📄 chemistry

Valorization of Pomelo Peel into a Carboxyl-Rich Adsorbent for Charge-Selective Removal of Cationic Dyes

This study demonstrates that a carboxyl-functionalized pomelo peel biosorbent, prepared through alkali treatment, oxidative bleaching, and esterification grafting, serves as a highly efficient, reusable, and charge-selective adsorbent for the removal of cationic dyes from wastewater via electrostatic attraction and hydrogen bonding.

Original authors: Linlin Du, Jiaxing Jiang, Dayong Xie, Yingbo Zhang, Bing Niu, Sisi Ma, Wenying Shi

Published 2026-08-03

📖 3 min read☕ Coffee break read

CC BY 4.0

Original authors: Linlin Du, Jiaxing Jiang, Dayong Xie, Yingbo Zhang, Bing Niu, Sisi Ma, Wenying Shi

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the world of water treatment as a giant, chaotic dance floor. On one side, you have the "clean" dancers, and on the other, a messy crowd of colorful, stubborn guests called dyes. These dyes are like glitter that refuses to wash off; they are complex, bright, and incredibly hard to get rid of once they get into our rivers. Scientists have been trying to build special "bouncers" (adsorbents) to kick the bad dancers out of the party. Some bouncers are expensive and hard to make, while others are cheap but not very picky—they might kick out the good dancers too. The big challenge is finding a bouncer that is cheap, super strong, and can tell the difference between a "good" guest and a "bad" guest, especially when they are all mixed together in a crowded room. This is where the science of turning waste into a weapon comes in: taking something we usually throw away and giving it a superpower to clean our water.

In this study, researchers Linlin Du and her team at Henan University of Engineering decided to take a common kitchen waste product—pomelo peel (the thick, spongy skin of a large citrus fruit)—and turn it into a high-tech water cleaner. They realized that while raw pomelo peel is a bit too dense and messy to be a great cleaner, it has a secret potential. By giving it a chemical "makeover," they transformed it into a material called CPP. Think of CPP as a sponge that has been electrified. The team used a process involving alkali, bleach, and a chemical called succinic anhydride to rip the peel apart into tiny, fluffy nanoparticles and cover them with a sea of negative electric charges (carboxyl groups).

The results were like magic. Because the CPP surface is now negatively charged, it acts like a magnet for positively charged (cationic) dye molecules, such as Methylene Blue and Methyl Violet. When these "bad" dyes try to swim by, they get stuck to the CPP with a strong grip. However, the CPP is picky; it ignores negatively charged (anionic) dyes, which are repelled by the surface, much like two magnets with the same pole pushing each other away. The paper shows that this new material can remove over 90% of these specific cationic dyes. It's incredibly efficient, with a maximum capacity of 954.20 mg·g-1 for Methylene Blue and 1170.96 mg·g-1 for Methyl Violet. The process is fast, working best in slightly alkaline water, and it even works well when other salts are present in the water, which usually messes up other cleaners.

Perhaps the most exciting part is that this "super-sponge" is reusable. The researchers tested it six times, washing the dye off with a simple acid solution each time. Even after six rounds, it still held onto a massive amount of dye (566.37 mg·g-1 for Methylene Blue and 809.10 mg·g-1 for Methyl Violet), proving it doesn't fall apart easily. The study suggests that the main reason this works is simple physics: the strong electrostatic attraction between the negative CPP and the positive dye, with a little help from hydrogen bonds. While the paper doesn't claim this is the final solution for every water problem on Earth, it strongly suggests that turning pomelo peel into a charge-selective adsorbent is a promising, low-cost, and sustainable way to tackle specific types of colorful water pollution.

Technical Summary: Valorization of Pomelo Peel into a Carboxyl-Rich Adsorbent for Charge-Selective Removal of Cationic Dyes

Problem Statement
The textile and printing industries discharge large volumes of wastewater containing synthetic dyes, which pose significant risks to aquatic ecosystems and human health due to their complex aromatic structures and resistance to biodegradation. While cationic dyes like methylene blue (MB) and methyl violet (MV) are widely used, their removal is challenging, particularly in multicomponent systems where anionic dyes, inorganic salts, and other dissolved species coexist. Existing adsorbents, such as activated carbon and metal-organic frameworks, often suffer from high preparation costs, complex synthesis procedures, limited regenerability, or insufficient selectivity in complex wastewater environments. Furthermore, unmodified biomass adsorbents typically possess dense matrices with low specific surface areas and insufficient active sites, limiting their practical application.

