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Characterization of Bayer Hydrotalcites Formed from Bauxite Refinery Residue Liquor Sara J. Palmer and Ray L. Frost* Chemistry Discipline, Faculty of Science and Technology, Queensland University of Technology, Queensland 4001, Australia ABSTRACT: The precipitate formed during the seawater neutralization (SWN) of Bayer liquors has been characterized by a variety of techniques, including X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), inductively coupled plasmaoptical emission spectroscopy (ICP-OES), infrared and Raman spectroscopy, and thermal analysis. Three mineralogical phases are detected: (1) hydrotalcite, (2) calcite (CaCO3), and (3) aragonite (CaCO3). It is proposed that two hydrotalcite structures form and have the formulas Mg8Al2(OH)12(CO32,SO42) 3 xH2O and Mg6Al2(OH)16(CO32,SO42) 3 xH2O. The Mg, Al molar ratio of the Bayer hydrotalcite is dependent on both the pH and the composition of the initial Bayer liquor. It is proposed that carbonate and sulfate ions are intercalated predominantly into the hydrotalcite interlayer; however, small amounts of arsenate, vanadate, and molybdate have been shown to be removed from solution. The formation of Bayer hydrotalcite assists in the removal of oxy-anions of transition metals from bauxite refinery residues, through a combination of intercalation and adsorption reactions involving the newly formed hydrotalcite. Infrared and Raman spectroscopy also confirmed the presence of hydrotalcite, calcite, and aragonite, showing characteristic wavenumbers of hydroxyl stretching modes for hydrotalcite, and antisymmetric stretching modes of carbonate for the calcium carbonate minerals. ICP-OES also confirmed the removal of oxy-anions from Bayer liquors.
1. INTRODUCTION The Bayer process produces large quantities of bauxite refinery residues (red mud), which requires treatment before it can be safely discharged into the environment or stored in tailing dams. The alkaline residue (45% liquor and 55% solid) consists primarily of iron oxides, aluminum oxides, silica oxides, titanium oxides, and trace heavy metals. Seawater neutralization is one such treatment that reduces both the pH and dissolved metal concentrations of the residue, through the precipitation of magnesium, calcium, and aluminum hydroxides and carbonate minerals.1 The formation of these hydrotalcite-like compounds removes oxy-anions of transition metals through a combination of intercalation and adsorption mechanisms. Hydrotalcites are a variety of layered double hydroxides (LDHs), which consist of stacked layers of metal cations (M2þ and M3þ) similar to brucite-like structures M(OH)2. Substitution of divalent cations for trivalent ones, of similar radii, gives rise to positively charged layers.2,3 The general formula for these structures is as follows: [M2þ1x M3þx(OH)2]xþAmx/m 3 nH2O, where M2þ is a divalent cation, M3þ is a trivalent cation, and A is an interlamellar anion with charge m. LDH phases exist with 0.2 e x e 0.33.4 The positive layer charge is neutralized through the intercalation and adsorption of a variety of anionic species, including halides, oxy-anions, oxy- and polyoxy-metallates, anionic complexes, and organic anions.5 The affinity of anions for the interlayer region is based on the anion’s charge density and size. An increase in anionic charge results in the electrostatic interactions between the positively charged hydroxide layer and the anion to become stronger, therefore rendering a more stable hydrotalcite. Hydrotalcites have been synthesized using Bayer refinery liquors under seawater neutralization (SWN) conditions. This investigation will enable the identification of the type of LDH r 2011 American Chemical Society
that forms during the SWN of bauxite refinery residues, and the mechanism of formation. Currently, there is no known method for the separation of hydrotalcite from the other components of SWN red mud.
