Uptake of Eu, Th, U, and Pu by granite and biotite gneiss in Korean fresh groundwater under oxidizing and reducing conditions

Abstract

The uptake of Eu, Th, U, and Pu by Korean granite and biotite gneiss was investigated in a series of batch experiments. Experiments were conducted under well-defined redox conditions, i.e., oxidizing (air), mildly reducing (Ar-atmosphere and buffered with hydroquinone, pe + pH ≈ 8–9), and strongly reducing (Ar-atmosphere and buffered with Na2S2O4, pe + pH ≈ 0.5–3). Radionuclide concentration, pH and Eh were systematically monitored up to t ≤ 113 days after the addition of the radionuclide. The natural content of Eu, Th and U in pristine granite and biotite gneiss materials was quantified by means of alkaline fusion. Eu exhibited moderate sorption on biotite gneiss displaying higher distribution ratios (Rd) compared to granite. This observation was possibly explained by the affinity of Ln(III)/An(III) towards biotite mineral absent in the investigated granite material. Strong sorption was observed for Th, U, and Pu in reducing systems where the predominance of the +IV oxidation state is expected. For these three systems, the strength of the uptake follows the order Rd(Pu(IV)) > Rd(U(IV)) > Rd(Th(IV)), consistent with the hydrolysis strength of the corresponding aquo-ions. A significantly weaker sorption was observed for U and Pu under oxidizing conditions, although Rd values are manifestly higher for Pu than U. Thermodynamic calculations for the oxidizing conditions predicted the predominance of U(VI) and Pu(V)/Pu(IV), explaining observed differences in retention under oxidizing conditions. These results contribute to a quantitative description and a better understanding of the retention of redox-sensitive radionuclides in crystalline host rocks. Emphasis is placed on the importance of utilizing both redox-stable probes (e.g., Eu, Th) and redox-sensitive actinides (e.g., U, Pu), as well as well-defined redox conditions for accurate predictions of radionuclide retention.