Influence of рН and Ca2+ Ions on Chemical Composition and Sorption of 137Cs.

The safety of near-surface and deep radioactive waste storage facilities is based on a system of engineered and natural barriers. Significant degradation of the engineered barrier system composed of cemented waste matrixes covered by cement mixture, concrete compartments, and structures at the basement of the storage facility may cause radionuclide transfer from the facility to groundwater. Mixing of the cement and concrete with water leads to the formation of several various hydration products with subsequent leaching of Ca²⁺ ions and formation of hydroxyl ions (OH^-), which affects the alkalinity of the water environment and the sorption properties of bentonite, as a component of the engineered barrier at the basement of near-surface facilities. The article presents the results of an experimental study of the influence of Ca²⁺ ion concentration and pH of the model solution (similar to groundwater composition at Vector Site in Chornobyl Exclusion Zone) on the elemental composition and sorption properties of natural (NB) and Na-modified (PBA-20) bentonites from the Cherkasy deposit concerning ¹³⁷Cs at the Solid: Solution ratio of 1: 100. Geochemical modeling suggests that addition of CaCl₂ to test solution and resulting alkaline pH leads to precipitation of solids, mainly oxides, hydroxides, Fe oxyhydroxides (hematite, goethite, limonite), and Ca carbonates (calcite, aragonite, dolomite). Their role in Cs adsorption was evaluated. The concentration of structural elements (Si, Al) in bentonites practically does not change with Ca²⁺ ion concentration increase in the model solution, demonstrating the bentonite structure's stability under these conditions. At the same time, an increase in the Ca concentration and a decrease in the Na concentration was found in the ion exchange complex of the bentonites if compared to the initial natural bentonite. This results in the transformation of Na-modified bentonite from Na, Ca-form to Ca, Na-form. The total sorption capacity of NB and PBA-20 bentonite concerning Cs⁺ ions at increased concentrations of Ca²⁺ ions and pH of the solution slightly decreases, though retaining high values of the degree of absorption (> 90 %). The total adsorption of Cs⁺ ions on NB and PBA-20 bentonites from model groundwater with the addition of CaCl₂ from 16 to 960 mg/dm³ and increase of рН from 7.4 to 11.8 decreases with the increase in ionic strength, in particular, due to competition with Са²⁺ and Na⁺ ions. NB and PBA-20 bentonites of the Cherkasy deposit remain a reliable component of the liner at the repository basement owing to their main functional property - high absorption capacity for ¹³⁷Cs, which is an important dose-forming radionuclide of short-lived low- and medium-level waste. [ABSTRACT FROM AUTHOR]