2. EXPERIMENTAL SECTION 2.1. Synthesis of Bayer Precipitate. The Bayer precipitate, containing hydrotalcite, was prepared by the addition of seawater (Inskip Point, QLD, Australia, 2008) to Bayer liquor (provided by an Australian aluminum refinery) at a volumetric ratio of 4.5:1. Bayer liquor refers to the combination of 1 part red mud liquor (RML) and 0.9 parts supernatant liquor (SNL). The compositions of the two Bayer liquors are provided in Table 1. The solution was stirred thoroughly for 2 h before being vacuum filtered and dried overnight in an oven (85 °C). The average pH and temperature of the solution was 9.1 and 25.7 °C, respectively. The pH of solution is dependent on the composition of the Bayer liquor. The seawater neutralization process is similar to the co-precipitation method used for the synthesis of hydrotalcite-like structures. The results presented in this paper are an average of three Bayer precipitates that were prepared. 2.2. Characterization of Bayer Precipitate. 2.2.1. X-ray Diffraction. X-ray diffraction (XRD) patterns were collected using a Philips X’pert wide-angle X-ray diffractometer, operating in step scan mode, with Cu KR radiation (1.54052 Å). Patterns were collected in the range from 3 to 90° 2θ with a step size of 0.02° and a rate of 30 s/step. Samples were prepared as a finely pressed powder into aluminum sample holders. The Profile Received: August 30, 2010 Accepted: March 18, 2011 Revised: January 17, 2011 Published: March 28, 2011 5346
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Table 1. Composition of Bayer liquors alumina (g/L Al2O3) caustic (g/L Na2O) carbonate (g/L Na2O) RML
5.4
14.6
n/a
SNL
2.8
3.0
9.9
Fitting option of the software uses a model that employs 12 intrinsic parameters to describe the profile, the instrumental aberration, and wavelength-dependent contributions to the profile. 2.2.2. Energy-Dispersive X-ray Spectroscopy. For X-ray microanalysis (EDX), samples were coated with a thin layer of evaporated carbon for conduction and examined in a JEOL 840A analytical SEM (scanning electron microscope) at 25 kV accelerating voltage. Microanalysis of the clusters of fine crystals was carried out using a full standards quantitative procedure on the JEOL 840 SEM using a Moran Scientific microanalysis system (Tokyo, Japan). Oxygen was not measured directly but calculated using assumed stoichiometries to the other elements analyzed. 2.2.3. Inductively Coupled PlasmaOptical Emission Spectrometry. Samples of the initial Bayer liquor and resulting solution after the SWN process were analyzed using a Varian inductively coupled plasmaoptical emission spectrometry (ICPOES) instrument. The samples were not diluted before analysis due to the low concentrations of anionic species. Standards containing aluminate, arsenate, vanadate, and molybdate were prepared to establish a calibration curve. Results were obtained using an integration time of 3 s with three replications. The relative amounts of each atom were recorded on a Varian Liberty 2000 ICP-OES at wavelengths of 394.400, 311.837, 202.032, and 188.980 nm for Al, V, Mo, and As, respectively. 2.2.4. Raman and Infrared Spectroscopy. The Fourier transform Raman spectroscopy (FT-Raman) analyses were performed on powder samples pressed in a sample holder suitable for the spectrometer using a Perkin-Elmer System 2000 Fourier transform spectrometer equipped with a Raman accessory, comprising of a Spectron Laser System SL301 and Nd:YAG laser operating at a wavelength of 1064 nm. Infrared spectra were obtained using a Nicolet Nexus 870 Fourier transform infrared spectrometer (FTIR) with a smart endurance single bounce diamond ATR (attenuated total reflectance) cell. Spectra over the 5254000 cm1 range were obtained by the coaddition of 128 scans with a resolution of 4 cm1 and a mirror velocity of 0.6329 m/s. Spectral manipulation such as baseline correction, smoothing, and normalization was performed using the GRAMS software package (Galactic Industries Corp., Salem, NH, USA). Band component analysis was undertaken using the Jandel “Peakfit” software package, which enabled the type of fitting function to be selected and allows specific parameters to be fixed or varied accordingly. Band fitting was undertaken using a LorentzGauss cross-product function with the minimum number of component bands used for the fitting process. The LorentzGauss ratio was maintained at values greater than 0.7 and fitting was undertaken until reproducible results were obtained with squared correlations of r2 greater than 0.995. 2.2.5. Thermogravimetric Analysis. Thermal decomposition of the hydrotalcite was carried out in a TA Instrument incorporated with a high-resolution thermogravimetric analyzer (series Q500) in a flowing nitrogen atmosphere (80 cm3/min). Approximately
Figure 1. XRD pattern of precipitate formed during the SWN of Bayer liquor.
50 mg of sample was heated in an open platinum crucible at a rate of 2.0 °C/min up to 1000 °C. The synthesized hydrotalcites were kept in an oven (85 °C) for 24 h before thermogravimetric analysis (TGA). Thus, the mass losses are calculated as a percentage on a dry basis.
