Diffusion of Na(I), Cs(I), Sr(II) and Eu(III) in smectite rich natural clay

Abstract

Diffusion of Na(I), Cs(I), Sr(II) and Eu(III) in smectite rich natural clay, proposed as a backfill material in the Indian geological repository, was studied using the out-diffusion method. Radiotracers 22Na, 137Cs, 85Sr and 154Eu were used; the first three are carrier-free enabling experimental work at sub-micromolar metal ion concentration, and Eu(III) tracer 154Eu was used at sub millimolar concentration. An out-diffusion methodology, wherein a thin planar source of radioactivity placed between two clay columns diffuses out, was used to obtain the apparent diffusion coefficient (Da) values. This methodology enabled determination of diffusion coefficient even for strongly sorbing 154Eu. Da values for 22Na, 137Cs, 85Sr and 154Eu were 2.35 (±0.14) × 10−11, 2.65 (±0.09) × 10−12, 3.32 (±0.15) × 10−11 and 1.23 (±0.15) × 10−13 m2 s−1, respectively. Da values were found to be in fair agreement with literature data reported for similar mineralogical sediments. Sorption of radionuclides on the clay was also determined in the present study and differences in Da values were rationalized on the basis of sorption data. Distribution ratios (Kd) for Cs(I) and Eu(III) were higher than that for Sr(II), which in turn was higher than that for Na(I).