Adsorption

Sorption processes are very important to the fate and transport of contaminants in the environment and for the removal of contaminants in engineered reactors. Sorption is most often defined as the concentration or movement of contaminants from one phase to another. Absorption involves the partitioning of a contaminant from one phase into another phase. Examples include the dissolution (absorption) of oxygen gas into water and the absorption of the pesticide DDT into the organic solvent hexane. Adsorption is the process by which ions or molecules present in one phase tend to condense and concentrate on the surface of another phase. Additional discussion of sorption processes is given in Sec. 4.9, Sec. 5.34, and Chap. 7. Adsorption is discussed in detail in this section.

Adsorption of contaminants present in air or water onto activated carbon is frequently used for purification of the air or water. The material being concentrated is the adsórbate, and the adsorbing solid is termed the adsorbent. There are three general types of adsorption, physical, chemical, and exchange adsorption. Physical adsorption is relatively nonspecific and is due to the operation of weak forces of attraction or van der Waals' forces between molecules. Here, the adsorbed molecule is not affixed to a particular site on the solid surface but is free to move about over the surface. In addition, the adsorbed material may condense and form several superimposed layers on the surface of the adsorbent. Physical adsorption is generally quite reversible; i.e., with a decrease in concentration the material is desorbed to the same extent that it was originally adsorbed.

Chemical adsorption (sometimes called chemisorption), on the other hand, is the result of much stronger forces, comparable with those leading to the formation of chemical compounds. Normally the adsorbed material forms a layer over the surface which is only one molecule thick, and the molecules are not considered free to move from one surface site to another. When the surface is covered by the monomolecular layer, the capacity of the adsorbent is essentially exhausted. Also, chemical adsorption is seldom reversible. The adsorbent must generally be heated to higher temperatures to remove the adsorbed materials.

Exchange adsorption is used to describe adsorption characterized by electrical attraction between the adsorbate and the surface. Ion exchange is included in this class. Here, ions of a substance concentrate at the surface as a result of electrostatic attraction to sites of opposite charge on the surface. In general, ions with greater charge, such as trivalent ions, are attracted more strongly toward a site of opposite charge than are molecules with lesser charge, such as monovalent ions. Also, the smaller the size of the ion (hydrated radius), the greater the attraction. Although there are significant differences among the three types of adsorption, there are instances in which it is difficult to assign a given adsorption to a single type.

Since adsorption is a surface phenomenon, the rate and extent of adsorption are functions of the surface area of the solids used. Activated carbon is used extensively for adsorptive purposes because of its tremendous surface area in relation to mass. It is generally made from a wood product or coal by heating to temperatures between 300 and 1000°C in one of a variety of possible gaseous atmospheres such as C02, air, or water vapor, and then quickly quenching in air or water. The interior of the wood cells is cleaned out by this procedure, leaving a structure with remarkably small and uniform pores. Surface areas in the range of 1000 m2 per gram of activated carbon result, with pore sizes in the general range of 10 to 1000 angstroms (A) in diameter. At a given temperature and pressure, a sample of activated carbon will adsorb a definite quantity of a gas. If the pressure is increased, it will adsorb more; if the pressure is decreased, it will adsorb less. If the quantities of adsorbed gas are plotted against pressure, curves of the sort shown in Fig. 3.12 are obtained.

Adsorption of solutes from solution follow the same general laws as gases. This is illustrated in Fig. 3.13, which shows data for the adsorption of acetic and benzoic acids. The curves are of the same nature as those shown in Fig. 3.12. From

Pressure, atm

Figure 3.12

Adsorption of gases on charcoal in relation to pressure at constant temperature.

Pressure, atm

Figure 3.12

Adsorption of gases on charcoal in relation to pressure at constant temperature.

Molal concentration

Figure 3.13

Adsorption of solutes on charcoal; temperature and pressure constant.

0.15

Molal concentration

Figure 3.13

Adsorption of solutes on charcoal; temperature and pressure constant.

these data, it may be concluded that the quantity of substance adsorbed by a given sample-of adsorbent depends upon the nature of the material and its concentration. Temperature is also a factor which is not demonstrated by the data presented.

