Tài liệu ADSORPTION OF ORGANIC COMPOUNDS doc

CHAPTER 13
ADSORPTION OF ORGANIC
COMPOUNDS
Vernon L. Snoeyink, Ph.D.
Ivan Racheff Professor of Environmental Engineering
Department of Civil and Environmental Engineering
University of Illinois at Urbana-Champaign
Urbana, Illinois
R. Scott Summers, Ph.D.
Professor of Environmental Engineering
Civil, Environmental, and Architectural Engineering
University of Colorado
Boulder, Colorado
Adsorption of a substance involves its accumulation at the interface between two
phases, such as a liquid and a solid or a gas and a solid. The molecule that accumu-
lates, or adsorbs, at the interface is called an adsorbate, and the solid on which
adsorption occurs is the adsorbent. Adsorbents of interest in water treatment
include activated carbon; ion exchange resins; adsorbent resins; metal oxides,
hydroxides, and carbonates; activated alumina; clays; and other solids that are sus-
pended in or in contact with water.
Adsorption plays an important role in the improvement of water quality. Acti-
vated carbon, for example, can be used to adsorb specific organic molecules that
cause taste and odor, mutagenicity, and toxicity, as well as natural organic matter
(NOM) that causes color and that can react with chlorine to form disinfection by-
products (DBPs). NOM is a complex mixture of compounds such as fulvic and
humic acids, hydrophilic acids, and carbohydrates.The aluminum hydroxide and fer-
ric hydroxide solids that form during coagulation will also adsorb NOM.Adsorption
of NOM on anion exchange resins may reduce their capacity for anions (see Chap-
ter 9),but ion exchange resins and adsorbent resins are available that can be used for
efficient removal of selected organic compounds. Calcium carbonate and magne-
sium hydroxide solids formed in the lime softening process have some adsorption

capacity, and pesticides adsorbed on clay particles can be removed by coagulation
and filtration (Chapters 6 and 8).
The removal of organic compounds by adsorption on activated carbon is very
important in water purification and therefore is the primary focus of this chapter.A
13.1
study conducted by two committees of the AWWA showed that approximately 25
percent of 645 United States utilities, including the 500 largest, used powdered acti-
vated carbon (PAC) in 1977 (American Water Works Association, 1977). In 1986, 29
percent of the 600 largest utilities reported using PAC (American Water Works
Association, 1986), predominantly for odor control. More attention is being given
now to granular activated carbon (GAC) as an alternative to PAC. GAC is used in
columns or beds that permit higher adsorptive capacities to be achieved and easier
process control than is possible with PAC.The higher cost for GAC can often be off-
set by better efficiency, especially when organic matter must be removed on a con-
tinuous basis. GAC should be seriously considered for water supplies when odorous
compounds or synthetic organic chemicals of health concern are frequently present,
when a barrier is needed to prevent organic compounds from spills from entering
finished water, or in some situations that require DBP control. GAC has excellent
adsorption capacity for many undesirable substances and it can be removed from
the columns for reactivation when necessary. The number of drinking water plants
using GAC,principally for odor control, increased from 65 in 1977 (American Water
Works Association, 1977) to 135 in 1986 (Fisher, 1986); in 1996, there were approxi-
mately 300 plants treating surface water and several hundred more treating contam-
inated groundwater. The promulgated as well as proposed DBP regulations will
drive many utilities to consider GAC for removal of organic compounds in the next
10 years. GAC is also used as a support medium for bacteria in processes to biologi-
cally stabilize drinking water before distribution.
This chapter also covers the use of ion exchange and adsorbent resins for the
removal of organic compounds. Removal of inorganic ions by ion exchange resins
and activated alumina is discussed in Chapter 9.

ADSORPTION THEORY
Adsorption Equilibrium
Adsorption of molecules can be represented as a chemical reaction:
A + B ⇔ A⋅B
where A is the adsorbate, B is the adsorbent, and A⋅B is the adsorbed compound.
Adsorbates are held on the surface by various types of chemical forces such as
hydrogen bonds, dipole-dipole interactions, and van der Waals forces. If the reac-
tion 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 reac-
tion (adsorption) equals the rate 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 it can accumulate. The constant-temperature equilibrium
relationship between the quantity of adsorbate per unit of adsorbent q
e
and its equi-
librium solution concentration C
e
is called the adsorption isotherm. Several equa-
tions or models are available that describe this function (Sontheimer, Crittenden,
and Summers, 1988), but only the more common equations for single-solute adsorp-
tion, the Freundlich and the Langmuir equations, are presented here.
13.2 CHAPTER THIRTEEN
The Freundlich equation is an empirical equation that is very useful because it
accurately describes much adsorption data.This equation has the form
q
e
= KC
e

1/n
(13.1)
and can be linearized as follows:
log q
e
= log K + log C
e
(13.2)
The parameters q
e
(with units of mass adsorbate/mass adsorbent, or mole adsor-
bate/mass adsorbent) and C
e
(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
e
and C
e
. Although the Freundlich equation was developed to empiri-
cally fit adsorption data, a theory of adsorption that leads to the Freundlich equation
was later developed by Halsey and Taylor (1947).
The parameter K in the Freundlich equation is related primarily to the capacity
of the adsorbent for the adsorbate, and 1/n is a function of the strength of adsorp-
tion. For fixed values of C
e
and 1/n, the larger the value of K, the larger the capacity
q
e

