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

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 Na-22, Cs-137, Sr-85 and Eu-154 were used; the first three are carrier-free enabling experimental work at sub-micromolar metal ion concentration, and Eu(III) tracer Eu-154 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 (D-a) values. This methodology enabled determination of diffusion coefficient even for strongly sorbing Eu-154. Da values for Na-22, Cs-137, Sr-85 and Eu-154 were 2.35 (+/- 0.14) x 10(-11), 2.65 (+/- 0.09) x 10(-12), 3.32 (+/- 0.15) x 10(-11) and 1.23 (+/- 0.15) x 10(-13) m(2) 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 D-a values were rationalized on the basis of sorption data. Distribution ratios (K-d) for Cs(I) and Eu(III) were higher than that for Sr(II), which in turn was higher than that for Na(I). (C) 2015 Elsevier Ltd. All rights reserved.