Chapter 14. Adsorption of Organic Compounds by Activated Carbon

14.2



CHAPTER Fourteen



Adsorption of Organic

Matter by Other

Adsorbents.................................. 14.87

Synthetic Adsorbent Resins............ 14.87

Carbonaceous Resins. ................... 14.88

.

Activated Carbon Fibers................ 14.88



High-Silica Zeolites .......................

Abbreviations...............................

Notation for Equations...........

References.....................................



14.89

14.89

14.91

14.91



Adsorption Overview

Adsorption of a substance involves its accumulation at the interface between two phases,

such as a gas and a solid or, more germane to this chapter, a liquid and a solid. The

molecule that accumulates, or adsorbs, at the interface is called the adsorbate, and

the solid on which adsorption occurs is the adsorbent. Adsorbents of interest in water

treatment include activated carbon; synthetic and carbonaceous adsorbent resins; ion

exchange resins; metal oxides, hydroxides, and carbonates; activated alumina; zeolites;

clays; and other solids that are suspended in or in contact with water. Adsorbates of

interest in water treatment include both organic and inorganic compounds of natural and

anthropogenic origins. The focus of this chapter is the adsorption of organic compounds

by activated carbon.

Dissolved organic matter (DOM) in drinking water sources can be divided into several

categories. For purposes of this chapter, we will use the categories shown in Fig. 14-1.

DOM contains specific organic compounds of both natural and synthetic origin. When these

compounds become of health or aesthetic concern to utilities and their consumers, they can be

referred to as micropollutants. Naturally occurring micropollutants include microbial metabolites that cause taste and odor (e.g., geosmin) or are toxic (e.g., microcystins). There are

also anthropogenic micropollutants, which are termed synthetic organic compounds (SOCs)

for this chapter. SOCs include pesticides, solvents, and pharmaceuticals and personal-care



Dissolved

Organic Matter

(DOM)



Natural Organic

Matter



Effluent Organic

Matter



(NOM)



(EfOM)



Humic substances

- Fulvic acids

- Humic acids



Non humic

substances



Micropollutants



Synthetic

organic

compounds



Figure 14-1  Categories of dissolved organic matter (DOM).



Taste & odor

compounds,

cyanotoxins







ADSORPTION OF ORGANIC COMPOUNDS BY ACTIVATED CARBON



14.3



products (PPCPs). Two other components of DOM are natural organic matter (NOM) and

wastewater treatment plant effluent organic matter (EfOM). NOM is a complex mixture

of ill-defined organic compounds, such as fulvic and humic acids, hydrophilic acids, and

specific compounds such as carbohydrates and proteins, all of natural origin (see Chap. 3

for coverage of NOM). EfOM includes background NOM, compounds from human activity, and metabolites from biological wastewater treatment.

Adsorption plays an important role in the improvement of water quality. Activated carbon, for example, is used most often to adsorb specific organic compounds that cause

taste and odor or are of health concern, as well as NOM, which can cause color and can

react with chlorine to form disinfection by-products (DBPs). Apart from activated carbon

adsorption, a number of other adsorption processes are important in water treatment. The

aluminum and ferric hydroxide solids that form during coagulation and the calcium carbonate and magnesium hydroxide solids that form in the lime softening process also adsorb

NOM. Pesticides adsorbed on clay particles can be removed by coagulation and filtration.

Ion exchange (IX) resins and synthetic and carbonaceous adsorbent resins are available

that can be used for efficient removal of selected organic and inorganic compounds, as

well as NOM. Removal of NOM by the coagulation process (see Chap. 8) and of organic

and inorganic ions by IX resins and activated alumina (see Chap. 12) are also discussed in

this book.

