The Sorption of Actinides in Igneous Rocks

Abstract

The sorption of americium(III) and neptunium(V) on some major minerals of igneous rocks (quartz, microcline, albite, bytownite, biotite, hornblende, augite, olivine, and kaolinite) and on two granites has been studied, the aqueous phase being on artificial groundwater and with pH varying from four to nine. The sorption was measured on crushed solids (0.044 to 0.063 mm) at ambient temperature by a batch technique, using ²⁴¹ Am (2 × 10^-9M) and ²³⁵Np (2 × 10^-11 M). For both americium and neptunium, sorption isotherms were obtained that seem to be related to the formation of hydrolyzed species of the elements in the aqueous phase, giving an increased sorption with an increase of the degree of hydrolysis (starting at pH 5 for americium and at pH 8 for neptunium). The sorption on the individual minerals seems to be qualitatively related to their specific surfaces (and cation exchange capacities), but the differences between high-sorbing biotite and low-sorbing quartz were not more than one order of magnitude (in terms of distribution coefficients) in the studied pH range. Distribution coefficients for the granites were equal to the weighted average values of the distribution coefficients for the individual minerals within a factor of three.