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<p>In this study, zinc oxide (ZnO) nanoparticles (NPs) were synthesized using <i>Allium cepa</i> (onion) peel extract and investigated for the removal of hexavalent chromium [Cr(VI)] from contaminated wastewater. Studies on <i>Allium cepa</i>-derived NPs have primarily focused on their antioxidant and antimicrobial activities; this study is novel in demonstrating the application of onion peel-mediated ZnO NPs synthesized via green co-precipitation, known as ZnO-O NPs, utilized for heavy metal remediation. Phytochemicals in onion peel extract acted as reducing and capping agents, enhancing NP stability and adsorption efficiency. In this study,&#xa0;different techniques, viz., FESEM, EDX, TEM, XRD, FTIR, BET, DLS, and point of&#xa0;zero charges (PZC), were employed to characterize the biosynthesized ZnO and ZnO-O&#xa0;NPs, and Cr(VI) adsorption was investigated. Both ZnO and ZnO-O (onion-assisted) exhibited rod-shaped, crystalline structures with average sizes of 28&#xa0;nm and 18.65&#xa0;nm, respectively. FTIR analysis confirmed the presence of hydroxyl, amide, and carbonyl functional groups in ZnO-O NPs, providing active sites for Cr(VI) binding&#xa0;onto their surfaces. The ZnO-based nano adsorbents had a mesoporous structure, with ZnO-O&#xa0;NPs showing a higher surface area (5.98 m<sup>2</sup>/g) and smaller pore diameter (19.84&#xa0;nm) than ZnO&#xa0;NPs (3.05 m<sup>2</sup>/g and 38&#xa0;nm). The point of zero charge was lower for ZnO-O NPs (5.1) than ZnO NPs (6.4), indicating more acidic surface properties. Batch adsorption at optimized conditions (pH 3, 0.5&#xa0;g/L adsorbent dosage, 30&#xa0;mg/L Cr(VI) concentrations, 90&#xa0;min contact time) show enhanced removal efficiency for ZnO-O&#xa0;NPs (95.7%) compared to ZnO&#xa0;NPs (89.5%). Adsorption followed pseudo-second-order kinetics, indicating chemisorption. Furthermore, adsorptive interactions of ZnO and ZnO-O NPs with Cr(VI) exhibited followed Freundlich isotherm, reflecting surface heterogeneity over Langmuir and Temkin isotherms. The removal efficiency of ZnO-O NPs was better than ZnO NPs after 5 consecutive cycles in reusability studies. This study presents an innovative and sustainable method for synthesizing ZnO-O NPs from onion peel agro-waste, demonstrating a promising, eco-friendly solution for biowaste valorization and efficient Cr(VI) removal. This highlights its potential for scalable application in green water purification and environmental remediation technologies.</p> Graphical Abstract <p></p>

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Green Synthesis of Zinc Oxide (ZnO) Nanoparticles from Allium cepa (onion) Peels for Sustainable Cr(VI) Removal: Adsorption Isotherms, Kinetics, Thermodynamics, and Reusability

  • 期刊论文

  • 发表日期：

    2025年7月3日

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  • Heena Chauhan,
  • Rakesh Kumar,
  • Renuka Gupta

摘要

In this study, zinc oxide (ZnO) nanoparticles (NPs) were synthesized using Allium cepa (onion) peel extract and investigated for the removal of hexavalent chromium [Cr(VI)] from contaminated wastewater. Studies on Allium cepa-derived NPs have primarily focused on their antioxidant and antimicrobial activities; this study is novel in demonstrating the application of onion peel-mediated ZnO NPs synthesized via green co-precipitation, known as ZnO-O NPs, utilized for heavy metal remediation. Phytochemicals in onion peel extract acted as reducing and capping agents, enhancing NP stability and adsorption efficiency. In this study, different techniques, viz., FESEM, EDX, TEM, XRD, FTIR, BET, DLS, and point of zero charges (PZC), were employed to characterize the biosynthesized ZnO and ZnO-O NPs, and Cr(VI) adsorption was investigated. Both ZnO and ZnO-O (onion-assisted) exhibited rod-shaped, crystalline structures with average sizes of 28 nm and 18.65 nm, respectively. FTIR analysis confirmed the presence of hydroxyl, amide, and carbonyl functional groups in ZnO-O NPs, providing active sites for Cr(VI) binding onto their surfaces. The ZnO-based nano adsorbents had a mesoporous structure, with ZnO-O NPs showing a higher surface area (5.98 m2/g) and smaller pore diameter (19.84 nm) than ZnO NPs (3.05 m2/g and 38 nm). The point of zero charge was lower for ZnO-O NPs (5.1) than ZnO NPs (6.4), indicating more acidic surface properties. Batch adsorption at optimized conditions (pH 3, 0.5 g/L adsorbent dosage, 30 mg/L Cr(VI) concentrations, 90 min contact time) show enhanced removal efficiency for ZnO-O NPs (95.7%) compared to ZnO NPs (89.5%). Adsorption followed pseudo-second-order kinetics, indicating chemisorption. Furthermore, adsorptive interactions of ZnO and ZnO-O NPs with Cr(VI) exhibited followed Freundlich isotherm, reflecting surface heterogeneity over Langmuir and Temkin isotherms. The removal efficiency of ZnO-O NPs was better than ZnO NPs after 5 consecutive cycles in reusability studies. This study presents an innovative and sustainable method for synthesizing ZnO-O NPs from onion peel agro-waste, demonstrating a promising, eco-friendly solution for biowaste valorization and efficient Cr(VI) removal. This highlights its potential for scalable application in green water purification and environmental remediation technologies.

Graphical Abstract