Activated carbon can be used as powdered activated carbon (PAC), which is added

directly to the water to be treated, typically as a slurry at the rapid mixer, or as granular

activated carbon (GAC), which is used in fixed-bed contactors. Historically, PAC has

been favored for use with seasonal taste and odor problems because of its low capital

cost and flexibility. However, when high PAC doses are required for long periods of

time, GAC is an economical alternative to PAC. GAC is used in rapid media filters (filter

adsorbers), where both adsorption and particle filtration can occur, in stand-alone units

after filtration (postfilter adsorbers), or for groundwater treatment. The use of fixed GAC

beds permits higher adsorptive capacities to be achieved and easier process control than

is possible with PAC. The higher capital cost for GAC often can be offset by better

efficiency, especially when the targeted adsorbate(s) must be removed on a continuous

basis. GAC should be seriously considered for water supplies when odorous compounds

or organic chemicals of health concern frequently are present, when a barrier is needed

against organic compounds from accidental (or deliberate) spills, or in some situations

that require DBP precursor removal. While GAC has excellent adsorption capacity for

many undesirable substances, it must be removed periodically from the adsorber bed and

replaced with fresh or reactivated GAC.

A 1977 study conducted by two committees of the American Water Works Association

(AWWA) showed that approximately 25 percent of 645 U.S. utilities, including the 500 largest,

added PAC (AWWA Committee Report, 1977). In 1984, 29 percent of the 600 largest

utilities reported using PAC (AWWA, 1986), predominantly for odor control. According

to the AWWA Water Stats 1996 database, PAC was available for use by 48 percent of the

543 responding surface water systems (AWWA, 1996). The U.S. Environmental Protection

Agency (USEPA) Community Water System Survey 2000 (USEPA, 2002) surveyed 1246

community water systems, and PAC was used in 7.6 percent of the 620 surveyed surface

water systems. Among surface water utilities serving more than 10,000 customers, more

than 17 percent of the responding utilities used PAC (USEPA, 2002).

The number of drinking water plants using GAC increased from 65 in 1977 (AWWA

Committee Report, 1977), principally for odor control, to 135 in 1986 (Snoeyink, 1990);

in 1996, there were approximately 300 plants treating surface water and several hundred

more treating contaminated groundwater (Snoeyink and Summers, 1999). Based on the

AWWA Water Stats 1996 database, GAC was being used by 12 percent of the 543 responding surface water systems, with another 4 percent listing it in the planned stage, and

5.3 percent of the 493 responding groundwater systems used GAC (AWWA, 1996).



14.4



CHAPTER Fourteen



In the USEPA Community Water System Survey 2000, approximately 14 percent of

surface water systems serving more than 3300 people used GAC. Among groundwater

systems, 8.6 percent of surveyed utilities serving between 100,000 and 500,000 people

employed GAC, but GAC use was less than 2 percent for groundwater systems in other

population categories (USEPA, 2002).



Adsorbent Characteristics

Activated Carbon

Production.  A wide variety of raw materials can be used to make activated carbon (Hassler,

1974), both PAC and GAC, and the substances used for drinking water treatment carbons

predominantly are subbituminous coal, lignite, coconut, and wood. Both the physical and

chemical manufacturing processes involve carbonization, or conversion of the raw material

to a char, and activation or oxidation to develop the internal pore structure. With physical

activation, carbonization or pyrolysis is usually done in the absence of air at temperatures less

than 700°C, whereas activation is carried out with oxidizing gases such as steam and carbon

dioxide (CO2) at temperatures of 800 to 900°C. Chemical activation combines carbonization

and activation steps by mixing a chemical activating agent, such as phosphoric acid, with the

base material and heating the mixture in the absence of oxygen. Patents describing carbonization and activation procedures are discussed by Yehaskel (1978). Both the base material and

manufacturing process affect activated carbon characteristics and performance.

Activated carbons are manufactured by either direct activation or a reagglomeration process. Direct activation means that the raw material (e.g., coal) is fed to the furnace as mined,

with only some crushing and screening prior to the activation process. The reagglomeration

process calls for crushing the raw material, adding some volatile material (such as coal tar

pitch), reagglomerating the mixture under high pressure (typically in a pocket briquettor),

crushing the material once again, and then activating it.

Physical Characteristics.  Activated carbon is a highly porous material with an internal

surface area that typically ranges from about 800 to 1500 m2/g (Bansal et al., 1988).

Typically, the manufacturer provides data that include the BET surface area. This parameter is determined by measuring the adsorption isotherm for nitrogen (N2) gas molecules

and then analyzing the data using the Brunauer‑Emmett‑Teller (BET) isotherm equation

(Adamson and Gast, 1997) to determine the amount of N2 to form a complete monolayer

of N2 molecules on the carbon surface. Multiplying the surface area occupied per N2

molecule (0.162 nm2 per molecule of N2) by the number of molecules in the monolayer

yields the BET surface area. Not all the BET surface area is accessible to aqueous adsorbates; because N2 is a small molecule, it can enter pores that are unavailable to larger

adsorbates.

