Dual-functionalized Fe(BTC)₈(PABA)₂ for ultrafast and ultrahigh-capacity capture of Cd(II) and Pb(II) with multiscale mechanistic insights

A mixed mono−/multidentate ligand strategy engineered defective MIL-100(Fe). • Coordinatively unsaturated Fe O sites promoted rapid heavy-metal adsorption. • Defect engineering integrated Fe O exposure and amino functionalization. • Synergistic Fe–O/NH₂/COO − sites enhanced adsorption kinetics and affinity. • A flexible MOF membrane achieved efficient and recyclable heavy-metal removal. To address Pb(II) and Cd(II)-dominated heavy-metal contamination in water, a series of amino-bearing, defect-regulated Fe-based metal-organic frameworks (MOFs) were constructed using ligand engineering. In the synthetic design, the tritopic linker 1,3,5-benzenetricarboxylic acid (H₃BTC) was coupled with the monotopic modulator p-aminobenzoic acid (PABA), enabling the simultaneous generation of coordinatively unsaturated Fe O centers and exposed -NH₂ functionalities within the framework. This dual functionalization substantially strengthened the affinity of the adsorbent for divalent heavy-metal ions. Among the obtained samples, 6 mg Fe(BTC)₈(PABA)₂, corresponding to an H₃BTC/PABA feed ratio of 8:2, delivered the best adsorption performance under the conditions of 25 °C, pH 5, and an initial concentration of 150 mg/L, with maximum uptake values of 732.03 mg/g for Pb(II) and 654.05 mg/g for Cd(II), and equilibrium was achieved within 15 min. Kinetic and equilibrium analyses followed the pseudo-second-order model and Langmuir isotherm, respectively, indicating that monolayer chemisorption dominates the capture process. To improve the handling, recovery, and reusability of Fe(BTC)₈(PABA)₂ particles, they were incorporated into a polyacrylonitrile (PAN) nanofibrous matrix via electrospinning to obtain a Fe(BTC)₈(PABA)₂@PAN membrane. The membrane retained good structural stability and pronounced selectivity, particularly for Pb(II), in both binary-ion media and multi-cationic aqueous systems. After five adsorption-desorption cycles, more than 83% of the initial adsorption capacity was retained. DFT (Density functional theory) calculations further supported the experimental observations, showing that the adsorption originated from cooperative coordination among unsaturated Fe-O-M sites, carboxyl oxygen atoms, and amino nitrogen atoms. Therefore this study provides a defect- and functionality-coupled strategy for developing MOF-based adsorbents with both rapid uptake and high capacity for heavy-metal remediation. Download: Download high-res image (331KB) Download: Download full-size image