The objective of this work was the study of the mechanism employed by hydroxyapatite (HAP), Ca5OH(PO4)(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(UO2)(PO4)(2) or as CaUO2(CO3)(2). The precipitates formed are extremely stable in acid and neutral solutions, but not in alkaline solutions where an amount of approximate to 32% of the precipitated uranium(VI) is dissolved. (C) 2004 Elsevier Ltd. All rights reserved.