Evaluation of Fluoride Adsorption Mechanism and Capacity of Different Types of Bone Char

The fluoride adsorption capacity of three types of bone char (BC), including cow BC (CBC), chicken BC (CKBC), and pig BC (PBC), was examined. At the optimum charring conditions (temperature and time), PBC had the highest hydroxyapatite (HAP) content (0.928 g-HAP/g-BC), while CBC had the highest specific surface area (103.11 m2/g-BC). CBC also had the maximum fluoride adsorption capacity (0.788 mg-F/g-HAP), suggesting that fluoride adsorption capacity depends more on the specific surface area of the BC than the HAP content. The adsorption data of CBC, CKBC, and PBC fit well with the pseudo-second-order model and the Langmuir isotherm. The maximum fluoride adsorption capacity of BC reached the maximum value when the solution had a pH of approximately 6.0. Lastly, the highest fluoride desorption occurred when the BCs were soaked in solutions with a pH higher than 11.0.