Analysis of polysaccharide, protein and humic acid retention by microfiltration membranes using Thomas’ dynamic adsorption model

Abstract

Thomas’ model has been applied in this study for the quantitative description and parameterization of the dynamic adsorption of organic foulants to membranes by analysis of the permeate concentration as a function of filtration time (or effluent volume). A hydrophilically enhanced polyvinylidene fluoride (PVDF) membrane was used with dextran (DEX), bovine serum albumin (BSA) and Aldrich humic acid (HA) that were used as model compounds representative of polysaccharides, proteins and humic substances, respectively. Thomas’ model stemming from Langmuirian adsorption kinetics was found to give a good description of physically irreversible membrane fouling as well as the corresponding adsorption constants. The simplification of Langmuirian kinetics to Linear kinetics was found to be reasonable though with no further mathematical simplifications or approximations valid for the case of fouling of a hydrophilically enhanced PVDF membrane. Ka, i.e. the product of the equilibrium constant and the total adsorption capacity, is considered to be a more practicable measure of adsorption affinity than K. The Ka values of 2.09, 2.83 and 3.44 for DEX, BSA and HA, respectively, confirm that the affinity exhibited by foulants typically follows the order: humic substances > proteins > polysaccharides for hydrophilically enhanced PVDF membranes. However, the final adsorbed amount (i.e. the fouling extent) is dependent upon not only the membrane-binding affinity but also the concentration of foulant in the solution.