. For fixed values of K and C
e
, the smaller the value of 1/n, the stronger is the
adsorption bond.As 1/n becomes very small, the capacity tends to be independent of
C
e
and the isotherm plot approaches the horizontal level; the value of q
e
then is
essentially constant, and the isotherm is termed irreversible. If the value of 1/n is
large, the adsorption bond is weak, and the value of q
e
changes markedly with small
changes in C
e
.
The Freundlich equation cannot apply to all values of C
e
, however. As C
e
increases, for example, q
e
increases (in accordance with Equation 13.1) only until the
adsorbent approaches saturation.At saturation, q
e
is a constant, independent of fur-
ther increases in C
e
, and the Freundlich equation no longer applies. Also, no assur-
ance exists that adsorption data will conform to the Freundlich equation over all

concentrations less than saturation, so care must be exercised in extending the equa-
tion to concentration ranges that have not been tested.
The Langmuir equation,
q
e
= (13.3)
where b and q
max
are constants and q
e
and C
e
are as defined earlier, has a firm theo-
retical basis (Langmuir, 1918).The constant q
max
corresponds to the surface concen-
tration at monolayer coverage and represents the maximum value of q
e
that can be
achieved as C
e
is increased.The constant b is related to the energy of adsorption and
increases as the strength of the adsorption bond increases. The Langmuir equation
often does not describe adsorption data as accurately as the Freundlich equation.
The experimentally determined values of q
max
and b often are not constant over the
concentration range of interest, possibly because of the heterogeneous nature of the
adsorbent surface (a homogeneous surface was assumed in the model develop-
ment), lateral interactions between adsorbed molecules (all interaction was

neglected in the model development), and other factors.
Factors Affecting Adsorption Equilibria. Important adsorbent characteristics
that affect isotherms include surface area, pore size distribution, and surface chem-
q
max
bC
e
ᎏ
1 + bC
e
1
ᎏ
n
ADSORPTION OF ORGANIC COMPOUNDS 13.3
13.4 CHAPTER THIRTEEN
FIGURE 13.1 Pore size distributions for different acti-
vated carbons. (Source: Lee, Snoeyink, and Crittenden,
1981.)
istry.The maximum amount of adsorption is proportional to the amount of surface
area within pores that is accessible to the adsorbate. Surface areas range from a few
hundred to more than 1500 m
2
/g, but not all of the area is accessible to aqueous
adsorbates.The range of pore size distributions in an arbitrary selection of GACs is
shown in Figure 13.1.A relatively large volume of micropores (pores less than 2 nm
diameter d) (Sontheimer, Crittenden,and Summers, 1988) generally corresponds to
a large surface area and a large adsorption capacity for small molecules, whereas a
large volume of mesopores (2 < d < 50 nm) and macropores (d > 50 nm) is usually
directly correlated to capacity for large molecules.The fulvic acid isotherms in Fig-
ure 13.2 are for the same activated carbons whose pore size distributions are shown

in Figure 13.1. Note that the activated carbons that have a relatively small volume
of macropores also have a relatively low capacity for the large fulvic acid molecule.
Lee et al. (1981) showed that the quantity of humic substances of a given size that
was adsorbed was correlated with pore volume within pores of a given size.The rel-
ative positions of the isotherms for the activated carbons in Figure 13.1 might be
entirely different than those in Figure 13.2 if the adsorbate were a small molecule,
such as a phenol, which can enter pores much smaller than those accessible to ful-
vic acid. Summers and Roberts (1988b) showed that if the amount adsorbed was
normalized for the available surface area, the differences in adsorption capacity of
different carbons for a humic acid could be attributed to the surface chemistry of
the carbon.
The surface chemistry of activated carbon and adsorbate properties also can
affect adsorption (Coughlin and Ezra, 1968; Gasser and Kipling, 1959; Kipling and
Shooter, 1966;Snoeyink and Weber,1972;Snoeyink et al., 1974). Several researchers
(Coughlin and Ezra, 1968; Gasser and Kipling, 1959; Kipling and Shooter, 1966)
demonstrated that extensive oxidation of carbon surfaces led to large decreases in
the amounts of phenol, nitrobenzene, benzene, and benzenesulfonate that could be
adsorbed. Oxidation of the activated carbon surface with aqueous chlorine was also
found to increase the number of oxygen surface functional groups and correspond-
ingly to decrease the adsorption capacity for phenol (Snoeyink et al., 1974). Thus,
oxygenating a carbon surface decreases its affinity for simple aromatic compounds.
The tendency of a molecule to adsorb is a function of its affinity for water as com-
pared to its affinity for the adsorbent. Adsorption onto GAC from water, for exam-
ple, generally increases as the adsorbate’s solubility decreases (Weber, 1972). As a
molecule becomes larger through the addition of hydrophobic groups such as
ᎏ
CH
2
ᎏ
, its solubility decreases and its extent of adsorption increases as long as

the molecule can gain entrance to the pores. When an increase in size causes the
molecule to be excluded from some pores, however, adsorption capacity may
decrease as solubility decreases. As molecular size increases, the rate of diffusion
within the activated carbon particle decreases, especially as molecular size
approaches the particle’s pore diameter.
The affinity of weak organic acids or bases for activated carbon is an important
function of pH. When pH is in a range at which the molecule is in the neutral form,
adsorption capacity is relatively high. When pH is in a range at which the species is
ionized, however, the affinity for water increases and activated carbon capacity
accordingly decreases. Phenol that has been adsorbed on activated carbon at pH
below 8, where phenol is neutral, can be desorbed if the pH is increased to 10 or
above, where the molecule is anionic (Fox, Keller, and Pinamont, 1973). If adsorp-
tion occurs on resins by means of the ion exchange mechanism, the specific affinity
of the ionic adsorbate for charged functional groups may also cause good removal.
The inorganic composition of water also can have an important effect on the
extent of NOM adsorption, as shown in Figure 13.3 for fulvic acids (Randtke and
Jepsen, 1982). After 70 days, a small GAC column was nearly saturated with fulvic
acid. Addition of CaCl
2
at this point resulted in a large increase in adsorbability of
fulvic acid, as reflected in the reduced column effluent concentration. After 140
days, elimination of the CaCl
2
resulted in desorption of much of the fulvic acid. Cal-
cium ion apparently associates (complexes) with the fulvic acid anion to make ful-
vic acid more adsorbable (Randtke and Jepsen, 1982; Weber, Voice, and Jodellah,
1983). Presumably many other divalent ions can act in similar fashion, but calcium
is of special interest because of its relatively high concentration in many natural
waters. Similar effects are expected for other anionic adsorbates, but salts are not
expected to have much effect on the adsorption of neutral adsorbates (Snoeyink,

