📄 chemistry

Oyster shell-derived adsorbent for simultaneous Pb²⁺ and Hg²⁺ removal: a thermochemical modification approach

This study demonstrates that a thermochemically modified oyster shell biosorbent (CMOS) is an effective, sustainable, and reusable material for the simultaneous removal of Pb²⁺ and Hg²⁺ from aqueous solutions, achieving high adsorption capacities and selectivity through chemisorption mechanisms.

Original authors: Phuong Thi Hoai Nguyen, Huynh Nhi Le, Phuong Anh Cao, Hang Thi Thu Nguyen, Anh Thi Kim Bui, Ba Cuong Nguyen, Manh Truong Nguyen, Duy Khiem Nguyen, Duc Duong La

Published 2026-07-08

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Phuong Thi Hoai Nguyen, Huynh Nhi Le, Phuong Anh Cao, Hang Thi Thu Nguyen, Anh Thi Kim Bui, Ba Cuong Nguyen, Manh Truong Nguyen, Duy Khiem Nguyen, Duc Duong La

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 giant, messy kitchen sink full of dirty water. Unfortunately, this water is poisoned with two invisible, dangerous guests: Lead (Pb²⁺) and Mercury (Hg²⁺). These are heavy metals that can make people very sick if they drink the water. Usually, cleaning this water requires expensive, high-tech machines or harsh chemicals that create their own kind of waste.

This paper introduces a clever, eco-friendly solution: turning oyster shells (the waste from seafood restaurants) into a super-sponge that can suck up these poisons.

Here is the story of how they did it and what happened, explained simply:

1. The Transformation: From Shell to Super-Sponge

The researchers didn't just throw raw oyster shells into the water. They gave the shells a "spa treatment" to make them much better at their job. Think of it like upgrading a plain brick house into a high-tech apartment complex.

The Heat Bath (Thermal): First, they baked the shells at a very high temperature (500°C). This was like popping the shells open, making them rougher and cracking them slightly to create tiny nooks and crannies.
The Acid Bath (Chemical): Next, they soaked the baked shells in a special acid (phosphoric acid). This was the real magic. It coated the shells with a new layer of "sticky" minerals (calcium phosphate).
The Result: The final product, called CMOS, looked very different under a microscope. While the original shells were smooth and flat, the CMOS was covered in tiny, needle-like crystals and flakes. It was like turning a smooth stone into a velcro-covered sponge.

2. The Test: Catching the Poisons

The researchers put this new "super-sponge" into water contaminated with Lead and Mercury to see how well it worked.

Speed: The sponge worked incredibly fast. Within an hour, it had grabbed almost all the Lead and most of the Mercury.
The "Velcro" Effect: The study found that the metals didn't just sit on the surface; they chemically bonded to the sponge's new "sticky" spots. It wasn't just a physical trap; it was a chemical handshake.

3. The Personality of the Sponge: Picking Favorites

Here is where it gets interesting. The sponge has a personality. It loves Lead more than Mercury.

For Lead (Pb²⁺): The sponge acts like a strict, single-person hotel. It has specific, perfect-sized rooms (called a "monolayer") where Lead fits perfectly. It can hold a lot of Lead—more than 165 mg for every gram of sponge. This is a huge amount compared to other materials tested.
For Mercury (Hg²⁺): The sponge acts more like a crowded, messy party. The Mercury sticks to all sorts of different, uneven spots on the surface (called "multilayer adsorption"). It still grabs a good amount of Mercury, but not as efficiently as it grabs Lead.

The "Party" Scenario:
When the researchers put both Lead and Mercury in the water at the same time (like a crowded room), the sponge showed its true colors. It was very picky. It grabbed the Lead almost immediately and completely (over 99% removal), leaving very little Lead behind. It also grabbed a lot of the Mercury (over 80%), but the Lead got there first and took the best spots.

4. Reusability: The Sponge That Keeps Giving

Usually, when you use a sponge to clean up a mess, you throw it away. But this oyster shell sponge is reusable.

The researchers washed the dirty sponge with a mild acid to "shake off" the metals, dried it, and used it again. They did this five times.

Lead: It kept working at over 90% efficiency.
Mercury: It kept working at over 80% efficiency.

This means the sponge is durable and can be used many times, which makes it very cheap and good for the environment.

The Bottom Line

This paper shows that we can take a waste product (oyster shells), give it a simple heat and acid makeover, and turn it into a powerful tool for cleaning water.

It's a Lead-Removal Champion: It is one of the best materials found so far for grabbing Lead out of water.
It's a Mercury Helper: It's also very good at grabbing Mercury, even if Lead is also present.
It's Sustainable: Instead of throwing away oyster shells, we are recycling them to solve a pollution problem.

In short, the researchers turned a kitchen waste product into a high-tech, reusable, and highly effective "magnet" for dangerous heavy metals, proving that sometimes the best solutions are hidden in plain sight (or in the shell pile).

