Diffusion of inorganic chemical species in compacted clay soil

Abstract

This research was conducted to study the diffusion of inorganic chemicals in compacted clay soil for the design of waste containment barriers. The effective diffusion coefficients ( D ∗ ) of anionic (Cl −, Br −, and I −) and cationic (K +, Cd 2+, and Zn 2+) species in a synthetic leachate were measured. Two clay soils were used in the study. The soils were compacted and pre-soaked to minimize mass transport due to suction in the soil. The results of the diffusion tests were analyzed using two analytical solutions to Fick's second law and a commercially available semi-analytical solution, POLLUTE 3.3. Mass balance calculations were performed to indicate possible sinks/sources in the diffusion system. Errors in mass balance were attributed to problems with the chemical analysis (I −), the inefficiency of the extraction procedure (K +), precipitation (Cd 2+ and Zn 2+), and chemical complexation (Cl − and Br −). The D ∗ values for Cl − reported in this study are in excellent agreement with previous findings for other types of soil. The D ∗ values for the metals (K +, Cd 2+, and Zn 2+) are thought to be high (conservative) due to: (1) Ca 2+ saturation of the exchange complex of the clays; (2) precipitation of Cd 2+ and Zn 2+; and (3) nonlinear adsorption behavior. In general, high D ∗ values and conservative designs of waste containment barriers will result if the procedures described in this study are used to determine D ∗ and the adsorption behavior of the solutes is similar to that described in this study.