Removal of Congo red by thermally and chemically modified halloysite: equilibrium, FTIR spectroscopy, and mechanism studies

Abstract

Unmodified halloysite and its heated and heat-processed/acid-leached forms were utilized for the adsorption of Congo red (CR) from aqueous medium. The samples were analyzed by ICP and nitrogen adsorption prior to the adsorption of the pollutant. We then studied the adsorption equilibrium of CR and the regenerating cycles of the most effective adsorbent. A special attention was paid to Fourier transform infrared analysis to elucidate the mechanism of CR adsorption onto halloysite materials. The thermochemical treatment results in the dehydroxylation of halloysite and an increase in SiO2 content. The modification increases specific surface area up to 503 against 63m(2)g(-1) for unmodified halloysite. The Redlich-Peterson isotherm yields the best fit of the Congo red adsorption. The best adsorbent was found to be the heat-treated/acid-leached sample. It interacts with Congo red molecules through an outer-sphere complexation mechanism by implying silanol species and amino and sulfoxide groups. Understanding the interactions between pollutants and halloysite materials is an essential approach to improving the use of these clays in sewage treatment.