Hydrotalcite Interlayer Research Articles

Characterization of Bayer Hydrotalcites Formed from Bauxite Refinery Residue Liquor

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 plasma−optical 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−)·xH2O and Mg6Al2(OH)16(CO32−,SO42−)·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 ...

The synthesis and spectroscopic characterisation of hydrotalcite formed from aluminate solutions

Raman spectroscopy has been used to characterise nine hydrotalcites prepared from aluminate and magnesium solutions (magnesium chloride and seawater). The aluminate hydrotalcites are proposed to have the following formula Mg 6Al 2(OH) 16(CO 3 2−)· xH 2O, Mg 6Al 2(OH) 16(CO 3 2−,SO 4 2−)· xH 2O, and Mg 6Al 2(OH) 16(SO 4 2−)· xH 2O. The synthesis of these hydrotalcites using seawater results in the intercalation of sulfate anions into the hydrotalcite interlayer. The spectra have been used to assess the molecular assembly of the cations and anions in the hydrotalcite structures. The spectra have been conveniently subdivided into spectral features based upon the carbonate anion, the hydroxyl units and water units. This investigation has shown the ideal conditions to form hydrotalcite from aluminate solutions is at pH 14 using a magnesium chloride solution at a volumetric ratio of 1:1. Changes in synthesis conditions resulted in the formation of impurity products aragonite, thenardite, and gypsum.

Effect of pH on the uptake of arsenate and vanadate by hydrotalcites in alkaline solutions: a Raman spectroscopic study

AbstractHydrotalcites have been synthesised using solutions of three different pH values to assess the effect of pH on the uptake of arsenate and vanadate. The ability of these hydrotalcites to remove vanadate and arsenate from solution has been determined by inductively coupled optical emission spectroscopy. Raman spectroscopy was used to monitor changes in the anionic species for hydrotalcites synthesised at different pH values. The results show a reduction in the concentration of arsenate and vanadate anions that are removed in extremely alkaline solutions. Hydrotalcites containing arsenate and vanadate are stable in solutions up to pH 10. Exposure of these hydrotalcites to higher pH values results in the removal of large percentages of arsenate and vanadate from the hydrotalcite interlayer. Copyright © 2010 John Wiley & Sons, Ltd.

SYNTHESIS OF HYDROTALCITE Zn-Al-SO<sub>4</sub> AS ANION EXCHANGER AND ITS APPLICATION TO TREAT OF POLLLUTANT CONTAINED HEXACYANOFERRAT(II)

Synthesis of Zn-Al-SO4 hydrotalcite and its application as anion exchanger for hexacyanoferrat (II) have been studied. Synthesis of Zn-Al-SO4 hydrotalcite was carried out by stoichiometric method and hydrothermal treatment. Sulphate in hydrotalcite interlayer was exchanged by hexacyanoferrat (II) that was assumed as pollutant. Kinetics of ion exchange was also investigated. The product of ion exchange was characterized by XRD, IR spectrophotometry and atomic absorption spectrometry. Zn-Al-SO4 hydrotalcite can be obtained by stoichiometric method at pH 8 and followed by hydrothermal treatment. The chemical formula of the hydrotalcite was Zn0.74Al0.26(OH)1.74(SO4)0.13.0.52H2O. The hexacyanoferrat (II) anion could exchange sulphate in hydrotalcite interlayer as showed by IR spectra and XRD patterns. The anion exchange capacity was 645.35 meq/100 g, and the kinetics of ion exchange reaction was first order with k = 5 x 10-5 s-1. It showed that Zn-Al-Fe(CN)6 could be regenerated easily, so it has been regenerate frequently. Keywords: hydrotalcite, anion exchanger, hexacyanoferrat

Synthesis and Raman spectroscopic study of Mg/Al,Fe hydrotalcites with variable cationic ratios

AbstractHydrotalcites of formula Mg6(Al,Fe)2(OH)16(CO3)·4H2O formed by intercalation with the carbonate anion as a function of divalent/trivalent cationic ratio have been successfully synthesised. The XRD patterns show variation in the d‐spacing attributed to the size of the cation. Raman and infrared bands in the OH stretching region are assigned to (1) brucite layer OH stretching vibrations, (2) water stretching bands and (3) water strongly hydrogen bonded to the carbonate anion. Multiple (CO3)2− symmetric stretching bands suggest that different types of (CO3)2− exist in the hydrotalcite interlayer. Increasing the cation ratio (Mg/Al,Fe) resulted in an increase in the combined intensity of the two Raman bands at around 3600 cm−1, attributed to MgOH stretching modes, and a shift of the overall band profile to higher wavenumbers. These observations are believed to be a result of the increase in magnesium in the structure. Raman spectroscopy shows a reduction in the symmetry of the carbonate, leading to the conclusion that the anions are bonded to the brucite‐like hydroxyl surface and to the water in the interlayer. Water bending modes are identified in the infrared spectra at positions greater than 1630 cm−1, indicating that water is strongly hydrogen bonded to both interlayer anions and the brucite‐like surface. Copyright © 2009 John Wiley & Sons, Ltd.

