There sorption models such as Kd-concept linear, ion-exchange, and adsorption models are presented and a nonlinear diffusion equation which includes a surface diffusion is derived. A semi-analytical method is used to solve the derived nonlinear difusion equation. The effects of sorption during a transient release period on the diffusion rate of an ion are found to be insignificant when surface diffusion is included. However, the effects of surface diffusion are found to be significant. Therefore, surface diffusion must be considered in order to estimate the performance of a compacted bentonite backfill material more precisely. As a system factor, $\alpha_0$ whose properties are similar to the apparent diffusivity is included in the nonlinear diffusion equation and it is highly dependent on Dp/Ds. The results of calculation obtained by the semi-analytical method are fairly consistent with existing experimental data presented by other authors.