3D-Printed Photocatalytic Spacers for Fouling Control and Pollutant Degradation in Membrane Water Treatment: Design Strategies and Prospects for Electrodialysis and Nanofiltration

Feed spacers are indispensable components of aqueous membrane processes, but conventional fabrication methods such as extrusion and injection moulding constrain spacer geometry to simple net-type designs, limiting further gains in mass transfer and fouling control. Additive manufacturing (3D-printing) has relaxed these geometric constraints and, more recently, enabled the integration of photocatalytic functionality directly onto the spacer structure, allowing a single component to simultaneously provide mechanical support, hydrodynamic mixing, and in situ pollutant degradation. This review describes the fundamentals of spacer hydrodynamics, including the roles of filament diameter, mesh angle, porosity, and channel height in governing concentration and temperature polarization before surveying the photocatalytic materials, such as TiO2, g-C3N4, iron oxides/oxyhydroxides, metal-organic frameworks and conducting polymers, and synthesis routes (hydrothermal, sol-gel/dip-coating, in situ polymerization, microwave-assisted) used to functionalize 3D-printed spacers. NF and ED are selected as the two spacer-dependent processes most widely used in water treatment and are discussed separately because of their different driving forces. Design strategies for integrating photocatalysts onto printed spacer architecture, the resulting fouling-control mechanisms, and reported performance in electrodialysis and nanofiltration systems are critically assessed, alongside emerging cross-cutting applications. Because direct data on photocatalytic spacers in NF and ED are still scarce, results obtained with spacers are distinguished from extrapolations from photocatalytic membrane studies. Key challenges, specifically photocatalyst loading stability under prolonged operation and light delivery within enclosed membrane modules, are identified among the principal barriers to scale-up, while 4D-printed stimulus responsive spacers, machine-learning-guided geometry optimization, and photothermal-photocatalytic coupling are highlighted as promising future directions. Furthermore, 3D-printed photocatalytic spacers represent a convergent technology capable of simultaneously addressing concentration polarization, organic fouling, biofouling, and micropollutant degradation within a single structural component. Three-dimensional-printed photocatalytic spacers may contribute to the mitigation of concentration polarization, organic fouling, biofouling, and micropollutant degradation within a single component, although these phenomena are governed by different mechanisms and their simultaneous control involves design trade-offs.