Hydrotalcite Compounds Research Articles

Comparison Between the Cr(VI) Adsorption by Hydrotalcite and Hydrotalcite-Gibbsite Compounds

Cr(VI) adsorption from aqueous solutions on Mg-Al hydrotalcite and Mg-Al hydrotalcite-gibbsite with Al/(Al+Mg) ratios (R) of 0.3 and 0.6 (MgAlHTR = 0.3 and MgAlHTGR = 0.6) was investigated as a function of the pH and chromium concentration. The results showed that the maximum Cr(VI) adsorption by Mg-Al compounds do not depend on the value of R at pH 7. In addition, the chromate equilibrium sorption capacity for MgAlHTGR = 0.6 is at least 4.8 times higher than that for MgAlHTR = 0.3 at pH 5. The Cr(VI) maximum capacities of MgAlHTR = 0.3 and MgAlHTGR = 0.6 at pH 7 were similar, 6.5 and 6.8 mgCr(VI)/g, respectively.

Selective Oxidation of Glycerol with O<sub>2</sub> Catalyzed by Fluorine-Modified CuNiAl Hydrotalcite

A fluorine anion modified CuNiAl hydrotalcite (CuNiAl-LDH (F-)) was prepared for the selective oxidation of glycerol (GLY) using O2 as oxidant. It was found that the obtained catalyst with typical layer structures of hydrotalcite compounds was an effective heterogeneous catalyst in GLY oxidation. Under the present conditions, the main product was the C3 oxygenated products of primary alcohol glyceric acid (GLYAC). The highest conversion of GLY reached 32.0% with 58.1% of the selectivity to GLYAC.

Adsorption Behavior in Dual-solutes of Cr(Ⅵ) and p-Cresol by Mg-Al Hydrotalcite Compounds

The adsorption behavior of Cr(Ⅳ) on Mg-Al hydrotalcite compounds(Mg-Al HTlc) was investigated in the metal-organic pollutant system of Cr(Ⅵ) and p-cresol.The results suggest that in the adsorption of Cr(Ⅵ) on HTlc is in the pseudo-second-order reaction and follows Langmuir isothermal adsorption equation;p-cresol can reduce the adsorption velocity but can increase the adsorption capacity of Mg-Al HTlc to adsorb Cr(Ⅵ) from water.Cr(Ⅵ) and p-cresol exhibit a competitive effect.On the one hand,the competitive effect is dependent to the concentration ratio of p-cresol to Cr(Ⅵ),the adsorption capacity of Mg-Al HTlc toward Cr(Ⅵ) can be increased by decreasing the pH value due to the adsorption of p-cresol on HTlc.The competitive ability of p-cresol decreases relatively with the increase of Cr(Ⅵ) concentration and the removal rate of Cr(Ⅵ) increases significantly when the mass concentration ratio of Cr(Ⅵ) to p-cresol is larger than 3.In addition,the adsorption capacity decreases with increasing temperature.

Adsorption of Abietic Acid from Colloidal Suspension by Calcined Mg/Al Hydrotalcite Compounds

Resin acids are major constituents of wood extractives and are known to cause a range of paper manufacturing problems including pitch deposition, poor paper machine runnability, and low paper quality. Abietic acid (AA) is one of major constituents of resin acids. Removing AA from colloidal suspension using calcined Mg/Al hydrotalcites (CHTs) was investigated in this study. The removal percentage of AA at the pH value of 6.8 was larger than those at the pH values of 7.8–9.8 and the adsorption capacity was 434.78 mg/g at 323 K. Kinetic data were followed pseudo-second-order kinetic model and adsorption followed the Langmuir isotherm. Adsorption process proceeded by physical adsorption. About 80% of the equilibrium adsorption capacity of the original CHT was achieved after five use cycles. The calcined Mg/Al hydrotalcites exhibited high adsorption capacity for AA and were efficient adsorbents for dissolved and colloidal substances (DCS) in papermaking process.

