Interaction of Uranium from Seepage Water with Hydroxyapatite

The ability of hydroxyapatite Ca10(OH)2(PO4)6 (HAP) to immobilize metal ions, particularly lanthanides and actinides, is well known. The long-term stability of this fixation is proven by natural analogue studies. Thus, HAP is a potential filling material in engineered barriers in abandoned mining areas as well as in the near-field of underground repositories for nuclear and toxic waste.
The interaction of U(VI) with HAP was studied in batch and unsaturated column experiments and by time-resolved laser-induced fluorescence spectroscopy (TRLFS). We investigated a seepage water from Schlema (Saxony) with a Uranium concentration of 10-5 M and a syn-thetic HAP packed in a matrix of purified quartz sand.
The batch sorption experiments show 100 % adsorption of UO22+. The break-through curves measured in the column experiments with continuous pulse injection yield a retardation factor Rf = 33.2 ± 1.5 compared to the conservative tracer tritiated water. This value corresponds to the strong adsorption behavior. An elution of the column with ten pore volumes of a uranium-free solution revealed a distinct tailing and a uranium recovery near unity. Thus, the sorption process proved reversible. The TRLFS measurements facilitate a differentiation of uranium bound to the HAP surfaces and to the quartz matrix. Furthermore, the surface speciation of uranium could be identified. Finally, a numerical model using the reactive transport code Crunch describes the transport behavior of uranium.