Activated carbon is one of the most important adsorbents in various industries. Due to its porous structure and large surface area, it is widely used in water and air purification systems, the food and pharmaceutical industries, and the energy sector. The most important parameter for the adsorption efficiency of activated carbon is its specific surface area (BET) . This parameter indicates the size of the internal and external surface area available for molecular adsorption per unit mass of the material.
In this article, we take a closer look at what the BET surface area is, how it is measured, how it relates to the performance of activated carbon, and why it is crucial in the selection and use of this material.
What is a private layer?
The specific surface area is the surface area available to solid particles per unit mass of a substance , usually expressed in square meters per gram (m²/g).
In porous materials like activated carbon, the specific surface area is not limited to the outer surface of the particles, but also includes the interior, which consists of pores and cavities. These tiny, interconnected pores give the activated carbon its active surface area, which ranges from hundreds to thousands of square meters per gram.
What is the BET method?
The BET method (Brunauer-Emmett-Teller) is a standard method for determining the specific surface area of porous materials. It is based on the physical adsorption of a gas (usually nitrogen at 77 K) on a solid surface .
The principle of the BET method:
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Nitrogen is adsorbed on the surface of the material.
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Record the adsorption curve (a graph of the dependence of the amount of adsorbed substance on the relative pressure of the gas).
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The area of the gas monolayer was calculated using the BET equation.
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Since the surface area is known, the total surface area of the substance (in square meters per gram) can be determined.
Betting equation:
1V(P0P−1)=C−1VmC⋅PP0+1VmC\frac{1}{V\left(\frac{P_0}{P}-1\right)} = \frac{C-1}{VmC}\cdot \frac{P}{P_0} + \frac{1}{VmC}
Where:
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VV : Volume of absorbed gas at relative pressure P/P0P/P_0
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VmVm : Gas volume in one layer
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CC : constant related to the absorbed energy
The specific surface area was calculated from the slope and width of the line in the linear region of the equator.
What is the specific surface area of activated carbon?
Depending on the type of raw material (wood, coal, coconut shell, lignin, etc.) and the activation method (chemical or physical), activated carbon can have a specific surface area in the following ranges:
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Microporous activated carbon : approx. 600-1200 m²/g
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Porous activated carbon : approx. 400-800 m²/g
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Large-pore industrial or special activated carbon : over 2000 m²/g
For comparison: The surface area of a tennis ball is only about 0.03 square meters; however, one gram of activated carbon has a surface area equivalent to several football fields!
Importance of specific surface area in activated carbon applications
The larger the specific surface area, the higher the absorption capacity . However, each application has different performance requirements:
1. Water and wastewater treatment
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This material has a large surface area and specific surface area, which allows it to effectively absorb heavy metals, organic compounds and toxic substances.
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Micropores are very important for the removal of ions and small particles.
2. Air and gas purification
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Surface and pore type play an important role in the removal of volatile organic compounds (VOCs), toxic gases and odors.
3. Food and pharmaceutical industries
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Activated carbon with a large specific surface area is used to remove color, odors and impurities from solutions.
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Due to their absorbing properties, activated charcoal tablets are also used in medicine to treat poisoning.
4. Energy and storage
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Activated carbon with a high specific surface area is used for the storage of hydrogen and methane and for the production of supercapacitors.
Factors that influence the specific surface area of activated carbon
1. Raw material type : Lignocellulosic materials such as coconut shells typically have a large surface area.
2. Activation method :
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Physical activation: A porous structure is formed by steam or carbon dioxide .
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Chemical activation : By using chemicals such as KOH or ZnCl₂, smaller pores and a higher specific surface area can be achieved.
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Processing temperature and time : Higher temperatures and longer times may lead to the opening of larger pores or damage to the structure.
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Particle size : The smaller the particles, the larger the surface area.
Limitations of the BET method for activated carbon
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The BET method is best suited for micro- and medium-porous materials; it is less accurate for macroporous materials.
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The presence of functional groups on the surface can influence gas adsorption and desorption.
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Selecting the wrong gas can distort the results (for example, nitrogen cannot penetrate some very small pores, in this case carbon dioxide is used).
Why is the BET value always low?
While a large surface area is a decisive advantage, the pore size distribution (micropores, medium pores, macropores) is also crucial. For example:
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When purifying drinking water, small pores remove dissolved substances more effectively.
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Medium and large pores are needed to store gas or accommodate larger molecules.
Therefore, when selecting activated carbon, you should pay attention not only to the BET value but also to the pore structure and the type of application.
Finally
The BET surface area is one of the most important indicators of the quality and efficiency of activated carbon . This parameter reflects the internal and external surface area available for adsorption and directly affects the adsorption capacity, performance, and efficiency of the activated carbon.
The nitrogen solubility adsorption (BET) method is the standard tool for measuring specific surface area, but for a complete analysis , the pore size distribution and surface chemistry must also be investigated.
In conclusion, the larger the BET surface area, the greater the adsorption potential of the activated carbon ; however, the type of activated carbon should be selected according to the application area, type of pollutant and operating conditions.