The mechanism of diffusion of a citric acid aqueous solution upon a sorption resin granule depends upon it structure and dimension of pore and distribution surface energy on sorption granule surface. Especially under the condition of high concentration liquid phase

diffusion in the solid phase usually proceeds as a diffusion controlled zone. Glueckauf and Hill suggested respectively linear driving force (LDF) model and modified linear driving force (MLDF) model. When ratio of liquid and solid phases is rather high and phase equilibrium model is nonlinear

we obtain analytical solution and plots of these models for the sorption and desorption steps.