Rigorous determination of pore size non-uniformity for nanofiltration membranes by incorporating the effects on mass transport

Abstract

The pore size uniformity of a nanofiltration (NF) membrane has an important impact on its rejection capacity and separation selectivity, which remains yet difficult to be scientifically and accurately characterized. In this study, a method was proposed for characterizing the pore size distribution of NF membranes, by assuming its obeyance of the log-normal distribution and incorporating the Donnan and steric pore model to describe water permeation and solute rejection. The distribution parameters are derived through data-fitting the experimental rejections of reference solutes in the model framework. The non-uniformity index is newly defined as the degree of deviation of a special pore size from the median pore size. The index makes that possible to quantitatively compare the non-uniformity among different NF membranes. The fast convergence of the method was verified. Furthermore, we put forward some suggestions on the use of the proposed method for pore size distribution characterization. The determination for the rejections of highly-rejected (∼90 %) solutes and the radii of the lowly-rejected (∼30 %) solutes must be sufficiently accurate. The proposed method is more advantageous over the conventionally adopted method due to the higher accuracy and more convenience in the assessment of membrane pore size distribution as well as membrane performance.