Selective ion permeation enables reduced combined gypsum-organic fouling of loose nanofiltration membranes

During the long-term operation of NF membranes, the occurrence of combined gypsum-organic fouling is a significant challenge. While numerous studies have focused on enhancing the fouling resistance of NF through various methods (e.g., antiscalants, pretreatment), there has been limited exploration into the correlation between its structural looseness and fouling resistance. Herein, we conducted an experimental study to evaluate the correlation by employing three NF membranes with different looseness degrees: a custom-developed loosely structured NF membrane (CLNF), the moderately loose NF270, and the tightly structured NF90. Our findings indicated that NF membranes with loose polyamide structure exhibited superior fouling resistance to gypsum scaling and combined gypsum-organic fouling, induced by model organic matters like bovine serum albumin (BSA), humic acid (HA), and sodium alginate (SA). After applying SA-coating to calibrate the surface properties of the membranes to exclude the influence of surface roughness, hydrophilicity, and functional groups, our study attributed the difference in fouling resistance to variations in ionic selectivity stemming from membrane looseness. Specifically, the primary advantage of looser polyamide NF membranes lies in enhanced Ca2+ permeation, which not only reduces the "egg-box" effect and surface foulant accumulation but also diminishes cake-enhanced concentration polarization, effectively mitigating gypsum-organic fouling. In real water testing scenarios, the enhanced fouling resistance of loose NF membranes was further affirmed. This research underscored the unexpected benefit of loose NF membranes in terms of fouling resistance, providing a novel insight for future application.