Defluoridation of wastewaters using HAP-coated-limestone

Hydroxyapatite (HAP)-coated-limestone was applied to remove fluoride from aqueous solution. The adsorbent was prepared by reaction of limestone and diammonium hydrogen phosphate with or without ethanol. The results of XRD and BET analyses both showed that with the addition of ethanol, the structure of HAP-coated-limestone has been changed, resulting in changes of its crystallinity and specific surface area. Results of removing fluoride indicate that adding small amounts of ethanol (3 vol%) to the formulation remarkably increases fluoride removal. The possible mechanisms for F− adsorption onto HAP-coated-limestone surface have been proposed in this paper.