Adsorption capacity of activated carbon

The adsorptive capacity of activated carbon for some common solvent vapors is shown in Table 25-27. 

We wUl now touch upon some of these factors. First, let s look at what we mean by system isotherm. Freundlich liquid phase isotherm studies can be used to establish the adsorptive capacity of activated carbon over a range of different concentrations. Under standard conditions, the adsorptive capacity of activated carbon increases as the concentration increases, until we reach a point of maximum saturation capacity. An example of an isotherm for phenol is shown in Figure 8. 

The Freundlich liquid phase isotherm can be used to determine the effect of solubility on the adsorptive capacity of activated carbon over a range of different concentrations. Phenol is highly soluble due to its polar nature whilst, in comparison, tetrachloroethylene (PCE) has a low solubility due to being non-polar. In the isotherms illustrated, the concentration of phenol is low relative to its solubility limit and consequently, the adsorptive capacity peaks at 18% maximum (see Figure 9). In comparison the concentration of tetrachloroethylene is relatively close to its solubility limit and, accordingly, the adsorptive capacity is exceptionally good. 

Comparison of H2 Adsorption Capacities of Activated Carbons and Metal-Organic Frameworks... 

The adsorption capacity of activated carbon for pyrene is very high (up to 0.6 g/g). The uptake decreases with increasing pressure but increases with temperature. This is in correspondence with results from literature found for the adsorption of DDT [10] or phenanthrene [11] on activated carbon. One important information is the shape of the adsorption isotherms. This kind of isotherm shape is generally favourable for adsorption. For that it is possible to regen-... 

The adsorption capacity of activated carbon may be determined by the use of an adsorption isotherm. The adsorption isotherm is an equation relating the amount of solute adsorbed onto the solid and the equilibrium concentration of the solute in solution at a given temperature. The following are isotherms that have been developed Freundlich Langmuir and Brunauer, Emmet, and Teller (BET). The most commonly used isotherm for the application of activated carbon in water and wastewater treatment are the Ereundlich and Langmuir isotherms. The Freundlich isotherm is an empirical equation the Langmuir isotherm has a rational basis as will be shown below. The respective isotherms are ... 

To use the isothenns, constants are empirically determined by running an experiment. This is done by adding increasing amonnts of the adsorbent to a sample of adsorbate solntion in a container. For each amonnt of adsorbent added, the equilibrium concentration [CJ is determined. The pairs of experiment trial values can then be used to obtain the desired parameter valnes from which the constants are determined. Once the constants are determined, the resnlting model is used to determine M, the amonnt of adsorbent (activated carbon) that is needed. From the derivation, the adsorption capacity of activated carbon is a = (XJM) i,. From this ratio, the absorption capacity of activated carbon is shown as the maximnm value of the XIM ratios. This ratio corresponds to a concentration equal to the maximum possible solute equilibrium concentration. 

The dynamic adsorption capacity of activated carbon containing monoliths has been shown to be equivalent to the micropore volume. However, this condition can only be met when the external area is above c. 100 m g" and the threshold diameter wide. In systems with no micropore volume or poor internal diffusion due to a low external surface area and narrow threshold diameter the breakthrough point is reached when c. 9% of the external area is covered. Future work will concentrate on using higher linear velocities and adsorption temperatures md different monolith geometries (wall thickness and channel width) in order to study the internal diffusion limitations of these types of adsorption units. 

It has long been known that, under appropriate conditions and especially in the liquid phase, synergistic associations can develop between microbiological systems and activated carbons or other support media (e.g.. in trickling bed filters for aerobic water treatment). In liquid phase applications, bacterial colonization of activated carbon can occur quite readily [76-79]. For example, the adsorptive capacities of activated carbon beds used in water treatment are often greatly enhanced by the presence of microorganisms, and the useful filter life is extended beyond that expected for a process of purely physical adsorption. Essentially, the... 

In a recent review of technological alternatives for NOM removal, Jacangelo et al. [558] presented field data that illustrate wide variability in adsorption capacities of activated carbon in a single location (e.g., exhaustion periods between 41 and 182 days). They concluded that these results are evidence of the site-specific nature of [dissolved organic carbon] removal by [activated carbon] and that the concerns regarding reliability of treatment practices to meet the new [regulations] have a sound basis. Clearly, much fundamental work remains to be done to understand fully the complex nature of these adsorbent/adsorbate interactions and thus be able to optimize both the physical and the chemical accessibility of the carbon surface to natural organic matter. 

This work intends to show the complexity of the dynamic adsorption process and to evaluate capacity of some granular carbons of various firms to remove pollutants from water. Adsorbents have been tested by various methods, and static and dynamic adsorption have been compared. Characteristics of carbons has been evaluated by the determination of porous structure, specific surface, content of ashes (mineral substances) and crushing strength and abrasion resistance. Adsorption capacity of activated carbon has been determined by means of phenol, iodide, methylene blue, sodium lauryl sulphate and molasses indicators for static conditions, and surfactant has been used for dynamic conditions. Analysis of some factors influencing adsorption has been accomplished and directions of further studies have been shown. 

The adsorption capacity of activated carbon was determined under static conditions by adsorption of organic matter from the microbiologically pretreated wastewater mixture. 

Chou, M-S. and Chiou, J-H. (1997). Modeling effects of moisture on adsorption capacity of activated carbon for VOCs. J. Environ. Eng., 1213, 437—43. 

