Activated carbon pore volume distribution

The extent of competition may also be a function of the adsorbate molecular size, correlated with the activated carbon pore size distribution. Activated carbon fibers tliat are exclusively microporous (more than 96 % of micropore volume) present a selectivity property for pesticides or phenol in the presence of higher molecidar weight compounds like humic substances, due to the direct connection of micropores to their external surface [41]. Using granular activated carbon, which does not have this molKUilar sieve property, a 20 to 70 % reduction in adsorption is obtained for atrazine in raw water compared with equilibria in distilled water [42]. 

Fig. 3.23 shows pore volume distributions of some commercially important porous materials. Note that zeolites and activated carbon consist predominantly of micropores, whereas alumina and silica have pores mainly in the me.sopore range. Zeolites and active carbons have a sharp peak in pore size distribution, but in the case of the activated carbon also larger pores are present. The wide-pore silica is prepared specially to facilitate internal mass-transfer. 

Figure 3.23. Pore volume distributions (Nt physi.sorption) of a. wide-pore silica, b. y-alumina, c. a-alumina, d. activated carbon, e. Raney Nickel and f. ZSM-5.

Figure 17.2. Typical pore volume distributions for three activated carbons used for liquid-phase processes(7)...

Kwong-Soave type). Epi is calculated from geometric considerations involving r, and H. Fa(H) can be related to the pore volume distribution fimction Fy(H) if the pore shape is known. In this work, we consider classical slit-like pores to describe the porous structure of the activated carbon and we take a bimodal gaussian for the mathematical expression of Fv(H). As a consequence ... 

Newcombe et al. 536] also concluded that the adsorption of four NOM ultrafiltration fractions on to activated carbon was consistent with the pore volume distributions of the carbons and the hydrodynamic diameters of the fractions. ... 

Bjelopavic, M., Newcombe, G., and Hayes, R. (1999). Adsorption of NOM onto activated carbon effect of surface charge, ionic strength, and pore volume distribution. J. Colloid Interface Sci., 210, 271—80. 

Newcombe, G., Drikas, M., and Hayes, R. (1997). Influence of characterized natural organic material on activated carbon adsorption II. Effect on pore volume distribution and adsorption of 2-methyhsobomeol. Water Res., 31, 1065—73. 

The HK micropore volume distribution for a slit-like microporous structure can be obtained by multiplying the adsorption potential distribution [see Eq. (24) and Fig. 10] by Eq. (77). For cylindrical and spherical micropore geometries another expressions for the derivative dAldx should be used [160]. An illustration of the HK pore volume distributions is shown in Fig. 12 for the WV-A900, BAX 1500 and NP5 active carbons. Similarly, the mesopore volume distribution can be calculated from the multilayer and capillary condensation range of the adsorption isotherm. In this case, the corrected Kelvin equation should be used to calculate the derivative dAldx. 

It is evident that the most valuable information concerning the adsorption capacity of a given activated carbon is its adsorption isotherm for the solvent being adsorbed and its pore volume distribution cinve. Figure 22.1.10 presents idealized toluene adsorption isotherms for three carbon types ... 

Figure 1.4. Differential pore volume distribution (dVp(r)/dr) per unit mass of activated carbon NoritRl Extra at 298 K (Hg-intrusion) [1.36].

The evolution of pore volume distributions, calculated from mercury porosimetry and helium densities, of activated carbons... 

Figure 2. Pore volume distributions of activated carbons from oxidized coal samples.

A major difficulty in testing the validity of predictions from the DR equation is that independent estimates of the relevant parameters—the total micropore volume and the pore size distribution—are so often lacking. However, Marsh and Rand compared the extrapolated value for from DR plots of CO2 on a series of activated carbons, with the micropore volume estimated by the pre-adsorption of nonane. They found that except in one case, the value from the DR plot was below, often much below, the nonane figure (Table 4.9). 

Fig. 2. Pore size distribution of typical samples of activated carbon (small pore gas carbon and large pore decolorizing carbon) and carbon molecular sieve (CMS). A / Arrepresents the increment of specific micropore volume for an increment of pore radius.

In addition to surface area, pore size distribution, and surface chemistry, other important properties of commercial activated carbon products include pore volume, particle size distribution, apparent or bulk density, particle density, abrasion resistance, hardness, and ash content. The range of these and other properties is illustrated in Table 1 together with specific values for selected commercial grades of powdered, granular, and shaped activated carbon products used in Hquid- or gas-phase appHcations (19). 

Activated carbons for use in Hquid-phase appHcations differ from gas-phase carbons primarily in pore size distribution. Liquid-phase carbons have significantly more pore volume in the macropore range, which permits Hquids to diffuse more rapidly into the mesopores and micropores (69). The larger pores also promote greater adsorption of large molecules, either impurities or products, in many Hquid-phase appHcations. Specific-grade choice is based on the isotherm (70,71) and, in some cases, bench or pilot scale evaluations of candidate carbons. 

Most of the adsorbents used in the adsorption process are also useful to catalysis, because they can act as solid catalysts or their supports. The basic function of catalyst supports, usually porous adsorbents, is to keep the catalytically active phase in a highly dispersed state. It is obvious that the methods of preparation and characterization of adsorbents and catalysts are very similar or identical. The physical structure of catalysts is investigated by means of both adsorption methods and various instrumental techniques derived for estimating their porosity and surface area. Factors such as surface area, distribution of pore volumes, pore sizes, stability, and mechanical properties of materials used are also very important in both processes—adsorption and catalysis. Activated carbons, silica, and alumina species as well as natural amorphous aluminosilicates and zeolites are widely used as either catalyst supports or heterogeneous catalysts. From the above, the following conclusions can be easily drawn (Dabrowski, 2001) ... 

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