Effect of Carbonate on the Migration Behavior of Strontium in Compacted Bentonite

Abstract

The apparent diffusion coefficients of strontium in compacted bentonites were investigated at various concentrations of NaHCO₃. Purified sodium bentonite Kunipia-F® was compacted with a jig into cylindrical pellets 10 mm in diameter and 10 mm high with dry densities of 1.0 to 1.6 Mg/m³. Each bentonite pellet was inserted into an acrylic resin column and saturated with carbonated water containing 0.1 to 1.0 M NaHCO₃ for more than 1 month. The face of the bentonite specimen was spiked with 5 µL of 1.0 M SrCl₂ tracer solution. After a few weeks, the strontium diffusion profiles were measured by inductively coupled plasma-mass spectrometry. The apparent diffusion coefficients of strontium decreased slightly with increasing dry density. NaHCO₃ concentrations of 0.5 M decreased the apparent diffusion coefficients of strontium by half at a dry density of 1.0 Mg/m³ and quarter at 1.6 Mg/m³. At a higher NaHCO₃ concentration of 1.0 M, no strontium diffusion profile was observed, whereas white precipitate was observed on the face of the bentonite specimen where it was spiked with strontium. This white precipitate could be strontianite, which is strontium carbonate. Diffusion experiments using cesium were carried out for comparison, and the presence of carbonate had no effect on the apparent diffusion coefficient.