A critical review of transport through osmotic membranes

Herein, we review mechanisms and models of solute transport relevant to nanofiltration (NF), reverse osmosis (RO), and forward osmosis (FO) membrane separation processes. We first consider state of the art polymeric NF, RO, and FO membrane properties and how these properties influence water and solute transport. Next, we critically review classical models (i.e., pore flow, solution-diffusion, and Nernst-Planck) and discuss the shortcomings of each, particularly focusing on their inability to explain fundamental relationships between water/solute transport and membrane properties, such as free volume, crystallinity, extent of swelling, and the presence of filler materials in mixed matrix/nanocomposite membranes. We then review modern structure-performance models that attempt to relate water/solute transport to quantifiable membrane structure and solute/water/polymer interactions. Finally, we consider both classical and modern mass transfer models that describe external, internal, and enhanced concentration polarization - all of which strongly influence water/solute transport through NF, RO, and FO membranes.