the lactate anion in the interlayer spacing that the stacking arrangement existing in the intercalated carbonate anion may be improved by the presence of ultrasound treatment, also causing a lowering of symmetry of carbonate anion of D_3h to C_3V symmetry. The interlayer region hosts the carbonate counter anions and water molecules that are placed in the intermediate plane between two adjacent sheets.

  Regarding the sample prepared at constant pH without sonication, the structure of the studied phase was refined as isotopic to the rhombohedra hydrotalcite [Mg_0.64 Al_0.36 (OH)₂](CO₃)_0.18, 0.46H₂O^[35]. The structure refinement was initiated by building octahedral coordination for metallic cations. The metals (Mg and Al) were distributed in the 3a (0, 0, 0) site of the R¯3m space group. Accordingly, one constraint, namely N (Co) + N (Al) = 1 was satisfied, i.e. 3a sites were totally occupied by Mg and Al cations.

  The oxygen atoms of hydroxyl groups are located in particular positions 6c (0, 0, z). As regards the carbon atoms of the carbonate groups in the interlayer domain species, they are located in 6c (1/3, 2/3, 0.5) positions. Besides, the oxygen atoms of the water molecules and those of the carbonate groups are all located in position 18h (x, -x, 0.5).

  The final refinement included the scale factor, profile shape and atomic parameters (positions, occupancies, and isotropic thermal displacements). The following good agreement factors are R_B = 5.2% and R_F = 3.50%. The results of refinement are given in Table 2 and the corresponding X-Ray diffraction pattern is shown in (Figure.2). The formula assigned to the compound must be Mg_0,74(4)A_l0,26(3)(OH)₂(CO₃)_0,13(2)(H₂O)_0,397(5). This composition corresponds to an Mg/Al ratio 3 that is identical to the initial mixture stoechiometri (0.75 /0. 25).

Table 2: Positional, occupancy and thermal paremeters with their standard deviations after Rietveld refinement of Co-Al-Co₃ LDH Mg_0,74(4)Al_0,26(3)(OH)₂(CO₃)_0,13(2)(H₂O)_0,397(5) in the R¯3m space group.

Figure 2: The X-ray diffraction pattern and the final Rietveld refinement plot of the Mg-Al-CO₃ LDH_S1. Points correspond to experimental values, and the continuous lines represent the calculated pattern ; vertical bars indicate the positions of Bragg peaks. The bottom trace depicts the difference between the experimental and the calculated intensity values.

Spectroscopic analysis
  The IR spectra of the two samples prepared without and under sonication are reported in (Figue.3). Each spectrum resembles those of other hydrotalcite – like phases^[18,38]. Typical for these spectra are the strong broad absorbance band vibration between 3000 and 3600 cm^-1 at around 3390 cm^-1 which are associated with OH-stretching vibrations from OH groups as well as interlayer water molecules.

Figure 3: (a) Infrared spectrum of Mg-Al-CO₃ LDH prepared without Ultrasound treatment (b) Infrared spectrum of Mg-Al-CO₃ LDH prepared under Ultrasound treatment.

  As for the sample prepared without ultrasound treatment, the bands in the range 400 - 900 cm^-1 provide evidence that the characteristic LDH network has been formed. The absorption band at 450 cm^-1 is due to the O-Al-O deformation mode (δ_O-Al-O), while those at 580 cm^-1 correspond to Al-O stretching vibrations (ν_O-Al-O)^[39,40]. The strong band observed at 1637.5 cm^-1 corresponds to the H-O-H deformation mode of intercalated water molecules(δ_(H2O)). The strong bands observed respectively at 1360 cm^-1 and 820 cm^-1 can be attributed to the anti-symmetric stretching mode ν₃ and the anti symmetric bending mode ν₂ of CO₃ groups, respectively. Moreover, the symmetric ν₄ (CO₃^2-) bending mode was observed at 611 cm^-1 which corresponds to intercalated CO₃^2- in D_3h symmetry^[41,42].