3. RESULTS AND DISCUSSION 3.1. X-ray Diffraction. The XRD pattern of the precipitate that formed by the SWN of Bayer liquors and the corresponding reference patterns is shown in Figure 1. Three mineralogical phases are detected: (1) hydrotalcite, (2) calcite (CaCO3), and (3) aragonite (CaCO3). Sodium chloride is present due to the excess of seawater used in the neutralization of bauxite refinery residues. NaCl does not precipitate but forms by the evaporation of water during the drying process. The full width at half-maximum (FWHM) of the d(003) hydrotalcite peak indicates that small crystallites form. The formation of hydrotalcite occurs through the reaction of magnesium cations and sulfate anions in seawater and aluminate (Al(OH)4), hydroxide (OH), and carbonate (CO32) ions in Bayer liquor. The d(003) reflection for the Bayer hydrotalcite was 7.76 Å. The d(003) spacing is in good agreement with literature.6,7 Changes in the 003 reflection indicated a slight change in the interlayer distance of the hydrotalcite, where an increase in interlayer space results in a larger d(003) spacing. Synthetic carbonate hydrotalcites, prepared using SWN conditions, had basal spacings of around 7.66 Å. The increase in the d(003) spacing for Bayer hydrotalcite suggests that anions larger than carbonate are intercalated into the structure, such as the oxy-anions of transition metals found in residue liquors and sulfate in seawater. The minimal increase of the d(003) spacings suggests that only a small quantity of anions other than carbonate are intercalated. 3.2. EDX Analysis. The major elements detected using EDX are magnesium, aluminum, sulfur, sodium, calcium and chloride. Deviations in the Mg, Al ratios are expected for different Bayer liquors because the aluminate concentration does not remain constant and differs between refineries and the bauxites used. An average Mg, Al ratio of 3.4:1 is obtained for this particular Bayer liquor, which had a low aluminate concentration, Table 2. It is thought a mixture of different hydrotalcite species forms during the SWN process. The formation of hydrotalcite is highly pHdependent, with low M2þ:M3þ hydrotalcite structures forming at high pH. The SWN process consists of a large pH range (pH 13 to pH 8), which suggests that a mixture of 3:1 (pH 10) and 5347
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Table 2. Percentage Removal of Al3þ, (AsO4)3, (VO4)3, and (MoO2)2 during the SWN of Bayer Liquors aluminate (Al3þ)
arsenate (AsO4)3
vanadate (VO4)3
molybdate (MoO2)2
initial concn (ppm)
1489 ( 5.0
6.700 ( 4.8
32.50 ( 5.1
1.650 ( 4.9
final concn (ppm)
0.2952 ( 5.0
0.4464 ( 4.8
14.04 ( 5.1
1.164 ( 4.9
% removal
99.98 ( 5.0
93.34 ( 4.8
56.80 ( 5.1
29.45 ( 4.9
Figure 3. Infrared spectrum of Bayer hydrotalcite in the carbonate vibrational region.
Figure 2. Infrared and Raman spectra of the Bayer precipitate in the hydroxyl stretching region.
4:1 (pH 8) hydrotalcite structures form. The formulas for the two hydrotalcite structures that form during the SWN process are as follows: Mg8Al2(OH)12(CO32,SO42-) 3 xH2O (4:1) and Mg6Al2(OH)16(CO32,SO42) 3 xH2O (3:1). The broadness of the 003 reflection in the XRD pattern, Figure 1, suggests that overlapping of similar types of hydrotalcites is possible. 3.3. ICP-OES Analysis. The concentrations of aluminate, arsenate, vanadate, and molybdate were analyzed before and after the SWN process to determine the removal percentage of each of the ions found in Bayer liquor. The removal percentage and initial and final concentrations for each of the ionic species are given in Table 2. The high removal percentage of aluminate from solution was due to the formation of the hydrotalcite hydroxyl layers. The removal of aluminate from Bayer liquors is essential for the safe disposal and storage of these refinery residues, and it appears that the SWN process is an effective way of reducing the concentration of aluminate in the residue. The pH of solution during the neutralization process suggests that vanadate exists as VO43, HVO42, and H2VO4, while arsenate can exist as AsO43 and HAsO42.8 It should also be noted that polyanions may also exist. However, these polyanions are considered to be physically too large to intercalate into the hydrotalcite interlayer.
