Experimental study of multicomponent diffusion in low permeable sediments

Faculty of Geology, Department of Hydrogeology, Lomonosov Moscow State University, GSP-1, Leninskie Gory, Moscow 119899, Russia

^* Corresponding author: v.lekhov@gmail.com

Abstract

Diffusion coefficients for Na⁺ and distribution coefficients for Cs⁺, Ba2⁺, Ni2⁺, CO2⁺, Sr2⁺ were measured on low permeable samples (3 cm diameter, average length 7 cm) from the deep disposal site Siberian Chemical Combine (SSC) using the end-diffusion technique. The direction of diffusion was perpendicular to bedding. Special equipment for experiment were designed and constructed. Two types of concentration observation were used. For non-sorbing Na⁺ in time used EC sensors and length distribution of sorbed elements. The synthetic solution used in the experiments was a model of low-activity contaminant of the SSC, and consist NaNO₃ (25 g/l) and nitrate compounds: Cs⁺, Ba2⁺, Ni2⁺, Co2⁺, Sr2⁺ (100 mg/l each). The measured values of the effective diffusion coefficients D_e) for Na⁺ from 7.60×10^-7 to 1.51×10^-5 m2/day and volumetric dimensionless distribution coefficients (K_dρ) are: Cs⁺ from 0.21 to 22.1, Ba²⁺ from 1.1 to 1.45, Ni²⁺ from 0.3 to 16.4, Co²⁺ from 1.98 to 24.7, Sr²⁺ from 1.9 to 14.3. The values of the diffusion coefficient show a large degree of variability. The diffusion coefficients of non-sorbing Na⁺ measured in this study are in good agreement with pore-scale diffusion simulation on microtomography data.