EVOLUTION OF BENTONITES IN THE PRESENCE OF Ca2+ IONS IN A CONTACT SOLUTION AND SORPTION CAPACITY OF BENTONITES FOR 137Cs AND 90Sr

The article presents the results of an experimental modelling of the influence of & Scy;& acy; 2+ ion concentration and pH of a model solution on the elemental composition and sorption capacity of natural (NB) and sodium-modified (SMB-20) bentonites from the Cherkasy deposit for 137 Cs and 90 Sr. The processes of the Cs and Sr sorption from multicomponent solutions with different pH, Eh and salt composition depend on the insoluble or sparingly soluble compounds precipitated on the bentonite surface. Based on the computer simulation data, it was found that small amounts of sediments, mainly oxides, hydroxides, Fe oxyhydroxides (hematite, goethite, limonite), and Ca carbo nates (calcite, aragonite, dolomite) were present in the model solution with alkaline pH after adding CaCl2 2 solutions. Oxides and hydrated oxides of Fe, and calcium carbonates play an important role in the Cs and Sr sorption processes on bentonites. They can concurrently sorb radionuclides and block the sorption centres in montmorillonites. The concentration of the main structural elements (Si, Al) in the bentonites practically does not change with the increased pH and Ca2+ 2+ ion concentration in the model solution compared to the initial samples of NB and SMB-20, which indicates their structural stability. At the same time, an increase in the calcium concentration and a decrease in the sodium concentration were found in the ion- exchange complex of montmorillonite - the main bentonite mine ral if compared to the initial samples. This can eventually lead to the transformation of SMB-20 bentonite from Na, Ca form to Ca, Na form. The 137 Cs and 90 Sr sorption on NB is shown to proceed more effectively, compared to SMB-20 in conditions of the increased pH and Ca2+ 2+ ion concentration. The results of this research may justify the bentonite clay application as a component of the isolating engineered barriers of radioactive waste disposal facilities.