Sorption behaviour of Am on precorroded HLW glass in water and brines

In the alteration layer of HLW glasses corroded in aqueous solutions, less soluble elements like REE, actinides and other elements are retained by sorption or by formation of secondary alteration products controlling the solution concentrations. In order to quantify the sorption effect on the retention of metal ions, first sorption tests with U, Th and Eu have been performed using precorroded, simulated HLW glass as substrate. To demonstrate that the sorption behaviour of Eu is also representative for homologue trivalent actinides additional sorption tests were performed with Am241 in deionized water, NaCl and MgCl2 brine. Distribution ratios Rs (Rs = Amsorb/Amsol x V/m) for Am were found independent of the starting concentration in the pH range 2 to 6. At higher pH, a high amount of Am-containing colloidal particles were detected. The Rs values of Am show a strong increase with pH, quite similar to the Rs values of Eu. In general, the sorption behaviour of Eu and Am is very similar in deionized water and in brines. Clay minerals formed in the gel layer as secondary phases during long-term glass corrosion are assumed to act primarily as strong sorbents. Therefore, the sorption of REE and Am onto synthetic poor crystalline smectite precursors and a ''well crystallized'' natural smectite is compared with the findings obtained from precorroded glass. The sorption of Am on the precorroded glass is found to be much stronger than the retention on the investigated pure clay phases, indicating a different number of active sites or occurance of a different sorption mechanism. (orig.)