Green synthesis of CaO-Fe₃O₄ composites for photocatalytic degradation and adsorption of synthetic dyes

Abstract

Abstract The efficient removal of synthetic dyes, such as methylene blue (MB) and malachite green (MG), continues to pose a significant challenge due to their high stability, toxicity, and resistance to conventional treatment methods. In this study, CaO-Fe₃O₄ compounds were synthesized using a sustainable ball-milling technique, utilizing calcium oxide derived from eggshells and Fe₃O₄. The compounds were calcined at temperatures ranging from 200 to 800 °C to optimize their structural and photocatalytic properties. The sample calcined at 400 °C exhibited the highest surface area (17.86 m 2 /g), the narrowest bandgap (2.10 eV), and the coexistence of CaO, Ca(OH)₂, and γ-Fe₂O₃ phases, making it an ideal candidate for achieving high dye removal efficiency. Under visible light, this sample completely degraded MB at 10 ppm within 30 min, following pseudo-first-order kinetics with a rate constant ( k ₐₚₚ) of 0.110 min −1 and a half-life ( t ₁ / ₂) of 6.30 min. At an MB concentration of 50 ppm, complete degradation was achieved in 90 min. Radical scavenging experiments indicated that superoxide radicals (·O₂ − ) played a key role in the degradation mechanism. For MG (100 ppm), the maximum adsorption capacity ( q ₑ) was 1111.11 mg/g, fitting the Langmuir model ( R 2 = 0.996) with an equilibrium constant ( K L ) of 0.6822 L/mg, indicating a highly favorable process. The adsorption kinetics followed a pseudo-second-order model ( R 2 ≈ 0.999), suggesting chemisorption as the rate-limiting step. Thermodynamic parameters confirmed that MG adsorption was spontaneous and endothermic, with negative Gibbs free energy, positive enthalpy, and increased entropy. This study proposes an eco-friendly and efficient approach for dye removal, integrating waste valorization. Graphical Abstract

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