Use of Hydrotalcites for the Removal of Toxic Anions from Aqueous

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Ind. Eng. Chem. Res. 2010, 49, 8969–8976

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Use of Hydrotalcites for the Removal of Toxic Anions from Aqueous Solutions Sara J. Palmer and Ray L. Frost* Chemistry Discipline, Faculty of Science and Technology, Queensland UniVersity of Technology, Queensland, Australia

The removal of toxic anions has been achieved using hydrotalcite via two methods: (1) coprecipitation and (2) thermal activation. Hydrotalcite formed via the coprecipitation method, using solutions containing arsenate and vanadate up to pH 10, are able to remove more than 95% of the toxic anions (0.2 M) from solution. The removal of toxic anions in solutions with a pH of >10 reduces the removal uptake percentage to ∼75%. Raman spectroscopy observed multiple A1 stretching modes of V-O and As-O at ∼930 and ∼810 cm-1, assigned to vanadate and arsenate, respectively. Analysis of the intensity and position of the A1 stretching modes helped to identify the vanadate and arsenate specie intercalated into the hydrotalcite structure. It has been determined that 3:1 hydrotalcite structure predominantly intercalate anions into the interlayer region, while the 2:1 and 4:1 hydrotalcite structures shows a large portion of anions being removed from solution by adsorption processes. Treatment of carbonate solutions (0.2 M) containing arsenate and vanadate (0.2 M) three times with thermally activated hydrotalcite has been shown to remove 76% and 81% of the toxic anions, respectively. Thermally activated hydrotalcite with a Mg:Al ratio of 2:1, 3:1, and 4:1 have all been shown to remove 95% of arsenate and vanadate (25 ppm). At increased concentrations of arsenate and vanadate, the removal uptake percentage decreased significantly, except for the 4:1 thermally activated hydrotalcite. Thermally activated Bayer hydrotalcite has also been shown to be highly effective in the removal of arsenate and vanadate. The thermal activation of the solid residue component (red mud) removes 30% of anions from solution (100 ppm of both anions), while seawater-neutralized red mud removes 70%. The formation of hydrotalcite during the seawater neutralization process removes anions via two mechanisms, rather than one observed for thermally activated red mud. 1. Introduction Hydrotalcites consist of stacked layers of metal cations (M2+ and M3+) similar to brucite-like structures. They are part of a common group of minerals known as layered double hydroxides (LDHs). The brucite-type layers are stacked on top of each other and are held together by weak interactions through the H atoms.1 Substitution of divalent cations for trivalent cations with similar radii, where a maximum of one in three trivalent sites are substituted by a divalent cation,2 gives rise to positively charged layers.3,4 The general formula for these structures is [M1-x2+ Mx3+(OH)2]x+Ax/mm- · nH2O, where M2+ is a divalent cation, M3+ a trivalent cation, and A an interlamellar anion with charge m-. Hydrotalcite phases exist for 0.2 e x e 0.33.5 The resultant positive charge, caused by the substitution of aluminum, is neutralized through the intercalation and adsorption of anions. Charge neutrality is not confined to the interlayer region, but also to the external surfaces of the hydrotalcite structure. Because there is no overall charge, hydrotalcites are quite stable. The interlayer region of LDHs are complex, consisting of anions, water molecules, and other neutral or charged moieties. A large variety of anionic species can be positioned between the hydroxide layers, including halides, oxy-anions, oxy and polyoxy-metallates, anionic complexes, and organic anions.6 The interlayer interactions of LDHs are mediated by Coulombic forces between the positively charged layers and the anions in the interlayer, and hydrogen bonding between the hydroxyl groups of the layer with the anions and the water molecules in the interlayer.6,7 Water molecules are connected through extensive hydrogen bonding to the hydroxyl ions of the metal hydroxide layers and interlayer anions.8-10 * To whom correspondence should be addressed. Tel.: +61 7 3138 2407. Fax: +61 7 3138 1804. E-mail: [email protected].

