Adsorption of natural organic matter and divalent cations onto / inside loose nanofiltration membranes: Implications for drinking water treatment from rejection selectivity perspective

Loose nanofiltration (LNF) membranes hold great promise for the selective rejection of natural organic matter (NOM) while maintaining mineral salts to produce high-quality drinking water. Nevertheless, the rejection selectivity performance is not only determined by the inherent properties of membranes but also influenced by the feed water compositions. This study explored the inevitable adsorption of NOM and inorganic ions onto and inside membrane materials, which in turn altered the charge properties of LNF membranes, thereby affecting the rejection selectivity. Zeta potential measurements, X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry technique were employed to characterize solute adsorption on the membrane surface and within the membrane pores. Filtration experiments using synthetic and natural waters were conducted to assess the contribution of electrostatic effects and evaluate the membrane rejection performance. Results revealed that LNF membrane surfaces during filtration were readily coated by NOM molecules, probably via hydrophobic interactions, which in turn adsorbed divalent cations that actually determined the net charge density on the membrane surface. Additionally, NOM adsorption within the membrane pores largely altered pore charge properties, particularly of the sulfonated polyethersulfone membranes (e.g. NTR7450), where deprotonated sulfonic groups otherwise contributed to a high charge density. These interactions among NOM, divalent cations and membrane materials greatly reduced charge density on the membrane surface and largely diminished charges in pores, leading to decreased rejection of both NOM and mineral salts, as well as the mitigation of co-ion competition effects. Nevertheless, the UA60 membrane, having a molecular weight cut-off of ∼1000 Da, rejected NOM by ∼70 % while maintaining ∼95 % bicarbonate and ∼65 % hardness ions in the treated water, demonstrating fairly good selectivity. These findings offer valuable insights for optimizing LNF membranes to improve the safety, chemical stability and palatability of treated drinking water.