At low concentrations, sorption to mineral surfaces will be the dominant mechanism retarding radiouclide migration in subsurface environments. However, radionuclides can also be removed from the solution phase through co-precipitation as new mineral phases are formed and, at higher radionuclide/ ligand concentrations, by precipitation. At lower concentrations, co­precipitation will be the more important process.45 For the actinides, hydrolysis can occur for all oxidation states and can lead to precipitation from solution.129 In particular, An(iV) has a strong tendency to hydrolyse.110

In natural waters, key mineral phases for (co-)precipitation will include carbonate and iron (oxy)hydroxide mineral phases. Parkman et al. (1998)72 investigated the interaction of Sr21 with calcite and found that at higher ( > 0.3 mM) concentrations Sr21 was precipitated as strontianite on the calcite surface, perhaps as a result of the existing mineral phase providing nucleation sites and higher localised carbonate concentrations at the calcite surface-water interface. NpO21 has also been reported to co-precipitate with calcite.130 Co-precipitation appeared to be independent of pH, even though pH did affect Np(v) solution speciation. Extended X-ray absorption fine structure (EXAFS) analysis suggested NpO21 was structurally incorporated into the calcite lattice, with Np and the axial O atoms substituting for one Ca21 and two CO3 groups, respectively. Co-precipitation of UO221 with calcite is reported to be limited,130 but natural aged calcite has been found with uranyl incorporated into the lattice131,132 and one study investigating uranium contamination in the Aral Sea suggests co-precipitation with calcite and gypsum is the dominant mechanism for U removal from the solution phase.133 U(vi) can also co-precipitate with iron oxides, with X-ray absorption spectroscopy (XAS) suggesting that it is incorporated into the oxide lattice as uranate (containing U-O single bonds and no axial U-O double bonds) rather than uranyl.134