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<p>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&#xa0;°C to optimize their structural and photocatalytic properties. The sample calcined at 400&#xa0;°C exhibited the highest surface area (17.86 m<sup>2</sup>/g), the narrowest bandgap (2.10&#xa0;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&#xa0;ppm within 30&#xa0;min, following pseudo-first-order kinetics with a rate constant (<i>k</i>ₐₚₚ) of 0.110&#xa0;min<sup>−1</sup> and a half-life (<i>t</i>₁<sub>/</sub>₂) of 6.30&#xa0;min. At an MB concentration of 50&#xa0;ppm, complete degradation was achieved in 90&#xa0;min. Radical scavenging experiments indicated that superoxide radicals (·O₂<sup>−</sup>) played a key role in the degradation mechanism. For MG (100&#xa0;ppm), the maximum adsorption capacity (<i>q</i>ₑ) was 1111.11&#xa0;mg/g, fitting the Langmuir model (<i>R</i><sup>2</sup> = 0.996) with an equilibrium constant (<i>K</i><sub>L</sub>) of 0.6822 L/mg, indicating a highly favorable process. The adsorption kinetics followed a pseudo-second-order model (<i>R</i><sup>2</sup> ≈ 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.</p> Graphical Abstract <p></p>

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

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2025年3月31日

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  • Odín Reyes-Vallejo,
  • Rocío Magdalena Sánchez-Albores,
  • José Escorcia-García,
  • Abumale Cruz-Salomón,
  • Pascual Bartolo-Pérez,
  • Ashok Adhikari,
  • Maritza del Carmen Hernández-Cruz,
  • Héctor Hiram Torres-Ventura,
  • Héctor Armando Esquinca-Avilés

摘要

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 m2/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 (kₐₚₚ) of 0.110 min−1 and a half-life (t₁/₂) 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 (qₑ) was 1111.11 mg/g, fitting the Langmuir model (R2 = 0.996) with an equilibrium constant (KL) of 0.6822 L/mg, indicating a highly favorable process. The adsorption kinetics followed a pseudo-second-order model (R2 ≈ 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