Minggu, 08 Februari 2015

Characteristics of Activated Carbon

Characteristics of Activated Carbon - The effectiveness of activated carbon as an adsorbent is attributed to its unique properties, including “large surface area, a high degree of surface reactivity, universal adsorption effect, and favorable pore size,” according to Bansal.

Activated carbon is often characterized by its extremely large surface area. In fact, the surface area per gram of material can range from 500 to 1400 square meters, and values as high as 2500 m2/g have been reported.

The complex internal surface area is usually divided into three components. Channels and pores with diameters less than 2 nm are commonly known as micropores; these micropores generally contain the largest portion of the carbon’s surface area. On the other hand, pores with diameters between 2 and 50 nm are known as mesopores, and pores with diameters greater than 50 nm are defined as macropores.

For comparison, a given type or sample of activated carbon is usually quantified based on four primary criteria: total surface area, carbon density, particle size distribution, and adsorptive capacity. Of course, all of these factors influence adsorption rate and capacity.

Total surface area is measured by the adsorption of nitrigen gas onto the carbon and is expressed in square meters of surface area per gram of carbon. Because the gas molecules used to measure adsorption are very small, it should be noted that this measurement of surface area may be misleading when considering the adsorptive capacity of a carbon for large organic macromolecules. Those types of compounds may have adsorption limited by pore size considerations.

Carbon density is the weight of one milliliter of carbon in air. Bulk density is also sometimes used for carbon as it is for soil, and is expressed in pounds per cubic foot or in kilograms per liter.
Particle sizes in carbons are measured using standard U.S. sieve sizes, as for soil constituents. Particle size distributions are important in carbon systems because they influence handling of the activated carbon material. For example, in granular carbon, the particle size affects hydraulic loading and backwash rates for a filter. On the other hand, particle size is often important because of its effect on adsorption rates as well.

Adsorptive capacity is characterized by the effectiveness of activated carbon in removing a given contaminant. For comparison, several standard compounds are used for these measurements. For example, the commonly-used ‘iodine number’ describes the carbon’s capacity to adsorb low-molecular-weight substances, while the ‘molasses number’ characterizes a carbon’s capacity for more complex compounds.

The structural properties of activated carbon are very important to its effectiveness as an adsorbent, though activated carbon’s structure is not fully understood and is difficult to explain with text. In general, activated carbon is sometimes described as having a ‘crumpled’ layered surface, in which flat sheets are broken and curved back upon themselves. This unique structure creates activated carbon’s very large surface area. It can be more properly visualized with the attached images, which provide both electron microscope photos and conceptual diagrams of the surface structure.(activated carbon indonesia)

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