Mass transport mechanism of nitrate selective nanofiltration membranes on the basis of the Donnan Steric Pore Model with Dielectric Exclusion (DSPM-DE)

Nanofiltration (NF) membranes can play an important role to mitigate the nitrate pollution, emphasizing the necessity for understanding its mass transport mechanism to enhance water purification strategies. This study focuses on the nitrate separation capabilities of thin film composite NF membranes having a polyamide (PA) selective layer. The selective layer of the membranes was fabricated through interfacial polymerization, with varying piperazine (PIP) and trimesoyl chloride (TMC) monomer concentrations on porous polyacrylonitrile substrates. We studied their retention on aqueous solutions of PEG 200, single salts (NaCl, NaNO₃, Na₂SO₄), and mixed salts (NaCl + NaNO₃ and NaNO₃ + Na₂SO₄). Through a new fitting scheme, the Donnan Steric Pore Model with Dielectric Exclusion (DSPM-DE) was utilized to correlate the PIP and TMC concentrations with the ion transport and exclusion mechanisms responsible for the membranes’ nitrate separation capabilities. The model could simulate the Na⁺/NO₃^-/SO₄^2- system but not Na⁺/NO₃^-/Cl^-. Dielectric exclusion and diffusion were the dominant partitioning and transport mechanisms, respectively, explaining the high retentions of sulfate (96 to 99 %) and the negative retentions of nitrate (−24 to 11 %). This study sheds light on the potential of NF membranes having PA selective layers for efficient nitrate/sulfate separation, providing valuable insights into the underlying transport and exclusion mechanisms.