Unraveling the competitive adsorption mechanisms of coexisting organic-Cr(Ⅲ) complexes on MIL-101(Fe): Roles of ligand properties and interfacial interactions

● The differences in the adsorption mechanisms of MIL-101(Fe) for small molecular ligands, larger molecular ligands, and mixed-ligand complexes were investigated. ● The adsorption capacity of MIL-101(Fe) for TA-Cr(Ⅲ) was superior to that of other ligands, with a maximum adsorption amount reaching 183.15 mg/g. ● The adsorption competitive of TA-Cr(Ⅲ) is stronger than that of EDTA-Cr(Ⅲ) and TA-EDTA-Cr(Ⅲ) in the mixed solution. ● The order of binding affinities for different interactions was clarified by combining DFT calculations. The ubiquity of diverse organic-Cr(Ⅲ) complexes in industrial effluents poses a significant challenge to water remediation due to their high stability and complex competitive behaviors. In this study, MIL-101(Fe) was employed to decipher the differential adsorption behaviors and mechanisms of single and co-existing Cr(Ⅲ) complexes, specifically targeting natural (Tannic acid (TA)) and anthropogenic (EDTA) ligands. The experimental results revealed a distinct selective hierarchy as TA-Cr(Ⅲ) > free Cr(Ⅲ) > EDTA-Cr(Ⅲ) > TA-EDTA-Cr(Ⅲ), with values of 183.15, 134.05, 93.02, and 73.80 mg/g, respectively. Among them, TA-Cr(Ⅲ) demonstrated an extremely strong adsorption capacity. Kinetic and isotherm analyses confirmed a chemisorption-dominated process. Crucially, the competitive mechanism was elucidated by integrating spectroscopic characterization with Density Functional Theory (DFT) calculations. The results revealed that while electrostatic interactions drive the initial approach, the specific affinity of phenolic hydroxyl groups in TA towards the Fe-clusters of metal organic frameworks (MOFs), combined with strong π-π stacking interactions, provides an advantage for TA-complexes. The differences of organic-metal complexes were revealed through analysis of electrostatic potential, frontier molecular orbitals and binding energy. This work provides a fundamental understanding of ligand-dependent competitive adsorption, offering a theoretical basis for the selective removal of complexed heavy metals in realistic aquatic environments. Download: Download high-res image (308KB) Download: Download full-size image