Mn-Co-BTC@MOF/S-MXene composite with superior efficiency for Pb(II) ion removal: Mechanistic and DFT study

The increasing severity of water pollution requires effective remediation solutions. In this study, a Mn-Co-BTC@MOF/S-MXene composite was synthesized and characterized by variety of analytical tools including FTIR, XPS, XRD, SEM, and Zeta Potential to confirm its successful fabrication. Zeta potential clarified that Mn-Co-BTC@MOF/S-MXene has a point of zero charge of 3.4 with a highly negative surface (-22.8 mV) at pH 6. Experimental results demonstrated an outstanding Pb²⁺ adsorption capacity with a q_max of 857.98 mg/g within 30 min under optimal conditions (pH 6, 298 K). The adsorption isotherms followed both Langmuir (R² = 0.990) and Freundlich (R² = 0.998) models, indicating the combination of both chemical and physical adsorption mechanisms along with a pseudo-second-order kinetic model. The composite exhibited superior selectivity toward Pb²⁺ over other competing ions, in the order: Pb²⁺ > Cd²⁺ > Zn²⁺ > Cu²⁺. Interestingly, DFT calculations and Mulliken atomic charge analysis indicated that sulfur and oxygen functional groups significantly contribute to the adsorption affinity by providing strong binding sites for Pb²⁺ ions. Moreover, Monte Carlo and molecular dynamics simulations further supported this selective adsorption behavior. In conclusion, these findings revealed that Mn-Co-BTC@MOF/S-MXene is a highly efficient, selective, and sustainable adsorbent for Pb²⁺ removal from water.