Exploring the interaction of decavanadate with methylene blue, toluidine blue and rhodamine B

Abstract

The interaction of decavanadate with the cationic organic dyes methylene blue (MB), toluidine blue (TB) and rhodamine B (RB) was explored. For that purpose, a novel decavanadate salt (2,3-ampH)₆[V₁₀O₂₈]·4H₂O (2,3-ampV₁₀), where 2,3-ampH⁺ = 2-amino-3-methylpyridinium, and two salts with the organic dyes (MB)₄[H₂V₁₀O₂₈]·10.8H₂O (MBV₁₀) and (TB)₄(H₂V₁₀O₂₈)·10H₂O (TBV₁₀) were synthesized. The compounds were characterized by elemental analysis, powder X-ray diffraction, energy dispersive X-ray spectroscopy, scanning electron microscopy and infrared spectroscopy. The salts 2,3-ampV₁₀ and MBV₁₀ were also characterized by single-crystal X-ray diffraction, while TBV₁₀ was isolated as an amorphous solid. The supramolecular features of MBV₁₀ showed columns of MB cations connected to V₁₀ through H-bond interactions that resulted in channels filled with water molecules. The absorption spectrum of MBV₁₀ was studied by density functional theory (DFT) calculations at the X3LYP/def2-TZVP level using the crystallographic data. The efficacy of 2,3-ampV₁₀ to bleach MB solutions at pH 6.2–6.5 was outstanding, achieving nearly 90% discoloration in only 5 min, presenting moderate efficacy towards TB and quite negligible towards RB. Moreover, a metachromatic effect was observed in the TB solution at pH 3. The IR spectra of the isolated solids and the mass spectra of the supernatants suggest both adsorption and cation exchange as the main responsible processes for the bleaching of MB and TB solutions. The present work has tackled many fronts related to the chemistry of POVs, mainly synthetic, structural, and environmental approaches.