Investigation of the role of anions in hydrotalcite for quasi-solid state dye-sensitized solar cells application

In recent research in clean energy applications, clay has gained significant interest, especially as a gelator in dye-sensitized solar cells (DSSCs), for its capability to resolve the leakage issue of liquid electrolyte. In this paper, anionic hydrotalcite is utilized as a gelator to assist the formation of gel electrolyte in DSSCs. Three types of hydrotalcite with exchangeable anions, viz. NO3− (CL-N), CO32− (CL-C) and SO42− (CL-S), were synthesized with similar morphologies via the co-precipitation method. It is observed that the gel formation of hydrotalcite strongly depends on the exchangeable anions present in the hydrotalcite. The objective of this work is to understand the effect of hydrotalcite anions on the photovoltage and the photocurrent in the gel electrolyte through electrochemical analysis. With increasing ion affinity, the Voc increases. This is attributed to Li+ intercalation with hydrotalcite compound resulting in the elevation of the conduction band of TiO2. With increasing ion affinity, the Jsc decreases. This is attributed to the decreasing diffusion coefficient of triiodide and the increasing difficulty in the injection process. For anions with low ion affinity in hydrotalcite, the diffusion of the redox couple is not significantly affected by the high viscosity of the gel. Furthermore, the study indicates that proper selection of hydrotalcite compounds not only produces a quasi-solid gel electrolyte, but also increases the efficiency of the solar cells: the device performance was improved from 7.8% (liquid electrolyte) to 8.4% (hydrotalcite gel electrolyte).

Ultrasound assisted synthesis of Ca–Al hydrotalcite for U (VI) and Cr (VI) adsorption

Adsorption of chromium (VI) and uranium (VI) nitrate on Ca–Al hydrotalcite (Ca–Al LDHs) compounds is investigated. The powdered materials are synthesized in a modified co-precipitation method under the ultrasonic treatment with different conditions for Cr (VI) and U (VI) adsorption. Factors that affect the compounds synthesis such as temperature, reaction time, calcination and aging conditions are reported. The effects of various parameters on adsorption process including adsorption/desorption kinetics, isotherms and influencing factors such as contact time, solution pH and effect of competing ions are studied. It is found that Ca–Al LDHs has the largest capacity for both Cr (VI) and U (VI), adsorbing 104.82±0.02mg/g and 54.79±0.02mg/g, and highest rate of adsorption up to 98.78±0.02% and 90.28±0.02% respectively. Cr (VI) and U (VI) adsorption decreased obviously with pH levels outside the range of 7–11. Adsorption/desorption kinetics of Cr (VI) and U (VI) shows that desorption K values of U (VI) are larger than Cr (VI) while adsorption K values U (VI) are smaller than Cr (VI), suggesting that Ca–Al LDHs efficiency for the removal of U (VI) is fast adsorption and slow release compare to Cr (VI). The effect of competing ions such as nitrate radical and chloridion was negligible.

Adsorption of Arsenic from Borated Water Using Mg-Fe-Hydrotalcite (FeHT)

High selectivity and removal capacity for an adsorbate ion are an important factor for the selection of an adsorbent. Hydrotalcite and hydrotalcite-like compounds as layered double hydroxide have received much attention for their potential application as adsorbents in various fields. Iron-based hydrotalcite compounds has a high selectivity for arsenic. Boron mineralization in Emet-Hisarcik region is one of the world's largest reserves of boron. Unlike the other boron mineralizations, it also contains arsenic minerals, which cause arsenic pollution in the surface and ground waters within the surrounding area. These waters have also significant levels of boron as dissolved oxyanion. The groundwaters associated with arsenical boron deposits was reported to contain up to 600 μg/L As ve 1500 mg/L B. Remediation of arsenic and boron pollution in this water is therefore required. In this study, the removal of arsenic from borated waters by Fe-Hydrotalcite (FeHT) as iron-based layered double hydroxide was investigated. The investigations showed that the adsorption of arsenic was not influenced by increasing the boron concentration. Fe- Hydrotalcite was found to remove 98% of arsenic from ground water containing high level of boron with its ability to reduce As levels below the limit (10 μg/L As) set for potable water by World Health Organization (WHO). The results of this study showed that FeHT, can be used as a potent adsorbant for the removal of arsenic from the borated waters.

