Investigation on solubility and speciation of valence-variable radionuclides in Beishan underground water

The borehole 28 (BS28) at Beishan is adjacent to the site of the proposed first underground laboratory of high-level radioactive waste repository in China. By combination of on-site continuous measurements and examination of the dissolved iron content in the groundwater and the Fe²⁺/Fe³⁺ ratios in the surrounding rock, we proposed recently a redox potential (Eh) range of -56 to 98 mV with a temperature of 18.4℃ and pH of 8.1 ± 0.2 for BS28 groundwater at 365-690 m depth. Based on the above conditions, the speciation and solubility of U, ⁹⁹Tc, ⁷⁹Se and Np are calculated by PHREEQC using thermodynamic data containing ternary complexes of uranyl and carbonate with Mg, Ca and Sr. When the pH of groundwater is 8.1, and Eh is between -56 mV and 98 mV, the solubility of U, Tc and Se range from 10^-9 to 10^-6 mol/L, 10^-14 to 10^-4 mol/L and 10^-14 to 10^-7 mol/L, respectively, while the solubility of Np remains below 10^-15 mol/L. In the above conditions, dissolved U, Se and Tc mainly exist in the forms of Ca₂UO₂(CO₃)₃, CaUO₂(CO₃)₃^2-, HSeO₃^-, SeO₃^2- and TcO^4-. On the condition of an invariable partial pressure of oxygen, we also investigated the influence of pH in the range of 5-11 assuming an Eh of -56 mV (minimum value), 21 mV (middle value) and 98 mV (maximum value) at pH 8.1. It is worth emphasizing that this work only simulates the homogeneous solution system, further studies on reductive immobilization of radionuclides are necessary in representative underground water-rock system, as the surrounding rock of Beishan granite is rich in ferrous iron. (authors)