Salinity Effect in Permeability of Salt in Nanofiltration and Reverse Osmosis Membranes

In Morocco, water resources are increasingly under threat due to population growth, economic expansion, and climate change. Among the proposed solutions, brackish water desalination using membrane technologies such as nanofiltration (NF) and reverse osmosis (RO) with low-pressure membranes presents a promising alternative. This study evaluated the impact of salinity on the performance of two nanofiltration membranes (NF270 and NF90) and one reverse osmosis membrane (TM710) using three semisynthetic brackish water samples with salinities of 2, 4, and 6 g L^-1. Ion transfer mechanisms, particularly for sodium (Na⁺) and chloride (Cl^-), were analyzed using the Spiegler-Kedem (SK) and Kedem-Katchalsky (KK) mathematical models. Additionally, the effects of salinity on diffusion flux (Jdiff), convection-induced concentration (Cconv), reflection coefficient (σ), and solute permeability (Ps) were examined. Results indicate that the NF270 membrane exhibits the highest permeate flux, while NF90 and TM710 perform similarly. For all three membranes, permeate flux decreases almost linearly as feed water salinity increases. Regarding total dissolved solids (TDS) rejection, the TM710 membrane achieves the highest removal efficiency, followed by NF90 and then NF270. The NF270 membrane shows greater convective transport than NF90, with both diffusive and convective fluxes increasing with salinity. In contrast, the TM710 membrane operates primarily through diffusion, with TDS having little effect on its diffusion flux. NF90 and TM710 exhibit similar σ and Ps values for sodium and chloride ions, independent of TDS, highlighting the NF90's similarity to a reverse osmosis membrane. In contrast, for NF270, the sodium reflection coefficient (σ) increases with TDS, while solute permeability (Ps) rises for both ions due to a decline in retention efficiency.