Surface areas of activated carbon

The effectiveness of activated carbon for the removal of organic compounds from fluids by adsorption is enhanced by its large surface area, a critical factor in the adsorption process. The surface area of activated carbon typically can range from 450 to 1,800 m /g, with some carbons observed to have a surface area up to 2,500 m /g. Some examples are given in Table 6. 

Table 6. Typical Surface Areas of Activated Carbons.

K0a as described in its simplified form in Eq. (1), assuming yocr/yoM MW0ct/ MWom to be equal to 1 and poor to be 0.820 kg/L./EC is the fraction of elemental carbon in atmospheric particles, which was estimated to be less than 5% in rural and remote sites [59], aac is the surface area of active carbon, acc is the specific surface of elemental carbon and 10-12 is a factor for unit correction. Assuming that the ratios /oct/ /om- MW0ct/MW0m and acc/a,AC are equal to 1, it is possible to predict Kp values from/oM and/EC [30-34, 52],... 

Many commercial activated carbons have been prepared with various sources of raw materials and different processing conditions. As a result, the micropore structures and specific surface areas of activated carbons, which are the most profound influences on the extent of adsorption, vary, and in general, activated carbons have a surface area of up to 3000 m2/g. The rate of adsorption increases with some function of the inverse of the radius of the activated carbon even though the adsorption capacity (i.e., equilibrium adsorption) is relatively independent of the particle diameter. However, for a highly porous adsorbent such as activated carbon, the... 

As already described in Section 6.5.2, the surface area of active carbons can be directly assessed by immersion calorimetry with non-polar liquids (e.g. n-hexane). Satisfactory agreement with BET-nitrogen areas has been found with the supermi-croporous carbons. As expected, because of the unreliability of the BET areas, the ultramicroporous carbons gave poor agreement. However, we consider that this does not invalidate the use of immersion calorimetry. 

One of the activated carbons applications is purification of air and removal of odoriferous substances, hazardous to human health, from the atmosphere. Acetaldehyde can be considered as such a compound. Adsorption of formaldehyde on activated carbons was studied by Domingo-Garcia et al. [10]. They found that it is strongly adsorbed and the retention volumes obtained in that study increased with an increase in the surface areas of activated carbons. 

Surface areas of activated carbons obtained from almond shells (C-series) and olive stones... 

Molina-Sabio, M., Salinas-Martinez de Lecea, C., et al. (1985). A comparison of different tests to evaluate the apparent surface area of activated carbons. Carbon, 23, 91-6. Femandez-Colinas, J., Denoyel, R., and Rouquerol, J. (1989). Adsorption of iodine from aqueous solutions onto activated carbons correlations with nitrogen adsorption at 77 K. Adsorp. Sci. TechnoL, 6, 18-26. 

A problem with active carbon is that the usual /-plot is not obtained for adsorption isotherms measured on carbon. At 77 K adsorption is often limited because migration of adsorbed molecules over the surface is required to enter narrow pores. At 77 K the mobility of adsorbed species is often not sufficient. Carbon dioxide adsorption is therefore employed to assess the surface area of activated carbon supports. 

The relatively high values of the surface areas of activated carbons are mainly dtie to the contribution of the micropores and most of the adsorption takes place in these pores. At least... 

K is Langmuir s constant and is surface area of activated carbon covered... 

Seidi and Kriska and Drozhalina and Bulgakova also observed that the adsorption of phenol from aqueous solutions was reversible and could be used to determine the surface area of activated carbons. Brand and coworkers used the adsorption of phenol as a measure of surface area of alumina, but the adsorption in this case was determined from solutions of phenol in heptane. Jankowska et al. °Gruszek et al. Koltesev et al. and Badnar and Nagi found that the adsorption of phenol from aqueous solutions was determined by the pore-size distribution of the activated carbon. The carbons with finer micropores had a better adsorption capacity, which could be as much as 34% by weight at an initial phenol concentration of 1 g/L. 

Figure 6. BET surface area of activated carbon as a function of bum-off (open symbols demineralized lignite, closed symbols raw lignite).

Guzel, F., Determination of surface areas of active carbons by retention of ethylene glycol and ethylene glycol monoethyl ether, Sep. Sci. Technol, 31(12), 1687-1694 (1996). 

Activation is now recognized as a simple increase in the internal surface area of the carbon material, resulting in the formation of a well-developed and readily accessible pore structure, with pores of controllable size. The internal surface area of activated carbons ranges from 500 to 1500 m /g. 

Jiang, Q. and Y. Zhao. 2004. Effects of activation conditions on BET specific surface area of activated carbon nanotubes. Microporous andMesoporous Materials 76(1-3) 215-219. 

The surface area and volume of micropore, total surface area of activated carbon decrease by 40% after loading ruthenium and then barium, which indicates barium ions can mainly enter into the micropores. 

Kaneko et al. (1992a) make the case that these considerations can be used to explain the high surface areas of activated carbon. A three-layered structure of molecule type B can possess a surface area >3000 m g assuming that adsorption occurs at the edges as well as on the surface and that the theoretical limitation of 2630 m g for a single layer of graphite need not be restrictive. (This argument is somewhat spurious.)... 

This confirms the convenience of determining the surface area of activated carbons using immersion calorimetry in benzene when a non-porous carbon is used as the reference, independently of the chemical nature of the carbon. 

How is activated carbon prepared What are the chemical reactions involved What is remarkable about the surface area of activated carbon ... 

The evolution of the surface area (CO, 273 K) of coals, chars and activated chars with the time of oxidation of the coal is shown in Figure 1. Coal preoxidation determines a big increase in the surface area of chars. A large and continuous increase in the CO2 surface area of activated carbons is also produced during gasification, as a consequence of a more intense air oxidation of the raw coal samples. 

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