Quantifying steric, non-steric, and adsorption contributions to solute rejection in covalent organic framework nanofiltration membranes

Solute transport in nanofiltration (NF) membrane systems is described with the Donnan Steric Pore Model with Dielectric Exclusion (DSPM-DE), which couples size- and charge-based solute partitioning mechanisms into and out of the membrane pores with flow through the pore, as described by the Extended Nernst-Planck equation. If membrane structural and chemical characteristics are well defined, the DSPM-DE can theoretically be used to identify solute rejection mechanisms, predict NF performance, and guide membrane design. However, the presence of additional separation mechanisms, like adsorption, and the heterogeneous, convoluted characteristics of traditional NF membranes challenge these goals. In this work, we apply covalent organic frameworks (COFs) as model NF materials to demonstrate control over the steric and non-steric partitioning and transport mechanisms in NF. We experimentally isolate and quantify the steric and non-steric contributions to solute partitioning and transport in NF via application of the DSPM-DE to COF membranes fabricated with tailored pore sizes, thicknesses, and charge properties. We also demonstrate enhanced non-steric solute rejection achieved through changes to COF membrane structure and chemistry, and we highlight the significant impact of adsorption on measured solute rejection by COF membranes.