large surface to volume ratio with a small diffusion distance. activated carbon (r 1) is defined as the activated carbon particles retained on a 50-mesh sieve (0.297 mm). powdered activated carbon (pac) material is finer material. pac is made up of crushed or ground carbon particles, 95–100% of which will pass through a designated mesh sieve. the astm classifies particles passing through an 80-mesh sieve (0.177 mm) and smaller as pac. it is not common to use pac in a dedicated vessel, due to the high head loss that would occur. instead, pac is generally added directly to other process units, such as raw water intakes, rapid mix basins, clarifiers, and gravity filters. granular activated carbon a micrograph of activated charcoal (gac) under scanning electron microscope granular activated carbon (gac) has a relatively larger particle size compared to powdered activated carbon and consequently, presents a smaller external surface. diffusion of the adsorbate is thus an important factor. these carbons are suitable for adsorption of gases and vapors, because gaseous substances diffuse rapidly. granulated carbons are used for air filtration and water treatment, as well as for general deodorization and separation of components in flow systems and in rapid mix basins. gac can be obtained in either granular or extruded form. gac is designated by sizes such as 8×20, 20×40, or 8×30 for liquid phase applications and 4×6, 4×8 or 4×10 for vapor phase applications. a 20×40 carbon is made of particles that will pass through a u.s. standard mesh size no. 20 sieve (0.84 mm) (generally specified as 85% passing) but be retained on a u.s. standard mesh size no. 40 sieve (0.42 mm) (generally specified as 95% retained). awwa (1992) b604 uses the 50-mesh sieve (0.297 mm) as the minimum gac size. the most popular aqueous-phase carbons are the 12×40 and 8×30 sizes because they have a good balance of size, surface area, and head loss characteristics. extruded activated carbon (eac) extruded activated carbon (eac) combines powdered activated carbon with a binder, which are fused together and extruded into a cylindrical shaped activated carbon block with diameters from 0.8 to 130 mm. these are mainly used for gas phase applications because of their low pressure drop, high mechanical strength and low dust content. also sold as cto filter (chlorine, taste, odor). bead activated carbon (bac) bead activated carbon (bac) is made from petroleum pitch and supplied in diameters from approximately 0.35 to 0.80 mm. similar to eac, it is also noted for its low pressure drop, high mechanical strength and low dust content, but with a smaller grain size. its spherical shape makes it preferred for fluidized bed applications such as water filtration. impregnated carbon porous carbons containing several types of inorganic impregnate such as iodine and silver. cations such as aluminium, manganese, zinc, iron, lithium, and calcium have also been prepared for specific application in air pollution control especially in museums and galleries. due to its antimicrobial and antiseptic properties, silver loaded activated carbon is used as an adsorbent for purification of domestic water. drinking water can be obtained from natural water by treating the natural water with a mixture of activated carbon and aluminium hydroxide (al(oh)3), a flocculating agent. impregnated carbons are also used for the adsorption of hydrogen sulfide (h2s) and thiols. adsorption rates for h2s as high as 50% by weight have been reported. polymer coated carbon woven activated carbon cloth this is a process by which a porous carbon can be coated with a biocompatible polymer to give a smooth and permeable coat without blocking the pores. the resulting carbon is useful for hemoperfusion. hemoperfusion is a treatment technique in which large volumes of the patient's blood are passed over an adsorbent substance in order to remove toxic substances from the blood. woven carbon there is a technology of processing technical rayon fiber into activated carbon cloth for carbon filtering. adsorption capacity of activated cloth is greater than that of activated charcoal (bet theory) surface area: 500–1500 m2/g, pore volume: 0.3–0.8 cm3/g). thanks to the different forms of activated material, it can be used in a wide range of applications (supercapacitors, [odor absorbers , cbrn-defense industry etc.). properties a gram of activated carbon can have a surface area in excess of 500 m2 (5,400 sq ft), with 3,000 m2 (32,000 sq ft) being readily achievable. carbon aerogels, while more expensive, have even higher surface areas, and are used in special applications. under an electron microscope, the high surface-area structures of activated carbon are revealed. individual particles are intensely convoluted and display various kinds of porosity; there may be many areas where flat surfaces of graphite-like material run parallel to each other, separated by only a few nanometers or so. these micropores provide superb conditions for adsorption to occur, since adsorbing material can interact with many surfaces simultaneously. tests of adsorption behaviour are usually done with nitrogen gas at 77 k under high vacuum, but in everyday terms activated carbon is perfectly capable of producing the equivalent, by adsorption from its environment, liquid water from steam at 100 °c (212 °f) and a pressure of 1/10,000 of an atmosphere. james dewar, the scientist after whom the dewar (vacuum flask) is named, spent much time studying activated carbon and published a paper regarding its adsorption capacity with regard to gases. in this paper, he discovered that cooling the carbon to liquid nitrogen temperatures allowed it to adsorb significant quantities of numerous air gases, among others, that could then be recollected by simply allowing the carbon to warm again and that coconut based carbon was superior for the effect. he uses oxygen as an example, wherein the activated carbon would typically adsorb the atmospheric concentration (21%) under standard conditions, but release over 80% oxygen if the carbon was first cooled to low temperatures. physically, activated carbon binds materials by van der waals force or london dispersion force. activated carbon does not bind well to certain chemicals, including alcohols, diols, strong acids and bases, metals and most inorganics, such as lithium, sodium, iron, lead, arsenic, fluorine, and boric acid. activated carbon adsorbs iodine very well. the iodine capacity, mg/g, (astm d28 standard method test) may be used as an indication of total surface area. carbon monoxide is not well adsorbed by activated carbon. this should be of particular concern to those using the material in filters for respirators, fume hoods or other gas control systems as the gas is undetectable to the human senses, toxic to metabolism and neurotoxic. substantial lists of the common industrial and agricultural gases adsorbed by activated carbon can be found online. activated carbon can be used as a substrate for the application of various chemicals to improve the adsorptive capacity for some inorganic (and problematic organic) compounds such as hydrogen sulfide (h2s), ammonia (nh3), formaldehyde (hcoh), mercury (hg) and radioactive iodine-131(131i). this property is known as chemisorption. iodine number 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. molasses 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. 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 concentration by 50%. 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). 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 reduces the overall activity of activated carbon and reduces the efficiency of reactivation: the amount is exclusively dependent on the base raw material used to produce the activated carbon (e.g. coconut, wood, coal, etc.). 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. astm (d2866 standard method test) is used to determine the ash content of activated carbon. 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