In nanofiltration membrane separation technology, the Donnan Effect is one of the most essential electrostatic mechanisms that explains how nanofiltration (NF) membranes achieve selective ion rejection. Nanofiltration membranes typically carry surface charges, and these positively or negatively charged functional groups create an asymmetric distribution of ions on both sides of the membrane. This directly influences salt rejection, divalent ion selectivity, and the removal performance for poorly biodegradable organics.
The Donnan Effect originates from electrostatic repulsion. When feedwater contains divalent ions such as Ca²⁺, Mg²⁺, and SO₄²⁻, their transport is restricted by both nanofiltration membrane pore size and membrane surface charge. Among these two factors, the surface charge plays a more dominant role. A negatively charged NF membrane increases repulsion against divalent anions, enhancing the rejection of sulfate, carbonate, and other multivalent ions. This is a key advantage of nanofiltration membrane systems in municipal water polishing, industrial wastewater reuse, and selective ion separation. In contrast, monovalent ions such as Na⁺ and Cl⁻ experience weaker electrostatic repulsion and thus permeate more easily through NF membranes.
Importantly, the Donnan Effect not only impacts ion transport but also alters the electrochemical equilibrium between the feed and permeate sides. Under high-TDS conditions, the need for electrical neutrality may drive certain ions to "counter-migrate" through the membrane to balance charge differences. This unique phenomenon provides valuable insight into evaluating nanofiltration membrane pore size, membrane surface potential, electric-field interactions, and NF membrane concentration polarization behavior.
In industrial applications, the Donnan Effect is highly relevant for brackish water desalination, metal ion separation, textile wastewater treatment, and pharmaceutical intermediate concentration. For example, in heavy-metal wastewater treatment, a negatively charged nanofiltration membrane can significantly enhance rejection of Cu²⁺ and Ni²⁺ ions through electrostatic repulsion. This allows the system to achieve high desalination efficiency and selectivity at relatively low operating pressure, reducing energy consumption and prolonging membrane module lifespan.
However, while the Donnan Effect improves separation performance, it may also intensify localized concentration polarization, increasing challenges in membrane fouling control. To address this, membrane engineering projects often integrate feedwater pretreatment, antiscalant dosing strategies, and continuous monitoring of transmembrane pressure to mitigate the side effects and maintain stable operation.
Overall, the Donnan Effect is a fundamental mechanism in nanofiltration membrane science. Understanding its influence helps engineers select the correct NF membrane model, optimize reverse osmosis membrane system design, enhance industrial water treatment membrane module performance, and achieve better cost-efficiency and operational stability-especially when dealing with complex water matrices.
