Fluoride removal by hydrothermally modified limestone powder using phosphoric acid

Limestone powder has been modified hydrothermally in presence of phosphoric acid (PA) and fluoride adsorption behaviour of the modified material has been studied. The hydrothermal modification was done with Ca/P molar ratio as 1.66 using limestone powder and 0.30 M, 0.60 M and 0.90 M aqueous PA. Analysis of FTIR and XRD confirmed formation of hydroxyapatite (HAP) during the modification. Batch experiments were conducted varying initial fluoride concentration of water, adsorbent dose, contact time and pH to examine the fluoride adsorption behaviour of the products. The adsorption kinetics showed that the adsorption process follows second order kinetics. The isotherm models indicated physisorption of fluoride along with exchange of OH ions of HAP by F ions. Thermodynamic results showed the adsorption process to be spontaneous, endothermic and irreversible. The product modified using 0.90 M PA, with an adsorption capacity of 6.45 mg g (1), has a potential for application in fluoride removal from water. (C) 2016 Elsevier Ltd. All rights reserved.