  Concerning the sample prepared under ultrasound treatment, the two vibration bands at around 1618 and 1638 cm^-1 are assigned, respectively, to the stretching vibrations and deformation of water molecules intercalated in the interlayer space. To our knowledge, a single band on the deformation mode (δH₂O) has been reported to LDH prepared by conventional coprecipitation without sonication^[41-44]. The appearance of two bands can be explained by the fact the ultrasound irradiation, also causing a lowering of symmetry of carbonate anion of D_3h to C_3v and we know that the oxygen atoms of water molecules and those of carbonates groups are located in same position in the interlayer spacing, this lowering of symmetry leads to formation of two, not equivalents groups of water molecule in the interlayer space, or one of two groups are assigned by the vibration bands at around 1618 cm^-1 and the other groups of water molecule are assigned by a vibration band at around 1638 cm^-1. The unperturbed carbonate ion is a planar triangle with point symmetry D_3h. The free ion, CO₃^2- with D_3h symmetry exhibits four normal vibrational modes: (I) asymmetric stretching vibration (ν₁), (II) an out-of-plane bend (ν₂), (III) a doubly degenerate asymmetric stretch (ν₃), and (IV) a doubly degenerate bending mode (ν₄).

  The symmetries of these modes are A1′ (R) + A2″ (IR) + E′ (R, IR) + E″ (R, IR) and Occur at 1063, 879, 1415 and 680 cm^-1, respectively. It can also be noted the presence of the IR active absorption band arising from the carbonate anion observed at 1360 – 1390 cm^-1 (ν₃), 800 cm^-1 (ν₂) and 628 - 670 cm^-1 (ν₄) providing evidence in this case, for the sample prepared under ultrasound treatment. The two bands observed in the spectra of the sample, prepared under ultrasound treatment in the region 1360 – 1390 cm^-1, can be attributed either to the disordered nature of the interlayer or to a lowering of the symmetry of the carbonate anions from D_3h to C_3v in the interlayer, which lifts the degeneracy of the ν₃ mode. Moreover, the vibration band ν₄ (CO₃^2-) which currency in two new bands at 628 and 669 cm^-1 well confirms this decline of symmetry that lifts the degeneracy of the ν₄ mode. The ν₁ mode (CO₃^2-) is usually inactive in infrared spectroscopy when carbonate anions are in symmetry D_3h. The appearance of this band at about 1000 cm^-1 suggests a lowering of symmetry for the carbonate ion of D_3h to C_3v^[37,38].

  The infrared absorption spectrum of the compound prepared under sonication confirming the results of structural studies by X-ray diffraction of this compound, such that the increase of the interlayer distance and thereafter the parameter c was expressed by the lowering of symmetry of the carbonate ion (D_3h to C_3v) and the presence of two types of water molecules in the interlayer spacing.

The thermal decomposition of Mg-Al- HTLcs
Differential thermal and thermo gravimetric analysis
  The thermal decomposition of the prepared synthetic Mg-Al-LDH_S1 compound was investigated by TGA-DTA analysis. According to the literature^[18-45] the thermal decomposition of LDHs includes three main stages which are the loss of adsorbed water, the decomposition of structural hydroxyl groups, and finally the decomposition of interlayer carbonate anions.

  The Thermal Analysis Curves (TGA/DTA) of the prepared LDH, displayed in Figure 4, are quite similar to those obtained and reported in previous research works^[46]. The TGA curve is characterized by a continuous mass loss without well-defined plateaux between the decomposition steps. The first mass loss of 2% at 50°C, corresponding to a broad endothermic peak, can be accredited to the loss of adsorbed water. It is followed by a second more pronounced and sharp endothermic phenomenon, around 230°C and corresponding to 14.5% of the total mass, due to the loss of hydration water from the interlayer region (decomposition of structural hydroxyl groups). A third step, extending up to 430°C and corresponding to 40.57% of the total mass , is assigned to the overlapped mass losses due to the dehydroxylation of the layers and the decomposition of the carbonates counter-anions as carbon dioxide. It is associated with a broad endothermic peak at 360°C. With respect to the TGA analysis, the Mg-Al-CO₃ LDH exhibits a total mass loss of 40, 57%. After decomposition at 550°C, the solid product is composed of a spinel oxide phase (MgAl₂O₄) and MgO oxide compound.

Figure 4: Thermal analysis curves (TGA/DTA) of Mg-Al-CO₃ LDH_s1.

