Al-Ca-Zn Trimetallic Compound Supported on Tobacco Residue: A Sustainable Alternative for the Removal of Fluoride Ions from Water Supply

Abstract

Abstract The presence of fluoride ions in groundwater used for public supply may expose a large portion of the world population to the deterioration of teeth and bones, among other health issues. Within this context, the agricultural activities related to tobacco imply the use of large amounts of phosphate fertilizers, endowed with large residual amounts of fluoride. Thus, fertilizers application combined with rain and irrigation of tobacco crops contributes to high fluoride levels in groundwater. In this study, a residue from the tobacco production chain, the tobacco stem, was used as a support medium for the synthesis of an Al-Ca-Zn trimetallic compound, being named Al-Ca-Zn-tab. Thereby, an alternative adsorbent based on the recycling of an agricultural residue is presented. The impregnation and pyrolysis at 500°C of the material were the main steps adopted in the synthesis of Al-Ca-Zn-tab. The Response Surface analysis showed a positive and increasing effect of the adsorbent concentration. The pH, however, did not affect the adsorption. The efficiencies of fluoride removal were up to 71.7%, with actual accumulations up to 2.78 mg g-1, and maximum accumulations of 13.59 mg g-1. The fluoride adsorption on Al-Ca-Zn-tab followed the Freundlich isotherm and the general order kinetic model. In addition, the thermodynamic data suggested the predominance of physisorption phenomena with exothermic characteristics. Finally, the proposed adsorption mechanism is based on ion exchange of the surface hydroxyl groupings of the oxides and hydroxides, in addition to hydrostatic interactions due to the bi and trivalent cations.