Enhanced Pb(II) Adsorption by a Ternary MnO2/NH2-MIL-101(Fe)/Graphitic Carbon Nitride Nanocomposite Through Complementary Interfacial Interactions

Severe heavy metal pollution in aquatic environments demands the engineered development of structurally optimized, highly selective adsorbents. Herein, we report the intentional fabrication of a novel ternary MnO2/NH2-MIL-101(Fe)/g-C3N4 nanocomposite via an integrated in situ interfacial growth pathway. The true architectural novelty of this multi-component assembly lies in utilizing the 3D mesoporous metal–organic framework (MOF) scaffolding to structurally isolate the 2D g-C3N4 layers and prevent the self-aggregation of redox-active MnO2 nanoparticles. Comprehensive characterization via PXRD, FTIR, TEM, Raman, and high-resolution XPS confirmed that this unique interfacial hybridization maximizes the spatial exposure of unblocked chemical binding sites. Batch extraction trials demonstrated a high Pb2+ removal efficiency of 99.5 ± 2.7% at an optimized pH of 6.0, yielding a superior maximum monolayer adsorption capacity (qmax) of 431.8 mg.g−1. Competitive selectivity matrices containing co-existing ions (Cu2+, Cd2+, Ni2+, and Cr3+) revealed noticeable selectivity toward Pb2+ ions, driven by soft Lewis’s acid-base affinities, while background electrolyte tests identified SO42− as the most influential competing anion. Non-linear isotherm modeling showed a better agreement with the Langmuir model, suggesting dominant monolayer adsorption on accessible surface sites, while kinetic data followed the pseudo-second-order model. Spectroscopic profiling proved that this heightened performance is dictated by a multi-modal integrated network operating via cooperative inner-sphere Mn-OH complexation, oxygen-vacancy trapping, exocyclic framework amine (-NH2) chelation, and g-C3N4 triazine dative configurations. These findings establish the ternary system as an advanced, highly recyclable benchmark for targeted heavy metal decontamination.