The pores that give rise to this large surface area can be envisioned as spaces between

irregularly arranged graphite-like platelets (Leon y Leon et al., 1992) or condensed polyaromatic sheets (Bansal et al., 1988) that are the building blocks of activated carbon. To

classify pores according to size, the International Union of Pure and Applied Chemistry

(IUPAC) differentiates between (1) micropores (<2 nm width), (2) mesopores (2–50 nm

width), and (3) macropores (>50 nm width) (Sing et al., 1985). PAC and GAC for water

treatment typically exhibit a heterogeneous pore structure, in which micropores, mesopores, and macropores are all present. Activated carbon pore size distributions in the micropores and mesopores are calculated using gas adsorption data, whereas mercury porosimetry

data are used to calculate pore size distributions in the macropores.



14.5



ADSORPTION OF ORGANIC COMPOUNDS BY ACTIVATED CARBON







Table 14-1  Physical Characteristics of Representative Activated Carbons and a

Carbonaceous Resin



Adsorbent

F400 (bituminous coal based)

HD4000 (lignite based)

CC-602 (coconut shell based)

Picazine (wood based)

Ambersorb 563 (carbonaceous resin)



BET

surface area

(m2/g)

940

525

1160

1680

550



Micropore

volumea

(cm3/g)



Mesopore

volumeb

(cm3/g)



0.340

0.148

0.437

0.496

0.201



0.160

0.430

0.060

0.655

0.318



a



Micropore volume calculated by density functional theory (DFT) for pores with widths less than 2 nm.

Mesopore volume calculated by Barrett, Joyner, and Halenda (BJH) method for pores with widths

ranging from 2 to 50 nm.

Sources:  Data from Knappe et al. (2007) and Mezzari (2006).

b



BET surface areas, micropore volumes, and mesopore volumes for representative activated carbons are shown in Table 14-1. For reference, data for a carbonaceous resin are

also shown.

The particle shape of crushed GAC is irregular, but extruded GACs have a smooth, cylindrical shape. Particle shape affects the filtration and backwash properties of GAC beds.

Particle size is an important parameter because of its effect on adsorption kinetics and

breakthrough, as well as filtration performance. Particle size distribution refers to the relative amounts of different-sized particles that are part of a given sample, or lot, of carbon.

Commonly used GAC sizes include 12 × 40 and 8 × 30 U.S. Standard Mesh, which range in

apparent diameter from 1.68 to 0.42 mm and 2.38 to 0.59 mm, respectively, and have average

diameters of about 1.0 and 1.5 mm, respectively. Customized particle size distributions are

available to meet site-specific needs. The uniformity coefficient (see Chap. 10) is often quite

large, typically on the order of 1.9, to promote stratification during backwashing. Usually,

commercially available activated carbons have a small percentage of material smaller than

the smallest sieve and larger than the largest sieve, which significantly affects the uniformity

coefficient. Extruded carbon particles all have the same diameter, but they vary in length.

The apparent density* is the mass of nonstratified dry activated carbon per unit volume

of activated carbon, including the volume of voids between grains. Typical values for

GACs manufactured from bituminous coal, lignite, and coconut shells are 350 to 650 kg/m3

(22–41 lb/ft3), whereas typical values for wood based GAC are in the range of 225 to

300 kg/m3 (14–19 lb/ft3). Distinguishing between the apparent density and the bed density,

backwashed and drained (i.e., stratified, free of water) is important for GAC. The former

is a characteristic of GAC as shipped. The latter is a characteristic of the GAC as placed in

a bed and is about 10 percent less than the apparent density. The bed density is typical of

GAC during normal operation unless the GAC becomes destratified during backwashing.

The bed density determines how much GAC must be purchased to fill a contactor of a given

size and therefore is an important GAC characteristic.

The particle density wetted in water is the mass of solid activated carbon plus the mass

of water required to fill the internal pores per unit volume of particle. Its value for GAC

typically ranges from 1300 to 1500 kg/m3 (81–94 lb/ft3), and it determines the extent of

fluidization and expansion of a given size particle during backwash.

