Al-Ca-Zn trimetallic compound supported on tobacco residue: a sustainable alternative for the removal of fluoride ions from water supply

ABSTRACT The presence of fluoride ions in groundwater used for public supply may expose a large portion of the world's 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, which contain significant residual amounts of fluoride. Thus, fertilizer application combined with rainfall and irrigation of tobacco crops contributes to high fluoride levels in groundwater. In this study, a residue from the tobacco production chain, the tobacco stems, was utilized as a support medium for synthesizing an Al-Ca-Zn trimetallic compound, referred to as Al-Ca-Zn-tab. It presents an alternative adsorbent based on the recycling of an agricultural residue. The main steps involved in the Al-Ca-Zn-tab synthesis were impregnation and pyrolysis of the material at 500°C. The response surface analysis demonstrated the positive and increasing effect of the adsorbent concentration, while the pH did not significantly affect the adsorption. The efficiencies of fluoride removal reached up to 71.7%, with actual accumulations of up to 2.78 mg g−1 and maximum accumulations of 13.59 mg g−1. The fluoride adsorption on the Al-Ca-Zn-tab followed the Freundlich isotherm and the general-order kinetic model. Moreover, thermodynamic data suggested the predominance of the physisorption phenomenon with exothermic characteristics. Finally, the proposed adsorption mechanism is based on the ion exchange of the surface hydroxyl groupings of the oxides and hydroxides, along with hydrostatic interactions resulting from the presence of bi- and trivalent cations.