Prediction of the Species and Solubility of Uranium in Beishan Groundwater

In order to understand the sorption, diffusion and migration behaviors of U in Beishan water-rock system, the speciation distribution of uranium in Beishan granite groundwater, along with its solubility influenced by calcite were calculated, using PHREEQC software with the most recent thermodynamic data published by OECD/NEA. The results suggest that under acidic conditions, uranium mainly occurs as UO₂F⁺, UO₂SO₄, UO₂²⁺, UO₂F₂ and UO₂(SO₄)₂^2-, while under nearly neutral to alkaline conditions, the dissolved uranyl predominantly exists as anionic UO₂(CO₃)^4-₃, UO₂(CO₃)₂^2-,UO₂(OH)₃^- and UO₂(OH)₄^2-, which are expected to enhance the mobility of uranium. Since the potential high level radioactive waste repository in China will be constructed at the depths of 500-1000 meters underground, the groundwater chemistry at such depths would be weakly alkaline, and the uranyl will be existed as anionic complexes that are mobile. When the pH of the groundwater is fixed at 7.56, the calculations demonstrate that uraninite is the main phase at Eh<24 mV. With the Eh elevation, uranyl and its various anionic complexes formed with CO₃^2- and OH^- become the predominant species. When the groundwater is exposed to air, uranyl and its various complexes are expected to be the predominant species due to the dissolution of O₂. In case the groundwater-host rock system contains calcite, uranium solubility can be enhanced significantly at pH<8.0, while the influence is negligible at higher pH.