Fluoride removal by hydrothermally modified limestone powder using phosphoric acid Sweety Gogoi, Robin K. Dutta* Department of Chemical Sciences, Tezpur University, Napaam, Tezpur 784028, Assam, India A R T I C L E I N F O Article history: Received 7 October 2015 Received in revised form 7 December 2015 Accepted 5 January 2016 Available online 6 January 2016 Keywords: Fluoride removal Limestone Phosphoric acid Hydrothermal Hydroxyapatite Adsorption A B S T R A C T 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. ã 2016 Elsevier Ltd. All rights reserved. 1. Introduction Fluoride contamination in groundwater creates a worldwide problem. Fluoride is essential for human health in small amounts especially for children of age below 8 years [1]. Hydroxyapatite (Ca 5 (PO 4 ) 3 OH, HAP) is the main constituent of tooth enamel and bone. Fluoride, at a small intake, replaces some hydroxyl ion within the crystal structure of HAP and produce some fluorapatite (Ca 5 (PO 4 ) 3 F, FAP), which strengthens the tooth enamel and bone [1–4]. However, excessive intake of fluoride can cause dental and skeletal fluorosis [1,2]. The acceptable fluoride concentration in drinking water is 0.5–1.5 mg L 1 . The World Health Organization (WHO) has set a guideline value for fluoride in drinking water as 1.5 mg L 1 and also advises to adjust the fluoride concentration at 0.7 mg L 1 in case of fluoridation of fluoride-deficient water for drinking [5]. The concentration of fluoride in drinking water above 1.5 mg L 1 causes fluorosis. The fluoride contamination in ground- water occurs naturally from fluoride-containing rocks, viz., fluorite, biotites, topaz, etc., as well as from industrial activities [3]. It has been estimated that over 200 million people are exposed to excess fluoride through drinking water globally, especially in India, China, Sri Lanka and rift valley countries in Africa [1,2,6–8]. Though alternative fluoride-free water such as surface water is the first option for mitigation of drinking water fluoride problem, in absence of any such alternative sources and in case of uneconomi- cal piped water supply in sparse villages, defluoridation of fluoride-contaminated water is necessary. However, fluoride being a difficult-to-remove contaminant, defluoridation of water is still considered as a challenge to environmental chemists and engineers. Several techniques, based on coagulation/precipitation [9], ion exchange [10], reverse osmosis [11], electrodialysis [12], nano- filtration [13], adsorption [3,14,15], etc., have been applied for defluoridation. Among them, adsorption is a common type of defluoridation techniques because of ease of operation and cost- effectiveness. Geomaterials such as limestone [16–20], pumice stone [21], bauxite [22], magnesite [22], gypsum [22] are some of the commonly used adsorbents as they are naturally available. HAP [23,24], alumina [25] and clay [26] are also used for adsorption of fluoride. Many researchers have used limestone for defluoridation of water [16–20]. Reardon et al. reported that bubbling of CO 2 through crushed limestone-bed column can reduce fluoride from 10 mg L 1 to 2 mg L 1 by precipitation of fluoride as CaF 2 [16]. Turner et al. reported that both precipitation of CaF 2 and adsorption of fluoride on limestone surface are dominant mechanisms of fluoride removal by limestone treatment in presence of HNO 3 and H 2 SO 4 [17]. Nath et al. reported fluoride removal by limestone treatment of contaminated water, pre- * Corresponding author. Tel.: +91 3712 267007x5055; fax: +91 3712 267005. E-mail addresses: sgogoi4@tezu.ernet.in (S. Gogoi), robind@tezu.ernet.in (R.K. Dutta). http://dx.doi.org/10.1016/j.jece.2016.01.004 2213-3437/ ã 2016 Elsevier Ltd. All rights reserved. Journal of Environmental Chemical Engineering 4 (2016) 1040–1049 Contents lists available at ScienceDirect Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/je ce