Anionic RR120 dye adsorption onto raw clay: Surface properties and adsorption mechanism

Clay minerals are efficient adsorbent for dyes due to their colloidal properties. A Tunisian raw clay (Fouchana) composed of different species of clay minerals (kaolinite, smectite and minor illite) has shown efficiency for removal of anionic dye Reactive Red 120 (RR120) from aqueous solutions. In order to provide a better understanding of the role played by the different clay species in Fouchana, as well as the nature of the adsorption processes which are involved, a comparative study was conducted using standard clays, namely kaolinite KGa-2, smectite SWy-1 and illite IMt-2. Batch adsorption experiments were performed at ambient temperature and pH of the dye solutions. Acido-basic properties of the solid surfaces, electrophoretic mobility of particles, cation exchange capacity, amount of exchangeable cations and specific surface were investigated. Principal component analysis was used to highlight the influence of the physicochemical characteristics of the clay minerals in the adsorption behavior. The results show that for the conditions set in this study, kaolinite in the Fouchana raw clay plays a major role in anionic dye adsorption, mainly due to its acidic surface sites, but also to exchangeable aluminum (III) cations acting as bridges between the clay mineral and the dye molecules. Smectite and illite most likely play a secondary role by adsorption on exchangeable calcium (II) and dissociated Al (III)OH groups.