1. Pore Size Classification of Activated Carbon
The pore structure of activated carbon is very complex and is generally classified into three categories based on pore size:
Macropores (> 50 nm): These primarily serve as transport channels, transporting dioxin molecules from the external environment to the smaller pores within the activated carbon. Their inherent adsorption capacity is very weak.
Mesopores (2-50 nm): Also known as mesopores, these serve as secondary transport and adsorption channels. They have a strong adsorption effect on some larger molecular weight organic compounds.
Micropores (< 2 nm): These are the primary sites for dioxin adsorption. Micropores provide a large surface area, and the interactions between their pore walls (van der Waals forces) effectively capture and immobilize dioxin molecules.
2. Molecular Size of Dioxins
Dioxins are a general term for a class of polychlorinated aromatic hydrocarbons, of which 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is the most toxic and common.
According to molecular model calculations, the kinetic diameter of dioxin molecules is approximately between 1.0 and 1.5 nanometers (nm).
This size falls squarely within the range of micropores.
3. How Pore Size Affects the Adsorption Process (Core Mechanism)
The adsorption process can be figuratively understood as "grab" and "fixation":
"Grab" stage (kinetically controlled):
Dioxin molecules in exhaust gas first pass through the macropore and mesopore network and rapidly enter the activated carbon particles.
If the macropores and mesopores are underdeveloped, the molecules cannot easily reach the adsorption sites, and the adsorption rate will be slow.
"Fixation" stage (thermodynamically controlled):
When a dioxin molecule (~1.0-1.5 nm) enters a micropore of a very similar size (e.g., 1.7-2.0 nm), it is simultaneously affected by van der Waals forces from multiple surrounding pore walls.
This "synergistic effect" results in extremely high adsorption energy, making the adsorption extremely strong and nearly irreversible. This phenomenon is known as the pore-filling effect.
If the pore size is much larger than the molecular size (for example, if it enters a mesopore), the molecule may only experience forces from the pore wall on one side, resulting in weak adsorption and easy desorption.
If the pore size is too small (for example, less than 1.0 nm), dioxin molecules cannot enter at all and therefore cannot be adsorbed.
Summary and Practical Applications
In flue gas purification systems in industries such as waste incineration and metallurgy, activated carbon (powdered activated carbon or granular activated carbon) specifically designed and activated for dioxin removal is characterized by:
A highly developed microporous structure with pore sizes in the 1.5-2.0 nm range.
A balanced ratio of mesopores to macropores ensures low resistance and high mass transfer efficiency.
Therefore, when selecting activated carbon for dioxin adsorption, one must not only consider the total specific surface area, but also its pore size distribution (PSD) to ensure a sufficiently high pore volume within the critical micropore range (1-2 nm). This is the key factor in determining adsorption performance.
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