The Formation of Different Mg – Al LDHs ( Mg / Al = 2 : 1 ) Under Hydrothermal Conditions and Their Application for Zn 2 + ions Removal

The formation of different Mg-Al LDHs (Mg/Al = 2:1) under hydrothermal conditions (200 °C; 4–24 h) was investigated in the 4MgCO3⋅Mg(OH)2⋅5H2O – γ-Al2O3/Al(OH)3 – H2O and Mg5(CO3)4(OH)2·4H2O – γ-Al2O3 – H2O systems. It was determined that chemical nature of the initial Mg containing components changes the formation mechanism of the synthesis products during isothermal curing. Magnesium aluminum hydroxide hydrate is crystallized by using 4MgCO3⋅Mg(OH)2⋅5H2O as starting material while hydroxide hydrate in the Mg5(CO3)4(OH)2·4H2O presence. The sequence of the compounds formation is presented. It should be noted that different modifications of Al containing components have only a slight influence on the meixnerite-type LDH hydrothermal synthesis. It was determined that after 45 min of sorption all Zn ions are incorporated into the crystal structure of hydrotalcite. It should be underlined that crystallinity of the latter compound do not change during ion exchange experiments. Thus, the latter compound can be used as adsorbent for Zn ion removal. Synthesized samples were characterized by powder X-Ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), simultaneous thermal analysis (STA) and scanning electron microscopy (SEM).