Absorption of the selenite anion from aqueous solutions by thermally activated layered double hydroxide

The presence of selenite or selenate in potable water is a health hazard especially when consumed over a long period of time. Its removal from potable water is of importance. This paper reports technology for the removal of selenite from water through the use of thermally activated layered double hydroxides. Mg/Al hydrotalcites with selenite in the interlayer were prepared at different times from 0.5 to 20 h through ion exchange. X-ray diffraction of the MgAlSeO 3 hydrotalcites indicates that the selenite anion entered the interlayer spacing of Mg/Al hydrotalcite and MgAlSeO 3 hydrotalcite was formed. Raman spectra proved the presence of selenite anion in the hydrotalcite interlayer as the counter anion. The band intensity and width of MgAlSeO 3 hydrotalcite in the region of 3800–3000 cm −1 increase with the adsorption of selenite by the Mg/Al hydrotalcite. The characteristic bands of free selenite anions in the MgAlSeO 3 hydrotalcites are located between the region between 850 and 800 cm −1. The Raman spectra of the lower wave number region of 550–500 cm −1 show a shift toward higher wave numbers with adsorption of the selenite. An estimation of the amount of selenite anion removed by the thermally activated layered double hydroxide was obtained through the measurement of the intensity of the selenite Raman bands at 814 and 835 cm −1 resulting from the amount of selenite anion remaining in solution. Thermally activated LDHs provide a mechanism for removing selenite anions from aqueous solutions.

Thermal treatment study of vanadium-loaded hydrotalcites employing in situ DXAS

A comparative study was made in which two hydrotalcites were loaded with vanadium employing different procedures: (1) aqueous phase impregnation, with vanadyl sulfate solution in pH 6.5 or 9.5; (2) solid phase mixing, with vanadyl sulfate or vanadium pentoxide physically mixed up. TPR investigation revealed that vanadium-loaded hydrotalcites obtained by aqueous phase impregnation in two distinct pH, resulted in different reduction profiles. The vanadium-loaded hydrotalcites were characterized by FTIR, XRD and TGA–DTA analysis. In situ dispersive XAS (DXAS) spectra obtained during thermal treatment in air showed that, for the physical mixture of solids, vanadium is oxidized above 350 °C. For the samples prepared by aqueous phase impregnation, the effect of alkaline pH was to produce polymeric vanadium species deposited onto the HT in comparison with the isolated vanadium species in acidic conditions. The influence of both, the hydrotalcite interlayer anion type and the vanadium loading procedure, on the reducibility of vanadium species is also discussed.

Mechanism for hydrotalcite decomposition: A controlled rate thermal analysis study

The mechanism for the decomposition of hydrotalcite remains unsolved. Controlled rate thermal analysis enables this decomposition pathway to be explored. Hydrotalcites containing carbonate, vanadate and molybdate were prepared by coprecipitation. The resulting materials were characterised by XRD, simultaneous TG–DTG–DTA and controlled rate thermal analysis (CRTA) to determine the stability and thermal decomposition pathway of the synthesised hydrotalcites. For the carbonate intercalated hydrotalcite dehydration takes place in three steps two of which are quasi-isothermal and one non-isothermal. Dehydroxylation and decarbonation occur separately over the 235–330 and 330–370 °C temperature range. A second non-isothermal decarbonation step is observed in the 371–541 °C range. In comparison the mixed carbonate–vanadate and carbonate–molybdate hydrotalcites show two dehydration steps and the dehydroxylation and decarbonation occur simultaneously. The observation of three dehydration steps is used to support the model of water molecules in three structurally distinct environments in the hydrotalcite interlayer. CRTA technology provides a mechanism for the decomposition of hydrotalcites.