Adsorption Isotherms

An adsorption isotherm is a quantitative relationship describing the equilibrium between the concentration of adsórbate in solution (mass/volume) and its sorbed concentration (mass adsorbate/mass adsorbent). The term isotherm is used to signify that the relationship is for a given temperature. Four commonly used isotherms are linear, Langmuir, Freundlich, and BET. Which isotherm should be used depends on a variety of factors such as situation (e.g., engineered reactor versus natural environment); nature, concentration, and number of adsorbates (e.g., hydrophobic versus hydrophilic, organic compound versus metal, neutral versus charged species, high versus low concentration, single contaminant versus multiple contaminants); type of adsorbent [e.g., granular activated carbon (GAC) versus ion exchange resin versus iron oxide minerals versus aquifer material]; type of fluid (e.g., gas versus water versus organic solvent); and other environmental factors (e.g., pH, ionic strength). The linear isotherm is a limited, special case of the Freundlich isotherm. It is often used to describe soiption of organic chemicals'in the natural environment and is discussed in more detail in Sec. 5.34.

Langmuir Isotherm This isotherm assumes that a single adsórbate binds to a single site on the adsorbent and that all surface sites on the adsorbent have the same affinity for the adsórbate. Surface complexation theory7 can be used to develop the Langmuir isotherm:

tM, M. Benjamin, "Water Chemistry," McGraw-Hiîî, New York, 2002.

where q — (some texts use F) sorbed concentration (mass adsorbate/mass adsorbent) (sometimes called adsorption density) qm = maximum capacity of adsorbent for adsorbate (mass adsorbate/mass adsorbent)

C = aqueous concentration of adsorbate (mass/volume) JCjds = measure of affinity of adsorbate for adsorbent

As C gets larger and larger, adsorption sites become filled and q approaches qm. Evaluation of the coefficients qm and K^ can be obtained using the linearized form of Eq. (3.82) as shown in Fig. 3.14:

The Langmuir isotherm can be modified to account for competitive adsorption by more than one adsorbate and for adsorbents that have sites with different affinities for a given adsórbate.8'®

Freundlich Isotherm Freundlich studied the adsorption phenomenon extensively and showed that adsorption from solutions can be expressed by the following equation:

The Freundlich isotherm can be derived from the Langmuir isotherm by assuming that there exists a distribution of sites on the adsorbent that have different affinities for different adsorbates with each site behaving according to the Langmuir isotherm.10 Here, K is a measure of the capacity of the adsorbent (mass adsorbate/mass adsorbent) and n is a measure of how affinity for the adsorbate changes with changes in adsorption density. When n — 1, the Freundlich isotherm becomes a linear isotherm and indicates that all sites on the adsorbent have equal affinity for the adsorbate(s). Values of n > 1 indicate that affinities decrease with increasing adsorption density. Evaluation of the coefficients K and n can be accomplished using the linearized form of Eq. (3.84) (see Fig. 3.15):

Evaluation of coefficients for both Langmuir and Freundlich isotherms can be done using the same experimental data (see Example 3.16). Typically, different masses of adsorbent are added to a solution containing the adsorbate(s) of interest. These solutions are mixed and allowed to come to equilibrium (times may range from an hour to tens of hours). The concentration of adsorbate(s) remaining is measured. By knowing the initial concentration, q (mass adsorbate removed/mass adsorbent) can albid.

®Weber and DiGiano, "Process Dynamics in Environmental Systems." "'Benjamin, "Water Chemistry."

Figure 3.14

Straight-line form of the Langmuir isotherm.

Figure 3.14

Straight-line form of the Langmuir isotherm.

be calculated. Equations (3.83) and (3.85) can then be used to determine values of qmt Ksäs, K, and n using techniques such as least-squares linear regression. Statistical analysis of these regressions should allow determination of which isotherm works best for a given situation. Details of such analyses are given in Chap. 10.

BET Isotherm The BET isotherm was developed by Brunauer, Emmett, and Teller as an extension of the Langmuir isotherm to account for multilayer adsorption (adsorption of multiple layers of adsórbate). This model assumes that a number

Log concentration

Figure 3.15

Logarithmic plot of adsorption data.

Log concentration

Figure 3.15

Logarithmic plot of adsorption data.

of layers of adsórbate accumulate at the surface and that the Langmuir isotherm applies to each layer, It is somewhat more complex than the Langmuir isotherm and takes the form

The value C, represents the saturation concentrations for the adsórbate in solution. Of course, when C exceeds Cs, the solute precipitates or condenses from solution as a solid or liquid and concentrates on the surface. The BET equation can be put into the form

With this equation, Cs and b can be obtained from the slope and intercept of the straight line best fitting of the plot of the left side of Eq. (3.87) versus C!CS. The shape of the BET isotherm and its straight line form are shown in Fig. 3.16.