Weber, and Mark, 1969).
ADSORPTION OF ORGANIC COMPOUNDS 13.5
FIGURE 13.2 Adsorption isotherms for peat fulvic
acid. (Source: Lee, Snoeyink, and Crittenden, 1981.)
Inorganic substances such as iron, manganese, and calcium salts or precipitates
may interfere with adsorption if they deposit on the adsorbent. Pretreatment to
remove these substances, or to eliminate the supersaturation, may be necessary if
they are present in large amounts.
Adsorption isotherms may be determined for heterogeneous mixtures of com-
pounds using group parameters such as total organic carbon (TOC), dissolved
organic carbon (DOC), chemical oxygen demand (COD),dissolved organic halogen
(DOX),UV absorbance, and fluorescence as a measure of the total concentration of
substances present. Because the compounds within a mixture can vary widely in
their affinity for an adsorbent, the shape of the isotherm will depend on the relative
amounts of compounds in the mixture. For example,isotherms with the shape shown
in Figure 13.4 are expected if some of the compounds are nonadsorbable and some
are more strongly adsorbable than the rest (Randtke and Snoeyink, 1983). The
strongly adsorbable compounds can be removed with small doses of adsorbent and
yield large values of q
e
. In contrast, the weakly adsorbable compounds can only be
removed with large doses of adsorbent that yield relatively low values of q
e
.The
nonadsorbable compounds produce a vertical isotherm at low C
e
values. In contrast
to single-solute isotherms, the isotherm for a heterogeneous mixture of compounds
will be a function of initial concentration and the fraction of the mixture that is
adsorbed. The relative adsorbabilities of compounds within a mixture have an

important effect on the performance of adsorption columns. The nonadsorbable
fraction cannot be removed regardless of the column design, whereas the strongly
adsorbable fraction may cause the effluent concentration to slowly approach the
influent concentration.
Competitive Adsorption in Bisolute Systems. Competitive adsorption is impor-
tant in drinking water treatment because most compounds to be adsorbed exist in
solution with other adsorbable compounds. The quantity of activated carbon or
other adsorbent required to remove a certain amount of a compound of interest
13.6 CHAPTER THIRTEEN
FIGURE 13.3 Effects of calcium chloride addition and withdrawal on column per-
formance (pH = 8.3; TOC = 5.37 mg/L, peat fulvic acid buffer = 1.0 mM NaHCO
3
).
(Source: Randtke and Jepsen, 1982.)
from a mixture of adsorbable compounds
is greater than if adsorption occurs with-
out competition, because part of the
adsorbent’s surface is utilized by the
competing substances.
The extent of competition on activated
carbon depends upon the strength of
adsorption of the competing molecules,
the concentrations of these molecules,
and the type of activated carbon. Some
examples illustrate the possible mag-
nitude of the competitive effect. Jain
and Snoeyink (1973) showed that as
p-bromophenol (PBP) equilibrium con-
centration increased from 10
−4

to 10
−3
M
(17 to 173 mg/L), the amount of
p-nitrophenol (PNP) adsorbed at an
equilibrium concentration of 3.5 × 10
−5
M
(∼ 5 mg/L) decreased by about 30 percent.
Displacement of previously adsorbed
compounds by competition can result in a
column effluent concentration of a com-
pound that is greater than the influent concentration, as shown in Figure 13.5. A
dimethylphenol (DMP) concentration about 50 percent greater than the influent
resulted when dichlorophenol (DCP) was introduced to the influent of a column
ADSORPTION OF ORGANIC COMPOUNDS 13.7
FIGURE 13.4 Nonlinear isotherm for a het-
erogeneous mixture of organic compounds.
(Source: Randtke and Snoeyink, 1983.)
FIGURE 13.5 Breakthrough curves for sequential feed of DMP and DCP to
a GAC adsorber (C
0
= 0.990 mmol/L, C
02
= 1.02 mmol/L, EBCT = 25.4 s).
(Source: W. E. Thacker, J. C. Crittenden, and V. L. Snoeyink, 1984. “Modeling of
Adsorber Performance: Variable Influent Concentration and Comparison of
Adsorbents,” Journal Water Pollution Control Federation 56: 243. Copyright ©
Water Environment Federation, reprinted with permission.)
saturated with DMP (Thacker, Crittenden,and Snoeyink, 1984).Similar occurrences

have been observed in full-scale GAC systems. Effluent concentrations in excess of
influent concentrations can be prevented through careful operation. Crittenden et
al. (1980) showed that the magnitude of the displacement decreased when the value
of C
eff
/C
inf
was lowered at the time the second compound was introduced. Thus, a
reasonable strategy to prevent the occurrence of an undesirable compound at a con-
centration greater than the influent is (1) to monitor the column for that compound
and (2) to replace the activated carbon before complete saturation at the influent
concentration occurs (i.e., before C
eff
= C
inf
).
A number of isotherm models have been used to describe competitive adsorp-
tion.A common model for describing adsorption equilibrium in multiadsorbate sys-
tems is the Langmuir model for competitive adsorption, which was first developed
by Butler and Ockrent (1930) and which is presented in the fourth edition of this
book (Snoeyink, 1990). This model is based on the same assumptions as the Lang-
muir model for single adsorbates. Jain and Snoeyink (1973) modified this model to
account for a fraction of the adsorption taking place without competition. This can
happen if the adsorbates have different sizes and only the smaller adsorbate can
enter the smaller pores (Pelekani and Snoeyink, 1999), or if some of the surface
functional groups adsorb one compound but not the other.Other models that can be
used to describe and predict competitive effects are the Freundlich-type isotherm of
Sheindorf, Rebhun, and Sheintuck (1981) and the ideal adsorbed solution theory of
Radke and Prausnitz (1972) described in the next section. The latter has proven to
be applicable to a large number of situations.