Heating in the Raman microscope
  In general, two essential stages can be observed during phase transformation of LDHs upon heating: (i) a shift of LDH basal spacing associated with the release of interlayer water and (ii) the disappearing of the LDH diffraction lines and the formation of oxides phases. This result confirmed by TGA analysis can also be confirmed by Heating in the Raman microscope for the synthetic hydrotalcite, Mg_0,74(4)Al_0,26(3)(OH)₂(CO₃)_0,13(2)(H₂O)_0,397(5). Figure 5 reveals the Raman spectra of this sample recorded at room temperature, the strong bands around 553 cm^-1 are assigned before as the M–OH translation and deformation modes of the hydroxide layers. The presence of carbonate, CO₃ ^2- is reflected by the relatively broad and weak bands around 1058 cm^-1(ν₁ (CO₃^2-)) and 1224 cm^-1 (ν₃ (CO₃^2-)). The weak band at around 1670 cm^-1 is assigned to the deformation mode of the interlayer water molecules δ(H₂O). Figure 6 shows the effects of heating from 50°C to 600°C on the Raman spectra of synthetic hydrotalcite, Mg_0,74(4)Al_0,26(3)(OH)₂(CO₃)_0,13(2)(H₂O)_0,397(5).

Figure 5: The Raman spectra of Mg-Al-CO₃ LDH_s1 recorded at room temperature.

Figure 6: The Raman spectra of Mg-Al-CO₃ LDH_s1 heating from 50°C to 600°C.

  At 50°C, (Figure 7a) displays that the presence of the strong bands around 553 cm^-1 is assigned to the M–OH translation and deformation modes of the hydroxide layers. These bands strongly decrease in intensity upon heating; they disappear above 150°C. The intensities of these two bands, attributed to the presence of carbonate anion (ν₁ and ν₃ mode), do not show significant changes upon heating to approximately 100°C. Although the bands become better visible due to a decrease in intensity ν₁ and ν₃ disappear around 230 – 320°C while a new band becomes visible around 700 cm^-1 up to 360°C, can be reasonably considered as arising from the A_1g symmetry mode which is usually ascribed to the stretching vibrations of oxygen atoms inside the octahedral BO₆ unit corresponding to the Al-O stretching vibration (ν_Al-O) of octahedral AlO₆ of MgAl₂O₄ like-spinal^[47,48]. The strong band observed at 480 cm^-1 is thought to be due to E_g symmetry Mg-O stretching vibrations (ν_Mg-O) and the line around 191 cm^-1 is assigned to the lowest frequency T_2g symmetry species corresponding to the (Mg, Al)-O bending vibration (δ_(Mg,Al)-O) in the tetrahedral A site for MgAl₂O₄ like-spinel^[47,48]. This behavior is in good agreement with the TGA/DTA pattern where dehydroxylation and decarbonisation of the hydrotalcite structure start above 230°C.

Figure 7: XRD patterns of Mg-Al-CO₃ LDH_s1 in the range from room temperature to 600°C.

In situ variable temperature XRD analysis
   Figure 7 shows in situ HT-XRD patterns in the range from room temperature to 600°C of samples obtained without ultrasound irradiation. The studied transformations involve changes in the sample composition, in particular as regards water and interlamellar carbonates groups. With increasing temperature up to 100°C, the position of the (003) reflection shifts to higher angle indicating a decrease in interlayer spacing. The heating causes a displacement of the diffraction lines (00l) to large angles and changes of their intensities. The gradual decline of the ratio of intensities of the lines (006) and (003), from room temperature to 100°C, is associated with the elimination of physisorbed and interlayer water and the loss of hydrogen bonding without collapse of the layered structure.

  Indeed, the elimination of water molecules causes a reduction of the electron density and, consequently, the reduction of the intensities of the diffraction lines relating to diffraction planes types (00l). Between 100 and 350°C, a contraction of the interlamellar distance and partial decarbonation is produced. This gives rise to an increase in intensity and a shift to the high angles of the diffraction lines (00l) connected to the stacking sequence of the layers. The line (003) from 11.4° (7.56 Å) to 12.0° (6.91 Å), the line (006) 22.9° (3.88 Å) to 27.4° (3.26 Å) and the line (009) 33.6° to 38.9°. From this contraction of the interlamellar distance corresponds to the end of dehydration phenomenon and partial decarbonisation of summer slip started by the loss of a portion of the anions intercalated carbonates reflecting the creation of a partial disorder of the structure. In the temperature range 350 - 600°C, the diffraction lines of the HDL phase disappear and new lines corresponding to new phases, weakly crystallized, appear. They are the kind MgO oxide phase and the spinel phase MgAl₂O₄. From 550°C, oxides obtained are better crystallized.

