INFLUENCE OF THE FUNCTIONALIZATION PARAMETERS ON THE PROPERTIES OF THE SEPIOLITE-BASED ANIONIC ADSORBENTS

The subject of this paper is comparative analysis of sepiolite - based adsorbent properties which used for adsorption of anionic species of pollutants from aqueous solution. Removal of anionic species of pollutants using adsorption, with the selection of a proper adsorbent, has been proposed for the treatment of anionic species of pollutants containing wastewater. Consequently, various types of adsorbents, a number of natural materials, which cost less and can be used as such or after some minor treatment, have been extensively searched and tested. Among clay minerals, sepiolite possesses certain properties which make it good choice for adsorption process such as large specific surface area, low cost, high availability. Depending on the target pollutants, this clay can be used either in its natural, activated or functionalized form. The sepiolite, a fibrous clay mineral of high specific surface area, is a very good adsorbent for cationic species, but its adsorption capacity for anionic species is very low, as in the case of other clay minerals. In order to use the sepiolite for the sorption of anionic species of pollutants from aqueous solutions, this natural clay mineral was acid activated and functionalized with organosilanes. Natural, acid activated and thermo-acid activated sepiolites were functionalized by covalent grafting (3-mercaptopropyl)trimethoxy-silane or mercapto-silane and [3-(2-aminoethylamino)-propyl]trimethoxy-silane or amine-silane to the silanol groups onto the sepiolite surface. Functionalization was proved by scanning electron microscopy, X-ray diffraction, Fourier transform infrared (FT-IR) spectroscopy, differential thermal analysis and nitrogen adsorption-desorption. Sepiolite as fibrous clay mineral with very high density of silanol groups (≡Si–OH) on its surface that can be increased by acid activation and thermo-acid activation, has suitable structure to be covalently functionalized with organic ligands. It has been established that the structure of parent material was preserved during the functionalization, and that the organosilanes were successfully grafted on the natural, acid activated and thermo-acid activated sepiolite surfaces. According to the results of infrared spectroscopic analysis and differential thermal analysis, the functionalization with mercapto-silane was best performed on the surface of acid activated sepiolite, and worse on the surface of natural and thermo-acid activated sepiolite. According to the results of differential thermal analysis, functionalization with amine-silane was best performed on the surface of acid activated sepiolite, and worse on the surface of natural and thermo-acid activated sepiolite, while according to the results of infrared spectroscopic analysis, functionalization with amine-silane was equally good on the surface of all three starting sepiolites. Based on the comparison of FT-IR spectra of aminosilanized and mercaptosilanized sepiolites, the organic matter content is higher in aminosilanized than in mercaptosilanized sepiolites, ie it indicates better functionalization of starting sepiolites with amino-silanes than with mercapto-silanes. The binding sites on the functionalized adsorbents surfaces were considered to be protonated mercapto and amine groups, which play a role in the electrostatic interactions with the anions species at lower pH-values of the aqueous solutions. The ability of sepiolites functionalized with organosilanes for adsorption anionic pollutants from aqueous solutions has potential importance for the removal of potentially toxic anionic pollutants from industrial effluents, surface and groundwater