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The removal of cadmium(II) and lead(II) from aqueous solution was investigated using thermally processed halloysite. The samples were previously heated in the 200–1000°C range at interval of 200°C. The resulting materials were characterized by thermal analysis, electron microscopy, X-ray diffraction, electrophoretic mobility measurement, and N2 adsorption. Metal adsorption was studied as a function of pH, contact time, temperature, metal and adsorbent concentrations, and correlated with the physicochemical properties of the materials. A particular interest has been focused on the spectroscopic study to elucidate the mechanism of the interaction of M2+ (M = Cd or Pb) cations with the best adsorbent. The kinetic and equilibrium data were adequately described by the pseudo-second order and Redlich–Peterson models, respectively. The mechanism mainly involved an electrostatic interaction between these metallic cations and the hydroxyl groups of surface. Whatever metal, maximum adsorption occurred for the material that preserved its structure, i.e. H200 (halloysite heated at 200°C) for Pb(II) and H400 for Cd(II). The intermediate adsorption of H600 and H800 was explained by their poorly organized structures due to dehydroxylation. H1000 was found to be the worst adsorbent due to its low specific surface area. Regardless of the material, the adsorption sequence was: Pb > Cd, which was correlated with the ionic properties of each metal. As long as it preserves its structure, halloysite clay proves to be an efficient adsorbent for removing heavy metals from aqueous solutions. [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 intercalated with dimethyl sulfoxide (DMSO) was prepared. The starting (H) and resulting (H-DMSO) materials were characterized by X-ray powder diffraction, Fourier transformed infrared spectroscopy, thermal analysis, transmission electron microscopy, pore-size distribution analysis, and employed as crystal violet (CV+) adsorbents from aqueous solutions. Intercalation reaches a rate of 95% and increases the basal spacing to 11.2 Å. (CH3)2SO interacts with the inner surface hydroxyls of halloysite through new hydrogen bonds with the S=O groups. The release of DMSO occurs in two phases: a partial elimination at 195 °C and a second part due to the DMSO combustion at 277 °C. The TEM image of H-DMSO reveals halloysite nanotubes (HNTs) polydisperse in length and diameter. A heterogeneous distribution in the nanotube size was highlighted with pore diameters of 10–11, 20.6, 28.6, and 37.0 nm, in correlation with transmission electron microscopy. The Redlich–Peterson equation describes efficiently the CV+adsorption onto the modified sample. H-DMSO adsorbs 93.6 against 50.9 mg g−1for the starting material. This improving of the adsorption capacity of DMSO-intercalated HNTs, was explained via the behavior of the intercalated DMSO molecules. Intercalation constitutes a key procedure for developing new nanocomposites, attractive in technological applications, such as effective adsorbents. [ABSTRACT FROM AUTHOR]