Organic Membrane Fouling in Advanced Water Purification: Mechanisms, Bulk-Phase Aggregation, and Control Strategies

Membrane separation has become a key technology for advanced water purification and control of emerging contaminants because of its high separation efficiency, low chemical demand, and ease of integration. However, organic membrane fouling induced by the coupling of dissolved organic matter and coexisting metal ions remains a major obstacle to stable and efficient membrane operation. This review focuses on metal ion-mediated formation of organic aggregates in the bulk solution and their governing role in membrane fouling behavior. The review summarizes how metal ions regulate organic aggregate formation through distinct dominant mechanisms. Na+ mainly screens electrostatic repulsion, Ca2+ promotes ion bridging and cross-linking, and Mg2+ often induces weaker bridging or hydration-mediated effects due to its stable hydration shell. The review further discusses the dual effects of mixed foulants and dynamic fouling layers on the rejection of emerging contaminants. In addition, current control strategies, including pre-coagulation, pre-oxidation, and catalytic functional membranes, are evaluated from the perspective of regulating aggregate structures and interrupting interfacial deposition. Finally, future research should shift from membrane-interface-centered analysis to bulk-phase aggregation, establish quantitative structure–effect relationships between aggregate properties and fouling behavior, and promote fouling-control strategies from mechanistic effectiveness toward engineering practicality.