Synthesis and Structural Characterization of Mg/Al Hydrotalcite-Like Compounds

Hydrotalcites consist of Brucite-like layers with positive charge and anionic compounds in the interlayer to form neutral materials. The general formula of hydrotalcite compounds (HTLc) was Mg-A1-HTLc. All samples were characterized by XRD, FT-IR spectra and TG. After calcination, the transformation of hydrotalcite structure can be characterized by vibration infrared bands corresponding to hydroxyl groups, octahedral layers and interlayer species. Moreover, the calcined samples show a relatively high surface area and total pore volume.

Effects of ion-exchange treatment on bromate formation and oxidation efficiency during ozonation

Ion-exchange treatment is a promising technique for removing hydrophilic compounds during drinking water treatment. In this study, we applied several different ion exchangers (i.e., anion exchange resins and a hydrotalcite compound) to bromide removal to minimize bromate formation during ozonation. It was found that ion-exchange treatment affected ozone and hydroxyl radical concentration profiles as well as bromate ion concentration after ozonation. Selecting an appropriate ion exchanger is important to achieve both the oxidation of target contaminants and the reduction of bromate ion during ozonation.

Adsorption of perchlorate from aqueous solution by the calcination product of Mg/(Al–Fe) hydrotalcite-like compounds

The calcination products containing Mg(II), Al(III), and Fe(III) in the brucite-like layers with varying Mg/Al/Fe molar ratios at 550°C were used as the adsorbent to remove perchlorate from aqueous solution, while the Mg/(Al–Fe) hydrotalcite compounds were synthesized by co-precipitation method at a constant pH value. The Mg/(Al–Fe) hydrotalcite compounds (HMAF) were characterized by XRD, FT-IR and TG–DTA. The characteristics showed that the layered double hydroxides structures in the HMAF were lost during calcination at 550°C, but were reconstructed subsequent to adsorption of perchlorate, indicating that the ‘memory effect’ appeared to play an important role in perchlorate adsorption. Batch adsorption studies were conducted under various equilibration conditions, such as molar ratios of Mg/Al/Fe, calcined temperature, different initial solution pH, adsorbent dose, initial perchlorate concentration, and co-existing anions. It was found that the existence of ferric iron in calcined Mg/(Al–Fe) hydrotalcite compound (CHMAF) was favorable to removal of perchlorate from water, and the best ratio of Mg/Al/Fe is 3:0.8:0.2 (CHMAF5%). This study demonstrated that the calcination product of Mg/(Al–Fe) hydrotalcite-like compound was a promising adsorbent for control of the perchlorate pollution in water.

Corrosion inhibition of aluminum by coatings formulated with Al–Zn–vanadate hydrotalcite

The hydrotalcite structure is an ionic lamellar solid with positively charged layers incorporating two kinds of metallic cations and hydrated gallery anions. The ability of these compounds to retain aggressive ions and simultaneously release a corrosion inhibitor is the main reason for the development of hydrotalcite compounds to replace hexavalent chromium compounds (chromates) as inhibitive pigments. In this study, alkyd coatings formulated with Al–Zn–vanadate hydrotalcite, at different pigment concentrations were applied on aluminum specimens. The painted panels were subjected to different accelerated tests (condensing humidity, salt spray and Kesternich) and atmospheric exposure in atmospheres of different aggressiveness. Corrosion performance was evaluated by electrochemical impedance spectroscopy (EIS). A traditional zinc chromate pigment was also tested for comparative purposes. The obtained results confirm that hydrotalcite compounds achieve corrosion inhibition of the underlying aluminum substrate. It has been shown that an increase of anticorrosive pigment content (for a constant pigment volume concentration) does not always improve the primer behavior, but a larger amount of vanadates released from Al–Zn–vanadate hydrotalcite particles does improve anticorrosive behavior.

