History of Activated Carbon
Activated Carbon was first known to treat water over 2000 years ago. However, it was first produced commercially at the beginning of the 20th century and was only available in powder form. Initially activated carbon was mainly used to decolorize sugar and then from 1930 for water treatment to remove taste and odor. Granular activated carbon was first developed as a consequence of WWI for gas masks and has been used subsequently for water treatment, solvent recovery and air purification. The unique structure of activated carbon produces a very large surface area: 1 lb of granular activated carbon typically provides a surface area of 125 acres (1 Kg =1,000,000 sq. m.). Activated carbon can be produced from a variety of carbonaceous raw material, the primary ones being coal, coconut shells, wood and lignite. The intrinsic properties of the activated carbon are dependent on the raw material source. The activated carbon surface is non-polar which results in an affinity for non-polar adsorbates such as organics. Adsorption is a surface phenomenom in which an adsorbate is held onto the surface of the activated carbon by Van der Waal’s forces and saturation is represented by an equilibrium point. These forces are physical in nature, which means that the process is reversible (using heat, pressure, etc.) Activated carbon is also capable of chemisorption, whereby a chemical reaction occurs at the carbon interface, changing the state of the adsorbate (dechlorination is an example of a chemisorption process).
Types of Activated Carbon
|Water Soluble Ash|
Adsorption Parameters of Activated Carbon
1. Capacity vs. Kinetics (Rate):
• Capacity parameters determine loading characteristics of activated carbon. Maximum adsorption capacity of activated carbon is only achieved at equilibrium
• Kinetic parameters only determine the rate of adsorption and have negligible affect on adsorption capacity.
2. Surface Area: Adsorption capacity is proportional to surface area (determined by degree of activation).
3. Pore Size: Correct pore size distribution is necessary to facilitate the adsorption process by providing adsorption sites and the appropriate channels to transport the adsorbate.
4. Particle Size: Smaller particles provide quicker rates of adsorption. Note: Total surface area is determined by degree of activation and pore structure and not particle size.
5. Temperature: Lower temperatures increase adsorption capacity except in the case of viscous liquids.
6. Concentration of Adsorbate: Adsorption capacity is proportional to concentration of adsorbate.
7. pH: Adsorption capacity increases under pH conditions, which decrease the solubility of the adsorbate (normally lower pH).
8. Contact Time: Sufficient contact time is required to reach adsorption equilibrium and to maximize adsorption efficiency.
Activated Carbon Properties
1. Iodine Number
• most fundamental parameter used to characterize activated carbon performance
• measure of activity level (higher number indicates higher degree of activation)
• measure of micropore (0 – 20 Å) content
• equivalent to surface area of activated carbon in sq m/g between 900 – 1100
• standard measure for liquid phase applications
2. Methylene Blue
• measure of mesopore structure (20 – 500 Å)
3. Caramel dp (Molasses No.)
• measure of macropore structure (>500 Å)
• important for decolorizing performance
4. Surface Area
• measure of adsorption capacity (Note: pore size distribution/pore volume is also important to determine ultimate performance)
5. Apparent Density
• higher density provides greater volume activity and normally indicates better quality activated carbon
6. Particle Size
• smaller size provides quicker rate of adsorption which reduces the amount of contact time required
• smaller size results in greater pressure drop
7. Hardness/Abrasion Number
• measure of activated carbon’s resistance to attrition
• important indicator of activated carbon to maintain its physical integrity and withstand frictional forces imposed by backwashing etc.
8. Dechlorination half-value length
• test to measure the dechlorination efficiency of activated carbon
• depth of activated carbon to reduce influent chlorine level from 5 ppm to 2.5 ppm
• lower half-value length indicates superior performance
9. Ash Content
• reduces overall activity of activated carbon
• reduces efficiency of reactivation
• metals (Fe2O3) can leach out of activated carbon resulting in discoloration
• acid/water soluble ash content is more significant than total ash content