The removal of arsenate, vanadate, and molybdate from Bayer liquors confirms that the formation of hydrotalcite during the SWN process removes a variety of anionic species. However, due to the presence of the higher affinity anions (small anionic size and charge density) such as carbonate and sulfate, it is proposed that arsenate, vanadate, and molybdate are removed primarily by adsorption reactions rather than intercalation reactions. Removal of these oxy-anions is essential before these refinery residues can be safely disposed or stored. 3.4. Raman and Infrared Spectroscopy. The infrared and Raman spectra of Bayer precipitate observed broad intense bands centered at around 3400 cm1, Figure 2. The broad bands are attributed to the stretching modes of hydroxyl groups in the hydroxyl layers and water molecules associated with the hydrotalcite structure. Band component analysis was used to help identify the different hydroxyl species in both the Raman and infrared spectra. Water hydroxyl stretching vibrations are intense in infrared spectroscopy because of the large change in dipole moment. The bands at lower wavenumbers in the infrared spectrum, 3047, 2908, and 2752 cm1, are suggested to be strongly hydrogen bonded water molecules to interlayer anions, such as carbonate and sulfate (predominately).9 The absence of these bands in the Raman spectrum is due to water being a very weak Raman scatterer. The bands at 3223 cm1 in the infrared and 3229 cm1 in the Raman spectrum are attributed to the OH stretching vibrations of water coordinated to the cations in the brucite-like layers.9 The bands at higher wavenumbers in the Raman (3387 and 3440 cm1) and infrared (3414 and 3595 cm1) spectra are assigned to the OH stretching vibrations of water bonded to M3OH units (where M might be Mg or Al and any combinational permutation of these metals).9 The infrared spectra of the (CO3)2 antisymmetric stretching region, Figure 3, showed four bands observed at 1491, 1459, 1403, and 1361 cm1. These bands are attributed to the different carbonate anions.8 Results from XRD revealed that calcite and 5348
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Figure 4. Raman spectrum of Bayer precipitate in the 1150 to 950 cm1 region.
Figure 5. Raman spectrum of Bayer precipitate in the 800 to 200 cm1 region.
aragonite (calcium carbonates) precipitated along with hydrotalcite. Therefore, the bands are assigned to carbonate in the two forms of calcium carbonate and carbonate in the hydrotalcite
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Figure 6. DTG curves of Bayer precipitate, hydrotalcite, calcium carbonate, and seawater.
interlayer. The band at 1491 cm1 is attributed to the ν3 mode of aragonite, 1459 cm1 is assigned to the ν3 mode of calcite, and 1403 cm1 is assigned to carbonate bonded to water in the hydrotalcite interlayer, while the band at 1361 cm1 is assigned to carbonate bonded to the hydroxyl surface of hydrotalcite.8 The ν1 mode of aragonite is observed at 1086 cm1, and at 1118 cm1 for calcite.8 Minerals and hydrotalcites containing physically adsorbed water give strong water deformation modes at around 1640 cm1.9 The numerous bands in this region indicate that there is a variety of water species present in the precipitate. Such water molecules may be hydrogen bonded to the cationic hydroxyl surface or to adjacent water molecules in the hydrotalcite interlayer. The position of the bands indicates that a number of anions were bonded with interlayer water. The larger bands at 1655 and 1631 cm1 are proposed to be due to sulfate and carbonate bridging bonds. The lower wavenumber region in the Raman spectrum is shown in Figure 4 (9501150 cm1) and Figure 5 (200800 cm1). The intense peak at 1085 cm1 is attributed to carbonate vibrations in phases of calcium carbonate (aragonite and calcite). Bands at 280 and 711 cm1 are assigned to calcite, while the band at 703 cm1 is due to aragonite. These bands are attributed to the ν4 planar bending mode of carbonate. Bands at 1062 and 1075 cm1 are assigned to the carbonate ν1 symmetric stretching modes.8 These carbonate vibrational bands are assigned to carbonate chemically bonded to the hydrotalcite hydroxyl surface (1075 cm1), and the band at 1062 cm1 is assigned to carbonate bonded to water in the interlayer of the hydrotalcite structure. In the lower wavenumber region, bands at 552 and 465 cm1 are believed to be attributed to the AlOAl and MgOMg linkage bonds in hydrotalcites, respectively. 5349
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in the works by Palmer et al.1012 The presence of these decomposition bands suggests that the bands obtained are associated with the dehydroxylation and decarbonation of the hydrotalcite structure. The ion current curve for the synthetic hydrotalcite showed the evolution of water vapor at 316 °C, confirming the loss of OH units at 313 °C, and the evolution of CO2 at 350 °C, which confirms the loss of carbonate anions in the interlayer. XRD of the thermal analysis products were conducted (not given), and from these results the following decomposition steps have been derived: 3:1 hydrotalcite : Mg6 Al2 ðOHÞ16 ðCO3 2 ; SO4 2 Þ 3 xH2 O T Mg6 Al2 ðOHÞ16 ðCO3 2 ; SO4 2 Þ þ xH2 O Figure 7. TG/DTG curve of the Bayer precipitate.
