Fluoride removal by a hybrid fluoride-selective adsorbent

High concentrations of fluoride in groundwater have become a global environmental challenge. Out of the various methods developed for effective removal of fluoride adsorption using activated alumina (AA) is popular as it has a low initial cost. AA offers excellent chemical and mechanical stability, selectivity for fluoride, favorable hydraulic properties, regenerability but offers lower adsorption capacity as there is reduction in material and capacity with an increased presence of aluminum. Hydroxyapatite nanoparticles have been found to possess high adsorption capacity for fluoride but they lack mechanical stability and favorable hydraulic properties. In this study, we synthesized a hybrid adsorbent, modified activated alumina, prepared by dispersing nanoparticles of hydroxyapatite inside AA granules. The hybrid adsorbent possesses a maximum adsorption capacity of 14.4 mg F/g which is at least five times higher than that of the virgin AA which has been used extensively for fluoride removal. A column run carried out on the hybrid adsorbent with a synthetic solution containing 3 mg/L fluoride lasted for 450 bed volumes before a breakthrough of 1.5 mg/L was observed. The exhausted adsorbent could be fully regenerated in a column within six bed volumes using commonly available innocuous chemicals. The study will contribute to the field of effective removal of fluoride from contaminated drinking water.