Competitive Adsorption in Natural Waters. Adsorption of organic compounds at
trace concentrations from natural waters is an important problem in water purifica-
tion. Essentially all synthetic organic chemicals that must be removed in water treat-
ment by adsorption must compete with natural or background organic matter for
adsorption sites. The heterogeneous mixture of compounds in natural waters
adsorbs on activated carbon and reduces the number of sites available for the trace
compounds, either by direct competition for adsorption sites or by pore blockage
(Pelekani and Snoeyink, 1999). The amount of competition and the capacity for the
trace compound depend on the nature of the background organic matter and its con-
centration, as well as the characteristics of the activated carbon. Also important is
the concentration of the trace compound, because this concentration affects how
much of this compound can adsorb on the carbon. For example, Figure 13.6 shows
that the adsorption capacity of 2-methylisoborneol (MIB), an important earthy/
13.8 CHAPTER THIRTEEN
100
10
1
1 10 100
C
e
(ng/L)
q
e
(ng/mg)
K = 9.56 (ng/mg)(L/ng)
I/n
K = 9.56 (ng/mg)(L/ng)
I/n
l/n = 0.492
1,000 10,000

C
0
= 1245 ng/L
C
0
= 150 ng/L
Single-Solute Isotherm
FIGURE 13.6 Effect of initial concentration on MIB capacity in Lake
Michigan water. (Source: Gillogly et al., 1998b.)
musty odor compound, is lower in natural water than in distilled water, and that this
capacity is further reduced as initial concentration decreases (Gillogly et al., 1998b).
It is important to have a procedure to predict capacity as a function of initial con-
centration, because the capacity of activated carbon depends in such an important
way on initial concentration and because the concentrations of trace organic com-
pounds vary widely in natural waters. The ideal adsorbed solution theory (IAST)
can be used for this purpose. The following two equations, based on the IAST as
developed by Radke and Prausnitz (1972) and modified by Crittenden et al. (1985)
to include the Freundlich equilibrium expression, describe equilibrium in a two-
solute system,
C
1,0
− q
1
C
c
−
΂΃
n
1
= 0 (13.4)

C
2,0
− q
2
C
c
−
΂΃
n
2
= 0 (13.5)
where q
1
and q
2
= equilibrium solid phase concentrations of compounds 1 and 2
q
1
= (C
1,0
− C
1,e
)/C
c
and q
2
= (C
2,0
− C
2,e

)/C
c
C
1,0
and C
2,0
= initial liquid phase concentrations of compounds 1 and 2
C
1,e
and C
2,e
= equilibrium concentrations of compounds 1 and 2
K
1
and K
2
= single-solute Freundlich parameters for compounds 1 and 2
1/n
1
and 1/n
2
= single-solute Freundlich exponents for compounds 1 and 2
C
c
= carbon dose
These equations show the relationship between the initial concentration of each
adsorbate, the amount of adsorbed compound per unit weight of carbon, and the
carbon dose. The Freundlich parameters are derived from single-solute tests in
organic-free water.
In natural waters, the organic matter present is a complex mixture of many dif-

ferent compounds; representing each of these compounds, even if they could be
identified, would be computationally prohibitive. Several researchers have modeled
NOM adsorption by defining several fictive components that represent groups of
compounds with similar adsorption characteristics, as expressed by Freundlich K
and n values (Sontheimer, Crittenden, and Summers, 1988). Extending Equations
13.4 and 13.5 to N components yields
C
i,0
− C
c
q
i
−
΂΃΄ ΅
n
i
= 0 (13.6)
where N = number of components in the solution
C
i,0
= initial liquid-phase concentration of compound i
C
c
= carbon dose
q
i
= equilibrium solid-phase concentration of compound i
n
i
and K

i
= single-solute Freundlich parameters for compound i
These equations can be solved simultaneously to determine the concentrations for
each component assumed to be in solution.
Crittenden et al. (1985) used this fictive component approach to describe the
adsorption of a target compound in the presence of NOM. With a single-solute
isotherm of the target compound and experimental results from isotherms measured
Α
N
i = 1
n
j
q
j
ᎏ
n
i
K
i
q
i
ᎏ
Α
N
j = 1
q
j
n
1
q

1
+ n
2
q
2
ᎏᎏ
n
2
K
2
q
2
ᎏ
q
1
+ q
2
n
1
q
1
+ n
2
q
2
ᎏᎏ
n
1
K
1

q
1
ᎏ
q
1
+ q
2
ADSORPTION OF ORGANIC COMPOUNDS 13.9
13.10 CHAPTER THIRTEEN
FIGURE 13.7 EBC model results for atrazine isotherms in Illinois ground-
water. (Source: Reprinted with permission from D. R. U. Knappe et al. 1998.
“Predicting the capacity of powdered activated carbon for trace organic com-
pounds in natural waters.” Environmental Science & Technology, 32:1694–1698.
Copyright 1998 American Chemical Society.)
using the natural water, parameters for each of the fictive components were found
through a best-fit search procedure. These results were then applied to describe the
adsorption of other compounds in that water.
The IAST was applied to the problem of trace organic adsorption in natural
waters by Najm, Snoeyink, and Richard (1991) using a procedure that was subse-
quently modified by Qi et al. (1994) and Knappe et al. (1998). These researchers
assumed that the background organic matter that competed with the trace com-
pound could be represented as a single compound, called the equivalent background
compound (EBC). This approach involved the determination of the single-solute
isotherm for the trace compound, and an isotherm in natural water for the trace
compound at two different initial concentrations. A search routine was used to find
the Freundlich parameters K and 1/n and the initial concentration C
0
for the EBC
that gave the observed amount of competition. For example, Figure 13.7 shows
isotherms determined for atrazine in organic-free water and in Illinois groundwater

