SUITABILITY OF THE FIRST-ORDER MASS-TRANSFER CONCEPT FOR DESCRIBING CYCLIC DIFFUSIVE MASS-TRANSFER IN STAGNANT ZONES

The concept of first-order mass transfer between mobile and immobile r egions, which mathematically simplifies the concept of Fickian diffusi on in stagnant areas, has often been used to describe physical nonequi librium transport of solutes into natural porous media. This study com pares the two concepts, using analytical expressions describing cyclic mass transfer into and out of stagnant layers. The results show that the first-order mass transfer concept cannot describe continuous diffu sion into the immobile zone during period of net outward diffusion if the immobile zone has not filled completely during the period of net i nward diffusion. This sets phenomenological limitations to the first-o rder mass transfer concept when short periods of relative time are inv olved; these limitations have to be compared with the practical Limita tions to the Fickian diffusion concept. (C) 1998 Elsevier Science B.V. All rights reserved.