Study on synthesizing Mg / Al layered double hydroxides at different pHs

Mg/Al layered double hydroxide (LDH) was successfully synthesized at different pHs values. The Mg/AL LDH was well characterized by X-Ray diffraction and Fourier transform infrared analysis. The morphology of the LDH was observed using Scanning electron microscopy with energy dispersive X-ray spectroscopy. The influence of pH values on the morphology of the Mg/Al LDHs were studied. The result showed that the well-synthesized Mg/Al LDHs could be obtained when the pH value was about 10.0 at room temperature.


INTRODUCTION
The terms layered double hydroxides (LDHs) or hydrotalcite-like compounds (HTLc) are used to designate synthetic or natural lamellar hydroxides with two or more kinds of metallic cations in the main layers and hydrated interlayer domains containing anionic species [1].This large family of compounds is also commonly referred to as hydrotalcites or anionic clays, the latter term indicating a complementarity with the more usual cationic clays whose interlamellar domains contain cationic species [2].Mg/Al layered double hydroxide (Mg/Al LDH: [Mg 2+  1-x •Al 3+ x (OH) 2 ](A n-) x/ n •mH 2 O) is widely recognized as a clay mineral exhibiting an anion exchange property [3][4][5].LDHs are a layered structure with positively charged brucite-like sheets, where Mg and Al are octahedrally coordinated.The excess charge is balanced by anions in the interlayer, together with water molecules [6].Recently, LDHs have attracted much attention because of their unique applications in many fields.For example, they can be used as catalysts, catalyst supports, ion exchangers, adsorbents, and pigments and also be used in sensor and magnetic technologies [6][7][8].Scotter and Ken B, prepared Ni/Al, Mg/Al, Co/ Al, Cu/Al LDHs and used them as the catalysts of the steam reforming of methanol to produce hydrogen [9].Alejandre et al. used Cu/Ni/Al LDHs as precursors of catalysts for the wet air oxidation of phenol aqueous solutions.Most of these advanced functions depend strongly on the composition, size, and morphology [10].Nowadays, an intensive economic growth of Mongolia is due to high price of copper mining product, gold production and other mineral resources [11].Therefore, ground and surface water are polluted with metal oxy anions caused by mining activities.To date, no previous report has described a study of the removal of metal oxy anions from water using LDH in Mongolia.Therefore, the LDH and its derivatives are able to use as one of the best adsorption materials removing metal oxy anions from water.The aim of this study was to synthesize primary Mg/Al LDH at different pHs and to investigate their physicochemical characterization.The synthesis and structure of the Mg/Al LDH was examined using X-Ray diffractometer (XRD), Fourier transform infrared (FT-IR) spectroscopy, Scanning electron microscopy with energy dispersive X-ray (SEM-EDX) spectroscopy.Measurements: FT-IR spectra (4000-400cm -1 , KBr disc) were recorded on a Bruker Alpha spectrometer.X-ray diffraction patterns were recorded using CuK α radiation (n = 1.5418Ǻ) on a Philips PW1800 diffractometer operating at 40 kV and 40 mA with 0.25 o divergence slit, 0.5 o anti-scattter slit, between 1.5 and 20  (2θ) at a step size of 0.0167 o .A scanning electron microscope (SEM, S-3400 N, Hitachi, Tokyo, Japan) equipped with energy dispersive X-ray (EDX) was used.pH values were measured using pH 300 (Hanna Instruments, Italy) digital pH meter.

RESULTS AND DISCUSSION
Synthesized Mg/Al LDH was examined by chemical analysis with mass percentage of Mg, Al, and precipitation yields of LDHs prepared at different pHs which were confirmed with EDX analysis (Table 1).As seen from the table, the Mg:Al ratio of 3:1 suggests the incomplete precipitation of Mg 2+ ions at pH of 8.The yield was dramatically increased while the pH level was increasing till 10.However, it was decreased gradually with pH level above 10.

Characterization of LDH:
Mg/Al layered double hydroxides with various pHs were successfully synthesized by co-precipitation method.As seen from the XRD patterns (Fig. 1), the hydrotalcite type phase was identified in all synthesized layered double hydroxides except LDH with pH value 8 [13].More intensive and sharper reflections with ( 003) and ( 006   .However, this band is not clear at the Figure 2 (a).Very strong peaks CO 3 2-were appeared at 1373-1365 cm -1 in spectra of the LDHs with pH value above 8.
The symmetry of the carbonate anions is lowered D 3h to C 2s resulting in activation of the IR inactive mode at 1050 cm -1 .This peak is observed Figure 2(c).They are also ascribed to the metal-oxygen vibration (500-1000 cm -1 ).

CONCLUSIONS
This study was dealt with the investigation synthesized the Mg/Al LDH and its physico-chemical properties.The confirmation of structures and morphology of the Mg/Al LDH with different pHs were performed by XRD, FTIR and SEM-EDX.The Mg/Al LDH was confirmed as hydrocalcite like compound using XRD.The morphology of the Mg/Al LDH was observed homogenously distributed nano particles.The molar ratio of Mg to Al was 2.57.The peaks corresponded to CO 3 2-groups and water molecules at 1360 cm -1 , and 3465 cm -1 were certified FTIR, respectively.This investigation leads further possibilities to study rest of derivatives of LDHs and their physicochemical characterizations.

Figure 2 (
c-f) also show the FT-IR spectra of Mg/Al LDH with increasing pH which were in the range of 4000 to 500 cm -1 region.In the investigation on all LDH, the spectra reveal the presence of coordinated water molecules that indicated by broad bands at 3400-3500 cm -1[12].This peak was attributed to stretching vibrations of the OH groups of water molecules.The peak around 1640 cm −1 can be attributed to vibration of H 2 O.The spectra in Figure2(b-e) are shown a broad band 3200-3000 cm -1 which is attributed to the bridging mode H 2 O-CO 3 2-

Fig. 2 .
Fig. 2. SEM image of Mg/Al LDH and their EDX results.