*

This definition is based on ASTM Standard D2854-09 (ASTM, 2009). It conflicts with ASTM Standard C128-07a

for apparent specific gravity, as used in Chap. 10, which does not include the volume of interparticle voids.



14.6



CHAPTER Fourteen



Particle hardness is important because it affects the amount of attrition during backwash, transport, and reactivation. In general, the harder the activated carbon is, the less is

the attrition for a given amount of friction or impact between particles. Activated carbon

hardness generally is characterized by an experimentally determined hardness or abrasion

number using a test such as the American Society for Testing and Materials (ASTM) ball

pan hardness test (ASTM Standard D3802), which measures the resistance to particle

degradation on agitating a mixture of activated carbon and steel balls (ASTM, 2009). The

relationship between the amount of attrition that can be expected when activated carbon

is handled in a certain way and the hardness number has not been studied extensively,

however. Comparing wood based GAC with an abrasion number of 47 and bituminous

coal based GAC with an abrasion number of 75, Grens and Werth (2001) showed that

filter performance and changes in GAC bed height were similar during 500 air-water

backwash cycles.

The primary difference between PAC and GAC is particle size. As specified by AWWA

Standard B600-05 (AWWA, 2005a), not less than 90 percent by mass of PAC shall pass

a 44-mm (325 U.S. Standard Mesh) sieve unless the activated carbon is wood based. For

wood based PACs, not less than 60 percent by mass shall pass a 44-mm sieve. The particle

size distribution is important because the smaller PAC particles adsorb organic compounds

more rapidly than large PAC particles (Najm et al., 1990; Adham et al., 1991). The apparent density of PAC ranges from 200 to 750 kg/m3 (12–47 lb/ft3) and depends on the base

material and the manufacturing process.

Chemical Characteristics.  At the edges of the condensed polyaromatic sheets that

constitute the building blocks of activated carbons, heteroatoms (i.e., atoms other than

carbon) are encountered that define the chemical characteristics of activated carbon surfaces. A typical elemental composition of activated carbon is approximately 88 percent

C, 6 to 7 percent O, 1 percent S, 0.5 percent N, and 0.5 percent H, with the remainder

being mineral matter (i.e., ash) (Bansal et al., 1988). However, the elemental composition

of activated carbons can vary substantially from these average values; for example, the

oxygen content can range from as low as 1 percent to as high as 25 percent (Bansal et al.,

1988), and the ash content can range from 1 to 20 percent (Jankowska et al., 1991).

Because of its abundance and profound effects on activated carbon hydrophilicity and

surface charge, oxygen is generally the most important heteroatom from a standpoint of

activated carbon surface chemistry. Oxygen commonly occurs in the form of carboxylic acid

groups (–COOH), phenolic hydroxyl groups (–OH), and quinone carbonyl groups (>C=O)

(Boehm et al., 1964; Puri, 1970; Mattson and Mark, 1971; Snoeyink and Weber, 1972; Leon

y Leon and Radovic, 1992; Boehm, 1994). Activated carbons assume an acidic character

when exposed to oxygen at between 200°C and 700°C or to oxidants such as hydrogen peroxide (e.g., Puri, 1970, 1983). The activated carbon acidity is explained primarily by the formation of carboxylic acid and phenolic hydroxyl groups (Leon y Leon and Radovic, 1992).

Oxidation of activated carbon surfaces also occurs during the exposure of activated carbon to

common oxidants used in water treatment, such as chlorine, permanganate, and ozone.

Adsorption Properties.  Both physical and chemical characteristics of activated carbon

affect its performance. Pore size distribution is one important characteristic of an activated

carbon. The size of adsorbent pores affects the adsorption of organic contaminants in two

important ways. First, size exclusion limits the adsorption of contaminants of a given size

and shape if pores are too small. Second, the strength of adsorbate-adsorbent interactions

increases with decreasing pore size because adsorption potentials between opposing pore

walls begin to overlap once the micropore width is less than twice the adsorbate diameter

(e.g., Dubinin, 1960; Sing, 1995). Therefore, adsorption of micropollutants preferentially

takes place in the smallest pores that are accessible to a given pollutant. As outlined in







