Evaluation of parameters describing flux decline in nanofiltration of aqueous solutions containing organic compounds

One of the major drawbacks for the introduction of membrane technology is the possible occurrence of fouling. Especially in newer processes such as nanofiltration, understanding of the mechanisms of flux decline and fouling is limited. Water fluxes obtained with nanofiltration of pure water are usually different from those obtained with real feed solutions. Depending on the feed composition, a flux decline ranging from a few percent to a complete loss of water flux can be found. For aqueous solutions containing organic components, in the absence of e.g. suspended solids or high concentrations of ions that may cause scaling, adsorption of organic material on the membrane surface is the major fouling mechanism. Identification of the parameters that play a role in the process of adsorption on the membrane surface should lead to a better understanding of the mechanism that results in a hindrance of the water flux, and eventually to pore blocking. In this study, the following parameters were selected for a detailed investigation of the adsorption process: the dipole moment, the polarisability, the dielectric constant, the solubility in water, the octanol-water partition coefficient, the contact angle membrane/water, the Small number, the modified Small number, molecular size, pKa, and the Taft parameter. All of these parameters were evaluated in the framework of adsorption on nanofiltration membranes from an aqueous solution. The use of each parameter for describing adsorption will be discussed. The pKa was rejected on theoretical grounds; other parameters such as the solubility in water proved to be impractical. Eventually, the dipole moment, the octanol-water partition coefficient, and the molecular size were selected. Correlations between adsorption on nanofiltration membranes and these interaction parameters show that there is a clear influence on adsorption.