Geochemical interactions between leachants from actinide-doped Synroc and three Australian granites

A study has been undertaken on the interactions of leachates from actinide-doped Synroc and three Australian granites. Leach testing was carried out on Synroc containing 237Np, 239Pu and 244Cm under MCC-1 conditions in close proximity to granite specimens at 70deg C for 28 days. The presence of the granites increased the pH and reduced the total activity released from the Synroc. Preferential uptake of actinides occurred onto specific secondary minerals, especially hematite, other Fe-oxides/hydroxides, leucoxene, sericite, chlorite and clays. The controlling mechanisms for fixation of actinides on the granites appeared to be adsorption, ionic exchange and reduction by Fe(II). The high sorptive capacity of the alteration minerals significantly reduced the concentration of actinides in solution indicating that hydrothermally-altered granites, with a significant proportion of secondary minerals present, are capable of removing actinides from groundwaters. (orig.)