LDHs have a so-called “memory effect”, whereby a hydrotalcite material can be thermally treated to remove water, hydroxyl, and carbonate units from its matrix, then rehydrated in an aqueous solution to return to its original structure.11,12 This memory effect can be used effectively to remove harmful anions, both organic13,14 and inorganic,15,16 from wastewater solutions. The calcination of hydrotalcite, at temperatures of 350-600 °C, removes interlayer water, interlayer anions (carbonate anions), and hydroxyls. The result is the collapse of the crystalline hydrotalcite to an amorphous magnesium oxide with dispersed Al3+ ions as a solid solution.11,15-17 The carbonate anions are decomposed to carbon dioxide (CO2) and O2-, leaving O2- anions between the layers.18-21 Rehydrating the calcined product regenerates the LDH, where water is absorbed to reform the hydroxyl layers, as well as being adsorbed into the interlayer, along with the anion in solution. Smith et al.22,23 found that the composition of hydrotalcite is dependent on the pH, where hydrotalcite formed at high pH (pH >13) had a Mg:Al ratio of 2:1 (Mg4Al2(CO3)(OH)12 · xH2O), while those precipitated at pH 8 had a Mg:Al ratio of 4:1 (Mg8Al2(CO3)(OH)20 · xH2O). At high pH, a more stable microcrystalline carbonate hydrotalcite forms, because of the readily adsorbed CO2 from the atmosphere, producing a saturated carbonate solution. At lower pH (pH 0.7 and fitting was undertaken until reproducible results were obtained with squared correlations of r2 > 0.995. 3. Results and Discussion 3.1. Removal of Anions by Hydrotalcite Formation (Coprecipitation Method). The coprecipitation of hydrotalcite in solutions containing toxic anions has the ability to remove the toxic anions provided certain criteria are met: (1) the anion has a high charge density, (2) the anion has a small particle size, and (3) there exists a low concentration of high-affinity anions in solution. Removal of the toxic anions can be facilitated via intercalation, adsorption, or a combination of both. Intercalation of the anionic specie involves a network of hydrogen bonds between the intercalated anion, water, cationic surface, and other anionic species between the hydroxyl layers. Adsorption, on the other hand, involves the bonding of anionic species to the hydroxyl units on the external surface of the hydrotalcite. Intercalation of the anionic specie is preferable due to more available bonding sites and increased stability, because of a larger number of bonds that are involved with the intercalated anion. This investigation looks at the removal of arsenate and vanadate from aqueous solutions. Synthetic hydrotalcites were prepared at pH 8, 10, and 13 to assess the effectiveness (percentage removal) of the removal of 0.1 and 0.2 M of arsenate and vanadate from solution (see Figure 1). The following results will examine the formation of hydrotalcite from four solutions:

