Hydrotalcite-type anionic clays are a group of important materials used in adsorption processes, mainly for organic pollutants removal due the layered double hydroxide structure. The layer-interlayer interactions provide a structural memory even after dehydration and dehydroxylation process, since a very stable interlayer anions are part of material composition, like the carbonate one. A limited numbers of trivalent modifier cations can replace the aluminium cation due the ionic radii mismatch or oxidation state restrictions. Transition metal cations can replace the aluminium one in octahedral site of hydroxide lamellas in order to improve the adsorptive behaviors. In this work, we have investigate three compositions of carbonated magnesium-aluminium hydrotalcite with different iron (III) contents through the co-precipitation method at pH 11 and aging step at 60˚C for 6 hours. Thermal analysis was performed aiming the determination of the hydration water and hydroxyl amounts in dried precipitate samples, taking in account the results obtained for X-ray diffractometry, infrared spectroscopy, and nitrogen adsorption-desorption characterization for several thermally treated samples. All of synthesized samples showed high surface areas, even for high temperature of thermal treatment. The co-substitution with iron (III) reduced the temperature of dehydration and dehydroxylation process, but the co-substitution at 5 mol% provides other desirables characteristics, like a more amount of rhombohedral HDL phase and higher porosity, even after the thermal treatment at 500˚C for 4 hours. This result makes that composition very applicable as a reusable adsorbent material in order to removal several types of micro-pollutant compounds in aqueous media.

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There are several purposes for investigating hydrotalcite clays and one of them is due to its importance as an anionic adsorbent and ion exchanger, which make them capable of removing various pollutants from water. The most common hydrotalcite structure is based on double lamellar hydroxide of magnesium and aluminum, containing carbonate anions in the interlayer space. However, it is common to investigate the co-substitution of aluminum with other trivalent cations, especially transition metal cations, due to their binding properties. In this work, it was investigate the co-substitution of chromium (III) and aluminum in a sample of magnesium-aluminum carbonated hydrotalcite, comparing it with the same composition without this co-substitution. Thermal stability, crystallinity and morphology were investigated, showing that chromium contributes to the adsorptive capacity at low temperature.

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