Enhanced fluoride adsorption using Al (III) modified calcium hydroxyapatite

► Al modified hydroxyapatite possessed a higher defluoridation capacity of 32.57mg/g. ► Hydroxyl groups on the surface of Al-HAP was the adsorption sites for F− removal. ► Enhanced F− removal over Al-HAP was attributed to the modification with aluminum. Aluminum-modified hydroxyapatite (Al-HAP) was prepared and characterized using XRD and BET analyses. Al-HAP possessed higher defluoridation capacity (DC) of 32.57mgF−/g than unmodified hydroxyapatite (HAP) which showed a DC of 16.38mgF−/g. The effect of Al/Ca atomic ratio in Al-HAP, solution pH and co-existing anions was further studied. The results indicated that the adsorption data could be well described by the Langmuir isotherm model and the adsorption kinetic followed the pseudo-second-order model. The pH changes during the adsorption process suggested that the OH on the surface of Al-HAP was the adsorption sites. The more adsorption sites were formed on Al modified HAP, which possessed abundant surface hydroxyl groups, resulting in higher efficiency of F− removal. Thermodynamic parameters such as ΔG°, ΔH° and ΔS° were calculated in order to understand the nature of adsorption process. The results revealed that the adsorption reaction was a spontaneous and endothermic process.