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(1) carbonate (0.1M) and vanadate (0.1 M), (2) carbonate (0.1 M) and arsenate (0.1 M), (3) vanadate (0.2 M), and (4) arsenate (0.2 M).These hydrotalcites will be referred to as (1) HT(CO32-,VO43-), (2) HT(CO32-,AsO43-), (3) HT(VO43-), and (4) HT(AsO43-). It is clearly observed that the pH of solution has a significant effect on the removal of toxic anions from solution. The removal of vanadate in solutions with pH 8 [(HT(CO32-,VO43-) and HT(VO43-)] is highly successful, with 99% of vanadate being removed, even in the presence of the high-affinity anion carbonate. The removal of arsenate also achieved high removal percentages; however, the presence of carbonate did reduce the amount of arsenate removed from solution slightly (97%). In the absence of carbonate, 99% of arsenate is removed from solution. Increasing the alkalinity of solution caused a reduction in the removal percentage of arsenate and vanadate. The removal of vanadate decreased from 99% to 97%, which is a minimal reduction. However, the removal of arsenate decreased to 88% [HT(CO32-,AsO43-)] and 89% [HT(AsO43-)]. The reduction in arsenate removal is due to the intercalation of hydroxide ions (OH-). Vanadate is proposed to be less affected by the pH increase due to a species change, whereby the H2VO4- anion at pH 8 transforms to the HVO42- anion at pH 10. The increase in anion charge and decrease in anion size increases the vanadate anions affinity for the interlayer allowing it to compete more strongly with the increased hydroxide concentration. At pH 8 and 10 arsenate is present in solution as the HAsO42- anion. Therefore, the protonated arsenate anion has the same anion affinity and is more vulnerable to competition of hydroxide ions at higher pH. Increasing the pH of solution to pH 13 significantly reduces the removal percentages for both vanadate and arsenate. At pH 13, vanadate exists as VO43- and arsenate exists as AsO43-. Even though the affinity of both anions increased for this pH range, the sheer concentration of hydroxide anions in solution reduced the amount of toxic anions that could be removed using hydrotalcite. At high pH, carbon dioxide (CO2) dissolution increases, which means carbonate also may have been competing. The removal of vanadate and arsenate were both between 72% and 78%. At high pH, it is also possible for the hydrotalcite to form the 2:1 structure [Mg4Al2(OH)12(anion) · xH2O], along with the impurity brucite. The formation of brucite will reduce the quantity of hydrotalcite, which therefore reduces the uptake of anions. The Raman spectra of hydrotalcites formed using pH 8 solutions containing (1) 0.2 M carbonate [HT(CO32-)], (2) 0.2 M vanadate [HT(VO43-)], (3) 0.2 M arsenate [HT(AsO43-)], and (4) 0.67 M of carbonate, vanadate, and arsenate [HT(CO32-,VO43-,AsO43-)] is given in Figure 2. The inclusion of the three anions is clearly observed: (i) carbonate (1100-1000 cm-1), (ii) vanadate (950-750 cm-1), and (iii) arsenate (925-800 cm-1). Bands attributed to carbonate are located at 1061, 1058, and 1030 cm-1, and are assigned to the symmetric stretching modes of carbonate. The presence of multiple bands in this region suggests that carbonate exists in different environments (slightly different bonding of the CO32- anion). It is proposed that carbonate is bonded to water in the interlayer or other anions in the hydrotalcite interlayer (1061 and 1058 cm-1), and also bonded to the external surface of the hydrotalcite structure (1030 cm-1).6,34 The Raman spectrum of HT(VO43-) displays four broad overlapping bands centered at 930 cm-1 and is assigned to the A1 stretching modes of V-O. The multiple V-O vibrational modes are believed to be due to different bonding strengths of the V-O bond. At pH 8, vanadate is most likely present as

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Figure 2. Raman spectra of hydrotalcite formed in arsenate and vanadate solutions at pH 8.

H2VO4-. It is proposed that the vibrational band at 939 cm-1 is due to the VdO symmetric stretching mode of the tetrahedral vanadate anion. Because of hydrogen bonding with the O-, the symmetry of the original tetrahedral vanadate structure is slightly obscured; thus, a shift to lower wavenumbers occurs. It is proposed that the lower wavenumber band at ∼815 cm-1 is due to V-OH bonds (weakest V-O bond in the protonated vanadate anion). Within the hydrotalcite structure, there are multiple bands (occurring at slightly different wavenumbers) because of this vibration, and this is shown by the broadness of the V-OH band at 815 cm-1. The bands at 907 and 879 cm-1 are believed to be due to the V-O- stretching modes, which are bonded to

