An α-MoO₃ hybrid with sulfuric acid-intercalated 4,4'-bipyridine for adsorptive dye removal

Herein, a polyoxometalate hybrid material, [MoVI3O₉(H₂SO₄)]·[(bpy)_0.5] (1), has been specifically designed for efficient dye adsorption. The molecular structure of 1 was elucidated via single-crystal X-ray diffraction (SC-XRD), revealing a well-defined coordination network. Comprehensive characterization through FTIR, P-XRD TGA, FE-SEM, EDX mapping, XPS and zeta potential measurements confirmed the uniform distribution of constituent elements and the robust stability of 1 under operational conditions. The adsorption performance of 1 was systematically evaluated against five dyes: methylene blue (MB), rhodamine B (Rh), Azur A, toluidine blue (TB), and methyl orange (MO). It exhibited markedly higher adsorption capacity for cationic dyes (MB, Rh, Azur A, and TB) even at elevated concentrations, with significantly lower affinity for the anionic dye (MO). In binary dye experiments, the hybrid selectively adsorbed MB from an MB-MO mixture, demonstrating its superior preference for cationic dyes. Mechanistic investigations show electrostatic and hydrogen bonding interactions which govern the selective dye uptake, with the negatively charged surface (zeta potential = -62.4 mV) driving cationic dye adsorption. The excellent reusability over multiple adsorption-desorption cycles underscores the potential of this hybrid material as a promising candidate for sustainable and selective cationic dye removal in advanced wastewater treatment applications.