The study of a particular adsorption process requires the knowledge of equilibrium data and adsorption kinetics [4]. Equilibrium data are obtained firom adsorption isotherms and are used to evaluate the capacity of activated carbons to adsorb a particular molecule. They constitute the first experimental information that is generally used as a tool to discriminate among different activated carbons and thereby choose the most appropriate one for a particular application. Statistically, adsorption from dilute solutions is simple because the solvent can be interpreted as primitive, that is to say as a structureless continuum [3]. Therefore, all equations derived firom monolayer gas adsorption remain vafid. Some of these equations, such as the Langmuir and Dubinin—Astakhov, are widely used to determine the adsorption capacity of activated carbons. Batch equilibrium tests are often complemented by kinetics studies, to determine the external mass transfer resistance and the effective diffusion coefficient, and by dynamic column studies. These column studies are used to determine system size requirements, contact time, and carbon usage rates. These parameters can be obtained from the breakthrough curves. In this chapter, I shall deal mainly with equilibrium data in the adsorption of organic solutes. 

Figure 6.13 Catalytic activity and adsorption capacity of activated carbons in the CWAO process of aqueous ammonia. C, original AC COX, C oxidized with HNO3 CH, C treated with H2 at 673 K CHO, CH oxidized with HNO3 COH, COX treated with H2 at 673 K. (Adapted from ref. 207.)...

It has already been pointed out that a high surface area and an adequate pore size distribution are necessary conditions for a carbon adsorbent to perform well in a particular application. However, there are many examples of carbons with similar textural characteristics, which show a very different adsorption capacity witih the same adsorbate [12]. The reason for these different behaviours is that an adequate porous texture is a necessary but not a sufficient condition for the optimization of the adsorption capacity of activated carbons. The nature and amount of surface groups that may be present on the carbon surfaces must also be taken into account. 

Despite a significantly lower adsorption capacity of activated carbons for the removal of SOj and NO from flue gases in comparison with the VOC removal capacity, there are many processes in which they (or activated cokes) are applied for purification of industrial fumes [171-174] from coal fired power plants and waste incinerators. Activated coke is a carbonaceous adsorbent manufactured from lignites or hard coals. Typically, the specific surface areas of commercially available activated cokes are relatively low (up to 400 m g" ) and the pore volumes are only up to ca. 0.25 cm g Depending on the material origin and the manufacturing process, either adsorptive or catalytic characteristics may play a dominant role in the removal of contaminants on this adsorbent. The majority of activated cokes is used for the removal of SO and dioxins fiom waste and flue gases. 

Table L Comparison of CEES and HD Adsorption Capacities of Activated Carbon Fabrics...

Figure 2. BET Normalized Adsorption Capacity of Activated Carbon Fabrics for CEES and Agent HD Volume Average Pore Diameter...

The adsorption capacity of activated carbon fabrics for CEES and HD dissolved in HFE-7100 increases with the specific surface area and as a function of the the volume average pore diameter, over the range of 0.2 nm to 2.9 nm. There is an excellent correlation between the HD adsorption values and the CEES adsorption values. Fabrics with a high mesopore concentration will exhibit significantly more liquid phase capacity for CEES and HD than microporous fabrics. Negligible off-gassing of CEES or HD is observed from activated carbon fabrics at low contaminant loadings when the contaminant is contained within the pores of the fabric. 

Besides the crystalline and porous structure, an active carbon surface has a chemical structure as well. The adsorption capacity of active carbons is determined by their physical or porous structure but is strongly influenced by the chemical structure. The decisive component of adsorption forces on a highly ordered carbon surface is the dispersive component of the van der Walls forces. In graphites that have a highly ordered crystalline surface, the adsorption is determined mainly by the dispersion component due to London forces. In the case of active carbons, however, the disturbances in the elementary microcrystalline structure, due to the presence of imperfect or partially burnt graphitic layers in the crystallites, causes a variation in the arrangement of electron clouds in the carbon skeleton and results in the creation of unpaired electrons and incompletely saturated valences, and this influences the adsorption properties of active carbons, especially for polar and polarizable compounds. 

Although the adsorption capacity of active carbons is determined by their physical or porous structure, it is strongly influenced by the chemical structure of their surface. In graphites, for example, which have a highly ordered crystalline stracture, the adsorption capacity is determined by the dispersion component of London forces. In the case of active carbons, however, the random ordering of the aromatic sheets causes... 

Adsorption Capacity of Active Carbon for Several Different Body Species... 

Guzel, F., The effect of surface acidity upon the adsorption capacities of activated carbons, Sep. Sci. Technol., 31(2), 283-290 (1996). 

Vidic, R. D. and M. T. Suidan. 1991. Role of dissolved-oxygen on the adsorptive capacity of activated carbon for synthetic and natural organic-matter. Environmental Science Technology 25(9) 1612-1618. 

Phenazone adsorption this is a test to estimate the adsorption capacity of activated carbon for pharmaceutical purposes. 

There are additional data that some producers supply to the users to provide a maximum of information about their products. The most common is the surface area, as deduced from the application of the BET equation to the adsorption data of nitrogen at 77 K (it can also be applied to other gases and vapors at different temperatures). Although the BET surface area has not much physical meaning in microporous solids (Section 4.1.3) it is widely used to give an idea of the adsorptive capacity of activated carbon. The adsorption capacity as measured by adsorption of different gases and vapors is also frequently given by some producers. 

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