the hydroxyl surface of the hydrotalcite, interlayer water, and other anionic species. The Raman spectrum for HT(AsO43-) also observed four bands: 911, 876, 841, and 808 cm-1, attributed to the A1 stretching modes of As-O. These different vibrational modes are attributed to different bonding strengths of the As-O bond, as previously mentioned for the vanadate anion. The band at 876 cm-1 is assigned to the AsdO bond of the tetrahedral arsenate anion and the As-O symmetric stretching mode. At pH 8, arsenate is predominantly present as the protonated arsenate anion (HAsO42-). It is proposed that the very broad band at 841 cm-1 is attributed to the symmetric stretch of As-O-, while at lower wavenumbers (808 cm-1), it is due to As-OH symmetric stretching vibrational modes. The broadness of the band at 841 cm-1 is due to multiple As-O- bands, originating from bonds that vary slightly in bond strengths caused by the bonding of the arsenate anion with other species in the hydrotalcite interlayer. The relative areas under the bands at 876 and 808 cm-1 indicates that there are approximately the same number of As-OH bonds present in the hydrotalcite structure as there are AsdO bonds, which suggests that the anion is in the HAsO42- form (1:1 ratio of As-OH to AsdO). The combination of XRD, thermal analysis, and Raman spectroscopy (Figures 3-5) has enabled determination of the mechanism involved in the removal of vanadate and arsenate from solution. Hydrotalcites with different cationic ratios (2:1, 3:1, and 4:1) have been analyzed to determine the mechanism that vanadate and arsenate anions are removed from solution for each type of hydrotalcite. XRD has been used to monitor the d(003) spacing for each hydrotalcite. A change in the d(003) spacing indicates a change in the interlayer distance, where an increase in d(003) indicates that the interlayer distance has increased. An increase in the interlayer distance suggests that larger anionic species have been intercalated between the hydroxyl layers. The d(003) spacings are provided in Table 1. Thermal analysis curves can be used to assess the thermal stability of the hydrotalcite. A delay in the decomposition temperature indicates that an anion other than carbonate has been intercalated into the hydrotalcite region (see Figure 4). The increased stability is due to a network of hydrogen bonds involving the intercalated anion as well as the reactivity of the anion being less than that of carbonate. Raman spectroscopy has been used to monitor the relative amounts of carbonate, vanadate, and arsenate in the hydrotalcite, whether the anion is

Figure 3. X-ray diffraction (XRD) patterns of hydrotalcites prepared with arsenate and vanadate with cationic ratios of 2:1, 3:1, and 4:1.

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Figure 4. Differential thermogravimetry (DTG) curves of hydrotalcites prepared with arsenate and vanadate with cationic ratios of 2:1, 3:1, and 4:1.

Figure 5. Raman spectra of hydrotalcites prepared with arsenate and vanadate with cationic ratios of 2:1, 3:1, and 4:1. Table 1. Interlayer Distances for Hydrotalcites Prepared with Arsenate and Vanadate with Cationic Ratios 2:1, 3:1, and 4:1 d(003) (Å) anion

2:1

3:1

4:1

HT(CO32-) HT(VO43-) HT(AsO43-)

7.67 7.71 7.66

7.66 7.79 7.80

7.93 7.84 7.79

intercalated or adsorbed (Figure 5). The term “inclusion” will be used to signify that the anionic specie may be intercalated, adsorbed, or a combination of both. The Raman spectra of 2:1 HT(VO43-) and 2:1 HT(AsO43-) clearly shows the inclusion of both vanadate (870 cm-1) and arsenate (830 cm-1). The intensity of the vanadate band for 2:1 HT(VO43-) is double the size of the carbonate band, compared to the arsenate band for 2:1 HT(AsO43-), which is almost half the size of the carbonate band. This indicates that the 2:1 HT(AsO43-) hydrotalcite has a considerable amount of carbonate in its structure. Carbonate was not added to these solutions, but originates from the original water source and due to the

dissolution of carbon dioxide (CO2). The synthesis of 2:1 HT(AsO43-) failed to cause an increase in the interlayer distance (compared to carbonate). This indicates that arsenate is not intercalated. Comparison of the differential thermogravimetric (DTG) curves [2:1 HT(AsO43-) and 2:1 HT(CO32-)] reveals that they are almost identical. Therefore, combining the fact that there is more carbonate in the structure, along with there being no changes in the d(003) spacing or the DTG curve, it is proposed that arsenate removed by 2:1 hydrotalcites occurs primarily through adsorption reactions. However, the 2:1 vanadate hydrotalcite showed an increase of 0.04 Å and observed slight changes in the DTG curve. This suggests that a very small amount of vanadate is intercalated into the 2:1 hydrotalcite, however, the main mechanism is believed to be adsorption reactions. The formation of 3:1 hydrotalcites using solutions containing vanadate and arsenate has caused an increase in the d(003) spacing, compared to 3:1 hydrotalcites containing carbonate. This indicates that an anion larger than carbonate has been

