Metal-Organic Frameworks in Water Treatment: A Review of Their Dual Role in Pollutant Sensing and Removal

The pollution of water with heavy-metal cations, synthetic dyes, antibiotics and other new contaminants is one of the most severe environmental challenges today, where the standard treatment chain rarely deals with the detection and removal of a contaminant in one go. Metal-organic frameworks (MOFs) are a remarkable type of crystalline porous materials constructed from metal nodes and organic linkers. The combination of ultrahigh surface area, variable pore chemistry and structural designability makes them interesting for dual purposes. This paper discusses the dual role that MOFs are increasingly expected to serve in water treatment, as sensitive probes reporting on the presence of a pollutant, and as adsorbents or photocatalysts that subsequently absorb or destroy it. First, we discuss the structural aspects governing MOF performance in aqueous conditions, and particularly highlight water stability, which has been the Achilles’ heel of the sector for a long time. We then survey the principal sensing modes luminescent, electrochemical and colorimetric together with the adsorptive and photocatalytic pathways exploited for removal. A dedicated section is devoted to bifunctional, “detect-and-remove” materials in which a single framework performs both tasks, often with parts-per-billion sensitivity and good recyclability. Representative performance data are tabulated, and the recurring mechanisms (photoinduced electron transfer, energy transfer, coordination and electrostatic interactions) are discussed. Finally, we consider the obstacles scalability, long-term stability and selectivity in real matrices that still separate laboratory demonstrations from field deployment, and we suggest where the field might usefully go next.