Characterization of surfactant/hydrotalcite-like clay/glassy carbon modified electrodes: Oxidation of phenol

The characteristics of hydrotalcite (HT)-like clay films containing ionic and nonionic surfactants and their ability to oxidize phenol have been examined. The HT clay (Co/Al–NO 3) was synthesized by coprecipitation techniques and then modified with surfactants such as sodium dodecylbenzenesulfonate (SDBS), octylphenoxypolyethoxyethanol (TX100) or cetylpyridinium bromide (CPB). X-ray diffraction analysis revealed that the interlayer basal spacing varied depending on the type of surfactant retained by the HT. The presence of SDBS and CPB expanded the HT interlayer, which in the presence of TX100 did not show an appreciable change. Phenol oxidation is favored at surfactant-HT-GC modified electrodes, after a preconcentration time, compared to phenol oxidation at HT-GC or GC electrodes. Surfactant-HT-GC modified electrodes display good stability in continuous electrochemical phenol oxidation. At pH values between 6 and 10.8, both SDBS-HT-GC and TX100-HT-GC modified electrodes seem to be promising electrodes for the detection of phenol in water; while the CPB-HT-GC modified electrode should be affected by the inorganic anions.

Memory effect-enhanced catalytic ozonation of aqueous phenol and oxalic acid over supported Cu catalysts derived from hydrotalcite

Mg/Al supported metal (Fe, Co, Ni and Cu) oxide catalysts were prepared by co-precipitation of hydrotalcite-like clay materials as precursors, calcined, and used for the ozonation reaction of phenol and oxalic acid. The reaction was carried out using the catalyst and aqueous solution of phenol or oxalic acid in an O 3/O 2 mixed gas-flow at 20 °C. In the ozonation of phenol, the combination of ozone and supported metal oxide catalysts was effective for the removal of total organic carbon (TOC). Also in the ozonation of oxalic acid as the main TOC component, Cu/Mg/Al catalysts showed the highest activity, followed by Ni/Mg/Al catalyst, while both Fe/Mg/Al and Co/Mg/Al catalysts were not active. Leaching of Cu and Ni, probably due to the chelation of metals by oxalic acid, was significantly observed at the beginning of the reaction. However the metal leaching disappeared at the end of the reaction possibly due to the entire consumption of oxalic acid during the reaction. The best result of oxalic acid mineralization was observed over Cu/Mg/Al catalyst calcined at 600 °C, on which least leaching of the metal was detected. Moreover, a “memory effect” of hydrotalcite accelerated the mineralization of oxalic acid over the Cu/Mg/Al catalyst; oxalate anions were captured and decomposed in the reconstituted hydrotalcite interlayer space on the surface of the Cu/Mg/Al catalyst, resulting in a remarkable enhancement in the catalytic activity of the ozonation.

Adsorption of acidic pesticides 2,4-D, Clopyralid and Picloram on calcined hydrotalcite

Calcined hydrotalcite, HT500, a Mg–Al mixed oxide with NaCl-structure of composition Mg 3AlO 4.5 was shown to be an effective adsorbent for the acidic pesticides: 2,4-D, Clopyralid and Picloram. Adsorption experiments were conducted using three procedures: time-dependent (kinetic), pH-dependent and concentration-dependent (isotherm). Results indicate that the intensity of the adsorption is related to the acidity of the pesticides. The characterization of the adsorption products by XRD and FT-IR spectroscopy confirm the adsorption of the three pesticides in the anionic form in the hydrotalcite interlayer by reconstruction of the layered structure. There was evidence of the adsorption of Picloram also in molecular form.

Intercalation of hydrotalcites with hexacyanoferrate(II) and (III)—a thermoRaman spectroscopic study

Raman spectroscopy using a hot stage indicates that the intercalation of hexacyanoferrate(II) and (III) in the interlayer space of a Mg, Al hydrotalcites leads to layered solids where the intercalated species is both hexacyanoferrate(II) and (III). Raman spectroscopy shows that depending on the oxidation state of the initial hexacyanoferrate partial oxidation and reduction takes place upon intercalation. For the hexacyanoferrate(III) some partial reduction occurs during synthesis. The symmetry of the hexacyanoferrate decreases from O h existing for the free anions to D 3d in the hexacyanoferrate interlayered hydrotalcite complexes. Hot stage Raman spectroscopy reveals the oxidation of the hexacyanoferrate(II) to hexacyanoferrate(III) in the hydrotalcite interlayer with the removal of the cyanide anions above 250 °C. Thermal treatment causes the loss of CN ions through the observation of a band at 2080 cm −1. The hexacyanoferrate (III) interlayered Mg, Al hydrotalcites decomposes above 150 °C.

