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In this study, a carbonate material (CM) and its form heated to 900°C (CM900) were used to remove Congo red (CR) from aqueous solutions. The objective was to combine the results of the characterization of the modified material, CR adsorption and spectroscopic study to propose a coherent mechanism of the CR-CM900 interaction. A bibliographical study on the adsorption of dyes by carbonates and their interactions revealed the scarcity of published works. This study will compensate somewhat for the lack of results in this field. Carbonate materials are abundant, low-cost and available in different countries around the world. The materials were characterized by X-ray diffraction and scanning electron microscopy. A solid/solution concentration of 1 g·L-1, pH of 6.9, equilibrium time of 2 h and temperature of 40°C, were found to be the optimum conditions for a maximum amount of CR adsorbed by CM900 of 288.2 against 32.7 mg·g-1 for CM. A full decarbonation of CM900 results in MgO and CaO, releases CO2 from the structure, and leads to a more porous structure. The pseudofirst-order model adequately described the kinetic data. The experimental isotherms were suitably fitted by the Redlich-Peterson model with determination coefficient and average relative error values =0.97% and <10.0%, respectively. Thermodynamic parameters suggested a spontaneous and endothermic process. Methanol easily desorbed the CR adsorbed by CM900 which maintained its adsorption capacity during three adsorption-desorption cycles. Fourier-transform infrared spectroscopy investigation before and after the dye adsorption showed that the CR-CM900 interaction involves a mechanism of outer-sphere complexation between the amine groups and the hydroxylated magnesium oxides. Understanding the interactions between dyes and carbonate materials is an important approach to develop the use of these materials in wastewater treatment. [ABSTRACT FROM AUTHOR]

Algerian halloysite was heated at 600 °C and treated with HCl 5N. The materials were characterized by chemical analysis, TEM, and FT-IR spectroscopy, and used in the elimination of Congo red (CR) from aqueous solution. The effects of contact time and temperature were investigated. The thermal treatment at 600 °C results in the formation of dehydroxylated structure. Acid attack involves an increase in SiO2 content, due to the leaching of Al ions from octahedral sheet. Thermo-chemical treatment also diminishes the percentage of impurities and maintains the tubular morphology. Kinetic data follow the pseudo-second order model, whilst thermodynamic parameters lead to a not spontaneous and endothermic process. Significant changes occur in the vibrational spectrum of H600-5N (halloysite treated at 600 °C and with HCl 5N), after adsorption of Congo red, with the involvement of amino and sulfoxide groups. The mechanism highlights an outer-sphere surface complexation of SiOH....H2O species, i.e., SiOH linked to H2O via H-bonds. [ABSTRACT FROM AUTHOR]

Algerian halloysite was processed at 600°C and with 5-N HCl (H600-5N), and compared with the forms treated thermally at 600°C (H600) and unmodified (H). The obtained samples were characterized by FTIR, TEM, and nitrogen adsorption, and employed as malachite green (MG) adsorbents from aqueous solutions. The effects of pH, contact time, solution concentration, and temperature were examined. A particular attention has been focused on desorption and regeneration. The ease in desorption and regeneration constitutes a major practical advantage for treating industrial effluents. Thermo-chemical activation leads to the leaching of Al ions from the octahedral sheet. As consequence, H600-5N presents a specific area of 503 m2 g−1, i.e. eight orders of magnitude higher than that of H (63 m2 g−1). For all halloysitic solids, the capacity in MG increases with decreasing adsorption temperature. The affinity sequence is H600-5N > H600 > H. The isotherms are found to be well represented by the Redlich–Peterson equation. The thermodynamic data show that the MG adsorption onto H600-5N is spontaneous and exothermic, consequence of the electrostatic attraction between positively charged molecules and negatively charged adsorption sites. Among the seven eluents used, methanol manifests the greatest desorption capacity from H600-5N, which increases overall with temperature. After four adsorption–desorption cycles, this material was easily desorbed and regenerated. On the basis of all these considerations and, also, its adsorption capacity (192.6 mg g−1at 25°C), H600-5N appears very effective for removing dyes from wastewaters. [ABSTRACT FROM AUTHOR]