Bone char modification by iron to improve its capacity for adsorbing fluoride from an aqueous solution

Abstract

High fluoride concentrations in groundwater have attracted great concern worldwide because many people are afflicted by fluorosis due to water consumption. In this work, the fluoride adsorption from water onto bone char (BC) modified with iron sulfate (BCM) was studied as an option for eliminating fluoride from drinking water. The experimental data for the adsorption equilibrium of fluoride on BCMs were procured in a batch adsorber. The synthesis conditions of BCMs were optimized for improving the adsorption capacity of BCMs, and the optimal BCM was designated as BCM2. The BCMs were characterized by different analytical techniques, the BCMs surface exhibited an irregular morphology and the chemical nature was basic, and the BCMs were mesoporous materials. The Langmuir isotherm satisfactorily interpreted the experimental data of the fluoride adsorption isotherms on BCMs. The basic sites of BC and BCMs were quantified and identified in this work. The main adsorption mechanism of fluoride on the BCMs was the electrostatic attraction between the fluoride and the basic sites of BCMs, and the adsorption capacities of BC and BCMs towards fluoride increased almost linearly with the concentration of basic sites so that the enhancement of the adsorption capacity of BCMs was attributed to the increase of basic sites during the modification. Furthermore, the BCM2 adsorption capacity was lessened by incrementing the pH from 5 to 9, and this behavior was ascribed to the reduction of the electrostatic attraction interactions between the BCM2 surface basic sites and fluoride in the solution. The adsorption process was endothermic because the adsorption capacities of BC and BCM2 toward fluoride were raised by incrementing the solution temperature from 15 to 35 °C. The BCM2 presented a high capacity for adsorbing fluoride of 14.4 mg g−1 at 25 °C and a pH of 5.