Figure 8: The X-Ray diffraction pattern and the final Rietveld refinement plot for the mixture oxides product obtained after decomposition of the Mg-Al-CO₃ LDH_S1 at 600°C. Points correspond to experimental values, and the continuous lines represent the calculated pattern ; vertical bars indicate the positions of Bragg peaks. The bottom trace depicts the difference between the experimental and the calculated intensity values.
• Corresponds to the MgO phase,
*Correspond to the spinel phase MgAl₂O₄.

  Rietveld refinement of the powder diffraction X-ray chart of HDL decomposition product calcined at 600°C was achieved. Above 600°C, the characteristic peaks of mixed metal oxides are apparent; a mixture of spinel MgAl₂O₄ and MgO phase’s evolves. This result is confirmed by the rietveld refinement of the sample calcined at 600°C. Figure 8 presents the experimental and refined X-Ray diffraction pattern and their differences for the sample calcined at 600°C. Rietveld refinement in profile matching results revealed a mixture of two phases: periclase cubic MgO (35%) phase with refined cell parameter a = 4.127(3) Å in Fm¯3m space group and spinel-like phase MgAl₂O₄ (65%) with refined cell parameter a = 8.355(2) Å, space group Fd¯3m. This can indicate that after the decomposition of the hydrotalcite phase, aluminum tends to remain in a mixed oxide phase, while the MgO structure (periclase or rock-salt) has the same oxygen packing of MgAl₂O₄ spinel (cubic close packing), the percentage of each phase was calculated by the quantitative phase analysis with the Reference Intensity Ratio (RIR) method^[49].

Adsorption of fluoride
  Figure 9 shows that the best efficiency of defluoridation is obtained with the compound prepared under ultrasound treatment, corresponding to an amount of fluoride absorbed equal to 91.55 mg / g of sample. It is 64.6% for the Mg-Al HDL calcined at 600°C and 61.9% in HDL prepared without sonication. Besides, the saturation time (50 minutes) is significantly less for the sample prepared under sonication than for the sample calcined at 600°C (150 min) and than for the HDL prepared by conventional method (170 min). This result indicates that a higher basal spacing and smaller crystallites size of hydrotalcite compound favor the fluoride removal in contaminated water.

Figure 9: Measurement of the removal efficiency (E) of fluoride ion in function of time for: (a) HDL prepared without sonication, (b) product calcined at 600°C. (c) HDL prepared under sonication.

Conclusion

  Mg-Al hydrotalcite-like compounds were synthesized by the coprecipitation method and under ultrasound treatment. The ultrasound treatment has a clear influence not only on the structural, textural and chemical properties of the produced LDH minerals, but also on the fluoride removal properties. Besides, the ultrasound treatment can cause an increase in basal spacing and a decrease in crystallites size compared to the sample prepared without ultrasound treatment. From the present study, it can be seen that the ultrasound treatment can be used effectively for the removal of the fluoride anions from aqueous solutions.

  The crystallographic formula shown for the sample prepared without ultrasound treatment after Rietveld refinement is [Mg_0,74(4)Al_0,26(3)(OH)₂(CO₃)_0,13(2)(H₂O)_0,397(5)]. Heating stages Raman microscopy for this sample were shown, in this paper, to be a very useful tool to study the structural changes and reactions like dehydration, dehydroxylation and decarbonisation in crystalline materials like hydrotalcites. During heat treatment along with the conventional techniques as XRD and TGA/ DTA. Further heating up to 600°C dehydroxylation is generally completed and new phases, including spinel (MgAl₂O₄) and MgO are formed. This result can be confirmed by the rietveld refinement of a sample calcined at 600°C.

Acknowledge:
  The authors are particularly grateful to Professor Mr. Alain Bulou (LUMAN Université, Université du Maine, CNRS UMR 6283, Institut des Molécules et Materiaux du Mans (IMMM)) for performing Raman and Infrared measurements. The authors wish to express their gratitude to M. Wassim Hriz from the Faculty of Science, Sfax, Tunisia for his valuable proofreading and language polishing services.

References