at initial concentrations of 176 and 36 µg/L (Knappe et al., 1998). These data were
used to determine the following EBC characteristics:
K
EBC
> 1.0 × 10
6
(µmole/g)(L/µmole)
1/n
,1/n
EBC
= 0.648, C
0,EBC
= 0.870 µmole/L
The K value for the EBC was arbitrary above 1.0 × 10
6
(µmole/g)(L/µmole)
1/n
.
These EBC parameters are specific for the type of carbon, the type and concentra-
tion of background organic matter, and the type of synthetic organic chemical
(SOC). They can be used in Equations 13.4 and 13.5 together with the initial con-
centration of the trace compound and its single-solute Freundlich parameters to cal-
culate the surface coverage of trace compound as a function of carbon dose C
e
.
Given the surface coverage q, the initial concentration C
0
, and the carbon dose C
c
,

the equilibrium concentration of the trace compound can be calculated from the
equation q = (C
0
− C
e
)/C
c
. This approach was used to determine the predicted
isotherm for atrazine at an initial concentration of 8.3 µg/L, shown in Figure 13.7,
which compares very well with the measured data. It is interesting to note that at an
equilibrium concentration of 1 µg/L, there is a 63 percent reduction in capacity for
atrazine as the initial concentration is reduced from 176 to 8.3 µg/L.
An important modification of the EBC model was developed by Knappe et al.
(1998), who found that the amount of adsorption on a unit mass of activated carbon
was directly proportional to the initial concentration of that trace compound in an
adsorption test if (1) the Freundlich exponents for the trace compound 1/n
1
and the
EBC 1/n
2
both fall between 0.1 and 1, as is generally the case, and (2) the solid-phase
concentration of the background organic matter is much in excess of that of the
trace compound, which also is often true.When these two approximations hold, the
IAST model simplifies to
q
1
= C
1,0
΄
C

c
+
΂΃
n
1
΅
−1
(13.7)
This equation shows that the surface loading of trace compound q
1
is directly pro-
portional to its initial concentration. The equation can also be manipulated to
show that the percent removal or percent remaining of a compound for any carbon
dose, when the two preceding assumptions are valid, is a constant. The implication
of this result is that only one isotherm need be determined for low concentrations
of trace compound in a natural water. This isotherm can be plotted as percent
remaining versus carbon dose, as shown in Figures 13.8 and 13.9 for the same data
shown in Figures 13.6 and 13.7, respectively. In Figure 13.9, the atrazine data for
C
0
≤ 36 µg/L plot on a single percent remaining versus carbon dose line, and the
C
0
= 176 µg/L line is only slightly higher. All the MIB data in Figure 13.6 plot on
one line in Figure 13.8.
n
2
q
2
ᎏ

n
1
K
1
1
ᎏ
q
2
ADSORPTION OF ORGANIC COMPOUNDS 13.11
FIGURE 13.8 Percent MIB remaining as a function of PAC dose.
(Source: Gillogly et al., 1998b.)
Graham et al. (1999) independently derived a different form of Eq. 13.7 and
applied it using the EBC approach to adsorption of MIB and geosmin from four nat-
ural waters. The same EBC parameters [K = 1.35 (µg/mg)(L/µg)
1/n
and 1/n = 0.20]
were found to be applicable to both MIB and geosmin and, at different initial con-
centrations (15 to 51 µg/L), to all four natural waters. For three of the four natural
waters, the EBC initial concentration was about 0.45 percent of the TOC initial con-
centration.
These results allow the following general procedure to be used to determine
adsorption capacity for a trace compound in natural water:
1. Determine one adsorption isotherm for the trace compound in natural water at a
sufficiently low initial concentration.
2. Plot the data on a log-log percent remaining versus carbon dose plot.
3. Use this isotherm plot to determine the carbon dose required for any desired per-
cent removal for any initial concentration that satisfies the assumptions.
An important question now is what concentration is sufficiently low that the
assumptions made in developing the percent remaining versus carbon dose plot are
valid. Research to date has shown that MIB (Gillogly et al., 1998b) and geosmin

(Graham et al., 1999) concentrations less than 1000 ng/L, and atrazine concentra-
tions less than about 50 µg/L
32
, will give a satisfactory plot. However, it is necessary
to expand this database for other trace compounds.
Desorption. Adsorption of many compounds is reversible, which means that they
can desorb. Desorption may be caused by displacement by other compounds, as dis-
cussed previously, or by a decrease in influent concentration. Both phenomena may
occur in some situations. An analysis of desorption by Thacker, Snoeyink, and Crit-
tenden (1983) showed that the quantity of adsorbate that can desorb in response to
a decrease in influent concentration increased as (1) the diffusion coefficient of the
13.12 CHAPTER THIRTEEN
FIGURE 13.9 Removal efficiency as a function of PAC dose for the
adsorption of atrazine from Illinois groundwater (GW). (Source: Re-
printed with permission from D. R. U. Knappe et al., 1998. “Predicting the
capacity of powdered activated carbon for trace organic compounds in
natural waters.” Environmental Science & Technology, 32: 1694–1698.
Copyright 1998 American Chemical Society.)
adsorbate increased, (2) the amount of compound adsorbed increased, (3) the
strength of adsorption decreased (e.g., as the Langmuir b value decreased or the
Freundlich l/n value increased), and (4) the activated carbon particle size decreased.
Volatile organic compounds are especially susceptible to displacement because they
are weakly adsorbed and diffuse rapidly. Summers and Roberts (1988a, b) have
shown that NOM only partially desorbs and for the desorbing fraction, the desorp-
tion diffusivity is lower than that during adsorption.
Adsorption Kinetics
Transport Mechanisms. Removal of organic compounds by physical adsorption
on porous adsorbents involves a number of steps, each of which can affect the rate
of removal:
1. Bulk solution transport Adsorbates must be transported from bulk solution to

