Preparation, characterisation and application of thermally treated Algerian halloysite
Résumé
Natural halloysite was treated at different temperatures in the range 200-1000 degrees C. The resulting materials were characterised by TGA, DTA, TEM, XRD, FTIR and nitrogen adsorption. The characterised halloysitic solids were employed as Pb(II) adsorbents from aqueous solutions. The crystal structure of halloysite is stable up to 400 degrees C. The thermal treatment at 600 degrees C results in the formation of dehydroxylated halloysite, due to -OH release from the structure. A poorly organised structure and a progressive amorphisation of the structure were obtained not only for H600 but also H800 (halloysite treated at 800 degrees C). For H1000, a recrystallisation process arose from amorphous substance, materialised by the formation of gamma-Al2O3 and amorphous SiO2. Nanotubular particles were obtained for H600 proving that the thermal treatment at 600 degrees C conserves the original morphology of halloysite. The size distribution of the tubes was found to be quite broad. H1000 presents both tubular and plate morphologies. The nanotubes were however damaged. The values of specific surface area remained more or less constant up to 800 degrees C, beyond which it drastically decreased from 63 m(2)/g for H200 to 17.8 m(2)/g for H1000. Whatever the sample, total volume was mainly represented by mesopores, while micropores volume was negligible. The adsorption isotherms of Pb(II) were of L-type. The affinity follows the sequence: H200> H400> H800> H600> H1000. This evolution was explained in terms of the surface characteristics of the adsorbents.