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Kinetic, equilibrium, and thermodynamic study of Methylene Blue adsorption on orange peel biochar prepared by microwave-assisted pyrolysis
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  • Published: 11 February 2026

Kinetic, equilibrium, and thermodynamic study of Methylene Blue adsorption on orange peel biochar prepared by microwave-assisted pyrolysis

  • Jhonny Correa-Abril1,
  • Elvia V. Cabrera1,
  • Nilo Robles1,2,
  • J. L. López Terán1 &
  • …
  • Ullrich Stahl1 

Scientific Reports , Article number:  (2026) Cite this article

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We are providing an unedited version of this manuscript to give early access to its findings. Before final publication, the manuscript will undergo further editing. Please note there may be errors present which affect the content, and all legal disclaimers apply.

Subjects

  • Chemistry
  • Engineering
  • Environmental sciences
  • Materials science

Abstract

This study presents a sustainable approach for Methylene Blue (MB) dye removal using pristine, non-activated biochar derived from orange peel waste via Microwave-Assisted Pyrolysis (MAP). The key novelty lies in the systematic comparison of the biochar’s adsorption performance under both pH-controlled (constant pH 4) and unregulated pH conditions, demonstrating that pH regulation is essential for optimizing adsorption efficiency. The resulting biochar exhibited a high fixed carbon content (60.89%), an alkaline surface (Point of zero charge (pHpzc) = 11.20, ZPotential = 0.1 mV @ pH 9), and oxygenated functional groups. Best MB removal of 83% was achieved at pH 4, despite the expected electrostatic repulsion. Kinetic studies showed the best fit with the Elovich model, indicating a heterogeneous surface. The Langmuir isotherm accurately described the equilibrium data, revealing a maximum adsorption capacity (qmax) of 20.57 mg g⁻1 under pH-controlled conditions, representing an 83% increase over the 11.24 mg g⁻1 obtained in the unregulated scenario. Thermodynamic analysis confirmed the process is spontaneous (ΔG° < 0), endothermic (ΔH° = + 4.88 kJ mol⁻1 at constant pH), and governed by physisorption mechanisms, including hydrogen bonding and π-π interactions. This work demonstrates that pristine orange peel biochar generated via MAP is a highly effective adsorbent and highlights the critical impact of pH control on improving adsorption capacity and elucidating the dominant physisorption mechanisms.

Data availability

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

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Acknowledgements

The authors would like to express their gratitude to all the individuals and institutional collaborators who have provided their support and assistance and to the Universidad Central del Ecuador for financing the Senior Project DI-CONV-2022-025 and DI-CONV-2023-029. We are also very grateful to Dr. Carsten Natzeck from the Institute of Functional Interfaces – Karlsruhe Institute of Technology for his valuable support on SEM images and EDX analysis, the laboratory of research activities (LISE) of the Faculty of Chemical Engineering, as well as to Dr. Pablo Bonilla of the laboratory of nanomaterials of the Faculty of Chemistry for his support on ZP measurements.

Funding

This work was financed by the Universidad Central del Ecuador and it´s Dirección de Investigación via the Senior Project “Desarrollo de un adsorbente basado en biocarbón de Theobroma cacao y marco metal orgánico (MOF) para la remoción de fármacos en aguas residuales sintéticas” (No. DI-CONV-2023-029).

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Authors and Affiliations

  1. Facultad de Ingeniería Química, Grupo de Investigación en Alimentos, Compuestos Orgánicos, Materiales, Microbiología Aplicada, y Energía – ACMME, Universidad Central del Ecuador, Enrique Ritter S/N y Bolivia, Quito, Ecuador

    Jhonny Correa-Abril, Elvia V. Cabrera, Nilo Robles, J. L. López Terán & Ullrich Stahl

  2. Instituto de Investigación Geológico y Energético - IIGE, Quito, 170518, Ecuador

    Nilo Robles

Authors
  1. Jhonny Correa-Abril
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  2. Elvia V. Cabrera
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  3. Nilo Robles
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  4. J. L. López Terán
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  5. Ullrich Stahl
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Contributions

Ullrich Stahl: Writing—review & editing, Writing—original draft, Methodology, Investigation, Formal analysis. Elvia V. Cabrera and Jhonny Correa-Abril: Writing—review & editing, Validation, Supervision, Formal analysis, Conceptualization. Nilo Robles: Writing, Validation, Supervision, Formal analysis, J. L. López Terán: Project administration, Funding acquisition.

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Correspondence to Ullrich Stahl.

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Correa-Abril, J., Cabrera, E.V., Robles, N. et al. Kinetic, equilibrium, and thermodynamic study of Methylene Blue adsorption on orange peel biochar prepared by microwave-assisted pyrolysis. Sci Rep (2026). https://doi.org/10.1038/s41598-026-36741-6

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  • Received: 05 August 2025

  • Accepted: 16 January 2026

  • Published: 11 February 2026

  • DOI: https://doi.org/10.1038/s41598-026-36741-6

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Keywords

  • Orange peel
  • Microwave assisted pyrolysis
  • Biochar
  • Adsorption dynamics
  • Methylene Blue
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