An experimental study of the dissolution rates of Nd-britholite, an apatite-structured actinide-bearing waste storage host analogue

摘要

The steady-state dissolution rates of Nd-britholite Ca9Nd(PO4)(5)SiO4F2 have been measured at 25 degrees C as a function of aqueous solution composition in open-system mixed flow reactors. Measured Nd-britholite dissolution rates based on Ca, P, and F release are found to be close to corresponding apatite dissolution rates; these 25 degrees C rates decrease from 10(-8) to 10(-12) mol/m(2)/S with increasing pH from 3 to 12. Measured Nd release rates are far slower than those of these other elements; Nd concentrations in solution appear to limited by precipitation of a secondary phosphate phase, which limits aqueous Nd concentrations to < 10(-12) mol/kg at near neutral conditions. Correspondingly, it appears likely that aqueous actinide release from analogous waste hosts will be similarly limited by the precipitation of sparingly soluble phases. (c) 2006 Elsevier B.V. All rights reserved.