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<p>In this study, a green synthesis approach using aqueous leaf extract of <i>Vigna unguiculata</i> was employed as both a reducing and stabilizing agent to fabricate copper oxide nanoparticles (CuO NPs), which were subsequently applied as an adsorbent for the removal of Congo red (CR) dye from aqueous solutions. The synthesized adsorbent was characterized using SEM, EDX elemental composition and mapping, XRD, zeta potential, optical analysis, and N<sub>2</sub> adsorption/desorption analysis, confirming successful formation with a surface area of 23.959&#xa0;m²/g and an average crystallite size of 15.2&#xa0;nm. Batch adsorption experiments revealed that the optimum conditions for CR adsorption onto CuO NPs were achieved at a solution pH of 5.0, an adsorbent dosage of 16&#xa0;mg, and an initial dye concentration of 28.0&#xa0;mg/L, with a contact time of 15.0&#xa0;min at 65&#xa0;°C. Under these conditions, a maximum adsorption capacity of 35.33&#xa0;mg/g was achieved, with adsorption following the Langmuir isotherm and pseudo-second-order kinetics, indicating monolayer chemisorption. Thermodynamic analysis confirmed that the process was spontaneous and endothermic. Furthermore, the adsorbent exhibited good reusability, maintaining removal efficiencies from 99.03% to 78.93% over five adsorption–desorption cycles. Mechanistically, adsorption is primarily governed by electrostatic interactions, in which the CuO NP surface becomes positively charged at pH 5.0 (below pHpzc = 6.1), promoting strong attraction to anionic Congo red molecules, as evidenced by the measured zeta potential (− 12.9 mV), along with possible π–π and hydrogen-bonding interactions. These findings demonstrate that the green-synthesized CuO NPs provide a cost-effective, efficient, and sustainable adsorbent material with high potential usage for wastewater treatment and encourage further studies on their large-scale application and regeneration performance.</p> Graphical Abstract <p></p>

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Green-Synthesized CuO Nanoparticles from Vigna unguiculata Leaf Extract for Adsorptive Removal of Congo Red: Characterization, Kinetics, Isotherm, and Mechanistic Insights

  • 期刊论文

  • 发表日期：

    2026年4月21日

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  • Sara A. Mohammed,
  • Jvan A. Saeed,
  • Helan Zeyad Sami,
  • Sameera Sh. Mohammed Ameen,
  • Khalid M. Omer

摘要

In this study, a green synthesis approach using aqueous leaf extract of Vigna unguiculata was employed as both a reducing and stabilizing agent to fabricate copper oxide nanoparticles (CuO NPs), which were subsequently applied as an adsorbent for the removal of Congo red (CR) dye from aqueous solutions. The synthesized adsorbent was characterized using SEM, EDX elemental composition and mapping, XRD, zeta potential, optical analysis, and N2 adsorption/desorption analysis, confirming successful formation with a surface area of 23.959 m²/g and an average crystallite size of 15.2 nm. Batch adsorption experiments revealed that the optimum conditions for CR adsorption onto CuO NPs were achieved at a solution pH of 5.0, an adsorbent dosage of 16 mg, and an initial dye concentration of 28.0 mg/L, with a contact time of 15.0 min at 65 °C. Under these conditions, a maximum adsorption capacity of 35.33 mg/g was achieved, with adsorption following the Langmuir isotherm and pseudo-second-order kinetics, indicating monolayer chemisorption. Thermodynamic analysis confirmed that the process was spontaneous and endothermic. Furthermore, the adsorbent exhibited good reusability, maintaining removal efficiencies from 99.03% to 78.93% over five adsorption–desorption cycles. Mechanistically, adsorption is primarily governed by electrostatic interactions, in which the CuO NP surface becomes positively charged at pH 5.0 (below pHpzc = 6.1), promoting strong attraction to anionic Congo red molecules, as evidenced by the measured zeta potential (− 12.9 mV), along with possible π–π and hydrogen-bonding interactions. These findings demonstrate that the green-synthesized CuO NPs provide a cost-effective, efficient, and sustainable adsorbent material with high potential usage for wastewater treatment and encourage further studies on their large-scale application and regeneration performance.

Graphical Abstract