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intercalated. The Raman spectra of the 3:1 vanadate and arsenate hydrotalcites both show that the V-O and As-O bands are approximately twice as intense as the carbonate band. Less carbonate in the structure suggests that the intercalation of vanadate and arsenate is successful. The thermal analysis patterns confirm the intercalation of both anions with considerable delays in decomposition temperature being observed (33 and 21 °C, respectively). These delays in decomposition are due to the intercalation of vanadate and arsenate. The intercalation of either one of these species increases the thermal stability of the structure because vanadate and arsenate anions are less reactive than the carbonate anion, as well as an increase in the number of hydrogen bonds. Therefore, the removal mechanism for 3:1 hydrotalcites is proposed to consist of a large degree of intercalation reactions. Note that adsorption reactions are also occurring. The Raman spectra of the 4:1 hydrotalcites [4:1 HT(AsO43-) and 4:1 HT(VO43-)] clearly show the inclusion of vanadate and arsenate. The ratio between vanadate (900 cm-1) and arsenate (840 cm-1) compared to carbonate (1060 cm-1) is approximately 2.5:1 and 1:1, respectively. The higher content of arsenate in the 4:1 hydrotalcite compared to the 2:1 hydrotalcite suggests that a small degree of arsenate is intercalated into the structure. Slight increases in the decomposition temperature reinforce that small amounts of arsenate are intercalated. However, adsorption is still proposed to be the primary mechanism as no increase in the d(003) is observed. The same appears to be true for the 4:1 vanadate hydrotalcite, with minimal changes being observed in both the DTG curves and XRD interlayer spacing values. Therefore, the 4:1 structure is proposed to remove anions primarily by adsorption reactions rather than the more favorable intercalation reactions. 3.2. Removal of Anions by Thermally Activated Hydrotalcite. The concept behind the thermal activation of hydrotalcites is to dehydrate the structure, removing high affinity anions (carbonate), while producing a dehydrated hydrotalcite which is chemically more reactive. The major limiting factor to the amount of anions that can be removed from solution is the concentration of carbonate. Any carbonate that is present in a contaminated aqueous system will be intercalated preferentially over the desired toxic anion. Therefore, by removing carbonate from the hydrotalcite structure, a larger concentration of anions can be removed, provided that the contaminated solution has a small concentration of carbonate. Contaminated solutions containing large quantities of carbonate must be treated several times with thermally activated hydrotalcite. The initial treatments will remove predominately carbonate and minor amounts of the desired anion, and then each sequential treatment afterward will remove a larger percentage of the desired anion until all carbonate is removed from solution. This is clearly shown in Table 2, where the initial treatment of an arsenate and vanadate solution removed only 18.2% and 37.2%, respectively. The second treatment of the contaminated solution then removed a further 20.5% and 42.0% of the remaining arsenate and vanadate in solution. As the carbonate content decreases, the amount of arsenate and vanadate removed in each sequential treatment increases. Once the solution had been treated three times, a total of 75.5% and 81.0% of anions are removed, compared to a single treatment of 18.2% and 37.2% (arsenate and vanadate, respectively). Four different hydrotalcites have been thermally activated (under the same heating conditions): (1) 2:1 hydrotalcite, (2) 3:1 hydrotalcite, (3) 4:1 hydrotalcite, and (4) hydrotalcite prepared from Bayer liquor (BHT). Both arsenate and vanadate

Table 2. Percentage Removal of 0.2 M Solutions of Vanadate and Arsenate Containing 0.2 M of Carbonate % removal of arsenate from each treatment