ADSORPTION OF ORGANIC COMPOUNDS BY ACTIVATED CARBON



14.7



more detail below, the adsorption of micropollutants and NOM occurs primarily in pores

with dimensions that match those of the targeted adsorbate. Thus many micropollutants

adsorb in small micropores (<1 nm), whereas the largest percentage of NOM preferentially

adsorbs in mesopores and large micropores (>1 nm)

With respect to surface chemistry, hydrophobic adsorbents (i.e., activated carbons with

low oxygen content) exhibit larger adsorption capacities for organic micropollutants than

hydrophilic adsorbents (i.e., activated carbons with high oxygen content) with similar physical characteristics (Knappe et al., 2003). This trend is principally attributable to enhanced

water adsorption on hydrophilic activated carbons (e.g., Kaneko et al., 1989, Li et al., 2002,

Quinlivan et al., 2005). More detailed information about surface chemistry effects on the

adsorption of micropollutants can be found in Knappe (2006).

A number of surrogate parameters are used to describe the adsorption capacity of activated carbon (Sontheimer et al., 1988). The iodine number (ASTM Standard D4607-94;

ASTM, 2009) measures the amount of iodine that will adsorb under a specified set of conditions, and it generally correlates well with the surface area available for small molecules.

The molasses number or decolorizing index is related to the ability of activated carbon to

adsorb large-molecular-weight color bodies from molasses solution and generally correlates well with the ability of the activated carbon to adsorb other large adsorbates. Other

surrogate parameters include the carbon tetrachloride activity, the methylene blue number,

the acetoxime number, the tannin value, and the phenol adsorption value.

Surrogate parameters give some insight into the possible performance of activated carbons in drinking water treatment. BET surface area and iodine number tend to correlate

well with adsorption capacity when high solid-phase concentrations can be reached, such

as in the treatment of highly contaminated waters or in solvent recovery operations (e.g.,

Manes, 1998). For the removal of micropollutants from drinking water sources; however,

activated carbon performance generally does not correlate well with BET surface area or

iodine number (e.g., Quinlivan et al., 2005). In all cases, data from bench- or pilot-scale

tests (e.g., isotherm tests, jar tests to evaluate PAC performance, and column tests to evaluate GAC performance) conducted with the specific compound(s) and water of interest are

much better indicators of field performance.

Other Adsorbents

Apart from PAC and GAC, a wide range of alternative adsorbents has been tested for their

effectiveness to remove organic compounds from water. Among the more frequently studied

alternative adsorbents are synthetic adsorbent resins, carbonaceous resins, activated carbon

fibers, and hydrophobic zeolites.

Synthetic Adsorbent Resins.  Synthetic adsorbent resins are polymeric beads with a large

internal surface area that is created during the polymerization process (Neely and Isacoff,

1982). Synthetic adsorbent resins, such as the styrene-divinylbenzene (SDVB) copolymer

resin and the phenol-formaldehyde (PF) resin, differ from IX resins because they lack

charged or ionizable functional groups. (See Chap. 12 for a discussion of the removal of

inorganic substances and NOM by IX resins.) In synthetic adsorbent resins, the degree of

cross‑linking between the polymers that constitute the matrix can be varied, and thus resins

with different pore size distributions can be prepared. In theory, pore size distributions can

be tailored to a specific target compound that needs to be removed from water.

Carbonaceous Resins.  Carbonaceous resins are prepared by heating macroporous polymer

beads in an inert atmosphere. This heat treatment, or pyrolysis, step creates micropores while

largely maintaining the macroporous structure of the starting polymer (Neely and Isacoff, 1982).



14.8



CHAPTER Fourteen



Thus the pore structure of carbonaceous resins can be tailored in a manner similar to that of

synthetic adsorbent resins. Furthermore, pyrolysis conditions can be chosen such that the carbonaceous adsorbents have molecular sieve properties (Neely and Isacoff, 1982). One commonly studied carbonaceous resin is prepared from macroporous SDVB beads, and properties

of a representative member (Ambersorb 563) are summarized in Table 14-1.

