Removal of fluoride from aqueous solution using acid and thermally treated bone char

In this work, bone char (BC) was synthesized at different calcination temperatures and modified by using HNO₃ solutions having distinct concentrations. Furthermore, the relationship between the crystalline structure and physicochemical properties of BC on the adsorption capacity of BC towards fluoride from aqueous solution was analyzed. It was noted that the calcination temperature and the acid modification of BC significantly influenced the crystalline phases, crystallite size, and contents of hydroxyapatite, monetite, and other calcium phosphates, which are the main constituents of the BC. Additionally, the surface area diminished by raising both the calcination temperature and acid concentration used in the modification. The adsorption capacity of BC linearly declined as the calcination temperature was increased from 400 to 800 °C. On the other hand, BC modified with HNO₃ solutions showed that the adsorption capacity of BC was enhanced when monetite and hydroxyapatite with low crystallinity were present in these materials. Hence, the adsorption capacity of BC treated with acid was independent of textural properties. The adsorption capacity of BC was enhanced by increasing the solution pH due to the electrostatic interactions between the fluoride in the solution and the surface charge of BC. Finally, DRX, FTIR spectroscopy, and XPS studies corroborated that the fluoride adsorption mechanism on BC modified with acid solutions was influenced by electrostatic interactions and chemisorption between fluoride ions in solution and calcium phosphates from hydroxyapatite and monetite.