The observation of bands at 981 and 992 cm1 suggests the presence of sulfate anions in the precipitate.8 The sulfate bands may have originated from sulfate anions intercalated and/or adsorbed into/onto the hydrotalcite structure. The absence of bands in the region 800900 cm1 suggests that the intercalation of arsenate, molybdate, and vanadate is limited.10,11 ICP analysis showed that only a small concentration of these anions (less than 35 ppm) are present in Bayer liquor, which is lower than the detection limit of FT-Raman spectroscopy. The intensity of these bands and the absence of any other sulfate species in the XRD pattern indicate that the sulfate detected is associated with the hydrotalcite structure. The appearance of two sulfate bands, suggests that the sulfate anions are present in two different chemical environments: (1) sulfate bonded to the cationic surface of the hydroxyl layer and (2) sulfate bonded to interlayer water. 3.5. Thermogravimetric Analysis. Due to the precipitate containing a variety of mineralogical phases, the TG/DTG curves are slightly more complex than pure samples of hydrotalcite. Due to the possibility of organic compounds being present, a mass spectrometer (MS) was not used to identify each component that was being evolved at the corresponding decomposition temperatures. Organic compounds block the capillary in the MS. Therefore, on the basis of the results determined by XRD, samples of the other components of the precipitate were analyzed to identify the decomposition steps, Figure 6. The thermal analysis of the precipitate, Figure 7, showed four main decomposition steps. The DTG curves for synthetic hydrotalcite (Mg6Al2(OH)16(CO32) 3 5H2O), calcium carbonate, and seawater were band component fitted to identify the different decomposition steps. The decomposition steps for the precipitate are as follows: (1) the loss of adsorbed water on the surface of the precipitate (up to 100 °C), (2) dehydroxylation and decarbonation of the hydrotalcite structure (between 200 and 400 °C), (3) the decomposition of the two calcium carbonates (500700 °C), and (4) the dehydration (melting) of sodium chloride/seawater (700900 °C). The largest mass loss, 24.73%, is assigned to the removal of adsorbed water on the external surface of the precipitate. The absence of a peak at slightly higher temperatures, 100200 °C, suggests that the hydrotalcite does not contain a large quantity of interlayer water. Two mass loss steps were observed at 360 °C and a shoulder at 313 °C. The decomposition temperatures obtained are in good agreement with the synthetic hydrotalcite, whose details of preparation and characterization are published
Mg6 Al2 ðOHÞ16 ðCO3 2 ; SO4 2 Þ f MgAl2 O4 þ 5MgO þ ðCO2 ; SO2 Þ þ 8H2 O þ O2 4:1 hydrotalcite : Mg8 Al2 ðOHÞ18 ðCO3 2 ; SO4 2 Þ 3 xH2 O T Mg8 Al2 ðOHÞ18 ðCO3 2 ; SO4 2 Þ þ xH2 O Mg8 Al2 ðOHÞ18 ðCO3 2 ; SO4 2 Þ f MgAl2 O4 þ 7MgO þ ðCO2 ; SO2 Þ þ 9H2 O þ 2O2 The delay in the decarbonation temperature for the Bayer precipitate, compared to the carbonate hydrotalcite, suggests that the Bayer hydrotalcite is thermally more stable. It is proposed that the intercalation of other anions into the Bayer hydrotalcite, such as sulfate, arsenate, and vanadate, increased the structure's thermal stability due to a substantial number of hydroxyl groups involved in a network of hydrogen bonds involving these more negatively charged anions. As previously mentioned, the increase in the d(003) spacing compared to the carbonate hydrotalcite suggests that these larger anionic species are intercalated into the structure. The thermal analysis of carbonate hydrotalcite, calcium carbonate, and seawater enabled the identification of the decomposition steps of the precipitate that formed during the SWN of Bayer liquor. The decomposition of calcite and aragonite is believed to occur at a temperature of around 600 °C. The combined mass loss of 9.42% between 500 and 650 °C is due to the releasing of CO2 by decomposing of calcium carbonate species. The decomposition temperatures observed in this study are in good agreement with the decomposition of calcium carbonate reported in the literature.13
4. CONCLUSIONS The results obtained by XRD determined the mineralogical composition of the precipitate to be comprised of four components, (1) hydrotalcite, (2) sodium chloride, (3) calcite, and (4) aragonite. Infrared and Raman spectroscopy also confirmed the presence of hydrotalcite, calcite, and aragonite, showing characteristic wavenumbers of hydroxyl stretching modes for hydrotalcite and antisymmetric stretching modes of carbonate for the calcium carbonate minerals. The formation of the hydrotalcite species is believed to have contributed to the removal of arsenate, vanadate, and molybdate from Bayer liquors. The mechanism for removal of these anions is believed to be due to 5350
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Industrial & Engineering Chemistry Research intercalation into the hydrotalcite structure and/or adsorption onto the external surface of the hydrotalcite structure. The Raman spectrum of the precipitate identified the presence of sulfate, represented by bands at 981 and 992 cm1. Due to the pH range the Bayer hydrotalcite forms in, two hydrotalcite structures are believed to form: Mg8Al2(OH)12(CO32, SO42) 3 xH2O (4:1) and Mg6Al2(OH)16(CO32,SO42) 3 xH2O (3:1). The Bayer hydrotalcite is proposed to be more stable than the carbonate-only hydrotalcite synthesized under the same conditions. This enhancement in thermal stability is proposed to be due to the large quantity of hydrogen bonds formed by the hydroxyl surface of the hydrotalcite and arsenate, vanadate, and molybdate.