the boundary layer of water surrounding the adsorbent particle. The transport
occurs through diffusion if the adsorbent is suspended in quiescent water such as
a sedimentation basin, or through turbulent mixing such as during turbulent flow
through a packed bed of GAC,or when PAC is being mixed in a rapid mix unit or
flocculator.
2. External (film) resistance to transport Adsorbates must be transported by
molecular diffusion through the stationary layer of water (hydrodynamic bound-
ary layer) that surrounds adsorbent particles when water is flowing past them.
The distance of transport, and thus the time for this step, is determined by the
flow rate past the particle. The higher the flow rate, the shorter the distance.
3. Internal (pore) transport After passing through the hydrodynamic boundary
layer, adsorbates must be transported through the adsorbent’s pores to available
adsorption sites. Intraparticle transport may occur by molecular diffusion
through the solution in the pores (pore diffusion), or by diffusion along the
adsorbent surface (surface diffusion) after adsorption takes place.
4. Adsorption After transport to an available site, an adsorption bond is formed
between the adsorbate and adsorbent.This step is very rapid for physical adsorp-
tion (Adamson, 1982) and as a result one of the preceding diffusion steps will
control the rate at which molecules are removed from solution. If adsorption is
accompanied by a chemical reaction that changes the nature of the molecule, the
chemical reaction may be slower than the diffusion step and thereby control the
rate of compound removal.
The transport steps occur in series, so the slowest step, called the rate-limiting step,
will control the rate of removal. In turbulent flow reactors, a combination of film dif-
fusion and pore diffusion very often controls the rate of removal for some of the
types of molecules to be removed from drinking water. Initially, film diffusion may
control the rate of removal, and after some adsorbate accumulates within the pore,
pore transport may control the rate of removal. The mathematical models of the
adsorption process, therefore, usually include both steps.
Both molecular size and adsorbent particle size have important effects on the

rate of adsorption. Diffusion coefficients, in particular, decrease as molecular size
increases, and thus longer times are required to remove the large-molecular-weight
humic substances than are needed for the low-molecular-weight phenols, for exam-
ple. Adsorbent particle size is also important because it determines the time
required for transport within the pore to available adsorption sites. If the rate of
ADSORPTION OF ORGANIC COMPOUNDS 13.13
adsorbate uptake is controlled by intraparticle diffusion, and the effective diffusion
coefficient is constant, the time to reach equilibrium is directly proportional to the
diameter of the particle squared. Calculations by Randtke and Snoeyink (1983) for
activated carbon illustrate these points. For the low-molecular-weight dimethylphe-
nol, nearly 8 days is estimated for near-equilibrium (C
final
= 1.01 C
e
) of 2.4-mm-
diameter activated carbon, but only about 25 min is required for 44-µm-diameter
activated carbon. For very large-molecular-weight (approximately 50,000) humic
acid, the 2.4-mm-diameter particle is expected to take much longer than a year to
equilibrate, but only 2 days is required for the 44-µm-diameter particle. Calcula-
tions for 10,000-molecular-weight fulvic acid showed only about 25 percent satura-
tion of 2.4-mm-diameter particles after 40 days of contact. Thus, the smaller the
particle, the faster equilibrium is achieved in both column and complete-mix
adsorption systems.
Some conclusions that can be drawn from these observations are that (1) granu-
lar carbon should be pulverized for isotherm measurement, especially when the
capacity of large molecular weight compounds is to be determined (pulverizing does
not affect the total surface available for adsorption if all of the GAC is pulverized)
(Randtke and Snoeyink, 1983); (2) the smallest activated carbon, consistent with
other process constraints such as head loss and loss during reactivation, should be
chosen for the best kinetics; and (3) depending on the compound, all the capacity of

large activated carbon particles in a column may not be used because the time inter-
val between activated carbon replacements is not sufficient for equilibrium to be
achieved.
Mass Transfer Zone and Breakthrough Curves for Packed Bed Reactors. The
region of an adsorption column in which adsorption is taking place, the mass trans-
fer zone (MTZ), is shown in Figure 13.10a. The activated carbon behind the MTZ
has been completely saturated with adsorbate at C
e
= C
0
, and the amount adsorbed
per unit mass of GAC is (q
e
)
0
.The activated carbon in front of the MTZ has not been
exposed to adsorbate, so solution concentration and adsorbed concentration are
both zero. Within the MTZ, the degree of saturation with adsorbate varies from 100
percent (q = [q
e
]
0
) to zero. The length of the MTZ, L
MTZ
, depends upon the rate of
adsorption and the solution flow rate. Anything that causes a higher rate of adsorp-
tion, such as a smaller carbon particle size, higher temperature, a larger diffusion
coefficient of adsorbate, and/or greater strength of adsorption of adsorbate (i.e., a
larger Freundlich K value), will decrease the length of the MTZ. In some circum-
stances, L

MTZ
will be reduced sufficiently that it can be assumed to be zero, yielding
the ideal plug-flow behavior, as shown in Figure 13.10b. If L
MTZ
is negligible, analy-
sis of the adsorption process is greatly simplified.
The breakthrough concentration C
B
for a column is defined as the maximum
acceptable effluent concentration. When the effluent concentration reaches this
value, the GAC must be replaced. The critical depth of a column L
critical
is the depth
that leads to the immediate appearance of an effluent concentration equal to C
B
when the column is started up. For the situation in which C
B
is defined as the mini-
mum detectable concentration, the critical depth of an activated carbon column is
equal to the length of the MTZ.The length of the MTZ is fixed for a given set of con-
ditions, but L
critical
varies with C
B
.The critical depth, the flow rate Q, and the area of
the column A, can be used to calculate the minimum empty bed contact time
(EBCT) (EBCT = Q/V, where V is the bulk volume of GAC in the contactor):
= EBCT
min
(13.8)

