Geochemical Reaction Modeling for Ferrihydrite Nano-Particles In the Lake Karachai Area, Russia

Nitrate-rich effluent containing radionuclides was discharged from a reprocessing plant into Lake Karachai, South Urals, Russia in 1951 to 1995, thus the lake has been contaminated with large amount of radioactive nuclides such as Sr, Cs, Ru, Zr, Ce, Co, Zn and Pu. The lake water infiltrated into weathered zone of bedrock of andesitic to basaltic porphyrite and volcaniclastics, and made nitrate-rich plume in the weathered zone, which was about 100 m deep and 4 km long in north-to-south in 1990. It was found that most Pu moves with ferrihydrite nano-particles (Novikov et al, 2006). Thus, the behavior of ferrihydrite nano-particles is important for expecting the transport of radionuclides in groundwater. Thermodynamic and kinetic considerations have clarified the mechanism of the formation and dissolution of ferrihydrite nano-particles. On the basis of the mechanism, the geochemical reaction model has been developed, which consists of the diffusion of Fe(II) from rocks, the oxidation of Fe(II) to Fe(III) by Nitrate, the formation and dissolution of ferrihydrite nano-particles, and the formation of goethite and hematite. The model has shown that the existence of nitrate is essential for ferrihydrite to exist in groundwater. The numerical calculation shows that without nitrate the ferrihydrite disappear in 30 years, and with nitrate the concentration of ferrihydrite becomes constant after 30 years, and exist in groundwater for more than 10 000 years in the Lake Karachai area.