Application of electron microscopy to understanding colloid-facilitated transport of radionuclides at the mayak production association facility, near Lake Karachai, Russia

Plutonium association and transport by ferrihydrite has been confirmed in the groundwater system of Lake Karachai at the Mayak Production Association, Russia (Novikov et al. 2006). Since this initial description, there has been substantial progress in understanding the mechanisms that control the concentrations and distribution of Pu at the Mayak site. This chapter is a concise review of work during the past 10 years that utilizes to advantage recent advances in nanoscale, analytical techniques. The structure of ferrihydrite (P63mc) was determined using a pair-distribution function analysis. Scanning transmission electron microscopy of Mayak colloids resolved the occurrence of mixed silica nano-colloids and ferrihydrite. Thermodynamic properties for aqueous Pu-species have been updated. Inner-sphere surface complexation was determined for adsorbed Pu using synchrotron-based X-ray absorption spectroscopy. The adsorbed Pu can transform into Pu-dioxide nanoparticles as revealed by high-resolution transmission electron microscopy. Plutonium polymerization can also proceed by aggregation of cubic Pu-dihydroxy building blocks of Pu(OH)22+6H2O. Plutonium desorption occurs associated with re-adsorption at neutral pH. To establish comprehensive model of Pu transport at the Mayak site, the detailed mechanisms described above must be considered. Rigorous characterization of colloids is necessary and these results must be compared with results obtained from experimental studies. The state-of-the-art knowledge on the Pu association with Fe hydroxides is now well enough understood that safety assessments no longer need to rely on the classical Kd approach.