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<p>The rapid and efficient removal of toxic dyes from wastewater remains a critical environmental challenge, motivating the development of sustainable and high-performance adsorbent materials. In this paper, manganese dioxide nanoparticles (MnO₂ NPs) were fabricated through an environmentally friendly approach using leaf extract from <i>Vigna unguiculata.</i> The plant extract, rich in various phytochemical constituents, served as both a natural reducing agent and a stabilizing (capping) agent during the nanoparticle formation process. This green synthesis produced nanocrystals with near-spherical shapes measuring 10–25&#xa0;nm, exhibiting high purity and a microporous framework. The material showed a surface area of 74.14&#xa0;m² g⁻¹, a pore volume of 0.027&#xa0;cm³ g⁻¹, and an average pore size of 1.46&#xa0;nm. Surface characterization revealed a point of zero charge at pH 6.1 and a zeta potential of − 20 mV, indicating excellent colloidal stability. These MnO₂ NPs displayed outstanding adsorption performance for Congo Red, achieving 99.5% dye removal within only 9&#xa0;min at 298&#xa0;K using 15&#xa0;mg of adsorbent, with a maximum adsorption capacity of 17.9&#xa0;mg/g. Equilibrium data were best described by the Langmuir model (q<sub>max</sub> = 25.13&#xa0;mg/g, KL = 0.203&#xa0;L mg⁻¹, R² = 0.9909), indicating monolayer adsorption on relatively uniform active sites. Kinetic analysis revealed that the adsorption closely obeyed a pseudo-second-order model, with an excellent fit (R² = 0.9963). Thermodynamic evaluation showed a negative enthalpy change (ΔH° = − 14.56&#xa0;kJ mol⁻¹) and Gibbs free energy values below zero (ΔG° &lt; 0), confirming the process is both spontaneous and releases heat. The modest ΔH° magnitude implies that physical interactions, rather than chemical bonding, are the primary drivers of adsorption. The rapid adsorption kinetics, high removal efficiency, and simple green synthesis without dopants or hazardous reagents highlight the strong potential of these MnO₂ nanoparticles for scalable, energy-efficient, and environmentally sustainable dye remediation applications.</p> Graphical Abstract <p></p>

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Rapid Adsorptive Removal of Congo Red Using Green-Synthesized MnO₂ Nanoparticles Derived from Biowaste Vigna unguiculata Leaves

  • 期刊论文

  • 发表日期：

    2026年4月22日

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  • Blind S. Ali,
  • Jwan S. Haji,
  • Sameera Sh. Mohammed Ameen,
  • Khalid M. Omer

摘要

The rapid and efficient removal of toxic dyes from wastewater remains a critical environmental challenge, motivating the development of sustainable and high-performance adsorbent materials. In this paper, manganese dioxide nanoparticles (MnO₂ NPs) were fabricated through an environmentally friendly approach using leaf extract from Vigna unguiculata. The plant extract, rich in various phytochemical constituents, served as both a natural reducing agent and a stabilizing (capping) agent during the nanoparticle formation process. This green synthesis produced nanocrystals with near-spherical shapes measuring 10–25 nm, exhibiting high purity and a microporous framework. The material showed a surface area of 74.14 m² g⁻¹, a pore volume of 0.027 cm³ g⁻¹, and an average pore size of 1.46 nm. Surface characterization revealed a point of zero charge at pH 6.1 and a zeta potential of − 20 mV, indicating excellent colloidal stability. These MnO₂ NPs displayed outstanding adsorption performance for Congo Red, achieving 99.5% dye removal within only 9 min at 298 K using 15 mg of adsorbent, with a maximum adsorption capacity of 17.9 mg/g. Equilibrium data were best described by the Langmuir model (qmax = 25.13 mg/g, KL = 0.203 L mg⁻¹, R² = 0.9909), indicating monolayer adsorption on relatively uniform active sites. Kinetic analysis revealed that the adsorption closely obeyed a pseudo-second-order model, with an excellent fit (R² = 0.9963). Thermodynamic evaluation showed a negative enthalpy change (ΔH° = − 14.56 kJ mol⁻¹) and Gibbs free energy values below zero (ΔG° < 0), confirming the process is both spontaneous and releases heat. The modest ΔH° magnitude implies that physical interactions, rather than chemical bonding, are the primary drivers of adsorption. The rapid adsorption kinetics, high removal efficiency, and simple green synthesis without dopants or hazardous reagents highlight the strong potential of these MnO₂ nanoparticles for scalable, energy-efficient, and environmentally sustainable dye remediation applications.

Graphical Abstract