Activated Carbon Fibers.  Activated carbon fibers (ACFs) are manufactured from woven

and nonwoven materials such as cross-linked phenolic fibers, polyacrylonitrile, and cellulose (Economy and Lin, 1976; Bahl et al., 1998). ACFs are commonly prepared by pyrolysis and steam activation, and preparation conditions can be controlled such that uniform

pore size distributions are obtained (Kasaoka et al., 1989a; Daley et al., 1996). Examination

of ACFs by scanning tunneling microscopy showed that both mesopores and micropores

are present at the ACF surface; the mesopores were oriented parallel to the fiber axis and

did not penetrate the bulk fiber to a depth greater than about 60 nm. Because of their narrower pore size distribution, ACFs are particularly suitable to elucidate pore size effects on

organic compound adsorption from aqueous solution (Kasaoka et al., 1989b; Pelekani and

Snoeyink, 1999, 2000, 2001; Li et al., 2002; Karanfil et al., 2006).

Zeolites.  In water treatment, hydrophilic low-silica zeolites are well known as cation

exchangers. These zeolites are aluminosilicates with relatively low SiO2/Al2O3 ratios that

give rise to negative charges in the zeolite framework (Dyer, 1984; Townsend, 1984). In

contrast, hydrophobic zeolites with high SiO2/Al2O3 ratios show promise for the removal of

organic compounds from aqueous solution (e.g., Ellis and Korth, 1993; Kawai et al., 1994;

Rossner and Knappe, 2008). Because of their highly ordered lattice structure, zeolites are

adsorbents with well-defined pore sizes. For example, ZSM-5 zeolites have straight, elliptical channels with minor and major axis dimensions of 0.53 × 0.56 nm. The straight channels intersect perpendicularly with sinusoidal, elliptical channels with minor and major axis

dimensions of 0.51 × 0.55 nm. From a drinking water treatment perspective, high-silica

zeolites are attractive because pore sizes can be selected that permit the targeted adsorption

of smaller organic contaminants while preventing the adsorption of competing substances

with larger molecular sizes. High-silica zeolites are marketed in the form of powders and

extrudates.



Adsorption  Theory

Adsorption Equilibrium

Adsorption of molecules can be represented as a chemical reaction:





A + B  A:B



(14-1)



where A is the adsorbate, B the adsorbent, and A:B the adsorbed compound. Adsorbates are

held on the surface by intermolecular forces such as van der Waal’s forces, dipole‑dipole

interactions, and hydrogen bonds. If the reaction is reversible, as it is for many compounds

adsorbed to activated carbon, molecules continue to accumulate on the surface until the

rate of the forward reaction (adsorption) equals that of the reverse reaction (desorption).

When this condition exists, equilibrium has been reached, and no further accumulation

will occur.

Isotherm Equations.  One of the most important characteristics of an adsorbent is

the quantity of adsorbate that it can accumulate. The constant‑temperature equilibrium



ADSORPTION OF ORGANIC COMPOUNDS BY ACTIVATED CARBON







14.9



relationship between the quantity of adsorbate per unit of adsorbent q and its equilibrium aqueous-phase concentration C is called the adsorption isotherm. Several equations or models are available that describe this relationship (Sontheimer et al., 1988),

but only the commonly used Freundlich model is presented here.

The empirical Freundlich model is useful because it effectively describes adsorption

isotherm data for many organic contaminants. The Freundlich isotherm equation has

the form





q = KC 1/ n



(14-2)



1



log q = log K + log C

n



(14-3)



and can be linearized as follows:





The parameters q (with units of mass adsorbate/mass adsorbent or mole adsorbate/mass

adsorbent) and C (with units of mass/volume or moles/volume) are the equilibrium surface

and solution concentrations, respectively. The terms K and 1/n are constants for a given

system; 1/n is unitless, and the units of K are determined by the units of q and C. Although

the Freundlich equation was developed to empirically fit adsorption data, the Freundlich

equation also can be developed from adsorption theory (e.g., Halsey and Taylor, 1947;

Weber and DiGiano, 1996).

The parameter K in the Freundlich equation is related primarily to the adsorption capacity of the adsorbent for the adsorbate, and 1/n is a function of the adsorbent heterogeneity.

