The ash content of an activated carbon is used to determine the raw material used to produce the activated carbon. Almost all shell carbon contains 10% - 20%
ash content and coal based contains 6% - 16% ash content. The ash content
reduces the efficiency of reactivation. It also reduces the overall activity
coefficient. The metals (Fe
2
O
3
) can leach out of activated carbon resulting in
discoloration – acid/water soluble ash content. The remaining content apart from the amount of the inorganic material or the carbonaceous material in the activated carbon is the percentage fixed carbon. The ash content is also known as loss on ignition. When a sample of known weight is heated in the furnace at the specified temperature for a given time, the weight obtained after heating corresponds to the ash content while the weight loss corresponds to the volatile solids (Okiemimen, et. al, 2007). The percentage fixed carbon is assumed to be approximately 58% of the volatile matter (Dioha, et. al, 2005).
ii. pH of The Carbon
The pH is the measure of the surface acidity or basicity o the oxygen containing group. This measurement is used to predict hydophilicity and anionic or cationic adsorptive preference of the carbon. The p
H
of the carbon particle determines the materials ironic or cationic preference .The pH can be measured by using the pH meter or a litmus paper and compared with the pH chart.
iii. Apparent Density and Bulk Density
Higher density provides greater volume activity and normally indicates better quality
activated carbon. The apparent density is equal to the mass (weight) of a quantity of
carbon divided by the volume it occupies (including pore volume and inter particle
voids, adjusted for the moisture content). Generally, bituminous – based GAC has a
density between 28-- 40 pounds per cubic foot (PCF), liqnite-based GAC has a density
of approximately 22-26 PCF, and wood-based GAC has a density of 15 – 19 PCF
(Department of the Army Engineering US. Army Corps of Engineers and Adsorption
Design Guide, 2001). The bulk density of activated carbon and its specific adsorptive
capacity for a given substance is used in determining the grade of carbon needed for
existing systems. The bulk density of the unit weight of the carbon within the
adsorber. Generally, the bulk density of the liquid phase applications is 80 - 95% of
the apparent density and for vapor phase applications, it is 80 – 100% of the
apparent density. Decolorized carbon has bulk density of around 0.5g/ml. apparent
density is used to determine the volumetric carbon usage rate since the carbon usage
rate is typically stated in mg.contaminant removal/ gram of carbon.
iv. Particle Size Distribution
The finer the particle size of an activated carbon, the better access to the surface area and the faster the rate of adsorption kinetics. For smaller particle sizes the rate of diffusion of an organic into the pore and its subsequent adsorption is significantly increased. The particle size that will be used in a filter. However, particle size may not be that important in all cases, as the porous nature of the carbon particle results in large surface areas in all sizes of carbon particles. Head loss through a carbon bed increases and as the uniformity increases (Department of the Army Corps of Engineers and Engineering Design Adsorption Guide, 2001).
The particle size distribution can be carried out in the laboratory using the mesh/sieve analysis. This is done using a rest of sieves of different mesh sizes.
The sieves may be mounted vibrator, which should be designated to give a degree of vertical movement in addition to the horizontal vibration, or may be hand shaken (Richardson, et. al, 2003). Careful consideration of particle size distribution can provide significant operating benefits.
v. Pore Volume
Activated carbon has a large volume of small pores which create a large surface area. This large surface area is a result of the space created by crystallites of micro porous structures with large internal surface area. Activated carbon has surface areas ranging from 250 m
2
/g to 2500m
2
/g. These internal pores are classified based on sizes in micropores (10A
o
to 1000A
o
). Adsorption occurs primarily in the micropores with macropore acting conduits. The relative proportions of these pores depend on the raw material used. However, particle size may not be that important in all cases as the nature of the pore of the carbon results in large surface area in all sizes of carbon particles. The pore volume is a measure of the total pore volume within the carbon particles in cubic centimeters per gram (cm
3
/g).
vi Molasses Number
Molasses number or molasses efficiency refers to the milligrams of molasses
adsorbed during the standard test. It is also a measure of the macropore content of
the activated carbon (greater than 28A
o
in diameter or larger than 2nm) by
adsorption of molasses from solution ahigh molasses efficiency is reported as a percentage.
A carbon with a high percentage of this pore size is suitable for adsorbing high molecular weight substances such as color bodies or other colloids – carbons with a high molasses number are generally only used in color removal applications, and is not a valid specification requirement for water treatment. This is a preparatory test, and should not be used in specifying granular activated carbon (GAC) (US. Army Corps Engineers and Engineering design Adsorption Guide, 2001).
vii. Hardness/Abrasion Number
The abrasion number measures the ability of carbon to withstand handling and slurry transfer and to resist attrition. It is important indicator of activated carbon to maintain its physical integrity and withstand frictional forces imposed to backwashing. There are large differences in the hardness of activated carbon, depending on the raw material and activity level. Two different tests are used, based on the type of carbon material. A Ro tap abrasion test is used for bituminous – coal based on GAC, and a stirring abrasion test is used for the softer, liqnite – coal based GAC. The abrasion number is the ratio of the final average (mean) particle diameter to the original mean particle diameter (determined by sieve analysis) multiplied by 100. The desired average particle size of the GAC retained should be greater than or equal to 70%. This is of limited value because measuring techniques are not reproducible.
viii Oil Composition
This is also called the percentage oil yield. The oil is separated from the activated carbon by the use of extraction. The solvent used must be immisible with the other components but the oil must be more soluble in the selected solvent. The extraction may have to be repeated several times to effect complete separation. Solvents that may be used for extraction include hexane, ether, dichloromethane, amongst others.
