📄 chemistry

Modification of H3PO4-activated Pomelo Peel Biochar with Cetyltrimethylammonium Bromide for Methyl Orange Removal: Adsorption Performance and Mechanism

This study demonstrates that modifying H3PO4-activated pomelo peel biochar with 2 wt% cetyltrimethylammonium bromide (CTAB) significantly enhances its specific surface area and creates a positively charged surface, resulting in a highly effective adsorbent (CTAB2@A-PBC) capable of removing methyl orange with a maximum capacity of 534.23 mg/g through mechanisms including electrostatic attraction, pore-filling, π-π interactions, and hydrogen bonding.

Original authors: Mai Lien Tran

Published 2026-07-08

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Mai Lien Tran

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 you have a very dirty swimming pool filled with bright orange dye. You want to clean it, but the dye is stubborn and won't just wash away. This paper is about building a super-efficient "sponge" to suck that orange dye out of the water, using something you might throw away: the skin of a pomelo fruit.

Here is the story of how the researchers turned pomelo peels into a high-tech cleaning tool, explained simply:

1. The Raw Material: Pomelo Peel

The researchers started with pomelo peels (the thick skin of a large citrus fruit). Usually, this is just waste. They turned it into biochar, which is basically a lightweight, black, charcoal-like powder made by burning the peel in a special way. Think of this as turning a fruit rind into a rough, porous sponge.

2. The Problem: The Sponge Was Too Picky

In a previous study, the authors found that this plain pomelo-charcoal was great at grabbing positive things (like a magnet picking up iron filings). However, the orange dye they wanted to remove (Methyl Orange) is negatively charged.

The Analogy: Imagine trying to stick a negative magnet to another negative magnet. They push each other away. The plain charcoal was pushing the orange dye away, so it couldn't clean the water well.

3. The Solution: The "Velcro" Coating

To fix this, the researchers dipped the charcoal into a solution of a chemical called CTAB.

The Analogy: Think of CTAB as a special "Velcro" coating. The researchers coated the charcoal with this chemical, which acts like a layer of positive magnets.
The Result: Now, instead of pushing the orange dye away, the coated charcoal loves to grab it. The negative dye sticks tightly to the positive coating.

4. Making the Sponge Bigger and Better

The researchers didn't just coat the charcoal; they found a "Goldilocks" amount of the coating.

They tried different amounts of CTAB.
The Magic Number: When they used a specific amount (2% of the weight), the charcoal transformed.
The Transformation: The surface area of the charcoal exploded. It went from having a surface area of about 1,200 square meters per gram to over 2,150 square meters per gram.
The Analogy: Imagine taking a standard brick and magically unfolding it until it became as large as a football field, but still weighing the same. This gave the dye millions of tiny nooks and crannies to hide in.

5. How Well Did It Work?

They tested this new "super-sponge" in a lab.

The Speed: It grabbed the dye very quickly.
The Capacity: It could hold a massive amount of dye. For every gram of this charcoal, it could trap 534 milligrams of orange dye.
The Comparison: When they compared their sponge to other sponges made by other scientists (using things like bamboo, wood, or magnetic materials), their pomelo-peel sponge was one of the best at holding onto the dye.

6. How Does It Actually Grab the Dye?

The researchers looked at the microscopic level to see how the dye stuck. It wasn't just one thing; it was a team effort of four different "grip" methods:

Static Electricity: The positive coating on the charcoal grabbed the negative dye (like a balloon sticking to hair).
Pore Filling: The dye molecules were small enough to crawl inside the tiny holes of the charcoal and get stuck there.
Molecular Handshakes: The chemical structures of the charcoal and the dye matched up like puzzle pieces (called $\pi$ - $\pi$ interaction).
Hydrogen Bonding: Tiny chemical "glue" points formed between the charcoal and the dye.

7. The Bottom Line

The paper concludes that by taking a common fruit waste (pomelo peel), treating it with acid, and then coating it with a specific chemical (CTAB), they created a highly effective tool for cleaning orange dye out of water.

What it did: It turned a negative surface into a positive one to attract the dye.
What it achieved: It created a sponge with a huge surface area that can hold a lot of dye very efficiently.
What it didn't claim: The paper focuses entirely on the lab creation and testing of this material. It does not claim to have cleaned a real river yet, nor does it discuss using this for medical purposes or other future applications beyond what was tested in the lab.

In short: They turned trash (pomelo peels) into a treasure (a super-sponge) that loves to eat orange dye.

Technical Summary: Modification of H3PO4-activated Pomelo Peel Biochar with CTAB for Methyl Orange Removal

Problem Statement
Organic dyes, particularly azo dyes like Methyl Orange (MO), represent a significant class of water pollutants due to their stability, resistance to biodegradation, and potential carcinogenicity. While various treatment methods exist, adsorption remains a preferred technique due to its cost-effectiveness and simplicity. However, traditional biochars often suffer from low specific surface areas and limited active sites, reducing their efficiency. Furthermore, pristine biochars derived from agricultural waste often possess negatively charged surfaces, which create electrostatic repulsion against anionic dyes like MO, thereby limiting adsorption capacity. This study addresses the need to enhance the physicochemical properties of pomelo peel biochar to improve its efficacy in removing anionic dyes.