Methodology
This study presents a strategy to valorize pomelo peel waste into a high-performance biosorbent (CPP) through a three-step chemical modification process:

Alkaline Treatment: Pomelo peel powder was treated with NaOH to remove impurities and partially degrade the biomass structure.
Oxidative Bleaching: The alkali-treated peel was subjected to oxidative bleaching using hydrogen peroxide to further modify the lignocellulosic matrix.
Esterification Grafting: The pretreated material (PP) was reacted with succinic anhydride under reflux. This step covalently anchored abundant carboxyl groups onto the biomass skeleton, converting the compact matrix into a porous nanoparticulate architecture.

The resulting CPP was characterized using scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), nitrogen adsorption-desorption, and zeta potential measurements. Adsorption performance was evaluated through batch experiments involving cationic dyes (MB, MV) and anionic dyes (methyl orange, tartrazine). The study investigated the effects of pH, initial concentration, contact time, coexisting ions, and temperature. Kinetic, isotherm, and thermodynamic models were applied to elucidate the adsorption mechanism. Additionally, competitive adsorption in binary systems, fixed-bed column experiments, and reusability tests (six cycles) were conducted to assess practical applicability.

Key Contributions and Results

Structural and Surface Transformation: The modification process successfully transformed the dense, lamellar structure of pomelo peel into well-dispersed micro-/nanoscale particles (100–200 nm). The specific surface area increased from 8.19 m²·g⁻¹ (unmodified PP) to 17.41 m²·g⁻¹ (CPP). The grafting of succinic anhydride introduced abundant carboxylate groups, rendering the surface superhydrophilic and strongly negatively charged across a pH range of 4.0–10.0.
Charge-Selective Adsorption: CPP exhibited exceptional selectivity for cationic dyes. In single-component systems, removal efficiencies exceeded 90% for MB and MV, while showing minimal affinity for anionic dyes (TTZ and MO) due to electrostatic repulsion.
Adsorption Capacity: The maximum adsorption capacities estimated via the Langmuir model were 954.20 mg·g⁻¹ for MB and 1170.96 mg·g⁻¹ for MV. These values represent a significant improvement over unmodified biomass and many reported synthetic adsorbents.
Kinetics and Thermodynamics: The adsorption process followed the pseudo-second-order kinetic model, indicating that chemisorption involving valency forces (electrostatic attraction and electron sharing) was the rate-controlling step. Thermodynamic analysis revealed the process to be spontaneous ( $\Delta G^0 < 0$ ) and endothermic ( $\Delta H^0 > 0$ ), with adsorption capacity increasing as temperature rose from 25 to 45 °C.
Robustness in Complex Environments: The adsorbent demonstrated excellent salt tolerance, with negligible performance loss in the presence of various interfering ions (Zn²⁺, Fe³⁺, Cu²⁺, Cl⁻, NO₃⁻, SO₄²⁻). In binary dye systems (e.g., MB/MO, MV/TTZ), CPP selectively removed cationic dyes while leaving anionic dyes in solution, confirmed by UV-vis spectroscopy and visual observation.
Reusability: After six adsorption-desorption cycles using 0.5 mol·L⁻¹ HCl, CPP retained high adsorption capacities (566.37 mg·g⁻¹ for MB and 809.10 mg·g⁻¹ for MV), demonstrating structural stability and regeneration potential.
Mechanism: The primary driving force for adsorption was identified as electrostatic attraction between the negatively charged carboxylate sites (-COO⁻) on CPP and the positively charged cationic dye molecules. Hydrogen bonding between residual hydroxyl/carboxyl groups on the biomass and the dye molecules provided an additional stabilizing contribution.

Significance
The paper claims that this work provides a simple, sustainable, and low-cost route for converting agricultural waste (pomelo peel) into a reusable, high-performance biosorbent. The study demonstrates that carboxyl functionalization effectively enhances the interfacial activity and selectivity of biomass materials, addressing the critical need for selective treatment technologies in multicomponent dye wastewater. By achieving high capacity, rapid kinetics, and effective separation of cationic dyes from mixed systems, CPP is presented as a promising candidate for practical, scalable water purification applications. The research highlights the potential of biomass valorization in developing environmentally compatible solutions for industrial wastewater treatment.

More like this