L
critical
ᎏ
Q/A
13.14 CHAPTER THIRTEEN
When C
B
is greater than the minimum detectable concentration, the critical depth is
less than L
MTZ
and its value can be determined as shown in a later section (see Fig-
ure 13.14 and related discussion).
The breakthrough curve is a plot of the column effluent concentration as a func-
tion of either the volume treated, the time of treatment, or the number of bed vol-
umes (BV) treated (BV = V
B
/V, where V
B
is the volume treated).The number of bed
volumes is a particularly useful parameter because the data from columns of differ-
ent sizes and with different flow rates are normalized. A breakthrough curve for a
single adsorbable compound is shown in Figure 13.11. The shape of the curve is
affected by the same factors that affect the length of the MTZ, and in the same way.
Anything that causes the rate of adsorption to increase will increase the sharpness of
the curve, while increasing the flow rate will cause the curve to “spread out” over a
larger volume of water treated. The breakthrough curve will be vertical if L
MTZ
= 0,
as shown in Figure 13.10b. As shown in Figure 13.11, the breakthrough capacity,
defined as the mass of adsorbate removed by the adsorber at breakthrough, and the

degree of column utilization, defined as the mass adsorbed at breakthrough/mass
adsorbed at complete saturation at the influent concentration, both increase as the
rate of adsorption increases.
The breakthrough curve can be used to determine the activated carbon usage
rate (CUR), the mass of activated carbon required per unit volume of water treated:
ADSORPTION OF ORGANIC COMPOUNDS 13.15
FIGURE 13.10 Adsorption columnMTZ.(a) Columnwith MTZ.(b) Column withoutMTZ.
(a) (b)
CUR
΂΃
= (13.9)
Breakthrough curves are strongly affected by the presence of nonadsorbable com-
pounds, the biodegradation of compounds in a biologically active column, slow
adsorption of a fraction of the molecules present, and the critical depth of the col-
umn relative to the length of the column. Immediate breakthrough of adsorbable
compounds occurs if the L
MTZ
is greater than the activated carbon bed depth (com-
pare curves A and B in Figure 13.12). Nonadsorbable compounds immediately
appear in the column effluent, even when the carbon depth is greater than the L
MTZ
(compare curves B and C in Figure 13.12). Removal of adsorbable, biodegradable
compounds by microbiological degradation in a column results in continual
removal, even after the carbon is saturated with adsorbable compounds (see curve
D in Figure 13.12). If a fraction of compounds adsorbs slowly, the upper part of the
breakthrough curve will be similar to that produced by biodegradation but will
slowly approach C
eff
/C
0

= 1. Breakthrough curves shown in later sections (see Fig-
ures 13.15 and 13.22, for example) also illustrate some of these effects.
GAC ADSORPTION SYSTEMS
Characteristics of GAC
Physical Properties. A wide variety of raw materials can be used to make acti-
vated carbon (Hassler, 1974), but wood, peat, lignite, subbituminous coal, and bitu-
minous coal are the substances predominately used for drinking water treatment
carbons in the United States. Both the physical and chemical manufacturing pro-
cesses involve carbonization (conversion of the raw material to a char) and activa-
tion (oxidation to develop the internal pore structure). With physical activation,
carbonization, or pyrolysis, is usually performed in the absence of air at tempera-
tures less than 700°C, while activation is carried out with oxidizing gases such as
mass of GAC in column
ᎏᎏᎏᎏ
volume treated to breakthrough V
B
mass
ᎏ
volume
13.16 CHAPTER THIRTEEN
FIGURE 13.11 Adsorption column breakthrough curve.
steam and CO
2
at temperatures of 800 to 900°C. Chemical activation combines car-
bonization and activation steps. Patents describing carbonization and activation pro-
cedures are given by Yehaskel (1978).
Various characteristics of activated carbon affect its performance.* The particle
shape of crushed activated carbon is irregular, but extruded activated carbons 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 rate of

adsorption, as discussed previously. Particle size distribution refers to the relative
amounts of different-size particles that are part of a given sample, or lot,of carbon,and
has an important impact on the filtration properties of GAC in GAC columns that are
used both as filters to remove particles and as adsorbers (i.e., filter-adsorbers) (Graese,
Snoeyink, and Lee, 1987). Commonly available activated carbon sizes are 12 × 40 and
8 × 30 US Standard mesh,which range in apparent diameter from 1.68 to 0.42 mm and
from 2.38 to 0.59 mm, respectively.The uniformity coefficient (see Chapter 8) is often
quite large, typically on the order of 1.9,to promote stratification during backwashing.
Commercially available activated carbons usually 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 diame-
ter, but vary in length.There is no method comparable to the sieve analysis procedure
to characterize the distribution of lengths, however.
ADSORPTION OF ORGANIC COMPOUNDS 13.17
FIGURE 13.12 Effect of biodegradation and the presence of nonadsorbable compounds
on the shapes of breakthrough curves.
* Note: Descriptions of the analytical procedures for testing activated carbon are given in ASTM Stan-
dards (American Society for Testing Materials, 1988) available from ASTM, 1916 Race St., Philadelphia, PA
19103, as well as in AWWA Standards B604-96 and B600-66, available from AWWA, 6666 West Quincy Ave.,
Denver, CO 80235.
The apparent density* of activated carbon is the mass of nonstratified dry acti-
vated carbon per unit volume of activated carbon, including the volume of voids
between grains. Typical values for GAC are 350 to 500 kg/m
3
(25 to 31 lb/ft
3
). Distin-
guishing between the apparent density and the bed density, backwashed and drained
(i.e., stratified, free of water) is important, however.The former is a characteristic of
activated carbon as shipped.The latter is about 10 percent less than the apparent den-

