A new method of characterizing mass transfer controlling mechanism in pollutant adsorption from aqueous solutions

Abstract

Pollutant adsorption from aqueous solutions is usually controlled by diffusion or mass transfer which can be external film diffusion or intraparticle diffusion or both, depending on the relative magnitude of the film and intraparticle mass transfer resistances. Traditionally the Biot number is used to characterize mass transfer controlling mechanism in adsorption. In this work a new quantitative characterization method is proposed. Specifically the average fractional film mass transfer resistance is defined and used as an indicator of the relative importance of film and intraparticle mass transfer resistances. The average fractional film mass transfer resistance varies from 0 to 1, corresponding respectively to complete intraparticle diffusion control and complete film diffusion control. Numerical simulations of batch adsorption processes are performed and the corresponding average fractional film mass transfer resistance calculated for a wide range of process parameters. Based on the simulation results a correlation is obtained between the average fractional film mass transfer resistance and the Biot number, the adsorbate partition ratio and the separation factor. It is shown that the Biot number alone is not sufficient to characterize mass transfer controlling mechanism in adsorption.