Stability of Plutonium Oxide Nanoparticles in the Presence of Montmorillonite and Implications for Colloid Facilitated Transport

ABSTRACT The physical and chemical stability of PuO2 nanoparticles (intrinsic colloids) in groundwater will control their transport and may affect the performance of high-level radioactive waste repositories. We examined the chemical stability of two types of PuO2 nanoparticles at both 25 and 80 °C. The “alkaline” PuO2 nanoparticles were prepared by neutralizing an acidic Pu(IV) solution with dilute NaOH to pH ∼9.5. The “acidic” PuO2 nanoparticles were precipitated from 0.1 M nitric acid by heating a Pu(IV) solution at 60-80 °C for 30 minutes. The chemical stability of these PuO2 nanoparticles was tested in the presence of montmorillonite, a common mineral in the environment and potentially relevant backfill material in some nuclear waste repository designs. The “alkaline” PuO2 nanoparticles were found to be unstable over a timescale of months at both 25 and 80 °C, with elevated temperature enhancing their dissolution rates and sorption to montmorillonite. PuO2 nanoparticle dissolution rates decreased with increasing Pu concentration, consistent with solution saturation. The “acidic” PuO2 nanoparticles appeared to remain stable for much longer than the “alkaline” PuO2 nanoparticles. The “alkaline” PuO2 nanoparticle dissolution rate constants were as high as 10-11.9±0.4 mol m-2 s-1 and 10-11.2±0.2 mol m-2 s-1 at 25 and 80 °C, respectively, while the “acidic” PuO2 nanoparticle dissolution rate at 80 °C was 10-13.5 mol m-2 s-1. Based on transmission electron microscopy, the “alkaline” and “acidic” PuO2 nanoparticles were of similar size (2.5-4.5 and 2-3 nm nanoparticles, respectively). However, the “acidic” PuO2 nanoparticles formed more ordered nanoparticle aggregates. Our results suggest that the specific conditions experienced during PuO2 nanoparticle formation could significantly affect the stability of PuO2 in the presence of competing sorption processes and, in turn, the relative importance of intrinsic versus pseudocolloid transport in the environment.