The adsorption isotherms are equilibrium equations and apply to conditions resulting after the adsorbate-containing phase has been in contact with the adsorbent for sufficient time to reach equilibrium. However, in any practical.process for the removal of a contaminant from a gas or liquid, the rate at which the material is adsorbed onto the solid becomes an important consideration. Essentially three steps can be identified in the removal of a contaminant by adsorption. First, it must move from the liquid or gaseous phase through a boundary layer in the fluid to the exterior of the adsorbent. Next, it must pass by diffusion into and through the pores of the adsorbent. Finally, it must become attached to the adsorbent. If the phase containing the adsorbent is quiescent, then diffusion through the boundary layer may be the slowest and rate-determining step. In this case, if the fluid is agitated,

Figure 3.16

Plots of the BET isotherm and its straight-line form.

Figure 3.16

Plots of the BET isotherm and its straight-line form.

the thickness of the boundary layer becomes reduced and the rate of adsorption will increase. At increased turbulence, however, a point will be reached where diffusion through the pores becomes the slowest step so that further increased turbulence will not result in greater rates of adsorption. Thus, depending upon the general characteristics of the material being adsorbed and the relative rates of diffusion through the boundary layer and into the pores, increased agitation of the fluid containing the material may or may not increase the rate of adsoiption.

For adsorption, say, of a contaminant in water onto activated carbon, both the rate and the extent of adsorption are dependent upon the characteristics of the molecule being adsorbed and of the adsorbent. The extent of adsorption is governed to some extent by the degree of solubility of the substance in water. The less soluble the material, the more likely it is to become adsorbed. With molecules containing both hydrophilic (water liking) and hydrophobic (water disliking) groups, the hydrophobic end of the molecule will tend to become attached to the surface. Next, the relative affinity of the material for the surface is a factor as already discussed in relation to the three general types of adsorption. Finally, the size of the molecule is of significance, as this affects its ability to fit within the pores of the adsorbent, and its rate of diffusion to a surface. Except for exchange adsorption, ions tend-to be less readily adsorbed than neutral species. Many or-ganics form negative ions at high pH, positive ions at low pH, and neutral species at intermediate pH ranges. Generally, adsorption is increased at pH ranges where the species at neutral in charge. In addition, pH affects the charge on the surface, altering its ability to adsorb materials. In water or wastewater samples, there are many different materials, each with different adsorption properties. Each competes in some way with the adsorption of the others. For this reason a given material may adsorb to a much less extent in a mixture of materials than if it were the only material in the solution.

One of the most important uses of adsorption in environmental practice has been for the removal of organic materials from waters, wastewaters, and air. Examples include removal of taste- and odor-producing organic materials and other trace organic contaminants such as trihalomethanes, pesticides, and chlorinated organic compounds from drinking waters and air, removal of residual organic contaminants from treated wastewater effluents, and treatment of leachates, industrial wastewaters, and hazardous wastes. Activated carbon, granular or powdered, may be mixed with the water and then removed along with the adsorbed materials by settling or filtration. When large quantities of organic material must be removed, more efficient usage of carbon and a higher quality of water can be obtained by passing the water through a carbon filter bed of large depth. This is a practical application of the principles expressed by the adsorption isotherms and, in some ways, can be compared to countercurrent extraction. Contaminants in air are removed by the passage of air through a bed of activated carbon. Use of such systems to manage a wide variety of contamination problems is now quite common. Development of special activated carbons that can be regenerated efficiently has improved process economics considerably.

EXAMPLE 3.16 [ .Granular activated carbon (GAC) was, tested for its lability to .remove soluble; organic lii-;^ ' trogeri (SON) from treated wastewater. Different masses of GAC were added to 1 liter of the wastewater (initial SON concentration 0.9 mg/L) and contacted for 2 h at 20°C .and .

0.14

---——r—--. , ;,..,,• ... -,:V... V ivviftV:

.'" —-——— ———" ~ : •.•••• - mg SON removed

'.••" 10.0 ■ 0.90 ., ; 0.14 : r:;:; - ,, 0.076

To check the Freundlich isotherm;

; need to plot log q versus log C. To check the Langmuir isotherm:

we need to plot Mq versus 1/C.

.-----1--:---■■■;-••■ .■•■;-•;■ . ; \i ■

s-SESSl

.-----1--:---■■■;-••■ .■•■;-•;■ . ; \i ■

. ¡. .0.650 '

0.77

r 1.54

1.30

-0.187

0.500 *

0:65

• 2.00 •

1.54

' -0.301

. : 0.290

0.32

: ; 3.45 '

3.13

-0.538

0.142 .

0.19

7.04 •

5.26

-0.848 ■

0.076 ■

0.14

13.2

7.14

— 1.12

0.041

0.09

24.4

11.1

-1.39

. 0.017

0.06

58.8

16.7

-1.77

chapter 3 Basic Concepts from Physical Chemistry

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