In this approach, Zn-M+3 layered double hydroxides (LDHs) with M+3 = Fe, Cr, or Al were synthesized by the co-precipitation method from the aqueous solution at a constant solution pH. The as-synthesized samples were characterized by XRD analysis, FTIR spectra, BET techniques and simultaneous thermogravimetric-differential scanning calorimetry (TGA/DSC). XRD analysis showed that Zn-Fe-CO3 had the greatest lattices parameters. BET surface area of Zn-Fe-CO3 was calculated as 52.24 m2.g-1 and was higher than Zn-Cr-CO3 and Zn-Al-CO3 with 46.70 and 49.99 m2.g-1, respectively. The FTIR spectra clearly confirmed the presence of carbonate anions in the structure of the LDHs. Adsorption experiments for Indigo Carmine (IC), as the main model organic pollutant in this study from aqueous solution onto synthetized samples were carried out in terms of solution pH, contact time and initial dye concentration. Experimental results indicate that the capacity of dye uptake augmented rapidly within the first 15, 40, and 55 minuts for Zn-Fe-CO3, Zn-Cr-CO3 and Zn-Al-CO3 respectively and then stayed practically the same regardless of the concentration. Adsorption kinetics studies revealed that the adsorption process followed pseudo-second order kinetics model instead of a pseudo-first-order model. The adsorption isotherm data follow the Langmuir equation in which parameters are calculated. The maximum Langmuir monolayer adsorption capacities were 94.87, 21.79, and 66.71 mg.g-1, respectively, for Zn-Fe-CO3, Zn-Cr-CO3, and Zn-Al-CO3. The adsorption capacities were slightly influenced by the pH variations from 5 to 10, showing the advantage of using these materials in water treatments in a wide pH range. Finally, the IC removal is proven by the presence of IC functional groups in IR spectra and thermograms. TGA/DSC of Zn-Fe-CO3 obtained after removal of IC indicate that the LDHs stabilizes IC and delays the combustion of adsorbed molecules. Copyright © 2020 BCREC Group. All rights reserved
Abstract__Hydrotalcite (HT) or hydrotalcite-like compounds (HTlc) are layered double hydroxides belonging to the class of anionic clays. The structure of these compounds are very similar to that of brucite, Mg(OH) 2 , where some of Mg 2+ represented as [M (II) ] are isomorphously replaced by Al 3+ represented as [M (III) ] and the net positive charge is compensated by the inter-layered exchangeable anion (A n− ). LDH has been receiving increasing attention in recent years, owing to its potential technological applications such as catalysis, electrode, optical memory, separator, adsorbent, precursor for composite materials, and ion exchange. The general formula of these compounds can be represented as: [M (II) 1−x M (III) x (OH) 2 ]·A n x/n ·mH 2 O with M (II) and M (III) as metal cations and A n− as exchangeable anion.LDHs also possess relatively large surface areas and high anionexchange capacities. Because of these properties, LDHs have been studied for removing toxic anionic species from aqueous systems. Thermal decomposition of these materials by calcination above 420°C results in the formation of high surface area basic mixed oxides which are reported to be potential candidates in catalyzing various reactions involved in the synthesis of a variety of fine chemicals. Indeed at this temperature, the as formed mixed oxides solid solution can regenerate upon rehydratation the HT structure with the suitable anions present in solution. Therefore, this so-called reconstruction process may be used to improve the sorption of anionic species. The LDH has been synthesized by the coprecipitation method at pH constant. The material has been obtained with [Mn/Al] molar ratio of 2.0. The product is characterized by X-ray diffraction (XRD), spectroscopy infra red (FTIR), and differential thermal analysis / thermo gravimetric analysis (DTA/TG). Dosages of polluted solutions have been realized by spectrometry UV visible.
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