Cysteine-Functionalized Magnetic Manganese-Based MOF Composite for Enhanced Removal of Pb²⁺ from Water

Among the multitude of toxic water pollutants, Pb²⁺ poses a significant threat to human health. A novel magnetic manganese-based metal-organic framework (Mn-MOF) composite, MFCys, was designed for the selective removal of Pb²⁺. MFCys was readily synthesized through the postsynthetic modification of magnetic Mn-MOF/Fe₃O₄ (MF) with l-cysteine (l-Cys), utilizing a prospective Mn-MOF, {[Mn₂(tzpa)(OH)(H₂O)₂]·DMA}. MFCys demonstrated superior adsorption of Pb²⁺, reaching equilibrium within 2 h compared to Mn-MOF and MF. The Langmuir model indicated that MFCys underwent chemisorption of Pb²⁺. Meanwhile, the adsorption process aligned with the pseudo-second-order (PSO) kinetic model, confirming that Pb²⁺ was adsorbed by MFCys mainly through monolayer chemisorption. Thermodynamic results revealed that the adsorption of Pb²⁺ by MFCys was spontaneous and endothermic. Moreover, even in the presence of competing Cd²⁺, Zn²⁺, Ca²⁺, Mg²⁺, Na⁺, K⁺, and Pb²⁺, MFCys exhibited exceptional selectivity for Pb²⁺ due to the electrostatic interaction and the electronic coupling effect between MFCys and Pb²⁺. The magnetic MFCys enabled easy separation from the suspension within 1 min by using an external magnet for recycling. MFCys retained 98.0% of the initial adsorption capacity for Pb²⁺ after five cycles, demonstrating excellent reusability. Notably, MFCys displayed exceptional adsorption of Pb²⁺ in simulated lead-acid battery wastewater, retaining 98.3% of its pristine adsorption capacity for Pb²⁺ even after five cycles. These findings suggest that an easily separated Mn-MOF composite has promising application prospects in Pb²⁺ wastewater treatment.