Removal of Cu²⁺, Cd²⁺, and Pb²⁺ from aqueous solution by fabricated MIL-100(Fe) and MIL-101(Cr): Experimental and molecular modeling study

In this study, two metal-organic frameworks [MIL-100(Fe) and MIL-101(Cr)] are fabricated and investigated to determine their ability to remove Cu²⁺, Cd²⁺, and Pb²⁺ from aqueous solution. MIL-100(Fe) and MIL-101(Cr) exhibited fast adsorption kinetics, achieving equilibrium in approximately 0.5 h. To evaluate the adsorption capacities of MIL-100(Fe) and MIL-101(Cr), the experimental data was fit to the Linear, Freundlich, and Langmuir isotherm models. Based on the sum of the squared error analysis, the experimental data fit most closely to the Freundlich model, followed closely by the Linear isotherm model. However, the values for the Freundlich parameter n were close to 1, which suggests that the adsorption followed the Linear isotherm model. The K_LIN adsorption affinity coefficient [(mg/g)/(mg/L)] for the Linear isotherm model was the largest for Cu²⁺ (K_{LIN, MIL-100(Fe)} = 14.9; K_{LIN, MIL-101(Cr)} = 60.3), followed by Cd²⁺ (K_{LIN, MIL-100(Fe)} = 12.9; K_{LIN, MIL-101(Cr)} = 11.5) and Pb²⁺ (K_{LIN, MIL-100(Fe)} = 4.44; K_{LIN, MIL-101(Cr)} = 8.33). Characterization data of MIL-100(Fe) and MIL-101(Cr) showed specific surface areas of 1586 m²/g and 2505 m²/g for MIL-100(Fe) and MIL-101(Cr), respectively, along with the presence of various functional groups, including carboxyl and phenyl groups. Considering this data alongside the local energy decomposition analysis that was performed using molecular modeling, electrostatic interactions were determined to be the dominant adsorption mechanism for the removal of Cu²⁺, Cd²⁺, and Pb²⁺ by MIL-100(Fe) and MIL-101(Cr), which is consistent with other, similar adsorption studies. This study shows that MIL-100(Fe) and MIL-101(Cr) are effective adsorbents for the removal of heavy metals from aqueous solution.