Thermal and chemical modification of a halloysite and application to the adsorption of a cationic dye

Abstract

Algerian halloysite 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 77 K. The modified halloysites were employed as malachite green (MG+) adsorbents from aqueous solutions. Thermal treatment at 600 °C results in the formation of dehydroxylated halloysite, due to –OH release from the structure. Acid treatment involves a relative increase in SiO2 content. 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 HCl concentration. Thermo-chemical treatments preserve the tubular morphology, though the obtained tubes are somewhat damaged. Specific surface area increased significantly from 60.5 to a maximum of 503 m2/g. Total volume is mainly represented by mesopores. Microporosity grows progressively with HCl treatment, reaching a maximum of 21 %. The MG+ adsorption onto the modified halloysites is rapid in the first 10 min. The adsorbed quantity decreased with increasing temperature. The affinity follows the sequence: H600-0.5N > H600-3N > H600-5N > H600.