Temperature-dependent KMnO₄ oxidative decrosslinking for enhanced permeance and scaling resistance of polyamide nanofiltration membranes

Despite ensuring high rejection properties, the dense and highly cross-linked network of polyamide (PA) nanofiltration (NF) membranes limits their ability to selectively distinguish between ions with different charges due to the dominance of physical sieving, resulting in the excessive rejection of minerals beneficial to human health. To overcome this performance limitation, this study proposes a temperature-controlled potassium permanganate (KMnO₄) post-treatment strategy. The controlled de-crosslinking of the PA network allows for the simultaneous regulation of the pore size, charge, and hydrophilicity of the membrane. KMnO₄ oxidation selectively attacks amide linkages within the PA matrix, incorporating oxygen-containing functional groups, increasing surface roughness, and reducing crosslink density. Under optimized conditions (50 °C, 5 wt% KMnO₄, 30 min), the resulting membrane (NFM-T50) exhibits a markedly more negative surface charge and enhanced hydrophilicity. These structural and chemical changes increase pure water permeance to 19.4 L m⁻² h⁻¹ bar⁻¹ while sustaining a high Na₂SO₄ rejection of 93.9% and enabling moderated passage of Ca²⁺ and Mg²⁺. The reduced divalent cation rejection mitigates gypsum supersaturation at the membrane interface, yielding substantially improved scaling resistance, with flux recovery exceeding 83.0% across repeated fouling-cleaning cycles. Overall, this study demonstrates that moderate KMnO₄-driven oxidative decrosslinking is a practical and effective approach to simultaneously enhance permeance, selective mineral retention, and anti-scaling performance in commercial PA nanofiltration membranes.