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(13) Frost, R. L.; Hales, M. C.; Martens, W. N. Thermogravimetric analysis of selected group (II) carbonate minerals. Implication for the geosequestration of greenhouse gases. J. Therm. Anal. Calorim. 2009, 95, 999–1005.
’ AUTHOR INFORMATION Corresponding Author
*Fax: þ61 7 3138 1804. Tel.: þ61 7 3138 2407. E-mail: r.
[email protected].
’ ACKNOWLEDGMENT The financial and infrastructure support of the Queensland Research and Development Centre (RioTintoAlcan) and the Queensland University of Technology Inorganic Materials Research Program of the School of Physical and Chemical Sciences are gratefully acknowledged. ’ REFERENCES (1) Hanahan, C.; McConchie, D.; Pohl, J.; Creelman, R.; Clark, M.; Stocksiek, C. Chemistry of seawater neutralization of bauxite refinery residues (red mud). Environ. Eng. Sci 2004, 21, 125–138. (2) Costantino, U.; Marmottini, F.; Nocchetti, M.; Vivani, R. New synthetic routes to hydrotalcite-like compounds. Characterization and properties of the obtained materials. Eur. J. Inorg. Chem. 1998, 50, 1439–1446. (3) Frost, R. L.; Ding, Z.; Kloprogge, J. T. The application of nearinfrared spectroscopy to the study of brucite and hydrotalcite structure. Can. J. Anal. Sci. Spectrosc. 2000, 45, 96–102. (4) Frost, R. L.; Erickson, K. L. Thermal decomposition of synthetic hydrotalcites reevesite and pyroaurite. J. Therm. Anal. Calorim. 2004, 76, 217–225. (5) Taylor, H. F. W. Crystal structures of some double hydroxide minerals. Mineral. Mag. 1973, 39, 377–389. (6) Kloprogge, J. T.; Frost, R. L. Fourier transform infrared and Raman spectroscopic study of the local structure of Mg-, Ni-, and Co-hydrotalcites. J. Solid State Chem. 1999, 146, 506–515. (7) Ruan, H. D.; Frost, R. L.; Kloprogge, J. T.; Duong, L. Infrared spectroscopy of goethite dehydroxylation: III. FT-IR microscopy of in situ study of the thermal transformation of goethite to hematite. Spectrochim. Acta, Part A 2002, 58, 967–981. (8) Farmer, V. C. The Infrared Spectra of Minerals; Mineralogical Society: London, 1974. (9) Rives, V. Layered Double Hydroxides: Present and Future; Nova Science: New York, 2001. (10) Palmer, S. J.; Frost, R. L.; Ayoko, G.; Nguyen, T. Synthesis and Raman spectroscopic characterization of hydrotalcite with CO32 and (MoO4)2 anions in the interlayer. J. Raman Spectrosc. 2008, 39, 395–401. (11) Palmer, S. J.; Nguyen, T.; Frost, R. L. Synthesis and Raman spectroscopic characterisation of hydrotalcite with CO32 and VO3 anions in the interlayer. J. Raman Spectrosc. 2007, 38, 1602–1608. (12) Palmer, S. J.; Frost, R. L. The effect of synthesis temperature on the formation of hydrotalcites in Bayer liquor—A vibrational spectroscopic analysis. Appl. Spectrosc. 2009, 63, 748–752. 5351
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