2.1 Properties of Activated Carbon

The surface area of a one gram of activated carbon can be in excess of 500 m2, with

1500 m2 being readily achievable. While being more expensive carbon aerogels have

much higher surface areas, and are applied for special applications. Activated carbon has

different properties and can be categorized in physical properties and chemical properties.

Ash content, moisture content, yield, bulk density, hardness number, and pore size

distribution can be categorized under the physical properties of the activated carbon.

Surface area, iodine number and surface chemistry (chemical reactivity) can be placed

under chemical properties. These are some principal properties that are used for making a

comparison of various activated carbons. There are also some other properties of activated

carbon and they can vary depending on the initial carbon source as well as the method of

activation.

2.1.1 Physical Properties

2.1.1.1 Ash Content

The ash content of a carbon is the residue that remains when the carbonaceous

materials is burned off. The inorganic constituents in a carbon are usually reported as

being in the form in which they appear when the carbon is ashed. Ash content was

measured by burning the produced activated carbon in a muffle furnace at 973 K. One

gram of dry carbon was transferred into a crucible and then placed in the furnace for four

hours. The difference between the original and final weight of the carbon represents the ash

content per gram

2.1.1.2 Moisture Content

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When hot air and steam over is applied to generate activated carbon then it is necessary

to wash the generated activated carbon with acid rain and then rinse and dried it. During

this process the activated carbon is bound with a little amount of moisture. Moisture

content was also obtained by weighing 10 grams of the carbon and placed in an oven at

105oC for 3 h. Then the carbon was cooled in the absence of humidity and reweighed

again. The difference between the initial and final mass of the carbon represents the water

content in the sample.

2.1.1.3 Yield

The activated carbon yield was calculated using the following equation:

Yield (% )=WCWO

∗100

Where WC is the dry weight (g) of final activated carbon and WO is the dry weight (g)

of precursor.

2.1.1.4 Bulk/Apparent Density

This property is needed to find out how many kgs of the activated carbon must be used

to fill up a certain volume of a tank. The bulk/apparent density of a material was mainly

determined by weighting five grams of the produced activated carbon and transferring it

into a 10 mL graduated cylinder. The cylinder was tamping with a rubber pad while

activated carbon was being added until the entire original sample was transferred to the

cylinder. This is usually measured in g/ml or pounds per cubic foot. Bulk Density is used

to determine the weight of a fixed volume of activated carbon.

Bulk /Apprel Density=weight of the sample (g )volume of the sample ( I )

2.1.1.5 Hardness number1

More hardness of the activated carbon to make it less crumbled into fine particles

during handling and use. It is a relative measurement of the ability of granular or pelletized

activated carbon. For describing this property, a specific amount of the activated carbon is

put into a pan with some steel balls and shaken for a limited period of time. The difference

in the weight of the carbon before and after the shaking gives the amount of loss. The

percentage of the activated carbon after the shaking is the hardness number of the activated

carbon.

2.1.1.6 Pore Size Distribution (PSD)

Different particle sizes of activated carbon offer different flow resistance to air or water

it is filtering. Typically, for filtering air or gases, the larger granule sizes are used, and for

filtering water or fluids, the smaller granules sizes are used. The finer the particle sizes of

an activated carbon, the better the access to the surface area and the faster the rate of

adsorption kinetics. In vapour phase systems this needs to be considered against pressure

drop, which will affect energy cost. Careful consideration of particle size distribution can

provide significant operating benefits.

2.1.2 Chemical Properties

For testing the efficiency and capability of activated carbon, some specified chemical

tests are performed by the certain amount chemicals that can be adsorbed by per weight of

used activated carbon.

2.1.2.1 Surface Area

Generally, the larger the specific surface area of the adsorbent, the better its adsorption

performance will be. The most widely used commercial active carbons have a specific

surface area of the order of 600- 1200 m2/g. The pore volume limits the size of the

molecules that can be adsorbed whilst the surface area limits the amount of material which

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can be adsorbed, assuming a suitable molecular size. The adsorptive capacity of adsorbent

is related to its internal surface area and pore volume.

The specific surface area (m2/g) of porous carbon is most usually determined from gas

adsorption measurement using the Brunauer-Emmett-Teller BET theory. The most

commonly employed method to characterize these structural aspects of the porosity is

based on the interpretation of adsorption isotherm (e.g., N2 at 77K). Nitrogen at its boiling

point of 77K is the recommended adsorptive, although argon at 77K also used.

2.1.2.2 Iodine Number

The iodine number is tested by ASTM4607 and it is mainly used for the measurement

of the porosity of the activated carbon by adsorption of iodine from solution. For this, a

specific properly known amount of activated carbon, this was grounded up into powder,

mixed with a standard solution of iodine in water. Some iodine is absorbed in the mixture

and some amount of iodine left in the mixture. By finding out this remaining amount of

iodine, we can calculate how much iodine has been absorbed by the activated carbon. Each

1.0 mg of adsorbed iodine is ideally considered to represent 1.0 m2 of activated carbon

internal surface area.

2.1.2.3 Surface Chemistry (Chemical reactivity)

The selection of activated carbons for adsorption is mainly depends upon their surface chemistry. Normally, the adsorptive surface of activated carbon is approximately neutral such as that polar and ionic species are less readily adsorbed than organic molecules. For many applications it would be advantageous to be able to tailor the surface chemistry of activated carbon in order to improve their effectiveness. The chemical composition of the raw material influence the surface chemistry and offer a potentially lower cost method for adjusting the properties of activated carbons. The surface chemistry of activated carbon is dominated by oxygen functionality. The affinity of activated carbon for oxygen is quite high, as chemisorption has been demonstrated at temperatures below –40 oC: the resultant oxides could only be removed as CO and CO2 at temperatures greater than 200 oC

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