Diffusion Behavior of Humic Acid in Compacted Bentonite: Effect of Ionic Strength, Dry Density and Molecular Weight of Humic Acid

Diffusion behavior of humic acid (HA) in the compacted bentonite was investigated by through diffusion method under various ionic strength and dry density conditions for 1 - 2 years. The breakthrough of HA is observed in case of 1.2-1.6 Mg/m(3) dry density in 1M NaCl, 1.2-1.5 Mg/m(3) in 0.1 M NaCl, however, no breakthrough is observed even with 1.2 Mg/m(3) in lower ionic strength than 0.01M within this experimental duration. Molecular weight of HA was evaluated by size exclusion chromatography (SEC). The HA diffusing through bentonite is of lower molecular weight than 5kD in 1M NaCl or 3kD in 0.1M NaCl, those are independent of dry density of bentonite. It indicates that the structure of diffusing path for HA in compacted bentonite is dominated by ionic strength rather than dry density. Diffusion behavior of Nd(III) is also investigated in the presence and absence of HA, and concluded that it is facilitated by HA. These diffusion behaviors are modeled by one dimensional diffusion equation taking the equilibrium Nd + HA <-> Nd-HA simultaneously into account. This model can explain the behavior of Nd in the presence of HA. The effective diffusion coefficient and distribution coefficient for Nd-HA are considered to be similar to those of HA.