Methodology
The study utilized pomelo peel biochar previously activated with phosphoric acid (A-PBC) as a base material. This biochar was modified using the cationic surfactant cetyltrimethylammonium bromide (CTAB) at varying mass ratios (0.05, 0.1, and 0.25 g/L), resulting in adsorbents designated as CTAB1@A-PBC, CTAB2@A-PBC, and CTAB5@A-PBC.

The research employed a comprehensive characterization suite:

Structural and Morphological Analysis: X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) were used to assess crystallinity, functional groups, and surface morphology.
Textural Properties: Brunauer–Emmett–Teller (BET) analysis determined specific surface area, pore volume, and pore size distribution.
Surface Charge: The point of zero charge ( $pH_{pzc}$ ) was determined via the salt addition method.

Adsorption experiments were conducted in batch mode to evaluate MO removal efficiency. Key variables included the type of biochar, solution pH, and contact time. The study utilized Langmuir, Freundlich, Temkin, Dubinin-Radushkevich (DR), and Sips isotherm models to describe equilibrium data. Kinetic behavior was analyzed using pseudo-first-order (PFO), pseudo-second-order (PSO), Elovich, and intraparticle diffusion models.

Key Results

Physicochemical Modifications: The modification with CTAB significantly altered the biochar's properties. The specific surface area increased from 1192.18 m²/g (A-PBC) to a maximum of 2150.61 m²/g for the sample modified with 2 wt% CTAB (CTAB2@A-PBC). The pore volume also increased from 1.998 cm³/g to 3.303 cm³/g. XRD analysis indicated a reduction in crystallinity, suggesting that CTAB reduced the aggregation of the biochar structure. FTIR confirmed the presence of CTAB through new C-N bond vibrations and the modification of carboxyl groups.
Surface Charge Reversal: The $pH_{pzc}$ of the modified biochar (CTAB2@A-PBC) was determined to be 6.49. This shift is critical as it allows the surface to carry a positive charge at pH values below 6.49, facilitating electrostatic attraction with the negatively charged sulfonic groups of MO.
Adsorption Performance: The CTAB2@A-PBC adsorbent demonstrated the highest removal efficiency (70.38%) compared to other CTAB concentrations and the unmodified A-PBC (55.26%). The removal efficiency was highly pH-dependent, with optimal performance observed at pH 4.01–6.14, where the adsorbent surface is positively charged.
Isotherm and Kinetic Modeling:
- Isotherms: The Langmuir model provided the best fit ( $R^2 > 0.99$ ), indicating monolayer adsorption on a homogeneous surface. The maximum adsorption capacity ( $q_{max}$ ) calculated was 534.23 mg/g at 30°C. The Temkin model also showed a strong fit, suggesting electrostatic interactions are significant.
- Kinetics: The Elovich kinetic model ( $R^2 = 0.9982$ ) described the adsorption process better than PFO or PSO models, suggesting that chemisorption involving energetic heterogeneity dominates the process. Intraparticle diffusion modeling indicated that while pore diffusion occurs, the process is not solely controlled by it; the non-zero intercept suggests that external diffusion also limits the adsorption rate.
Mechanism: The study proposes a multi-mechanism adsorption process involving:
1. Electrostatic Attraction: Between the positively charged biochar surface (at pH < 6.49) and anionic MO.
2. $\pi$ - $\pi$ Interaction: Between the graphene-like structures of the biochar and the aromatic rings of MO.
3. Pore Filling: Enabled by the increased pore volume and appropriate pore size distribution.
4. Hydrogen Bonding: Between surface hydroxyl groups and the nitrogen/oxygen atoms of the MO molecule.

Significance and Claims
The paper claims that the modification of H3PO4-activated pomelo peel biochar with CTAB is an effective strategy to transform a low-cost agricultural waste derivative into a high-performance adsorbent for anionic dyes. The study highlights that the CTAB modification not only reverses the surface charge to attract anionic pollutants but also nearly doubles the specific surface area and significantly increases pore volume.

With an adsorption capacity of 534.23 mg/g, the CTAB2@A-PBC adsorbent performs competitively against other modified biochars reported in recent literature (e.g., magnetic chitosan-functionalized biochar at 644.0 mg/g, N-doped biochar at 449 mg/g). The authors assert that the simplicity of the preparation method, combined with the high efficiency and the dual enhancement of surface area and surface charge, makes this modified biochar a promising candidate for the practical treatment of anionic dye pollutants in wastewater. The research concludes that such modifications can broaden the applicability of biochar beyond cationic pollutants to effectively target negatively charged organic contaminants.