% removal of vanadate from each treatment

1st Treatment with TA HT 18.2%

37.2% 2nd Treatment with TA HT

20.5%

42.0% 3rd Treatment with TA HT

24.3%

47.9%

Total Removal from Initial Solution after Three Treatments 75.5%

81.0%

are present in all solutions tested (x ppm arsenate + x ppm vanadate), and note that no precautions were taken to limit the carbonate content. Examination of the plots in Figure 6 shows that all thermally activated hydrotalcites remove up to 95% for each of the anions with a concentration of 25 ppm. At increased concentrations, the removal ability of the 2:1 and 3:1 hydrotalcite decreased significantly, with the 3:1 hydrotalcite performing the worse. At 100 ppm, the removal percentage of these two hydrotalcites was reduced to ∼50%. A larger percentage of arsenate is removed from solution than vanadate. It is proposed that arsenate may have a slightly higher affinity for the interlayer, or the presence and intercalation of vanadate anions assists in the intercalation of arsenate anions. The thermally activated 4:1 hydrotalcite and Bayer hydrotalcite showed essentially 100% removal of both arsenate and vanadate anions, for all concentrations tested. XRD of the 4:1 hydrotalcite and Bayer hydrotalcite both had d(003) spacings equal to 7.93 Å, compared to 7.67 Å for 2:1 and 3:1 synthetic hydrotalcites. The increased d(003) value indicates that these two hydrotalcite structures are able to remove a larger amount of anions from solution (as observed). The similarity of the graphs and the XRD d(003) spacings of the 4:1 and Bayer hydrotalcite suggest that the chemical characteristics of the hydrotalcite interlayer regions are similar. The increased removal ability of the thermally activated 4:1 and Bayer hydrotalcite is believed to be due to the higher magnesium content, resulting in a larger number of strong chemical bonds between anions and Mg2+ cations, compared with weaker bonding of anions with Al3+ anions. This is caused by increased anionic polarization by the higher charge density of Al3+ ions versus Mg2+, thus reducing the ionic character of the bonds. Based on these results, either the synthetic 4:1 hydrotalcite or the Bayer hydrotalcite should be thermally activated for the removal of arsenate and vanadate from solutions. 3.3. Removal of Anions by Thermally Activated Seawater Neutralized Red Mud. Red mud contains a minimal amount of hydrotalcite; therefore, any significant removal of anionic species from solution is due to adsorption onto the external surfaces of the red mud particles and other Bayer-derived components. Red mud residue is highly complex with 15 or more mineralogical phases, dependent on the source of the bauxite and refinery processes that are used.27 The seawater neutralization of red mud residues (SWN-RM) results in the formation of hydrotalcite-like structures, which are believed to be similar to Bayer hydrotalcite. Therefore, the thermal activation (TA) of SWNRM causes the dehydration of the red mud particles and the newly formed hydrotalcite. Exposure of the TA SWN-RM to solutions containing toxic anions should remove the anionic species via a combination of adsorption reactions (same as TA RM) and intercalation reactions (rehydration of the dehydrated

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Figure 6. Percentage removal of vanadate and arsenate from the same solution, using different thermally activated hydrotalcites.

tion reactions observed for TA SWN-RM makes this material a potential adsorbent material. Red mud residue is produced in large scale, and the potential use of this waste product for toxic anion removal from solutions is promising. Conclusions

Figure 7. Percentage removal of vanadate and arsenate from the same solution using thermally activated RM and SWN-RM.

hydrotalcite). This is clearly observed in Figure 7, where TA RM removes no more than 55% of anions, while SWN-RM removes almost 100% of the anions, up to 50 ppm. The capacity for red mud to remove anions through adsorption reactions is dependent on the initial concentration of the contaminated solution and the surface area of red mud particles. It has been clearly shown that the combination of adsorption and intercala-

