Rapid Removal of Azo Cationic Dyes Using a Cu(II) Hydrogen-Π-Bonded Organic Framework and Its Derived Oxide: A Combined Adsorption and Photocatalysis Study

Abstract

Azo dyes, prevalent in various industries, including textile dyeing, food, and cosmetics, pose significant environmental and health risks due to their chemical stability and toxicity. This study introduces the synthesis and application of a copper hydrogen-π-bonded benzoate framework (Cu-HBF) and its derived marigold flower-like copper oxide (MFL-CuO) in a synergetic adsorption-photocatalytic process for efficiently removing cationic azo dyes from water, specifically crystal violet (CV), methylene blue (MB), and rhodamine B (RhB). The Cu-HBF, previously available only in single crystal form, is prepared here as a crystalline powder for the first time, using a low-cost and facile procedure, allowing its application as an adsorbent and also serving as a precursor for synthesizing well-structured copper oxide (MFL-CuO). Characterization techniques, including XRD, SEM-EDX, FTIR, XPS techniques, point of zero charge (pH_PZC), N₂ adsorption–desorption, and UV–vis analysis, elucidated the structural and morphological properties of both materials. The synergy between adsorption and photocatalysis highlights the potential of both Cu-HBF and MFL-CuO. This combined process achieved impressive final removal rates under optimal conditions, nearly 100% removal of CV and MB in ∼5 min, and ∼85% removal of RhB in ∼8 min of total contact time. Computational density functional theory calculations revealed the implications of hydrogen interactions, π-π interactions, and electrostatic forces in the adsorption mechanisms along with the corresponding reactive sites. Photoelectrochemical measurements including cyclic voltammetry, electrochemical impedance spectroscopy (EIS), and Mott–Schottky analysis elucidate the photocatalytic mechanism, demonstrating that the rapid photodegradation of the three dyes is primarily related to the MFL-CuO’s ability to in situ generate H₂O₂, initiating a Fenton-like reaction (advanced oxidation process). This work contributes to the development of efficient materials for dye removal, addressing the pressing need for innovative solutions for wastewater treatment.