Loose nanofiltration membranes enabled by adaptive pore shrinkage for selective dye/salt separation

Simple and low-cost membrane fabrication strategies can enable resource recovery from dye-containing wastewater by selectively separating dyes from salts. Here, we develop a novel surfactant-regulated, adaptive pore-shrinkage strategy for preparing a series of loose nanofiltration (LNF) membranes via a two-step process. Microstructural characterization showed reduced cross-sectional thickness and more compact finger-like pores, decreasing the molecular weight cut-off (MWCO) from 20,000 Da (OM) to 4,209 Da (SDSM-0.025). The decrease in water contact angle from 69.3° to 40.2° supports a surfactant-mediated regulation mechanism that mitigates capillary pressure during liquid-air interface formation. The optimized SDSM-0.025 membrane delivered high water permeance (40.3 L m⁻² h⁻¹ bar⁻¹) with excellent dye rejection (99.1% for Congo red), while exhibiting low salt rejections of 6.04% for NaCl and 10.94% for Na₂SO₄, confirming typical dye/salt selectivity. Notably, the modified material cost is only 0.2 USD·m⁻² and the separation performance remains stable for 150 days, supporting the feasibility of scalable manufacturing and practical deployment.