Structure-Tailored Mn-Doped MIL-88A via Controlled Calcination for Enhanced Electrochemical Separation of Se(IV) from High-Salinity Brine

Abstract

This study develops a controlled calcination strategy to fabricate topologically evolved Mn-MIL-88A composites for the efficient electrochemical separation of Se(IV) from hypersaline brine. By tuning the Mn doping ratio and calcination temperature, the structure–performance relationship was systematically elucidated, revealing that the optimal N2–Mn1-MIL-88A1 composite (calcined at 350 °C) possesses a well-developed porous architecture and abundant Fe/Mn redox active sites. Process parameter optimization demonstrated that the applied voltage, kinetics, and thermodynamics synergistically govern the adsorption behavior, with the material maintaining exceptional stability against ionic interference. The optimized material achieved maximum Se(IV) adsorption capacities of 89.7 mg Se/g in simulated brine and 43.75 mg Se/g in real saline lake brine. After 10 consecutive adsorption–desorption cycles, the electrode retained >88% and 78% of its initial capacity in these respective matrices, confirming its structural robustness. Notably, the electrochemical adsorption performance was 41.91% higher than that of natural adsorption. Comprehensive characterization (X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS)) coupled with isothermal and kinetic modeling confirmed that the enhanced performance originates from the synergistic effects of Fe/Mn redox cycling, surface complexation (via Fe/Mn–O–Se linkages), electrostatic attraction, and the partial reduction of Se(IV) to Se(0). Density Functional Theory (DFT) calculations further validated that SeO32– adsorbs primarily via O-coordination, with the Mn–O–Se configuration exhibiting a significantly lower adsorption energy (−2.67 eV) compared to Fe–O–Se (−2.58 eV). This work provides a fundamental understanding of metal site regulation in MOF-derived electrocatalysts for the intensified separation of heavy metals in complex saline systems.