Mechanism of aqueous uranium(VI) uptake by hydroxyapatite A. Krestou, A. Xenidis, D. Panias * Section of Metallurgy and Materials Technology, Laboratory of Metallurgy, School of Mining and Metallurgical Engineering, National Technical University of Athens (NTUA), 9, Heroon Polytechniou str., Zografou Campus, 15780 Athens, Greece Received 16 October 2003; accepted 20 November 2003 Abstract The objective of this work was the study of the mechanism employed by hydroxyapatite (HAP), Ca 5 OH(PO 4 ) 3 , for the removal of hexavalent uranium from water in open to the atmosphere systems. The work showed that the attenuation mechanism employed by HAP can be attributed to bulk precipitation with almost 95% removal of U(VI) in a very short time, regardless of the applied conditions. A theoretical study of the HAP-U(VI) system showed that, depending on the pH, uranium (VI) can be precipitated either in the form of Ca(UO 2 )(PO 4 ) 2 or as CaUO 2 (CO 3 ) 2 . The precipitates formed are extremely stable in acid and neutral solutions, but not in alkaline solutions where an amount of 32% of the precipitated uranium(VI) is dissolved. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Industrial Minerals; Environmental; Pollution 1. Introduction Apatite minerals form naturally and are stable across a wide range of geologic conditions for hundreds of millions of years (Niagu, 1974; Wright et al., 1990). Apatites can be also produced synthetically by calcium and phosphate precipitation reactions or high tempera- ture solid-state processes, obtained from apatite ores, or derived from animal bones by heat treatment (Joschek et al., 2000), or treatment with hydrogen peroxide (Erts et al., 1994) to remove the organic fraction of the bone. Heat treatment has the added advantage of producing a more crystalline apatite. Hydroxyapatite is a calcium phosphate mineral that is also found in rocks and sea coral (Chen et al., 1997). It is chemically similar to the components of bones and hard tissues of mammals while is one of the few materials that are classed as bioactive. Hydroxyapatite is a thermally unstable com- pound, decomposing at temperatures between 800–1200 °C depending on its stoichiometry (www.azom.com). The apatite mineral group has been shown to be effective both in sequestering dissolved metals and in transforming soil-bound metals to less soluble phases. Many researchers document the strong affinity of apa- tite for uranium (Arey and Seaman, 1999), strontium (Laxic and Vukovic, 1991), and other metals (Gauglitz and Holterdorf, 1992). Apatite is an ideal material for long term containment of contaminants because of its high sorption capacity for actinides and heavy metals (Chen et al., 1997), low water solubility of metal phos- phates that form by interaction with phosphate released from apatite (Fuller et al., 2002), high stability under reducing and oxidizing conditions, availability and low cost. Uranium is a heavy metal that naturally occurs in the +2, +3, +4, +5, +6 valence states, but it is most com- monly found in its hexavalent form. In nature, hexava- lent uranium is found as the mobile, aqueous uranyl ion, UO 2þ 2 . Uranium is likely to occur as a contaminant in the environment as a result of emissions from the nu- clear industry, release in mill tailings, the combustion of coal and other fuels, the use of phosphate fertilizers that contain uranium, machining of depleted-U munitions and natural weathering of igneous rocks and ore bodies which can produce groundwater seeps in the hundreds to thousands of lg/l (ppb) range of dissolved U. Only in Europe, 14 countries face the problem of uranium contamination, which represents a particularly serious danger where drinking water resources might be affected (www.perebar.bam.de). * Corresponding author. Tel.: +30-210-7722276; fax: +30-210- 7722168. E-mail address: panias@metal.ntua.gr (D. Panias). URLs: http://www.azom.com, http://www.perebar.bam.de. 0892-6875/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2003.11.019 Minerals Engineering 17 (2004) 373–381 This article is also available online at: www.elsevier.com/locate/mineng