A 1/n value of 1 is obtained for homogeneous adsorbents (i.e., adsorbents with a uniform

pore size and surface chemistry); 1/n values typically are less than 1 for activated carbons,

which exhibit a broad distribution of adsorption site energies; much of the heterogeneity

results from the variety in pore sizes and shapes in activated carbons. For fixed values of C

and 1/n, the larger the value of K, the larger is the adsorption capacity q. Figure 14-2 depicts

an example isotherm; note that both the x- and y-axis scales are logarithmic, in which case

100



q, mg/g



y = 4.9152x0.5494

R2 = 0.998



10

K



1

0.1



1



10

C, µg/L



Figure 14-2  Example isotherm data and Freundlich isotherm model fit.



100



14.10



CHAPTER Fourteen



the Freundlich model yields a straight line. In Fig. 14-2, equilibrium solid-phase concentrations are presented in units of milligrams of adsorbed compound per gram of activated

carbon (mg/g) and equilibrium aqueous-phase concentrations in units of micrograms per

liter (mg/L). In this case, the parameter K represents the adsorption capacity (in mg/g) at an

equilibrium liquid-phase concentration of 1 mg/L. The parameter 1/n represents the slope

of the line; for the example isotherm in Fig. 14-2, log q increases by 0.549 for each unit

increase in log C.

The Freundlich equation cannot apply to all values of C, however. As C increases, for

example, q increases (in accordance with Eq. 14-2) only until all adsorbent pore surfaces

are occupied. At saturation, q is a constant, independent of further increases in C, and the

Freundlich equation no longer applies. Also, no assurance exists that adsorption data will

conform to the Freundlich equation over all concentrations less than saturation, so care must

be exercised in extending the equation to concentration ranges that have not been tested.

Tabulations of single-solute isotherm constants are useful when only rough estimates

of adsorption capacity are needed to determine whether a more intensive analysis of

the adsorption process is warranted. The Freundlich isotherm constants of Speth and

Miltner (1990, 1998) are reproduced in Table 14-2 for this purpose. Freundlich isotherm

Table 14-2  Freundlich Adsorption Isotherm Parameters for Organic Compounds

Compound

Cyanazine

Trimethoprim (pH 7.8)a

Metolachlor

2,4-Dinitrotoluene

Glyphosate

Alachlor

1,1,1-Trichloropropanone

1,3,5-Trichlorobenzene

Acifluorfen (pH 6.9)

Metribuzin

Hexachlorocyclopentadiene

2,4,5-Trichlorophenoxy acetic acid

Pentachlorophenol

Atrazine

p-Chlorotoluene

Dicamba

Simazine

Dinoseb

Chloropicrin

Sulfamethoxazole (pH 7.8)

Picloram

o-Chlorotoluene

o-Dichlorobenzene

Chloral hydrate

Bromobenzene

Carbofuran

Lindane

p-Xylene

Styrene



K (mg/g)(L/mg)1/n

102

98.9

98.2

96.1

87.6

81.7

74.4

63.8

60.2

48.7

43.0

43.0

42.6

38.7

35.9

33.1

31.3

30.4

30.2

29.4

23.4

23.2

19.3

18.9

17.2

16.4

15.0

12.6

12.2



1/n



Source



0.126

0.148

0.125

0.157

0.119

0.257

0.110

0.324

0.198

0.193

0.504

0.210

0.339

0.291

0.34

0.147

0.227

0.279

0.155

0.234

0.18

0.378

0.378

0.051

0.364

0.408

0.433

0.418

0.479



*

‡

*

*

*

*

†

*

†

*

*

*

*

*

*

*

*

*

†

‡

*

*

*

†

*

*

*

*

*

(Continued )



ADSORPTION OF ORGANIC COMPOUNDS BY ACTIVATED CARBON







14.11



Table 14-2  Freundlich Adsorption Isotherm Parameters for Organic Compounds (Continued )

Compound

Trichloroacetic acid (pH 5.6)

Diquat (pH 6.7)

Isophorone

Ethyl benzene

Chlorobenzene

Methyl isobutyl ketone

Aldicarb

Dibromochloropropane (DBCP)

2-Methylisoborneol

m-Dichlorobenzene

Toluene

p-Dichlorobenzene

m-Xylene

Dalapon

Methomyl

Tetrachlorethene (PCE)

1,1-Dichloropropene

Methyl ethyl ketone

Endothall (pH 7.1)

Trichloroethene (TCE)

Oxamyl

Dichloroacetic acid (pH 5.7)

