Metal -phenolic network-based NF membranes for nitrate removal and the utilization of DSPM-DE model

The rising nitrate levels in polluted surface and groundwater, often exceeding permissible limits, have created a pressing need for effective removal methods. High-throughput, low pressure nanofiltration (NF) membranes with high permselectivity are the ideal candidates for the removal of such pollutants. This work develops thin-film composite (TFC) NF membranes via supramolecular self-assembly between polyphenolic molecule of tannic acid (TA) and metal ions (Fe³⁺) to achieve a high-performance nitrate retention. The TFC membranes exhibited an excellent NO₃^– retention property ranging from 83 – 94 %, outperforming the commercial NF membranes, and meeting the requirement for drinking water from surface water and groundwater bodies. Among the membranes, the 1TA-4.5Fe variant achieved a pure water permeance of 13.6 L·m⁻²·h⁻¹·bar⁻¹ and an 83 % NO₃^– rejection for a 100 mg/L feed solution. Nitrate retention improved with increased feed pressure and declined with increased nitrate concentrations in the feed solution. The thin film composite membranes having TA-Fe³⁺ selective layers also showed a good monovalent/divalent ion (NO₃^–/SO₄^2–) separation with a selectivity between 4.5 and 38.1. The Donnan Steric Pore Model with Dielectric Exclusion (DSPM-DE) model was used to describe the transport mechanism and rejection mechanism of ions for the feed solutions containing the ternary ion mixture (Na⁺/NO₃^–/SO₄^2–). Experimental and theoretical results indicated that nitrate ion flux was primarily diffusion-driven, while ion partitioning was governed by dielectric exclusion and Donnan effects. This comprehensive evaluation underscores the high potential of these TFC membranes for advanced nitrate removal. For the first time it has been demonstrated that the DSPM-DE model can sufficiently predict the selective transport of NO₃^– over SO₄^2– ions through TFC membranes having TA-Fe³⁺ selective layers.