Tuning the Adsorption Properties for Selective Capture of Perfluorinated Compounds Using Multivariate Metal-Organic Frameworks

Per- and polyfluoroalkyl substances (PFAS) represent a class of emerging anthropogenic pollutants due to their widespread use in consumer and industrial products. Their interfacial nature leads to accumulation in humans and animals via direct and indirect exposure routesAs a result, PFAS exhibit toxicity at very low levels, typically parts per billion, prompting recent legally enforceable limits by the U.S. EPA as low as 4.0 parts per trillion (ppt, ng L-1) in drinking water. The inherent stability of the carbon-fluorine bond classifies PFAS as persistent pollutants, presenting a fundamental challenge for remediation methods, which must be highly selective and efficient in the presence of other chemical species often found in relative excess. Current degradation techniques require high concentrations, necessitating a pre-concentration step for removal from sources like groundwater and enrichment for subsequent degradation reactors. Structurally, the amphiphilic nature of PFAS, with a hydrophobic fluorinated tail and a polar, hydrophilic head, leads to complex environmental distribution and challenges selective sorbent development. Adsorption can occur via weak physical interactions or stronger chemical binding (e.g., hydrogen or ionic bonds). Metal-organic frameworks (MOFs), porous materials constructed from inorganic nodes and organic linkers, offer tunable internal surface chemistry potentially tailored for specific guest molecules. While several studies report efficient PFAS adsorption capacities in MOFs, most investigations focus on high concentrations (> mg L-1 range), making extrapolation to environmentally relevant, low-concentration conditions difficult due to factors like competition from dissolved species and weak interactions, especially for short-chain PFAS. More work is needed to establish structure-selectivity relationships for trace-level removal. We hypothesised that constructing multi-variate MOFs (MV-MOFs), integrating different linkers within the same framework, could enhance adsorption selectivity through potential synergistic effects. Specifically, we targeted the simultaneous interaction of the framework with the PFAS head and tail by combining hydrophobic and polar/ionisable functional groups. We prepared a series of MV-MOFs based on the UiO-67 topology, varying the ratio of 2,2′-diamino[1,1′-biphenyl]-4,4′-dicarboxylic acid (H2L(NH2)2) and 2,2′-trifluoromethyl[1,1′-biphenyl]-4,4′-dicarboxylic acid (H2L(CF3)2) linkers. Herein, we describe the synthesis and characterization (PXRD, elemental analysis, vibrational spectroscopy) of these materials. Adsorption kinetics, isotherms, and regeneration were evaluated using LC-MS for accurate quantification at parts-per-billion levels ranging from sub-ppm to high concentrations. Comparisons with UiO-67 indicate that combining polar (-NH2) and hydrophobic (CF3) functionalities within the MV-MOFs increases adsorption efficiency and binding strength, specifically at low PFAS concentrations.