Raman spectroscopy of hydrotalcites with sulphate, molybdate and chromate in the interlayer

Raman microscopy has been used to characterize the interlayer anions in synthesized hydrotalcites of formula Mg6Al2(OH)(16)(XO4)(.)4H(2)O, where X is S, Mo or Cr. The Raman spectrum shows that both the chromate and molybdate anions are not polymerized in the hydrotalcite interlayer. This lack of polymerization is attributed to the effect of pH during synthesis. A model of bonding is proposed for the interlayer anions based upon the observation of two symmetric stretching modes and symmetry lowering of the chromate, molybdate and sulphate anions. Two types of anions are present, hydrated and hydroxyl surface-bonded. Copyright (c) 2005 John Wiley & Sons, Ltd.

The role of water in synthesised hydrotalcites of formula Mg xZn 6− xCr 2(OH) 16(CO 3) · 4H 2O and Ni xCo 6− xCr 2(OH) 16(CO 3) · 4H 2O—an infrared spectroscopic study

Infrared spectroscopy has been used to characterise synthesised hydrotalcites of formula Mg x Zn 6− x Cr 2(OH) 16(CO 3) · 4H 2O and Ni x Co 6− x Cr 2(OH) 16(CO 3) · 4H 2O. The infrared spectra are conveniently subdivided into spectral features based (a) upon the carbonate anion (b) the hydroxyl units (c) water units. Three carbonate antisymmetric stretching vibrations are observed at around 1358, 1387 and 1482 cm −1. The 1482 cm −1 band is attributed to the CO stretching band of carbonate hydrogen bonded to water. Variation of the intensity ratio of the 1358 and 1387 cm −1 modes is linear and cation dependent. By using the water bending band profile at 1630 cm −1 four types of water are identified (a) water hydrogen bonded to the interlayer carbonate ion (b) water hydrogen bonded to the hydrotalcite hydroxyl surface (c) coordinated water and (d) interlamellar water. It is proposed that the water is highly structured in the hydrotalcite interlayer as it is hydrogen bonded to both the carbonate anion, adjacent water molecules and the hydroxyl surface.

Infrared and Raman study of interlayer anions CO32−, NO3−, SO42−and ClO4−in Mg/Al-hydrotalcite

The difference in the local environment of CO32−, NO3−, SO42−, and ClO4− in Mg/Al-hydrotalcite compared to the free anions was studied by infrared and Raman spectroscopy. In comparison to free CO32− a shift toward lower wavenumbers was observed. A band around 3000–3200 cm−1 has been attributed to the bridging mode H2O-CO32−. The IR spectrum of CO3− hydrotalcite clearly shows the split ν3 band around 1365 and 1400 cm−1 together with weak ν2 and ν4 modes around 870 and 667 cm−1. The ν1 mode is activated and observed as a weak band around 1012 cm−1. The Raman spectrum shows a strong ν1 band at 1053 cm−1 plus weak ν3 and ν4 modes around 1403 and 695 cm−1. The symmetry of the carbonate anions is lowered from D 3 h to C 2 s resulting in activation of the IR inactive ν1 mode around 1050–1060 cm−1. In addition, the ν3 shows a splitting of 30–60 cm−1. Although NO3-hydrotalcite has incorporated some CO32− the IR shows a strong ν3 mode at 1360 cm−1 with a weak band at 827 cm−1, and the ν4 band is observed at 667 cm−1, although it is largely obscured by the hydrotalcite lattice modes. The Raman spectrum shows a strong ν1 mode at 1044 cm−1 with a weaker ν4 band at 712 cm−1. The ν3 mode at 1355 cm−1 is obscured by a broad band due to the presence of CO32−. The symmetry of NO3− did not change when incorporated in hydrotalcite. The IR spectrum of SO4-hydrotalcite shows a strong ν3 at 1126, ν4 at 614 and a weak ν1 mode at 981 cm−1. The Raman spectrum is characterized by a strong ν1 mode at 982 cm−1 plus medium ν2 and ν4 bands at 453 and 611 cm−1; ν3 cannot be identified as a separate band, although a broad band can be seen around 1134 cm−1. The site symmetry of SO42− is lowered from T d to C 2 v . The distortion of ClO4− in the interlayer of hydrotalcite is reflected in the IR spectrum with both ν3 and ν4 bands split around 1096 and 1145 cm−1 and 626 and 635 cm−1, respectively. A weak ν1 band is observed at 935 cm−1. The Raman spectrum shows a strong ν1 mode at 936 cm−1 plus ν2 and ν4 bands at 461 and 626 cm−1, respectively. A ν3 mode cannot be clearly recognized, but a broad band is visible around 1110 cm−1. These data indicative a lowering of symmetry from T d to C s .

