Transport phenomena in reverse osmosis/nanofiltration membranes

A general unique structural vision of transport phenomena in Reverse Osmosis (RO) and Nanofiltration (NF) membranes is discussed. The most common models for RO (such as the solution-diffusion and/or the Spiegler-Kedem model) and for NF membranes (the extended Nernst-Planck equation) are introduced as particular cases of the general “statistical-mechanical theory” of membrane transport developed by Mason and Lonsdale in 1990. The use of that approach is recommended to develop a structural model when the physical meaning of the parameters is desired. The typical trends of solute rejection and of the total volume flux in RO processes are discussed and the meaning of the model parameters is explained. NF modelling is presented according to the conditions of the porous vision of the Donnan-Steric-Pore-and-Dielectric-Exclusion model (DSPM-DE). The complexity of the physical phenomena involved in the partitioning mechanisms is widely discussed: mechanisms of charge formation and of dielectric exclusion (image forces and Born partitioning) are described in detail. The general DSPM-DE model is adapted for the case of neutral solutes and for electrolyte mixtures: for each case, the basic equations are developed and the typical approximations are presented. The procedures for membrane parameters calculations are introduced and a detailed discussion about the recommended correct method for data elaboration is presented. The final discussion is focused on the problems not yet completely solved as well as on the possible future trends.