Removal of fluoride ions from aqueous solution by the calcination product of Mg–Al–Fe hydrotalcite-like compound

The calcination product of Mg–Al–Fe hydrotalcite compound at 500 °C (HTlc500) was used as the adsorbent to remove fluoride ions from aqueous solution, while the Mg–Al–Fe hydrotalcite compound was synthesized by co-precipitation method. The hydrotalcite-like compounds (HTlcs) were characterized by XRD, FT-IR, TG-DTA, SEM, TEM, BET and pH PZC analysis. The characteristics showed that the HTlc500 had a porous structure, while the Mg–Al–Fe hydrotalcite has a “memory effect”. Batch adsorption studies were conducted under various equilibration conditions, such as different pH, calcined temperature, adsorbent dose, co-existing anions, contact time and initial fluoride ions concentrations. It was found that the HTlc500 had the largest adsorption capacity of 14 mg/g at pH 6 with adsorbent dose of 0.2 g/L and the adsorption data were well described by the Langmuir isotherm model and the pseudo-first order kinetic model. According to these studies, the HTlc500 has potential application in fluoride ions removal field.

Production and characterisation of LDH meixnerite powders

Meixnerite, which is a magnesium-aluminium hydrotalcite compound, is a hydrotalcite group mineral material with double layer structure. Recently, to prevent the environmental pollution caused by greenhouse gases, especially CO2, studies for production of synthetic meixnerite have been carried out. Meixnerite releases water vapour to the environment and reduces the CO2 emission by replacing airborne CO2 with OH groups present in its double layered structure through the adhesion of CO2 on itself. The present study covers the synthesis and characterisation of meixnerite by sol-gel method. Aluminium-isopropoxide and magnesium-chloride-hexahydrate were used as initial materials. First, boehmitic sol was prepared using sol-gel method. The mixture was stirred vigorously after adding magnesium-chloride-hexahydrate into the boehmitic sol. The mixture obtained after the gelation process was heat-treated at 550, 1300, 1400 and 1500°C. The material which has [Mg5Al3(OH)16][(OH)3.4(H2O)] chemical structure was determined as meixnerite which is a member of LDH group and used as CO2 reducer.

The effect of co-existing solutes on arsenate removal with hydrotalcite compound

Hydrotalcite (HTAL-Cl), an inorganic anion exchanger, is of use as an adsorbent for the removal of arsenate (As(V)) in water systems. The adsorption properties of HTAL-Cl for As(V) and the effects of co-existing anions on the As(V) removal performance were investigated in this work. Under the conditions of pH>or=4, the adsorption capacity for As(V) gradually decreased with an increase of pH, but As(V) was removed effectively within the range of pH = 5-8. Co-existing anions interfered As(V) removal, and the effect decreased in the order of HPO(4)(2-) > HCO(3)(-) > SO(4)(2-) > Cl(-). In binary solute systems containing phosphate and As(V), the maximum adsorption capacity of HTAL-Cl was 0.95 mmol g(-1) for phosphate and 0.65 mmol g(-1) for As(V): the total of these values corresponded to the maximum adsorption capacity for As(V) in single solute systems. The adsorption isotherms in these binary solute systems were approximated by the following modified Langmuir equations:As(V): q(As) = 18.7 radicalC(As)/(1 + 21.5 radicalC(P) + 12.8 radicalC(As)), phosphate : q(P) = 33.1 radicalC(P)/(1 + 21.5 radicalC(P) + 12.8 radicalC(As)). The column adsorption experiments showed that the adsorbed As(V) was released by the phosphate adsorption, because phosphate was adsorbed more strongly on HTAL-CL than As(V).

Hydrotalcite-type precursors of active catalysts for hydrogen production

The effects that the composition of hydrotalcite (HT)-type precursors and their method of preparation have on the chemical–physical properties and catalytic performances of systems which are active in the partial oxidation and steam reforming of methane to produce H 2 were investigated. The key role of the structure of the HT compounds in the development of active catalysts is reported for Ni- and noble metal-based catalysts, which were prepared by controlled thermal decomposition of carbonate- or silicate-containing precursors. We highlight specific features which appear to be a function of the divalent to trivalent atomic ratio and/or nature of the anion present. Furthermore, we also report the advantages of coating ceramic and metallic supports with HT phases using coprecipitation and a novel type of electrochemical synthesis, respectively.

