Migration of Uranium(VI) in a Phosphate Environment: Column Experiments and Modeling

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 un-derground 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 concentra-tion of 10-5 M and a synthetic HAP packed in a matrix of purified quartz sand.
In common batch sorption experiments, a strong interaction of uranium with the HAP surface was observed. The sorption of uranyl ions onto phosphate groups is eventually as important as their sorption onto silanol groups, outweighing the low phosphate content. From the interpretation of the TRLFS spectra, two surface spe-cies could be distinguished. On the basis of their fluorescence decay time and the shift of the peak maxima related to the free uranyl ion, these two species could be associated with uranium adsorbed to phosphate and silanol surface groups, respec-tively.
The breakthrough curves (BTC’s), measured at different vertical positions of the column, exhibit a strong retardation of uranium compared to the conservative tracer. The breakthrough (i.e., exceeding 50 % of the starting concentration) occurs after about 30 pore volumes. However, the elution part of the BTC’s features a pro-nounced tailing. The release of uranium is controlled by two processes of different rates. We assign the faster sorption/desorption scheme to the uranyl silanol surface species, exhibiting an almost full reversibility. The second observed process creating the long and pronounced tailing in the BTC’s can be explained by the much stronger binding of the uranyl moiety to the phosphate sites.