From this investigation, it seems that removing anions from the solution is dependent on the Mg:Al ratio used and the preparation methods used. For the coprecipitation method, the formation of a hydrotalcite with a Mg:Al ratio of 3:1 is favored over the 2:1 and 4:1, whereas for thermally activated hydrotalcites a Mg:Al ratio of 4:1 is most favorable. A combination of techniques has determined that the mechanism of removal for the coprecipitation method is predominately adsorption reactions for 2:1 and 4:1 hydrotalcites, while the 3:1 hydrotalcites intercalate anions into the structure. Highly alkaline contaminated solutions have been found to have an impact on the removal of arsenate and vanadate, because of an increase in hydroxide and carbonate anions. It has been shown that treating carbonate solutions three times with thermally activated hydrotalcite can remove substantial amounts of toxic anions. The initial treatments remove carbonate, which then allows for the removal of toxic anions in sequential treatments. This investigation has also shown that Bayer refinery residues may be used for the treatment of solutions that contain toxic anions. Thermally activated Bayer hydrotalcite has been shown to be highly effective in the removal of arsenate and vanadate, with 100% removal being observed. The solid component of the refinery residue can also be used to remove toxic anions from solution. The thermal activation of red mud (RM) removed 55% of the arsenate and vanadate from the solution. Neutralizing the RM residue with seawater has been shown to be a promising adsorbent material, with removal percentages up 70% and greater being observed. The formation of hydrotalcite during

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the seawater neutralization (SWN) process removes anions via two mechanisms rather than one observed for thermally activated RM. This investigation has shown the versatility of hydrotalcite for the removal of toxic anions, where it can be used to remove toxic anions by (1) forming in the contaminated solution or (2) treating contaminated solutions with thermally activated hydrotalcite. It has also been shown that the formation of hydrotalcite during the SWN of bauxite refinery residues has the potential to turn the residue into an adsorbent material. The addition of solid hydrotalcite to a solution that contains arsenate and vanadate will remove these anions as long as minimal carbonate is present; otherwise, the contaminated solution will require multiple treatments with hydrotalcite. After hydrotalcite has been used to remove toxic anions from solution, it can be simply removed by filtration. Acknowledgment The financial and infrastructure support of the Queensland Research and Development Centre (QRDC-RioTintoAlcan) and the Queensland University of Technology, Faculty of Science and Technology, is acknowledged. Literature Cited (1) Trifiro, F.; Vaccari, A. ComprehensiVe Supramolecular Chemistry. Solid-State Supramolecular Chemistry: Two-and Three-Dimensional Inorganic Networks; Pergamon Press: Oxford, U.K., 1996. (2) Van Oosterwyck-Gastuche, M. C.; Brown, G.; Mortland, M. M. Mixed magnesium-aluminum hydroxides. I. Preparation and characterization of compounds formed in dialyzed systems. Clay Miner. 1967, 7, 177–192. (3) 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. (4) 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. (5) Frost, R. L.; Erickson, K. L. Thermal decomposition of synthetic hydrotalcites reevesite and pyroaurite. J. Therm. Anal. Calorim. 2004, 76, 217–225. (6) Rives, V. Layered Double Hydroxides: Present and Future; Nova Science: New York, 2001. (7) Taylor, H. F. W. Crystal structures of some double hydroxide minerals. Mineral. Mag. 1973, 39, 377–389. (8) Marcelin, G.; Stockhausen, N. J.; Post, J. F. M.; Schutz, A. Dynamics and ordering of intercalated water in layered metal hydroxides. J. Phys. Chem. 1989, 93, 4646–4650. (9) Miyata, S. The synthesis of hydrotalcite-type compounds and their structures and physiochemical properties. Clays Clay Miner. 1975, 23, 369– 375. (10) Pesic, L.; Salipurovic, S.; Markovic, V.; Vucelic, D.; Kagunya, W.; Jones, W. Thermal characteristics of a synthetic hydrotalcite-like material. J. Mater. Chem. 1992, 2, 1069–1073. (11) Erickson, K. L.; Bostrom, T. E.; Frost, R. L. A study of structural memory effects in synthetic hydrotalcites using environmental SEM. Mater. Lett. 2005, 59, 226–229. (12) Stanimirova, T.; Kirov, G. “Structural memory” of hydrotalcite metaphases. Geol.-Geografski Fakultet, Kniga 1: Geol. 2000, 92, 121– 130.

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ReceiVed for reView May 16, 2010 ReVised manuscript receiVed August 4, 2010 Accepted August 12, 2010 IE101104R