Nanofiltration (NF) and Reverse osmosis (RO) are applied for separation of salts and low molecular weight solutes from a solvent, usually, water. Predicting the separation performance of NF membranes for mixed electrolytes solutions poses a significant challenge due to the complex transport mechanism. In the current work I try to analyze the way these membranes should be modeled and to gain a better understanding of the physical mechanism behind RO and NF membrane separations.

The experimental part of the work had two main directions of focus:  multicomponent filtration experiments (sections 4.1, 4.2 and 4.5) and electrochemical impedance spectroscopy (EIS) measurements (sections 4.3 and 4.4). The combination of both types of experiments enabled obtainment of a wider picture of membrane separation behavior with information dominated by either slow (filtration) or fast (EIS) ions in the membrane.

A widespread approach used in modeling membrane transport uses a mean field theory. Models assume that transport of salt and water takes place in nanopores or a homogneous gel, from which the salt is excluded by a mean uniform potential caused by a combination of Steric, Donnan, and diElectric exclusion mechanisms (SDE model). This approach predicts certain dependence of salt permeability on salinity, however, filtration data and other independent measurements of ion partitioning indicate that it is not consistent with experiments and fails to predict membrane performance for different salts and concentrations.

 A surprisingly good agreement between filtration experiments and model was obtained simply by assuming constant ion permeabilities, e.g., separation of seawater ions by nanofiltration (section 4.1). A still better agreement was obtained if some variation may be allowed for divalent cations only, provided coupling of transport of all ions is appropriately addressed.  Using a similar model, pH variations in a desalination process were modeled (section 4.2), and an extremely high permeability of hydronium and hydroxyl ions to the RO membrane was demonstrated, indicating a different exclusion mechanism for the latter ions. The dependence of salt permeability on solution composition was further examined in a system with different combinations of NaCl and CaCl₂ at different pH values (section 4.5).  Results indicated that ion permeability was affected by ion specificity and different membrane affinity towards different ions.  These data suggest that treatment of ionic exclusion mechanisms using conventional mean-field approaches is problematic.

In order to further understand ion transport in polymeric membranes, EIS was used to directly measure polymer permeability to ions in single-salt solutions without any water flow (section 4.3 and 4.4). Experiments clearly show deviations from the standard SDE model.  These findings indicate high importance of ion-specific effects on membrane conductance, in particular, an exceptionally high affinity of polyamide to protons, in agreement with filtration results. In addition, they indicate that the effective RO membrane structure is much thinner than previously assumed and non-porous. This small thickness is explained by a polyamide sponge-like structure, containing voids separated by thin polymer films, which enables exceptional performance of polyamide membranes.