Speciation Distribution and Sorption Characteristics of ⁹⁰Sr and ⁹⁹Tc in Groundwater of Near Surface Disposal Facility in China

This study focuses on the speciation distribution and sorption characteristics of ⁹⁰Sr and ⁹⁹Tc at a near surface disposal facility in Northwest China. The primary geological medium in the study area is metamorphic quartz sandstone, whose main mineral components are quartz and mica. The surface potential of metamorphic quartz sandstone is negative when pH ranges from 3 to 11, and the surface potential decreases as the pH increases. The groundwater in the study area has a pH of 7.37 and an Eh of 125.7 mV; the water type is of SO₄•HCO₃-Na•Ca type. Based on the chemical compositions of groundwater in the study area, geochemical simulation software GWB was employed to simulate the chemical species of ⁹⁰Sr and ⁹⁹Tc in groundwater, and to evaluate the effect of pH, Eh and major anion concentrations on their speciation distribution. The simulation results indicate that speciation distribution of ⁹⁰Sr is mainly governed by groundwater pH, with Sr²⁺ being the predominant form in the groundwater. Besides, among the major anions(Cl⁻, \mathrmSO_4^2- , \mathrmHCO_3^- and F⁻) of groundwater, \mathrmSO_4^2- has the most significant impact on Sr speciation distribution. In contrast, speciation distribution of ⁹⁹Tc is mainly controlled by Eh, with \mathrmTcO_4^- being the primary species in oxidized environments. Batch sorption tests were performed to determine the adsorption equilibrium time and distribution coefficient K_d for ⁹⁰Sr and ⁹⁹Tc in metamorphic quartz sandstone. The test results show that adsorption equilibrium time for both ⁹⁰Sr and ⁹⁹Tc are approximately 30 days. ⁹⁰Sr exhibits a higher K_d value(1.68 mL/g) than ⁹⁹Tc(0.68 mL/g), which indicates that metamorphic quartz sandstone has a stronger adsorption capacity for ⁹⁰Sr than ⁹⁹Tc. This is attributed to the negative charge on the surface of the metamorphosed quartz sandstone, making it prone to adsorbing Sr²⁺ but difficult to adsorb \mathrmTcO_4^- . Batch sorption tests were also carried out to study the effect of aqueous pH on nuclide adsorption. The results show that trend in K_d values for ⁹⁰Sr and ⁹⁹Tc as a function of pH is closely related to the pH-dependent speciation distribution of nuclide. K_d values for ⁹⁰Sr firstly increase and then decrease with rising pH, reaching a maximus at pH of 8. This is primarily due to the presence of a large amount of H⁺ in the solution under low pH conditions, which occupies a portion of the surface adsorption sites of the geological medium; under high pH conditions, the concentrations of OH⁻ and \mathrmCO_3^2- in the solution increase significantly, forming complexes SrOH⁺ and SrCO₃(aq) with Sr²⁺, thereby hindering the adsorption of ⁹⁰Sr and leading to a decrease in its K_d. However, K_d values for ⁹⁹Tc decrease with increasing pH under the experimental conditions of oxidizing environment. This is due to that ⁹⁹Tc primarily exists as \mathrmTcO_4^- across most of the pH range in the oxidizing condition and the surface electron egativity of metamorphic quartz sandstone increases with rising pH. This leads to enhanced electrostatic repulsion between \mathrmTcO_4^- and the surface of the metamorphic quartz sandstone as pH increases, further hindering the adsorption of ⁹⁹Tc and causing its K_d to decrease as pH increases.