Technical Summary: Oyster Shell-Derived Adsorbent for Simultaneous Pb²⁺ and Hg²⁺ Removal

Problem Statement
Heavy metal pollution, specifically from lead (Pb²⁺) and mercury (Hg²⁺), poses severe risks to aquatic ecosystems and human health due to their persistence, bioaccumulation, and toxicity. While conventional remediation methods like chemical precipitation and membrane filtration exist, they often suffer from high operational costs, sludge generation, and incomplete removal. Although biosorption using waste-derived materials offers a sustainable alternative, most existing research focuses on the removal of single metal ions. Real-world wastewater often contains multiple competing metal species, yet there is a limited understanding of how modified oyster shells perform in simultaneous dual-ion systems, particularly regarding the competitive adsorption of Pb²⁺ and Hg²⁺.

Methodology
The study developed a thermochemically modified biosorbent (CMOS) derived from oyster shells, a calcium-rich aquaculture byproduct. The preparation involved a two-step process:

Thermal Activation: Raw oyster shells were calcined at 500°C for 2 hours to induce partial decomposition of calcium carbonate.
Chemical Modification: The calcined shells were treated with 0.5 M phosphoric acid at 65°C for 2 hours to induce partial phosphatization, forming a calcium phosphate-carbonate hybrid structure.

The resulting material was characterized using Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), and BET analysis to assess morphology, crystallinity, functional groups, and porosity. Batch adsorption experiments were conducted to evaluate the effects of contact time, initial metal concentration, and co-ion interference in both single and binary (Pb²⁺/Hg²⁺) systems. Kinetic and isotherm models were applied to elucidate adsorption mechanisms, and regeneration tests were performed using 0.1 M HCl over seven cycles.

Key Contributions and Results

Material Characterization:
- Morphology & Structure: SEM analysis revealed that chemical modification transformed the shell surface from compact plate-like particles to a structure with needle-like and flake-like crystals, indicating the formation of calcium phosphate phases.
- Crystallinity: XRD confirmed that while the original shells consisted of calcite (CaCO₃), the CMOS contained new peaks corresponding to brushite [CaPO₃(OH)·2H₂O], alongside residual calcite.
- Surface Chemistry: FTIR spectra showed the introduction of phosphate groups (PO₄³⁻) and the retention of carbonate and hydroxyl groups, which serve as active binding sites.
- Porosity: BET analysis demonstrated that CMOS possessed a significantly higher specific surface area (40.51 m²/g) and pore volume compared to raw (22.15 m²/g) and thermally treated shells (17.09 m²/g), with a well-developed mesoporous structure.
Adsorption Kinetics and Isotherms:
- Kinetics: Both Pb²⁺ and Hg²⁺ adsorption followed the pseudo-second-order model (R² > 0.99), indicating that chemisorption is the rate-limiting step.
- Isotherms:
  - Pb²⁺: Adsorption data fitted the Langmuir model best (R² = 0.9948), suggesting monolayer adsorption on a homogeneous surface with a maximum capacity ( $q_{max}$ ) of 165.56 mg·g⁻¹.
  - Hg²⁺: Adsorption data fitted the Freundlich model best (R² = 0.9980), suggesting multilayer adsorption on heterogeneous sites. The $q_{max}$ for Hg²⁺ was 30.86 mg·g⁻¹.
Simultaneous Removal and Selectivity:
- In binary systems, CMOS exhibited strong selectivity toward Pb²⁺. At an initial concentration of 30 mg/L, Pb²⁺ removal remained high (>99%), even in the presence of Hg²⁺.
- Hg²⁺ removal was slightly more affected by competition, decreasing from ~93% to ~89% in the binary system, but still maintained removal efficiencies above 80% across tested concentrations.
- The material demonstrated superior affinity for Pb²⁺ compared to Hg²⁺, likely due to differences in ionic radius and binding preferences with the surface functional groups.
Regeneration:
- Desorption studies using 0.1 M HCl showed that CMOS retained over 90% of its Pb²⁺ adsorption efficiency and over 80% of its Hg²⁺ efficiency after five adsorption-desorption cycles, confirming its reusability.

Significance and Claims
The authors claim that the thermochemically modified oyster shell (CMOS) is a cost-effective, sustainable, and reusable biosorbent for heavy metal remediation. The study highlights the material's exceptional capacity for Pb²⁺ removal, which surpasses many reported adsorbents in the literature, while maintaining effective simultaneous removal of Hg²⁺. By valorizing oyster shell waste through a simple thermochemical modification, the approach aligns with circular economy principles. The research provides a viable solution for wastewater treatment scenarios dominated by lead contamination or requiring the co-removal of lead and mercury, leveraging the material's high surface area, modified crystalline structure, and reversible interaction with phosphate, carbonate, and hydroxyl groups.