Surface diffusion of direct dyes in porous cellulose membranes

Abstract

Adsorption of C.I. Direct Yellow 12 and C.I. Direct Blue 15 from aqueous solution onto cellulose membranes was carried out in an ultrafiltration-type cell. The dyeing process was analysed on the basis of the parallel transport mechanisms of surface and pore diffusion using Langmuir and Freundlich isotherms. The equilibrium isotherm was represented by Langmuir-type adsorption more accurately than Freundlich-type adsorption when the solution concentration of the dye was less than 0.3 mol m −3. The differences between the theoretical concentration profiles obtained on the assumption of surface diffusion control for the Langmuir and Freundlich isotherms was negligibly small. In contrast there is a difference between the theoretical concentration profiles of pore diffusion control for Langmuir and Freundlich isotherms. The experimental uptake curves and the concentration profiles showed better agreement with the theory for surface diffusion control than for pore diffusion control. The concentration of NACl influenced the equilibrium isotherm but had little effect on the surface d