There are many different properties and characteristics of activated carbon, and understanding these factors will help you understand how the material is measured and analyzed.
Iodine Number
Iodine adsorption is used to measure the effectiveness of activated carbon. During this test, activated carbon is added to a liquid holding a specific amount of iodine. The carbon is mixed thoroughly until it has dissolved into the solution. After a few minutes, the solution is filtered into another container, removing the charcoal particles and allowing the liquid to pass through. The iodine number is a gauge of the amount of iodine removed from the liquid. Essentially, the higher the number, the more iodine was removed.
Many carbons preferentially adsorb small molecules. Iodine number is the most fundamental parameter used to characterize activated carbon performance. It is a measure of activity level (higher number indicates higher degree of activation,) often reported in mg/g (typical range 500–1200 mg/g). It is a measure of the micropore content of the activated carbon (0 to 20 Å, or up to 2 nm) by adsorption of iodine from solution. It is equivalent to surface area of carbon between 900 and 1100 m2/g. It is the standard measure for liquid-phase applications.
Iodine number is defined as the milligrams of iodine adsorbed by one gram of carbon when the iodine concentration in the residual filtrate is at a concentration of 0.02 normal (i.e. 0.02N). Basically, iodine number is a measure of the iodine adsorbed in the pores and, as such, is an indication of the pore volume available in the activated carbon of interest. Typically, water-treatment carbons have iodine numbers ranging from 600 to 1100. Frequently, this parameter is used to determine the degree of exhaustion of a carbon in use. However, this practice should be viewed with caution, as chemical interactions with the adsorbate may affect the iodine uptake, giving false results. Thus, the use of iodine number as a measure of the degree of exhaustion of a carbon bed can only be recommended if it has been shown to be free of chemical interactions with adsorbates and if an experimental correlation between iodine number and the degree of exhaustion has been determined for the particular application.
Pore Diameter
The diameter of the pores on and inside activated carbon will make a significant difference in how the materials performs. Pore diameter can determine the specific use of a carbon, as activated carbon with more micropores (smaller pores) can be effective for removing low concentrations of organic matter found in water. Activated carbon with both small and large pores are very versatile and can be used to remove both chlorine and a wide variety of organic matter at the same time.
Surface Area
The surface area is another important property that is often cited on activated carbon. Depending on the raw material, the activation process, and other factors, the surface area will vary, giving the charcoal more or less adsorption potential. Surface area for activated carbon is often measured using a BET nitrogen adsorption test.
Density
Density will affect the volume activity. Generally, a higher density will indicate a higher-quality activated carbon. There are numerous ways to define density, including real density, which is the density excluding the voids of the material, as well as particle density, which is the measured density of the carbon particles alone. There is also wetted density, apparent density, bed or bulk density, and tamped density. All of these density measurements provide specific data on activated carbon performance.
Tannin
Tannins are a mixture of large and medium size molecules. Carbons with a combination of macropores and mesopores adsorb tannins. The ability of a carbon to adsorb tannins is reported in parts per million concentration (range 200 ppm–362 ppm).
Methylene blue
Some carbons have a mesopore (20 Å to 50 Å, or 2 to 5 nm) structure which adsorbs medium size molecules, such as the dye methylene blue. Methylene blue adsorption is reported in g/100g (range 11–28 g/100g).
Dechlorination
Some carbons are evaluated based on the dechlorination half-life length, which measures the chlorine-removal efficiency of activated carbon. The dechlorination half-value length is the depth of carbon required to reduce the chlorine level of a flowing stream from 5 ppm to 3.5 ppm. A lower half-value length indicates superior performance.
Apparent density
The solid or skeletal density of activated carbons will typically range between 2000 and 2100 kg/m3 (125–130 lbs./cubic foot). However, a large part of an activated carbon sample will consist of air space between particles, and the actual or apparent density will therefore be lower, typically 400 to 500 kg/m3 (25–31 lbs./cubic foot).[19]
Higher density provides greater volume activity and normally indicates better-quality activated carbon. ASTM D 2854 -09 (2014) is used to determine the apparent density of activated carbon.
Hardness/abrasion number
It is a measure of the activated carbon’s resistance to attrition. It is an important indicator of activated carbon to maintain its physical integrity and withstand frictional forces. There are large differences in the hardness of activated carbons, depending on the raw material and activity levels.
Ash Content
Ash content is an important measurement for activated carbon and can drastically change the effectiveness and specific use for the product. Ash in the activated carbon reduces the speed and reliability of reactivation and metal oxides can be released from charcoal with high ash content, resulting in discoloration when used to purify water. Carbon with high ash content is not good for fish tanks, as they can lead to heavy metal poisoning in the aquatic life, including fish and coral species. The type of ash can vary as well. For example, activated carbon made from coconut shells often has a higher concentration of alkali earth metals, while carbon made from coal is often loaded with heavy metals.
Ash reduces the overall activity of activated carbon and reduces the efficiency of reactivation. The metal oxides (Fe2O3) can leach out of activated carbon resulting in discoloration. Acid/water-soluble ash content is more significant than total ash content. Soluble ash content can be very important for aquarists, as ferric oxide can promote algal growths. A carbon with a low soluble ash content should be used for marine, freshwater fish and reef tanks to avoid heavy metal poisoning and excess plant/algal growth. Standard method D 2866-2011 is used to determine the ash content of activated carbon.
Mesh
The size of granular activated carbon (activated carbon that is in the form or a powder or fine grains) is measured using a Mesh system. It is measured by shaking a sample of the granulated carbon through a series of fine sieves. Imagine sieves like a window screen only much finer, with far smaller holes between the wires. Using a system that measures how much of the carbon passes through the screens, the activated carbon can be measured for general size.
Molasses Number
The molasses number for activated carbon is a measurement of the charcoal’s effectiveness for removing large molecules. This is done by allowing the activated carbon to adsorb a molasses solution. The higher the molasses number, the better the activated charcoal is at removing these large molecules.
Some carbons are more adept at adsorbing large molecules. Molasses number or molasses efficiency is a measure of the mesopore content of the activated carbon (greater than 20 Å, or larger than 2 nm) by adsorption of molasses from solution. A high molasses number indicates a high adsorption of big molecules (range 95–600). Caramel dp (decolorizing performance) is similar to molasses number. Molasses efficiency is reported as a percentage (range 40%–185%) and parallels molasses number (600 = 185%, 425 = 85%). The European molasses number (range 525–110) is inversely related to the North American molasses number.
Molasses Number is a measure of the degree of decolorization of a standard molasses solution that has been diluted and standardized against standardized activated carbon. Due to the size of color bodies, the molasses number represents the potential pore volume available for larger adsorbing species. As all of the pore volume may not be available for adsorption in a particular waste water application, and as some of the adsorbate may enter smaller pores, it is not a good measure of the worth of a particular activated carbon for a specific application. Frequently, this parameter is useful in evaluating a series of active carbons for their rates of adsorption. Given two active carbons with similar pore volumes for adsorption, the one having the higher molasses number will usually have larger feeder pores resulting in more efficient transfer of adsorbate into the adsorption space.
Carbon tetrachloride activity
Measurement of the porosity of an activated carbon by the adsorption of saturated carbon tetrachloride vapour.
Particle size distribution
The finer the particle size 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. However, in the case of using activated carbon for adsorption of minerals such as gold, the particle size should be in the range of 3.35–1.4 millimetres (0.132–0.055 in). Activated carbon with particle size less than 1 mm would not be suitable for elution (the stripping of mineral from an activated carbon).