Contribution of the surface diffusion on mass transfer of Ba²⁺ in granite matrix

Abstract

  The effective diffusivity (D_e) and the distribution ratio (K_d) of Ba²⁺ in Inada granite have been determined by the through-diffusion method. Stable BaCl₂ solutions of the equal concentration were placed in two reservoirs across a granite sample and the self-diffusion of Ba was observed using radioactive ¹³³Ba. Experiments were performed in triplicate for 10^-1, 1 and 10 mol m^-3 BaCl₂ solutions at 25°C. The D_e value obtained at a BaCl₂ concentration of 10 mol m^-3, agreed to the estimated value based on the pore diffusion model. The lower BaCl₂ concentrations yielded higher D_e values and higher K_d values. The variation in D_e was neither due to the speciation of barium in the solutions nor due to variation in physical properties of the pore structure in the rock because diffusivity of I was the same between 10^-1, 1 and 10 mol m^-3 BaCl₂ solutions. Contribution of diffusion in sorbed state should be responsible for the variation in D_e.