Glyphosate-Hydrotalcite Interaction as Influenced by pH

AbstractAdsorption and the mechanism of adsorption of the herbicide glyphosate [N-(phosphonomethyl) glycine] by hydrotalcite (HT) and by hydrotalcite calcined at 500°C (HT500) was studied. The values of the Freundlich K constant of the adsorption isotherms and the distribution coefficients, Kd, depend on the pH of the adsorption system and vary between 55.54–9603 (K) and 36.82–6252 (Kd) for HT and between 10,106–23,242 (K) and 17,801–26,558 (Kd) for HT500. X-ray diffraction and infrared spectroscopy studies showed that glyphosate is not adsorbed into the interlayer of HT. The adsorption mechanisms are (1) electrostatic attraction between the surface of HT, charged positively below pH 12 according to its point of zero charge and the glyphosate anion, and (2) ligand exchange between the P-OH and/or C=O groups of the herbicide and the Al and Mg atoms of the HT surface. The occurrence of one or both mechanisms depends on the pH of the adsorption system. These adsorption mechanisms and the amphoteric nature of glyphosate account for the variation in K and Kd values with respect to pH. Previous studies on adsorption of different organic anions by HT have only considered the mechanism of anion exchange as important.

131I− sorption from aqueous solutions by nitrated hydrotalcites

The sorption of radioactive iodide anion from aqueous solutions on hydrotalcite in the nitrate form has been investigated as a function of the Al/(Mg+Al) ratio. It was found that I− can replace only a small fraction of the nitrates in hydrotalcites and that I− sorption depends highly on Al/(Mg+Al) ratio.131I− was sorbed by anion exchange in the interlayer of hydrotalcite with an Al/(Mg+Al) ratio of 0.20 and, for higher ratios,131I− sorption increased more than 3 times. Hydrotalcites in the nitrate form were prepared using microwave irradiation in the hydrotreatment step. The radionuclide content was determined by Γ-spectrometry. Solids were characterized by BET, X-ray diffraction and atomic absorption analyses.

Intercalation of Aliphatic Carboxylates in Hydrotalcite Interlayers: Selective Photochemical Hydrogen Abstraction by Benzophenonecarboxylate

Abstract The photochemical hydrogen abstraction of p-benzoylbenzoate ions (BPC) from aliphatic carboxylate ions in LDH (hydrotalcite) interlayers yielded a series of their 1:1 adducts. The product distributions were rather selective as well as distinctive from those of sodium salts and methyl esters in homogeneous solutions without LDH. These results have been discussed on the basis of their mutual disposition on the layer surfaces of the LDH.

Mono- and Bilayer Equilibria of Stearate Self-Assembly Formed in Hydrotalcite Interlayers by Changing the Intercalation Temperature

Abstract The self-assembly of sodium stearate and its homologs ( >C12) in hydrotalcite (LDH) interlayers was found to change reversibly between mono- and bilayer structures with a ratio changing continuously within a range of 70 °C and 5 °C in stoichiometric intercalations in aqueous dispersion. The bilayer structure was more stable than the monolayer, judging from the fact that bilayer aggregations were formed even in high temperatures with the addition of excess amounts of guest stearates over the anion exchange capacity (AEC). Hydrophobic interaction was determined to be the key factor for the formation of a bilayer assembly.

Benzophenone-sensitized cyclodimerization of cinnamic acid in hydrotalcite interlayers : A study of the sensitization behavior

Aromatic ketone-sensitized cyclodimerization was studied in the presence of an anion exchange, clay, hydrotalcite. Uv irradiation of the clay suspension coadsorbing trans-cinnamate and 4-benzoylbenzoate ions resulted in the efficient formation of a syn-head-to-head dimer. The ratio of the cis/dimer was significantly affected by the introduction of heavy atom substituents of the sensitizers in high concentrations but not in lower concentrations. Dimerization was revealed to be 10 times more efficient than E - Z isomerization by extrapolating to infinitely smaller concentrations of the sensitizers. The sensitization mechanism through the formation of an exciplex between the excited singlet 4-benzoylbenzoate and trans-cinnamate is presented on the basis of the efficient formation and stereoselectivity of the cyclodimer. A fluorescent probe, the 2-naphthoate ion, intercalated in the clay interlayers was shown to increase its fluorescence intensity in the presence of a sensitizer, implying the validity of a singlet sensitization mechanism via an exciplex formation. The turnover number for the energy transfer exceeded 40 per sensitizer molecule, thus the cyclodimerization was found to proceed catalytically with the sensitizers.