Nonsymmetric band spreading

Nonsymmetrical band spreading may be attributed to nonlinearity of the equilibrium adsorption isotherm (i.e., deviation of isotherm from Henry s Law). This causes the sorbate to move through the column at different rates (rate dependent upon sorbate concentration). 

Peak diffuseness may be a result of the kinetics of the sorption-desorption process (i.e., slow mass transfer or exchange at sorbent surfaces). Peak diffusion in this case is usually nonsymmetric because the rates of sorption and desorption are not the same. Band spreading due to the final rate of mass exchange is closely related to the diffusion phenomenon. Physical adsorption, for all practical purposes, is instantaneous. The overall process of sorption, however, consists of several parts (a) the movement of sorbate molecules toward the sorbent surface, resulting fi om intergrain diffusion (outer diffusion), (b) movement of sorbate molecules to the inside of pores (i.e., internal diffusion of the sorbate molecules in the pores and surface diffusion in the pores), and (c) the sorption process in general. 

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