Introduction
Layered double hydroxides (LDHs) can be described by general formula [M(II) 1x M(III) x (OH) 2 ] x+ (A n-) x/n ⋅mH 2 O, where M(II) -divalent (e. g.Zn 2+ , Fe 2+ , Mg 2+ , Ni 2+ , Cu 2+ ) and M(III) -trivalent metals (e. g.Al 3+ , Ga 3+ , Fe 3+ , Cr 3+ ), and A n-is an anion (e. g.Cl -, CO 3 2-, OH -, NO 3 -, SO 4 2-) [1,2].x has usually values between 0.20 and 0.33 [3].Their structure is based on brucite-like layers, in which a divalent metal cation is located in the center of oxygen octahedra and two-dimensional infinite layers are formed by edge-sharing of the octahedra.Partial isomorphous substitution of trivalent cations for divalent ones results in a positive charge on the layers.Organic or inorganic anions are intercalated between the layers in order to maintain charge balance, and crystallization water is also generally found in the interlayer galleries [4].Recently, LDHs attracted more attention due to their unique properties which are widely applied in anion exchange reactions and basic catalysis [5][6][7][8].
Layered double hydroxides may be synthesized by using various methods.The most common way to obtain these compounds is simple coprecipitation and its modifications (e. g. precipitation at low and high supersaturation, urea hydrolysis and etc.) [9][10][11][12][13].Meanwhile, mentioned methods have some disadvantages, such as pH value, i. e. alkaline solution should be maintained during preparation, temperature and time for LDHs to precipitate (the addition of metal salt solutions takes from one to two hours) [14].S. K. Sharma and the others claimed that the pH value mostly varies from 7 up to 10 in the temperature range of 60 -80 °C [15].
The important parameters determining the applications of LDHs are degree of crystallinity and textural properties, which depend on the used synthesis method [15,16].These properties are influenced by various parameters such as the nature of the bivalent and trivalent cations and their ratio, agining time, temperature and etc.As a result, the hydrothermal treatment received much attention in the preparation of LDHs, because it allows to control crystallite size, crystallinity, the shape of crystals by changing temperature, synthesis time or divalent and trivalent metals molar ratio [17,18].However, the literature concerning this LDHs synthesis method is limited.
Hydrotalcite has important characteristics that make it interesting for various applications.Firstly, hydrotalcite was widely studied and successfully used as basic catalyst for several reactions such as self condensation, cross-aldol condensation of aldehides and ketones, Knoevenagel condensation, Claisen-Smidth condensation, Michael addition.Secondly, this compound has a good ion-exchange capacity which leads to use it also for the removal of heavy metals from waste water.
H. Wang and co-workers [19] studied the defluoridation of drinking water by the latter compound.The results showed that calcined hydrotalcite samples exceeded greater F - uptake than uncalcined samples [19].Other researchers [2] also compared the uptake of Br - anions between calcined and uncalcined HT samples and got the same results.M. Park and co-workers [20] investigated the adsorbtion of Cu 2+ and Pb 2+ cations by hydrotalcite and found that removal of mentioned cations from aqueous solutions resulted in precipitation of Cu 7 Cl 4 (OH)10•H 2 O, PbCO 3 PbCl 2 and Pb(OH)Cl compounds on the surface of HT, i. e. the surface adsorption proceeded.
In previous work was observed that during isothermal curing an intermediate compounds (boehmite and magnesium carbonate) was formed when Mg/Al ratio was equal to 3:1 [21].
In order to decrease the amount of undesirable compounds and the cost of hydrothermal treatment, this work is focused on investigation of different Mg-Al LDHs with a Mg/Al ratio 2:1 under hydrothermal conditions.The application for removing Zn 2+ ions is also studied in this paper.
It should be underlined that Mg containing raw materials (magnesium carbonate hydroxide hydrate and basic magnesium carbonate) have the similar empirical formula, however the thermal properties of these compounds are different as it is shown in Fig. 1.It was determined that Mg(OH) 2 dehydration begins at 277 °C temperature.Moreover, the decarbonization was observed at 449 °C temperature (Fig. 1, a).Meanwhile, only two endothermic effects reflecting dehydration (257 °C) and decarbonization (456 °C ) are visible in the Mg 5 (CO 3 ) 4 ⋅(OH) 2 ⋅4H 2 O STA curve (Fig. 1, b).It should be noted that the recrystallization to magnesium oxide at 520 °C temperature is not characteristic to basic magnesium carbonate (Mg 5 (CO 3 ) 4 ⋅(OH) 2 ⋅4H 2 O).Dry primary mixtures having a Mg/Al ratio equal to 2:1 were mixed with water (water/solid ratio of the suspension W/S = 10.0).Hydrothermal synthesis was carried out in unstirred suspensions under the saturated steam pressure at 200 ºC temperature; the duration of isothermal curing was 4 and 24 hours.The products of the synthesis were filtered, rinsed with ethyl alcohol to prevent carbonization of material, dried at 50 ºC ± 5 temperature, and sieved through a sieve with a mesh width of 50 μm.
The X-ray powder diffraction (XRD) data were collected with a DRON-6 X-ray diffractometer with Bragg-Brentano geometry using Ni-filtered Cu K α radiation and graphite monochromator, operating with the voltage of 30 kV and emission current of 20 mA.The step-scan covered the angular range 2-60 o (2θ) in steps of 2θ = 0.02 o .
Simultaneous thermal analysis (STA: differential scanning calorimetry -DSC and thermogravimetry -TG) was also employed for measuring the thermal stability and phase transformation of synthesized products at a heating rate of 15 o C/min, the temperature ranged from 30 o C up to 1000 o C under air atmosphere.The test was carried out on a Netzsch instrument STA 409 PC Luxx.The ceramic sample handlers and crucibles of Pt-Rh were used.
FT-IR spectrum have been carried out by using a Perkin Elmer FT-IR Spectrum X system.The specimen was prepared by mixing 1 mg of the sample with 200 mg of KBr.The spectral analysis was performed in the range of 4000-400 cm -1 with spectral resolution of 1 cm -1 .
Scanning electron microscopy (SEM) (FEI Quanta 200 instrument) of the samples was performed using 30 kV accelerating voltage and a working distance of 9.2 mm, detector -ETD.
Ion exchange experiments were carried out at 25 °C in a Grant SUB14 thermostatic adsorber by stirring 1 g of hydrotalcite in 100 ml of Zn(NO 3 ) 2 aqueous solution, containing 0.3 g/dm 3 of Zn 2+ ions, for 45 min.The percentage of Zn 2+ ions exchange was determined using a Perkin-Elmer Analyst 4000 spectrometer based on the changes of the concentrations of the cations in the solution and the cations in the hydrotalcite.
It should be underlined that the intensity of diffraction peaks characteristic to Mg 6 Al 2 (OH) 18 ⋅4.5H 2 O increased in 1.5 times after prolonging the hydrothermal treatment duration up to 24 hours (Fig. 2, b).These data were confirmed by FT-IR spectrum: the absorption maximum reflecting Mg-OH and Al-OH bond vibration at 3479 cm -1 is broader (Fig. 3, b).Meanwhile, the absorption band at 554 cm -1 , which can be ascribed to Al-O bond vibration, is more visible.Moreover, STA analysis data showed the increase of endothermic effects at 238 and 315 °C temperatures characteristic to Mg-Al-OH LDH dehydroxylation (Fig. 4, b).It is interesting that after 24 h of hydrothermal synthesis the intensity of diffraction peaks of both magnesium carbonate and magnesium hydroxide is decreased (Fig. 2, b).It is clearly visible in FT-IR spectrum: only a shoulder at 3697 cm -1 reflecting Mg-OH bond vibrations and also the decrease in absorption bands intensity at 1430, 1414, 881 and 748 cm -1 characteristic to Simultaneous thermal analysis data is in good agreement with XRD and FT-IR (Fig. 4).After 24 h of isothermal curing, the intensity of endothermic effect at 629 °C temperature reflecting MgCO 3 decarbonization is decreased.It should be noted that it is a two-step process, e. g. decarbonization also identified at 431 °C temperature and overlaps with dehydration of Mg(OH) 2 (Fig. 4, b

