In the relentless pursuit of clean, safe water, nanofiltration (NF) membranes have emerged as pivotal tools in separating contaminants at the molecular level. Recent advances have spotlighted membranes engineered with exceptional charge properties, pushing the frontier of selective ion sieving and organic micropollutant (OMP) removal. A groundbreaking study by Xu, Chen, Wang, and their colleagues, published in Nature Water (2025), introduces ultrahigh negatively charged polyamide (PA) nanofiltration membranes with unprecedented separation performance, shedding new light on the intricate relationship between membrane surface chemistry and filtration efficiency.
At the heart of this innovation lies the precise control over the exposure density of oxygen-containing functional groups—often referred to as "[O] site exposure density"—within the PA layer. By manipulating this specific chemical motif, the researchers succeeded in fabricating membranes with a negative surface charge density reaching an extraordinary magnitude of −32.6 mC m⁻². This charge density substantially surpasses that of conventional NF membranes, enabling superior electrostatic interactions with feed water components, crucial for enhancing the selectivity of target ions and organic micropollutants.
Such an intense negative surface charge alters the conventional dynamics of monomer diffusion during membrane synthesis, particularly impacting the behavior of piperazine (PIP), a primary amine monomer used in interfacial polymerization. The team observed a marked 73.1% reduction in PIP diffusion rates under the influence of intensified hydrogen bonding and intermolecular forces, including induction and dispersion forces. This slowdown was quantitatively confirmed through Einstein’s relationship by analyzing the slope of mean square displacement (MSD) curves derived from diffusion simulations, adding a robust mechanistic layer to their experimental observations.
This carefully tuned monomer diffusion has profound implications on membrane morphology and chemistry. It dictates the relative proportions of PIP and trimesoyl chloride (TMC) at the polymerization interface, leading to formation of a uniform, thin incipient layer that is rich in carboxyl groups. Remarkably, the resulting membranes displayed a carboxylate group ratio (–COO–) of 45.7%, a key factor contributing to their ultra-high negative charge density and consequent functional properties. This carboxyl-enriched surface topography forms a charged barrier, finely regulating permeation pathways for water and solutes alike.
Performance metrics further underscore the phrase “ultra-high” when describing these membranes. They delivered an exceptional water permeance rate of 41.5 liters per square meter per hour per bar (l m⁻² h⁻¹ bar⁻¹), a benchmark that indicates not only efficient water flux but also energy-saving potential in filtration applications. Complementing this high permeability was the membrane’s outstanding ability to discriminate between anions in solution, yielding an anion selectivity coefficient (α_Cl⁻/SO₄²⁻) as high as 144.5 — a level of selectivity rarely reported in the field, signifying an enhanced sieving effect favored for monovalent chloride ions over divalent sulfate ions.
The membranes’ potential in tackling organic micropollutants offers a significant stride towards environmental and public health safety. Various commonly encountered OMPs — including bisphenol A (BPA), ofloxacin (OFL), tetracycline (TC), and chlorpheniramine (CP) — were efficiently rejected, with improved water-to-OMP selectivity ratios compared to conventional membranes. This advancement highlights the membrane’s versatility and robustness in complex aqueous matrices, presenting a transformative solution for water treatment facilities challenged by emerging contaminants.
The underlying chemistry responsible for these achievements transcends mere surface charge considerations, delving into molecular level interactions and diffusion kinetics. Enhanced hydrogen bonding not only reduces PIP diffusivity but also stabilizes the nascent polymer network during interfacial polymerization. This stabilization fosters the growth of a dense, functional layer, which prevents undesirable pore expansion and contributes to the selective sieving mechanism essential for discriminating between similarly sized ions and organic molecules.
Electrostatic repulsion, governed by the ultra-high negative charge density, synergizes with size exclusion to reject targeted solutes effectively. Monovalent and divalent ions interact differently with the membrane surface due to their charge and hydration properties, and the membrane’s peculiar charge profile exacerbates these disparities, enabling superior separation performance. The findings also imply potential tunability in membrane design; by adjusting [O] site densities and consequently the charge ratios, membranes could be custom-tailored for specific separation tasks ranging from industrial wastewater treatment to brackish water purification.
Another notable aspect is the membrane’s thin selective layer, which ensures minimal hydraulic resistance, contributing to elevated water permeance without compromising rejection rates. Traditionally, enhancements in selectivity often come at the cost of reduced permeance; however, the approach by Xu et al. circumvents this trade-off by leveraging interfacial polymerization control combined with molecular diffusion engineering. This finding could recalibrate how future membrane materials are conceptualized, aiming for optimized permeability-selectivity synergy.
The authors also employed advanced simulation techniques, likely molecular dynamics or Monte Carlo simulations, to elucidate the diffusion behaviors of monomers — an approach that integrates theoretical insight with experimental validation. Such comprehensive multidisciplinary methodology reflects the complexity of membrane science, where transport phenomena, surface chemistry, and polymer physics converge to influence final membrane performance.
In addition to performance, the membranes’ stability under operational conditions is implicitly promising given their dense, functionalized layer and strong electrostatic character. Longevity and fouling resistance, although not explicitly detailed, are critical factors for real-world deployment, suggesting fertile ground for subsequent research focusing on membrane durability and regeneration potential.
Moreover, the implications of this research transcend water treatment applications alone. The fundamental understanding of how controlled monomer diffusion and surface functional group exposure dictate membrane properties could inform the development of membranes for gas separations, energy applications like fuel cells, and even sensor technologies where surface charge modulation plays a critical role.
Future studies inspired by these findings could explore scaling the membrane fabrication process while maintaining the precision in monomer diffusion control. The integration of such ultrahigh charge-density membranes in pilot-scale or full-scale water treatment systems would be the logical next step to evaluate operational feasibility, fouling propensity, and economic viability, potentially revolutionizing the industry’s approach to selective separation.
In conclusion, the work presented by Xu and colleagues offers a paradigm shift in membrane science by marrying ultra-high negative charge density with precisely engineered polymerization chemistry. Their innovative manipulation of [O] site exposure density and the subsequent modulation of monomer diffusion dynamics have culminated in a next-generation nanofiltration membrane that achieves previously unattainable levels of anion selectivity, water permeance, and micropollutant rejection. These membranes not only underline the fundamental importance of surface charge in filtration science but also chart a clear course toward sustainable, efficient, and tailored water purification technologies.
This seminal study stands as a beacon for future explorations aiming to dissect the subtle molecular interactions dictating membrane behavior and guides the translation of such knowledge into real-world applications to safeguard precious water resources globally.
Subject of Research: Nanofiltration membranes with ultra-high negative charge density for enhanced separation of anions and removal of organic micropollutants.
Article Title: Nanofiltration membranes with ultra-high negative charge density for enhanced anion sieving and removal of organic micropollutants.
Article References:
Xu, X., Chen, Y., Wang, Z. et al. Nanofiltration membranes with ultra-high negative charge density for enhanced anion sieving and removal of organic micropollutants. Nat Water (2025). https://doi.org/10.1038/s44221-025-00440-9
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