Double mode of sorption.
It may be the case of water molecules. Absolute energy of sorption on strong interaction sites is strongly higher then in other parts of the membrane. The absorbed molecules may be trapped and released step by step in the membrane. The equilibrium concentration profile is reached step by step too. The concentration of the occupied sites (or trapped molecules) is high. The measurement of the initial slope of the kinetic curve does not give a proper estimation of the diffusion coefficient by the use of Fick law and another more complicate formulae should be used.
Example 3, Case of strong interaction sites and a rapid kinetics of trapping :