Scalable robust nano-porous Zr-based MOF adsorbent with high-capacity for sustainable water purification

To find sustainable water solutions, the development of high capacity, scalable robust adsorbents and mechanistic insight about their performance offers the potential to effectively address the global challenges of water scarcity and water contamination. We herein rationally design Zr-cluster defective MOF-808 (MOF-808def) with exposed carboxyl groups, a robust zirconium metal–organic framework (Zr-MOF), exhibiting high adsorption capacity (qmax ∼ 296 mg·g−1) coupled with high selectivitity for tetracycline (TC) antibiotics, outperforming other water-stable MOFs, commercial and inorganic nano-adsorbents. MOF-808def functions well across a wide range of contaminant concentrations (from trace to high-concentration) and even in harsh conditions (e.g., high acidity and salinity). Both experimental and simulation results indicate that the mechanism of adsorption involves both physisorption and chemisorption via hydrogen bonding, electrostatic interactions (EIs) and C-O-C covalent bonding via esterification. Computational studies confirm that hydrogen bonding plays a key role in strong guest–host interactions between TCs and MOF-808def. Further, defects resulting from missing-Zr-clusters in MOF-808def are confirmed to enhance adsorption performance. Specifically, the defect sites present exposed carboxyl groups from MOF-808def linker ligands that selectively react with –OH groups (phenol and tertiary alcohol moieties) in TC via esterification. These defects drive highly selective adsorption even at low concentrations of TCs (e.g., 500 ppb). Aiming for more than enhanced performance, economic estimation and scalable engineered reactor tests revealed that MOF-808def and its nano-composites are free of environmental risks and offer promise for sustainable water treatment at pilot scale. The use of defect-engineering rationales is a molecule-level design concept that could be generally useful for the development of the next generation of MOF-based nano-adsorbents for sustainable water treatment applications.