Activated carbon surface area

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. 

Technology has been developed by the Mega-Carbon Company, USA, to incorporate powders such as high surface area, KOH-activated carbons into robust... 

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. 

Lu, A.H. and Zheng, J.T. (2001). Study of microstructure of high surface area polyacrylonitrile activated carbon fibers./ Colloid Interface Sci., 236, 369-74. 

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. 

Figure 5.5 Uptake of [Ni(CN4) ] ions on BV46-S at pH 7 (squares), Ni + ions on BV46 at pH 6 (triangles), Cr + ions on BV46 at pH 6 (asterisks), and [Cr(C204) ] ions on HSl at pH 7 (diamonds) as a function of ionic strength. Sample BV46 is an activated carbon from olive stones. Oxidation of this sample with (NH4)2S20s yielded sample BV46-S. Sample HSl was a low-surface-area oxidized active carbon. (From ref. 26, with permission.)...

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... 

The chemical and physical methods of activation allow to produce carbon materials with a highly developed specific surface area. Therefore, activated carbons are extensively used as electrode materials for supercapacitors [11,25,30-37]. Considering Eq. (1) showing that capacitance depends on the surface area of the electrodes, it should be expected that higher the specific surface area of an activated carbon, higher the capacitance. However, practical... 

The electrochemical storage capacity of various nanotextured carbons, including high surface area graphites, activated carbons, single wall and multiwalled nanotubes, poorly correlates with the BET specific surface area [125,129], From the slope of the capacity vs surface area plot (Fig. 32), an uptake of 1.5 wt% for 1000 m g" is estimated. However, Fig. 32 shows clearly that it is only a trend, several points being completely out of proportionality. 

The results showed that the supports did not present significant catalytic activity (Table 1.7, entry 11-13). On the other hand, the catalytic activity of TPA supported on high-surface-area Si02, active carbon, and PMP has been increased with respect to the use of TPA alone (Table 1.7, entry 10). The optimized reaction conditions were used for the synthesis of several 2-oxazoline derivatives (19) starting from various aromatic (or heteroaromatic) nitriles and 2-aminoethanol (or 2-amino-2-methylpropanol). As reported in Table 1.8, in all entries, high to excellent yields were obtained. Notably, the authors reported that both mono- and bis-oxazolines can be obtained by this... 

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. 

A vast amount of research has been carried out in the area of activated carbon adsorption during the past four or five decades, and research data are scattered in different journals published in different countries and in the proceedings and abstracts of the International Conferences and Symposia on the science and technology of activated carbon adsorbents. This book critically reviews the available literature and tries to offer suitable interpretations of the surface-related interactions of the activated carbons. The book also contains consistent explanations for surface interactions applicable to the adsorption of a wide variety of adsorbates that could be strong or weak electrolytes. 

---