Grafting amino groups to enhance the adsorption of antimonate by MIL-100(Fe) for from natural water: Performance and mechanism

MIL-100(Fe) was modified by amino groups (MIL-100(Fe)-NH2) to enhance the removal of antimonate (Sb(V)) with low concentration (50−200 μg/L) from natural water through batch adsorption experiments. With the initial Sb(V) concentration up to 200 μg/L, the residual Sb(V) concentration after treated by MIL-100(Fe)-NH2 was only 3.52 ± 0.131 μg/L, lower than the drinking water standards (5 μg/L) in China, while that was much higher (94.2 ± 3.52 μg/L) after pristine MIL-100(Fe) treatment. Based on results of XPS analysis, and adsorption kinetics and isotherms, we concluded that Sb(V) substituted the terminal −OH groups of MIL-100(Fe) to form inner-sphere complexes by coordination reactions with Fe nodes and be adsorbed by hydrogen bonding as well. The grafted amino groups (−NH2) adsorbed Sb(V) by hydrogen bonding and facilitated the coordination reactions between Sb(V) and Fe nodes. The higher KL value (0.0116 ± 0.00,118 L/μg) obtained by fitting with the Langmuir isotherm model, while that of unmodified MIL-100(Fe) was 4.74 × 10−4 ± 5.85 × 10−5 L/μg, also confirmed that the −NH2 groups greatly enhanced the affinity between MIL-100(Fe) and Sb(V). Furthermore, MIL-100(Fe)-NH2 still worked nicely in a wide pH range (2 − 12), with varied ionic strength (0.05 µmol/L−1.5 mol/L), coexisting ions (CO32−, NO3−, SO42−, Cl−, H2PO4−), and even in the real water bodies. Additionally, MIL-100(Fe)-NH2 exhibited good reusability and low Fe leaching (99.92 μg/L). We hope MIL-100(Fe)-NH2 become a candidate for effective removal of Sb(V), even other heavy metals from natural waterbodies.