Bentonite alteration and retention of cesium and iodide ions by Ca-bentonite in alkaline and saline solutions

Abstract

The nuclide retention capacity of bentonite can be affected by changes in the repository environments, such as alkaline and saline conditions. This study investigated the effects of bentonite alteration on the nuclide retention capacity of Ca-bentonite. A series of batch experiments was conducted at room temperature (25°C) for 7 d using Ca-bentonite and alkaline and saline solutions spiked with 10 mM each of cesium (Cs) and iodide (I). Cation exchange and a decrease in the basal spacing (d001) of montmorillonite were observed, as was the formation of zeolite minerals in bentonite samples reacted with an alkaline solution (0.1 M KOH and 0.1 M NaOH). Illitization, partial dissolution, and aluminum ions substitution for silicon ions in the tetrahedral sheets of montmorillonite were also observed in the bentonite samples reacted with an alkaline solution (0.1 M KOH). However, the Cs retention capacity of bentonite was not significantly affected by the mineral and physicochemical changes of montmorillonite in alkaline and saline solutions. Competing ions, including Mg, Ca, K, and Na ions in solutions with high ionic strength, all played a crucial role in Cs retention. Further, the cation exchange capacity of bentonite samples increased due to the formation of zeolite minerals under alkaline conditions.