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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]

A series of new organic-inorganic nanohybrid materials were prepared by intercalating dimethylsulfoxide (DMSO) into the interlamellar space of halloysite at different contact times and DMSO/clay ratios. The X-ray diffraction and thermogravimetry/differential thermal analysis studies confirmed the insertion of DMSO into the interlayer space. The increase of the basal spacing to 1.12 nm was thus highlighted for an intercalation rate of 95%. All materials were used to remove crystal violet (CV) from aqueous media. We focused particularly on the mechanism of CV interaction with halloysitic nanohybrids by means of an infrared spectroscopy study. Understanding such an interaction is a fundamental approach for the effective use of halloysitic nanohybrids in wastewater sanitation. pH influence, kinetic, isotherm, and thermodynamic data have been examined. The equilibrium and kinetic data were appropriately adjusted, respectively, by the Redlich- Peterson, and pseudo-second-order models. The amount adsorbed by each nanohybrid depends on the intercalation rate. The larger the intercalated fraction, the better the amount adsorbed, so DMSO is involved in the CV adsorption. The mechanism would involve mainly a hydrogen bond between the electron lone pairs of oxygen of the sulfoxide function and the hydrogen of methyl bound to the tertiary amine of the dye molecule. [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]

Abstract: Halloysite clay was treated at 600°C and with hydrochloric acid solutions of various concentrations. The resulting materials were characterised by elemental analysis, TG, TDA, TEM, XRD and nitrogen adsorption at 77K. The modified halloysites were employed as crystal violet (CV+) adsorbents from aqueous solutions. The thermal treatment at 600°C results in the formation of dehydroxylated halloysite, due to e from the structure. The acid treatment involves a relative increase in the content of SiO2. The ratio of SiO2/Al2O3 increased from 1.92 to 27.75, consequence of the leaching of Al ions from the octahedral layer. This phenomenon is accompanied by the progressive amorphisation of the structure with increasing the HCl concentration. The thermo-chemical treatments preserve the tubular morphology, though the obtained tubes are somewhat damaged. The specific surface area increased significantly from 60.5 to a maximum of 503m2/g. Total volume is mainly represented by mesopores. Microporosity grows progressively with HCl treatment, reaching a maximum of 21%. The CV+ adsorption onto the modified halloysites is rapid in the first 10min. The adsorbed quantity decreased with increasing temperature. The affinity follows the sequence: H600-0.5N>H600-3N>H600-5N>H600. The overall mechanism of CV+-modified halloysites interactions follows pseudo-second-order kinetics with the contribution of the intraparticle diffusion. [Copyright &y& Elsevier]