Turning orange waste into a fast, reusable dye trap for wastewater

The optimized material, Fe/Zn-OPBC500, reached an adsorption capacity of 237.53 mg g⁻¹ and removed 96.8% of the dye within 60 minutes. It also maintained strong performance after repeated reuse and under a broad range of pH and common ionic conditions.

Synthetic dyes remain a major challenge in industrial wastewater treatment because they are highly soluble, structurally stable, and difficult to degrade once released into aquatic systems. Among them, methylene blue is widely used in textiles, staining, indicators, and pharmaceuticals, making its removal environmentally important. Adsorption has emerged as one of the most practical treatment approaches because it is simple, cost-effective, and efficient, but conventional biochar often suffers from limited adsorption capacity and poor structural optimization. Previous work has shown that ZnCl₂ activation can improve pore development, while Fe-based modification can introduce reactive adsorption sites. However, single-modification strategies usually cannot optimize pore architecture and surface chemistry at the same time. This gap led the researchers to investigate whether co-activation with ZnCl₂ and FeCl₃ could produce a more effective orange peel-derived biochar for dye removal.

A study (DOI:10.48130/bchax-0026-0001) published in Biochar X on 30 January 2026 by Lei Zhang’s & Junkang Guo’s team, Shaanxi University of Science & Technology, reports that Fe/Zn co-modification of orange peel produced a biochar with markedly enhanced porosity, richer active surface chemistry, rapid and high-capacity methylene blue adsorption, and strong regeneration performance.

To build the adsorbent, the team first washed, dried, ground, and sieved orange peel, then impregnated the biomass with FeCl₃ and ZnCl₂ before pyrolyzing it under nitrogen at controlled temperatures. They compared materials prepared at 500 °C and 900 °C, along with unmodified controls, and then characterized the products using SEM, XRD, Raman spectroscopy, FTIR, XPS, and nitrogen physisorption. These analyses showed that the dual-activation strategy reconstructed the biochar into a three-dimensional hierarchical porous structure while introducing iron oxide species and preserving oxygen-containing functional groups. Compared with pristine biochar, the optimized Fe/Zn-OPBC500 displayed a 16.1-fold increase in specific surface area and a 5.7-fold rise in total pore volume, creating a structure better suited for mass transfer and dye capture. The adsorption experiments then linked structure to performance. Under standard conditions, Fe/Zn-OPBC500 achieved 194.5 mg g⁻¹ at the selected dosage and reached a 96.8% removal rate within 60 minutes, outperforming Fe/Zn-OPBC900 and the pristine samples. Kinetic modeling showed that adsorption followed a pseudo-second-order model, indicating chemisorption-dominated behavior, while isotherm fitting favored the Langmuir model, consistent with predominantly monolayer adsorption. The maximum adsorption capacity reached 237.53 mg g⁻¹, and thermodynamic analysis indicated that the process was spontaneous. Performance also remained strong across pH 3 to 11, with adsorption increasing as the surface became more negatively charged above the point of zero charge. Tests with coexisting ions showed that multivalent cations such as Fe³⁺ and Ca²⁺ interfered more strongly than Na⁺, while anions had weaker effects overall. In regeneration experiments, the material retained more than 100 mg g⁻¹ over seven cycles, confirming good operational stability. Post-adsorption spectroscopic analysis further revealed that methylene blue removal arose from multiple cooperating pathways, including electrostatic attraction, hydrogen bonding, π–π interactions, pore confinement, covalent amide-like bonding, and surface complexation with iron sites. Together, these results showed that the material’s performance did not depend on a single factor, but on the synergy between engineered pore structure and multifunctional surface chemistry.

Overall, the study shows that orange peel waste can be converted into a robust, reusable, and high-performance adsorbent for dye-contaminated water. By combining accessible feedstock, moderate-temperature synthesis, fast kinetics, strong capacity, and multicycle reuse, the work advances both wastewater treatment and agricultural waste valorization.

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References

DOI

10.48130/bchax-0026-0001

Original Source URL

https://doi.org/10.48130/bchax-0026-0001

Funding information

This work was funded by the National Natural Science Foundation of China (42207464), Young Talent Fund of Xi'an Association for Science and Technology (959202413043), Natural Science Foundation of Shaanxi Provincial Department of Education (24JK0353), Key Research and Development Program of Shaanxi (2025SF-YBXM-511), and the Key Industrial Chain Project of Shaanxi Province (2024NC-ZDCYL-02-15).

About Biochar X

Biochar X is an open access, online-only journal aims to transcend traditional disciplinary boundaries by providing a multidisciplinary platform for the exchange of cutting-edge research in both fundamental and applied aspects of biochar. The journal is dedicated to supporting the global biochar research community by offering an innovative, efficient, and professional outlet for sharing new findings and perspectives. Its core focus lies in the discovery of novel insights and the development of emerging applications in the rapidly growing field of biochar science.