Benzene

1,2,3-Trichloropropane

1,1,1,2-Tetrachlorethane

Bromoform

1,3-Dichloropropane

1,2 Dibromoethane

Ethylene thiourea

trans-1,2-Dichloroethene

Dibromochloromethane

1,1 Dichloroethene

Carbon tetrachloride

1,1,2-Trichloroethane

1,1,1-Trichloroethane

1,2-Dichloropropane

Bromodichloromethane

Methyl tertiary-butyl ether (MTBE)

cis-1,2-Dichloroethene

1,2-Dichloroethane

Chloroform

Dibromomethane

1,1-Dichloroethane

Methylene chloride

a



K (mg/g)(L/mg)1/n



1/n



Source



11.7

11.2

9.75

9.27

9.17

8.85

8.27

6.91

6.01

5.91

5.01

4.97

4.93

4.92

4.78

4.05

2.67

2.53

2.28

2.00

1.74

1.63

1.26

1.08

1.07

0.929

0.897

0.888

0.716

0.618

0.585

0.47

0.387

0.365

0.335

0.313

0.241

0.218

0.202

0.129

0.0925

0.0722

0.0646

0.00625



0.216

0.325

0.271

0.415

0.348

0.279

0.402

0.501

0.64

0.63

0.429

0.691

0.614

0.224

0.290

0.516

0.374

0.295

0.329

0.482

0.793

0.462

0.533

0.613

0.604

0.665

0.497

0.471

0.669

0.452

0.636

0.515

0.594

0.652

0.531

0.597

0.655

0.479

0.587

0.533

0.669

0.701

0.706

0.801



†

†

*

*

*

†

*

*



When available, pH values are specified for weak acids and bases and ionic compounds.

From Speth and Miltner (1990).

†

From Speth and Miltner (1998).

‡

From Rossner (2008).

§

From Chen et al. (1997).

*



§



†

*

*

†

*

†

*

*

†

†

*

*

†

*

*

*

*

*

*

†

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*



14.12



CHAPTER Fourteen



contants for 2-methylisoborneol (MIB), a taste and odor compound, and sulfamethoxazole and trimethoprim, two antimicrobial compounds, are also shown in Table 14-2.

The adsorption isotherm parameters in Table 14-2 were derived from adsorption isotherm experiments conducted with coal based activated carbons with BET surface areas

of approximately 1000 m2/g. Furthermore, adsorbent-adsorbate contact times in these

studies were sufficient to reach adsorption equilibrium (5 days for MIB, ≥3 weeks for all

other compounds), and the tested aqueous-phase concentrations in were most cases environmentally relevant. Additional isotherm parameters can be found in Sontheimer and

colleagues (1988). The parameters in Table 14-2 can be used to judge relative adsorption

efficiency. The K values of isotherms that have nearly the same values of 1/n show the

relative capacity of adsorption. For example, if a GAC column is satisfactorily removing

benzene [K = 1.26(mg/g)(L/mg)1/n and 1/n = 0.533], the removal of compounds with

larger values of K and approximately the same concentration likely will be better. (An

exception might occur if the organic compounds adsorb to particles that pass through the

adsorber.) If the 1/n values are much different, however, the capacity of activated carbon

for each compound of interest should be calculated at the equilibrium concentration of

interest using Eq. 14‑2 because the relative adsorbability will depend on the equilibrium

concentration.

Adsorption isotherms for representative micropollutants are shown in Fig. 14-3. Among

the depicted contaminants, the herbicides metolachlor and atrazine exhibit the largest

adsorption capacities, while MTBE exhibits the lowest. At an equilibrium concentration

of 1 mg/L, adsorption capacities for MIB and TCE are similar; however, at concentrations

that are relevant for MIB (tens of ng/L), the adsorption capacity of activated carbon is considerably lower. The use of isotherm values to estimate GAC adsorber life and PAC usage

rate is discussed in the sections “GAC Performance Estimation” and “PAC Adsorption,”

respectively.



1000

Metolachlor

Atrazine

100



MIB



q, mg/g



TCE

10



MTBE



1



0.1



0.01

0.001



0.01



0.1



1

C, µg/L



10



100



Figure 14-3  Adsorption isotherms for representative trace organic contaminants.



1000