Activated carbon solution is widely applicable as adsorbent in water treatment and air pollution control. This chapter examines the use of this sorbent for organic compounds removal. The manufacture of activated carbon is discussed. Pyrolysis of raw materials and activation methods (physical and chemical) are given. In addition, the physical features such as surface area, pore volume and pore size distribution, and chemical surface structure (surface functional groups) are described. The porosity and surface chemistry of activated carbons strongly affect their adsorption capacity. The removal of organic compounds is exposed and particular attention is given to the effect of porous texture and chemical structure of activated carbon on the adsorption of organic pollutants.
AC is a porous material exhibiting amphoteric characteristics, and is usually used for adsorption of organic and inorganic compounds. There are some remarkable advantages of activated carbon such as, reduced operation cost, high surface area, significant stability, and tunability of the surface and structure. AC supported catalysts can be found in two different forms. The adsorption capacity of these supports is substantially affected by the nature of the carbon sorbent and the preparation methods. The CWPO process using AC supported catalysts is quite favorable compared with the conventional homogeneous Fenton process. https://www.yrdcarbon.com/
PAC is costly and is discarded after one use whereas GAC, although about two to three times the cost of PAC, can be reactivated after exhaustion and reused. Since the introduction of more rigorous standards for drinking water quality the use of GAC has become the predominant process for the removal of organic matter including micropollutants. GAC adsorbers are commonly installed downstream of rapid gravity filters used for turbidity removal.
Activated carbon can be made from wood, coal, coconut shells or peat. (If you want to find high quality activated carbon manufacturers, please click here) The material is first carbonized by heating and then is ‘activated’ by heating to a high temperature whilst providing it with oxygen in the form of a stream of air or steam. Sometimes chemical activation by phosphoric acid is used. It is then ground to a granular or powdered form. It is a relatively pure form of carbon with a fine capillary structure which gives it a very high surface area per unit of volume. The adsorption capacity of GAC is described by various parameters including Iodine Number and BET surface area.
GAC adsorbers are of conventional rapid gravity or pressure filter design and the basic design parameters are the EBCT and bed depth or hydraulic loading (m3/h.m2). Bed depths up to 2.5 m for rapid gravity filters and 3 m for pressure filters are used.GAC characteristics vary according to the base material used. For example, the adsorptive capacity for the pesticide atrazine varies in the order wood>coconut shell>peat>coal. However, coal-based GAC finds wide use for most water treatment applications as it has a distribution of both mesopores (2–50 nm diameter) and micropores (up to 2 nm diameter), a structure suitable for medium to large (colour, taste and odour) and small organic molecules micropollutants, respectively. Pilot plant work or Rapid Small Scale Column tests should be used to optimize the GAC type and other design parameters such as adsorption capacity (by Freundlich adsorption isotherm) and to determine the life of carbon between reactivation. EBCT varies for different micropollutants and is usually in the range 5–30 minutes; for pesticides it is 15–30 minutes and for DBPs and VOCs it is about 10 minutes.
Powdered activated carbon (PAC) can be added before coagulation, during chemical addition, or during the settling stage, prior to sand filtration. It is removed from the water during the coagulation process, in the former cases, and through filtration, in the latter. As the name implies, PAC is in particulate form, with a particle size typically between 10 and 100 μm in diameter. One of the advantages of PAC is that it can be applied for short periods, when problems arise, then ceased when it is no longer required. With problems that may arise only periodically such as algal toxins or tastes and odours, this can be a great cost advantage. A disadvantage with PAC is that presently it cannot be reused and is disposed to waste with the treatment sludge or backwash water.
PAC added for mercury control presents different challenges for ash use in concrete. Activated carbon is much more adsorptive than unburned coal with a high affinity to adsorb AEAs that are used in concrete production. These characteristics are the result of activated carbon's complex pore structure (Fig. 13.7). Mercury is sequestered in very small portions of the carbon structure, but there remain ample pores and surface area available to adsorb other compounds, including AEAs from fresh concrete mixtures.
The increased variability of content, type, and adsorption capacity of carbon in fly ash has raised the need to develop more accurate adsorption test methods for determining the impact of activated carbon on ash. There are several test methods working toward utilizing existing activated carbon adsorption tests to measure the adsorption capacity of ash and its impact on concrete. For example, IN, which has been traditionally used to determine the adsorption capacity of carbon black, is now being considered as a potential test method to determine the adsorption capacity of fly ash. In this test method, carbon black/fly ash is first boiled in a 5% HCl solution to remove any sulfur that may interfere with the results. After drying and filtering (and crushing, if necessary), a known amount of carbon black (or fly ash, as may be the case) is mixed with a standard iodine solution. Subsequently, the solution is filtered to separate the solids. Lastly, the concentration of the remaining iodine in the filtrate is measured by titration. The results are expressed in grams of iodine adsorbed per kilogram of carbon black/fly ash (Ahmed, David, Sutter, & Watkins, 2014). Although the IN test provides a relative indication of surface area, it may not measure the capacity of fly ash to adsorbed other chemical species. Vistit our website: https://www.yrdcarbon.com/