A Viologen-Based Metal-Organic Complex for Highly Efficient and Selective Dye Flocculation

The continuous discharge of industrial dye wastewater poses severe threats to aquatic environments, urgently demanding the development of efficient and selective treatment technologies. This study designed and synthesized a zero-dimensional (0D) viologen-based metal-organic complex (VIO-MOC, formula [Cd2(μ2-Cl)Cl2Br2bpyen]) that achieves exceptional Congo Red removal performance: 99.9% removal efficiency within 30 min and a maximum adsorption capacity of 3080.6 mg/g, significantly surpassing existing flocculants. The 0D structure provides abundant exposed active sites, while electron-deficient viologen moieties interact with anionic dye molecules through π-π stacking, electrostatic interactions, and hydrogen bonding to form a potent synergistic coagulation-flocculation mechanism. Zeta potential measurements, FTIR spectroscopy, and density functional theory (DFT) calculations confirm this multi-mechanism process. VIO-MOC maintains stably high performance across pH 5-11 and remains unaffected by inorganic ion interference, demonstrating excellent practicality. This work presents a novel high-efficiency material for dye wastewater treatment and establishes key principles for designing viologen-functionalized metal-organic complexes.