b) a)
The images of SEM analysis showed three different crystal morphology: an magnesium aluminum layered double hydroxide aggregates with indefinite structure, thin Mg(OH) 2 plates and rhombohedral MgCO 3 crystals.The crystals size varied from 2 to 10 μm (Fig. 5).
Thus, due to a low reactivity of both magnesium containing component and intermediates (MgCO 3 , Mg(OH) 2 ) Mg 6 Al 2 (OH) 18 ⋅4.5H 2 O is dominant in the synthesis products.Unfortunately, a fair amount of MgCO 3 and Mg(OH) 2 were found.
On purpose to exceed better interaction between Mg and Al containing components, γ-Al 2 O 3 is used as initial Al containing component.
It was determined that the intensity of diffraction peaks characteristic to Mg-Al-OH LDH remain almost the same after 4 hours of hydrothermal treatment compared with results obtained by using Al(OH) 3 (Fig. 6, a).Meanwhile, the areas of endothermic effects at 229 °C and 314 °C temperatures, are significantly increased from 26.89 to 72.41 μV/ mg (Fig. 4, a, 217 °C) and from 6.94 to 18.61 μV/ mg (Fig. 4, a, 306 °C) in the STA curve (Fig. 7, a).Furhermore, FT-IR spectrum data showed that absorption maximum at 3483 cm -1 is broader (Fig. 8, a).The basic diffraction maximum (d-spacing -0.780 nm) markedly increased in 1.5 times after prolonging the synthesis duration up to 24 hours (Fig. 6, b).It should be underlined that after 24 hours of isothermal curing only a slight decrease in magnesium hydroxide (Fig 7, b; endothermic peak at 432 °C temperature) and magnesium carbonate ( Fig. 7, b; endothermic peaks at 432 °C and 633 °C temperatures) amount were observed (Fig. 6, b).Mg(OH) 2 is not fully reacted during all experimental conditions, though the absorption band at 3696 cm -1 decreases (Fig. 8, b).The same tendency noticed in the case with MgCO 3 (Fig. 8, b; absorption bands at 1400, 884, 748 cm -1 ).Meanwhile, H O − bond vibrations at 3055 cm -1 can be assigned to 4MgCO 3 ⋅Mg(OH) 2 ⋅5H 2 O (Fig. 8, b).
SEM analysis data shows the same crystal morphology as in the samples with aluminum hydroxide (Fig. 9).Furthermore, the prolonging of the hydrothermal synthesis duration causes the increase in crystal size from small to larger crystals.During hydrothermal synthesis products formation can be represented as: As seen from the reaction, in the beginning of it, the initial Mg component not fully decomposes into MgCO 3 and Mg(OH) 2 .During interaction between formed products and Al containing compounds Mg-Al-OH LDH is formed, as a result the prolonging of synthesis duration (24 h) causes the increase of latter compound amount in synthesis products.It should be noted that only a part of magnesium hydroxide is combined to Mg 6 Al 2 (OH) 18 ⋅4.5H 2 O. ) ( CO H HO vibrations) and can be ascribed to the Mg-Al-CO 3 LDH.‫-‬ The absorption bands of carbonates are observed at 1403, 882 cm -1 and can be assigned to magnesium carbonate.It should be noted, that these vibrations dissapear after prolonging the synthesis duration.Moreover, the bands at 1366 and 684 cm -1 is also due to Simultaneous thermal analysis data confirms the formation of hydrotalcite.These thermal conversions were identified after 4 h of hydrothermal synthesis (Fig. 12 It should be underlined that the crystallinity of hydrotalcite increased after prolonging the synthesis duration up to 24 hours.Furthermore, a fair amount of boehmite still remain.Meanwhile, magnesium carbonate is fully reacted (Fig. 5).It is clearly visible from SEM images: after 24 hours of hydrothermal synthesis rhombohedral MgCO 3 crystals are undetected and a hexagonal hydrotalcite plates are more visible.

Characterization of hydrotalcite in the
During hydrothermal synthesis products formation can be represented as: Unfortunatelly, a fair amount of boehmite still remain even after 24 hours of isothermal curing.It should be noted that magnesium carbonate is fully reacted.As a result, during synthesis more CO 3 2--free anions, which intercalate in LDH interlayer, are produced.