27Al Solid-state NMR Structural Studies of Hydrotalcite Compounds Calcined at Different Temperatures

Hydrotalcites are anionic clays that are quite prevalent in nature and their importance is growing more and more because of their very wide range of potential applications and uses. Understanding the structural and compositional changes that occur on the molecular scale during the thermal decomposition of hydrotalcite compounds is essential for the basic prediction and comprehensive understanding of the behavior and technical application of these materials. In this study, several hydrotalcite compounds calcined at different temperatures for applications in a chlorine resistant textile were prepared and 27-Aluminm solid-state nuclear magnetic resonance (NMR) spectroscopy was used as a tool to study their local structure and behavior. The changes in the Al coordination of the hydrotalcite compounds were investigated with one dimensional (1D) solid-state magic angle spinning (MAS) NMR spectroscopy. The two broad resonances arising from the structurally different Al coordinations of these compounds were clearly resolved by two dimensional (2D) triple quantum magic angle spinning (3QMAS) NMR spectroscopy.

Nitrate adsorption from aqueous solution by MII-Al-CO3 layered double hydroxide

Hydrotalcite compounds were found to have high anion adsorption capacity that enhances their capability to remove anionic contaminants from aqueous systems. In this study, the adsorption of nitrates by various layered double hydroxides (LDHs), such as Mg-Al and Zn-Al, was investigated. The nature and content of divalent cations in LDHs have strong influence on the adsorption process. Calcined Mg-Al LDH with an Mg/Al molar ratio of 3.0 showed higher adsorption capacity compared to other calcined LDHs. The adsorption phenomenon has been well supported by x-ray diffraction and infrared studies.

FT-IR investigation of NOx storage properties of Pt–Mg(Al)O and Pt/Cu–Mg(Al)O catalysts obtained from hydrotalcite compounds

NOx storage-reduction catalysts based on Pt and Pt/Cu supported on Mg(Al)O mixed oxides obtained from hydrotalcite-type compounds were studied. In particular, NOx storage capability upon admission of NO and NO2 in the absence or in the presence of excess oxygen at temperatures up to 623 K was investigated by in situ FT-IR spectroscopy. For comparative purposes, the bare mixed oxide support was also investigated. Pure NO2 is adsorbed according to a dismutation reaction with the simultaneous formation of nitrates and nitrites that progressively evolve to nitrates (“dismutation route”), the nitrite evolution to nitrates is promoted by the metal phase. When metal phase is present, the nitrites oxidation is further accelerated by the oxygen presence. Pure NO is not stored. When O2 is present, NO is stored according to two different pathways, both promoted by the metal phases: (i) the NO oxidation to nitrites followed by their oxidation to nitrates (“nitrite route”); (ii) the NO oxidation to NO2, followed by the “dismutation route”.

X-ray diffraction and Raman spectroscopic studies of Zn-substituted carrboydite-like compounds

Hydrotalcite-like compounds of the formula Ni x Zn 6− x Al 2(OH) 16(SO 4)·4H 2O where x varies from 0 to 6, equivalent to a zinc-substituted carrboydite have been synthesised and characterised by X-ray diffraction, electron microscopy and vibrational spectroscopy. Both the d (0 0 3) spacing and the crystallite size are a function of the amount of zinc replacement for nickel in the carrboydite-like compounds. Transmission electron microscopy (TEM) shows the clay-like crystal structure for these hydrotalcite compounds. These compounds were characterised by vibrational spectroscopic techniques and a comparison made with the naturally occurring minerals. Additional bands in the sulphate antisymmetric stretching and bending region leads to the conclusion that the symmetry of the sulphate anion is reduced inferring the bonding of the sulphate anion to the hydrotalcite hydroxyl surface.

Macroscopically Ordered Hydrotalcite-Type Materials Using Self-Assembled Colloidal Crystal Template

Hydrotalcite compounds have been successfully prepared by successive infiltrations into the interstices of an ordered array of polystyrene spheres, subsequently removed by either dissolution or thermal decomposition. This method using a template leads to the formation of a three-dimensional open macroporous framework in which the walls are formed by the agglomeration of nanoparticles of two-dimensional hydrotalcite.