Carbon Adsorption

Solids are formed because of the attraction of the component atoms within the solid toward each other. In the interior of a solid, attractive forces are balanced among the various atoms making up the lattice. At the surface, however, the atoms are subjected to unbalanced forces—the ones toward the interior are attracted, but the ones at the

FIGURE 8.7 Raw carbon material on the left transforms to the carbon on the right after activation.

surface are not. Because of this unbalanced nature, any particle that lands on the surface may be attracted by the solid. This is the phenomenon of adsorption, which is the process of concentrating solute at the surface of a solid by virtue of this attraction.

Adsorption may be physical or chemical. Physical adsorption is also called van der Waals adsorption, and chemical adsorption is also called chemisorption. In the former, the attraction on the surface is weak, being brought about by weak van der Waals forces. In the latter, the attraction is stronger as a result of some chemical bonding that occurs. Adsorption is a surface-active phenomenon which means larger surface areas exposed to the solutes result in higher adsorption. The solute is called the adsorbate; the solid that adsorbs the solute is called the adsorbent. The adsorbate is said to be sorbed onto the adsorbent when it is adsorbed, and it is said to be desorbed when it passes into solution.

Adsorption capacity is enhanced by activating the surfaces. In the process using steam, activation is accomplished by subjecting a prepared char of carbon material such as coal to an oxidizing steam at high temperatures resulting in the water gas reaction: C + H2O ^ H2+CO. The gases released leave behind in the char a very porous structure.

The high porosity that results from activation increases the area for adsorption. One gram of char can produce about 1000 m of adsorption area. After activation, the char is further processed into three types of finished product: powdered form called powdered activated carbon (PAC), the granular form called granular activated carbon (GAC), and activated carbon fiber (ACF). PAC is normally less than 200 mesh; GAC is normally greater than 0.1 mm in diameter. ACF is a fibrous form of activated carbon. Figure 8.7 shows a schematic of the transformation of raw carbon to activated carbon, indicating the increase in surface area.

8.3.1 Activation Techniques

Activation is the process of enhancing a particular characteristic. Carbon whose adsorption characteristic is enhanced is called activated carbon. The activation techniques used in the manufacture of activated carbons are dependent on the nature and type of raw material available. The activation techniques that are principally used by commercial production operations are chemical activation and steam activation. As the name suggests, chemical activation uses chemicals in the process and is generally used for the activation of peat- and wood-based raw materials. The raw material is impregnated with a strong dehydrating agent, typically phosphoric pen-toxide (P2O5) or zinc chloride (ZnCl2) mixed into a paste and then heated to temperatures of 5QQ-8QQ°C to activate the carbon. The resultant activated carbon is washed, dried, and ground to desired size. Activated carbons produced by chemical activation generally exhibit a very open pore structure, ideal for the adsorption of large molecules.

Steam activation is generally used for the activation of coal and coconut shell raw materials. Activation is carried out at temperatures of 8QQ-11QQ°C in the presence of superheated steam. Gasification occurs as a result of the water-gas reaction:

This reaction is endothermic but the temperature is maintained by partial burning of the CO and H2 produced:

The activated carbon produced is graded, screened, and de-dusted. Activated carbons produced by steam activation generally exhibit a fine pore structure, ideal for the adsorption of compounds from both the liquid and vapor phases.

8.3.2 Adsorption Capacity

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 Freundlich 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:

X is the mass of adsorbate adsorbed onto the mass of adsorbent M; [C] is the concentration of adsorbate in solution in equilibrium with the adsorbate adsorbed; n, k, a, and b are constants.

The Langmuir equation may be derived as follows. Imagine a particular experiment in which a quantity of carbon adsorbent is added to a beaker of sample containing pollutant. Immediately, the solute will be sorbed onto the adsorbent until equilibrium is reached. One factor determining the amount of the sorbed materials has to be the number of adsorption sites in the carbon. The number of these sites may be quantified by the ratio XIM. By the nature of equilibrium processes, some of the solutes adsorbed will be desorbed back into solution. While these solutes are desorbing, some solutes will also be, again, adsorbed. This process continues on, like a seesaw; this "seesaw behavior" is a characteristic of systems in equilibrium.

The rate of adsorption rs is proportional to the concentration in solution, [C], (at equilibrium in this case) and the amount of adsorption sites left vacant by the desorbing solutes. Now, let us determine these vacant adsorption sites. On a given trial of the experiment, the number of adsorption sites filled by the solute may be quantified by the ratio X M, as mentioned previously. The greater the concentration of the solute in solution, the greater this ratio will be. For a given type of solute and type of carbon adsorbent, there will be a characteristic one maximum value for this ratio. Call this (XIM)ult. Now, we have two ratios: XI M, which is the ratio at any time and (XIM)ult, which is the greatest possible ratio. The difference of these two ratios is proportional to the number of adsorption sites left vacant; consequently, the rate of adsorption rs is therefore equal to k,s[C][(XJ M)ult - (XIM)], where ks is a proportionality constant.

For the desorption process, as the ratio (XIM) forms on the adsorbent, it must become a driving force for desorption. Thus, letting kd be the desorption proportionality constant, rd = kd(XIM), where rd is the rate of desorption. The process is in equilibrium, so the rate of adsorption is equal to the rate of desorption. Therefore, ks[ C ]

Solving for XIM produces Equation (8.24), where a = (XIM)ult and b = ksIkd. Note that in the derivation no mention is made of how many layers of molecules are sorbed onto or desorbed from the activated carbon. It is simply that solutes are sorbed and desorbed, irrespective of the counts of the layers of molecules; however, it is conceivable that as the molecules are deposited and removed, the process occurs layer by layer.

The straight-line forms of the Freundlich and Langmuir isotherms are, respectively, X1

Because the equations are for straight lines, only two pairs of values of the respective parameters are required to solve the constants. For the Freundlich isotherm, the required pairs of values are the parameters ln (XIM) and ln [C]; for the Langmuir isotherm, the required pairs are the parameters [C] (X M) and [C].

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

The value of X is obtained as follows: Let [C0] be the concentration of solute in a sample of volume V before adsorption onto a mass of adsorbent M. Then

8.3.3 Determination of the Freundlich Constants

Using the techniques of analytic geometry, let us derive the Freundlich constants in a little more detail than used in the derivation of the constants in the discussion of reverse osmosis treated previously. As mentioned, the straight-line form of the equation requires only two experimental data points; however, experiments are normally conducted to produce not just two pair of values but more. Thus, the experimental results must be reduced to just the two pairs of values required for the determination of the parameters; therefore, assuming there are m pairs of values, these m pairs must be reduced to just two pairs. Once the reduction to two pairs has been done, the isotherm equation may be then be written to just the two pairs of derived values as follows:

X in M = X ink + 1X in[ C ] = Unk + \ X k [ C ] for the first pair (8.29)

X in — = X ink + "X in [C] = (m - i) ink + -X in [ C] for the second pair

The index l is the number of data points for the first group. These equations may then be solved for n and k producing lSr+il«[C] - (m -1)Ylmln[C]

Xj in M-n X (in [ C ] -----------------------i-----------------------

8.3.4 Determination of the Langmuir Constants

As was done with the Freundlich constants, the Langmuir equation may be manipulated in order to solve the Langmuir constants. From the m pairs of experimental data

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Responses

  • Maximilian
    Why chemical adsorption latter also called activated adsorption?
    2 years ago

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