A test reaction for Zn 2+ ion removal
In order to evaluate sorption capacity of hydrotalcite (200 °C, 3h), adsorption of Zn 2+ ions from Zn(NO 3 ) 2 solution was performed.It was determined that in alkaline solution the sorption proceeds effectively at the beginning of reaction.After 30 s, the amount of adsorbed Zn 2+ ions in the crystal lattice of hydrotalcite is equal to 19.54 Zn 2+ mg /g when the initial concentration of Zn 2+ ions is 0.3 g/dm 3 (Fig. 14, a).Meanwhile, ion-exchange equilibrium is reached already after 5 minutes and 75 % of Zn 2+ ions are incorporated into the crystal structure of hydrotalcite (Fig. 14, b).It was observed that the crystallinity of hydrotalcite does not change during the ion exchange experiments.It should be noted that the cation exchange reactions are not reversible process.In order to confirme this fact, after sorption process hydrotalcite substituted with Zn 2+ ions was dryed up and immersed in distilled water.It was proved that Zn 2+ ions did not appear in the solution after 240 minutes at 25 o C.
Thus, hydrotalcite can be used as adsorbent for Zn 2+ ion removal.The results presented here show that the latter compound has a good ion-exchange capacity and further research is needed to understand better its exchange and selectivity properties.In the future work, the synthesized hydrotalcites will be used for cleaning heavy metals from waste water.

Conclusions
1. Different modifications of Al containing components has only a slight influence on the course of meixnerite-type LDH formation by using hydrothermal treatment.For this reason, the synthesis of the latter compound is a little more efficient when γ-Al 2 O 3 is used as initial Al containing component.2. It was determined that partial soluble magnesium carbonate hydroxide hydrate is not stable under hydrothermal conditions and decomposes into MgCO 3 and Mg(OH) 2 compounds, which reacted very heavily with Al containing components.As a result, a fair amount of lowreactive intermediates (MgCO 3 , Mg(OH) 2 ) were remain in the synthesis products even after 24 hours of isothermal curing.3. The crystallinity of magnesium carbonate hydroxide hydrate markedly increased in 1.5 times after prolonging the synthesis duration up to 24 hours.4. Chemical nature of the initial Mg containing components changes the formation mechanism of the synthesis products.It was determined that magnesium aluminum hydroxide hydrate are formed by using 4MgCO 3 •Mg(OH) 2 •5H 2 O as starting material.Meanwhile, the hydrotalcite are observed in the Mg 5 (CO 3 ) 4 (OH) 2 •4H 2 O presence.This is due to the different properties of formed MgCO 3 .During hydrothermal reaction Mg 5 (CO 3 ) 4 (OH) 2 •4H 2 O produces magnesium carbonate of CO 3 2--free anions.These anions intercalate in LDH interlayer to form hydrotalcite while the other Mg containing raw material forms more stable, low-reactive MgCO 3 .5. After 24 hours of isothermal curing the intensity of hydrotalcite diffraction peaks only slightly increases while the crystallinity of boehmite significantly increased in 2 times.It should be noted that magnesium carbonate is fully reacted after prolonging the synthesis duration.6.It was determined that after 45 min of sorption ~75 % of the Zn 2+ ions are incorporated into the crystal structure of hydrotalcite.It was determined that the cation exchange reactions are not reversible and specific to chemisorption process.It should be underlined that crystallinity of the latter compound does not change during ion exchange experiments.

Fig. 5 .
SEM images of synthesis products when the duration of hydrothermal synthesis at 200 °C is equal to 24 h and the magnitude is: a -2000, b -5000.Raw materials are 4MgCO 3 •Mg(OH) 2 •5H 2 O and Al(OH) 3 .

Fig. 12 .
can be ascribed to hydrotalcite.‫-‬ The absorption bands at 1067, 778 and 552 cm -1 reflecting Al-O bond vibrations proceed in boehmite crystal structure.STA curves of synthesis products when raw materials are Mg 5 (CO 3 ) 4 (OH) 2 •4H 2 O and γ-Al 2 O 3 .The duration of hydrothermal synthesis at 200 °C is equal to 4 (a) and 24 h (b).
): ‫-‬ At 245 °C temperature the interlayer water of Mg-Al-CO 3 LDH is lost.‫-‬ 323 °C temperature reflects a partial dehydroxylation of hydrotalcite brucite-like sheets.It should be underlined that OH -group bonded with Al 3+ is lost.This process overlaps with boehmite dehydration.‫-‬ At 439 °C temperature the decarbonization of hydrotalcite and magnesium carbonate is observed.‫-‬ At 592 °C temperature, the recrystallization of boehmite to γ-Al 2 O 3 takes place.It should be noted that it overlaps with the formation of mixed metal oxides and can be assigned to hydrotalcite.‫-‬ 846 °C temperature reflects mixed metal oxides recrystallization to spinel phase, which is characteristic to Mg-Al-CO 3 LDH.a) b) Fig. 13.SEM images of synthesis products when the duration of hydrothermal synthesis at 200 °C is equal to 24 h and the magnitude is: a -5000, b -10000.Raw materials are Mg 5 (CO 3 ) 4 (OH) 2 •4H 2 O and γ-Al 2 O 3 .