Materials derived from natural kaolinite for the adsorption of pollutants in water and gas phase

As it is well known, pollution represents one of the greatest environmental challenges that mankind has to face. Among the different types of pollutants, toxic dyes represent a threat for the integrity of waterbodies [1], while greenhouse gases (such as CO2) are the main responsible of greenhouse effect [2]. To limit the continuous spreading of these noxious compounds in the surrounding environment, several methods can be employed. In this respect, adsorption deserve particular attention thanks to its low costs, good performances and versatility [1,2,3]. Among the most used sorbents, (i.e., silicas, zeolites, activated carbons etc. [1,2,3]), natural clays have proved to be excellent candidates for the removal of pollutants thanks to their low costs, environmental sustainability and high natural abundance [1]. This work deals with the transformation of natural kaolinite into two different materials with enhanced adsorption capacity towards pollutants from both aqueous solution and gas phase. The first treatment of kaolinite involves a calcination at 700°C followed by a treatment with HCl to remove the aluminium fraction thus obtaining a silica. The second one consists in an optimized alkaline fusion that leads to the conversion of kaolinite into a zeolite X. The structural changes and the physico-chemical properties of the materials before and after the chemical activations were investigated with several techniques including XRPD, MAS-NMR, SEM-EDX, FT-IR and N2 physisorption at 77K. To test their adsorption capacity, both kaolinite and silica were used for the removal of the organic dye Rhodamine B from aqueous solution. The adsorption process was studied over time by means of UV-Vis spectroscopy and the functional stability of silica was tested over three cycles of regeneration and adsorption. On the other hand, CO2 adsorption performances in gas phase of zeolite X and kaolinite were investigated at 25°C and 1bar of pressure using a prototype volumetric apparatus available in our laboratories