sity and is typical of activated carbon during normal operation unless it becomes
destratified during backwashing. The latter is an important parameter because it
determines how much activated carbon must be purchased to fill a filter of given size.
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/m
3
(90 to 105 lb/ft
3
) and it determines
the extent of fluidization and expansion of a particle of given size during backwash.
Particle hardness is important because it affects the amount of attrition during
backwash, transport, and reactivation. In general, the harder the activated carbon,
the less the attrition for a given amount of friction or impact between particles.Acti-
vated carbon hardness is generally characterized by an experimentally determined
hardness or abrasion number, using a test such as the ASTM Ball Pan Hardness Test
that measures the resistance to particle degradation upon agitation of a mixture of
activated carbon and steel balls (American Society for Testing Materials, 1988). 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 deter-
mined, however.
Adsorption Properties. A number of parameters are used to describe the adsorp-
tion capacity of activated carbon (Sontheimer, Crittenden,and Summers, 1988).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 adsor-
bates. The iodine number (American Society for Testing Materials, 1988) measures
the amount of iodine that will adsorb under a specified set of conditions, and gener-
ally correlates well with the surface area available for small molecules. Other num-
bers have been developed for specific applications, such as the carbon tetrachloride

activity, the methylene blue number, and the phenol adsorption value. The values of
these numbers give useful information about the abilities of various activated car-
bons to adsorb different types of organics. However, isotherm data for the specific
compounds to be removed in a given application, if available, are much better indi-
cators of performance.
Two of the more important characteristics of an activated carbon are its pore size
distribution (discussed previously) and surface area.The manufacturer provides typ-
ical data that usually include the BET surface area. This parameter is determined by
measuring the adsorption isotherm for nitrogen gas molecules and then analyzing
the data using the Brunauer-Emmett-Teller (BET) isotherm equation (Adamson,
1982) to determine the amount of nitrogen required to form a complete monolayer
of nitrogen molecules on the carbon surface. Multiplying the surface area occupied
per nitrogen molecule (0.162 nm
2
/molecule of N
2
) by the number of molecules in the
monolayer yields the BET surface area. Because nitrogen is a small molecule, it can
13.18 CHAPTER THIRTEEN
* This definition is based on ASTM Standard D2854-83 (American Society for Testing Materials, 1988).
It conflicts with ASTM Standard C128-84 for apparent specific gravity, as used in Chapter 8,which does not
include the volume of interparticle voids.
enter pores that are unavailable to larger adsorbates.As a result, all of the BET sur-
face area may not be available for adsorbates in drinking water.
Tabulations of single-solute isotherm constants are very 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 Dobbs and Cohen (1980), as tabulated by Faust and Aly (1983), are reproduced in
Table 13.1 for this purpose.* Values from Speth and Miltner (1998) have also been
listed. Additional values can be found in Sontheimer, Crittenden, and Summers

(1988). These data 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 2-chlorophenol
[K = 51 (mg/g)(L/mg)
1/n
and 1/n = 0.41], the removal of compounds with larger val-
ues of K and approximately the same concentration will very likely 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 equilib-
rium concentration of interest using Equation 13.1, because the relative adsorbabil-
ity will depend on the equilibrium concentration. The use of isotherm values to
estimate adsorber life and PAC usage rate will be discussed later.
GAC Contactors
GAC contactors can be classified by the following characteristics: (1) driving
force—gravity versus pressure; (2) flow direction—downflow versus upflow;
(3) configuration—parallel versus series; and (4) position—filter-adsorber versus
postfilter-adsorber.
GAC may be used in pressure or gravity contactors. Pressure filters enclose the
GAC and can be operated over a wide range of flow rates because of the wide vari-
ations in pressure drop that can be used. An advantage of these filters is that they
can be prefabricated and shipped to the site.A disadvantage is that the GAC cannot
be visually observed with ease. Gravity contactors are better suited for use when
wide variations in flow rate are not desirable because of the need to remove turbid-
ity, when large pressure drops are undesirable because of their impact on operation
costs, and when visual observation is needed to monitor the condition of the GAC.
For many systems the decision between pressure or gravity contactors is made on
the basis of cost. Medium-size and large systems normally use gravity contactors.
Water may be applied to GAC either upflow or downflow, and upflow columns
may be either packed bed or expanded bed. Downflow columns are the most com-

mon and seem best suited for drinking water treatment. McCarty, Argo, and Rein-
hard (1979) found that carbon fines were produced during packed-bed upflow
operation and not during downflow operation.The pulsed-bed contactor can also be
used to decrease carbon usage rate from that of a single contactor. The flow is
applied upward through the column; the spent GAC, a fraction of the total amount
present, is periodically removed from the bottom of the column and an equal
amount of fresh GAC is applied to the top.
ADSORPTION OF ORGANIC COMPOUNDS 13.19
* Dobbs and Cohen (1980) and Speth and Miltner (1998) should be consulted to determine the type of
activated carbon and the experimental conditions that were used. The data of Dobbs and Cohen were not
determined in a way that would ensure that equilibrium was achieved for all adsorbates, but are suitable to
show relative absorbability of compounds and to make rough estimates of activated carbon life. If precise
values are needed, new isotherms should be determined using the water to be treated.
TABLE 13.1 Freundlich Adsorption Isotherm Parameters for Organic Compounds
Compound K (mg/g)(L/mg)
1/n
1/n Reference
PCB 14,100 1.03 *
Bis(2-ethylhexyl phthalate) 11,300 1.5
†
Heptachlor 9,320 0.92 *
Heptachlor epoxide 2,120 0.75 *
Butylbenzyl phthalate 1,520 1.26
†
Hexachlorocyclopentadiene 1,400 0.504 *
Dichloroacetonitrile 1,300 0.232 *
Toxaphene 950 0.74 *
Endosulfan sulfate 686 0.81
†
Endrin 666 0.8

†
Fluoranthene 664 0.61
†
Aldrin 651 0.92
†
PCB-1232 630 0.73
†
β-Endosulfan 615 0.83
†
Dieldrin 606 0.51
†
p-Dichlorobenzene 588 0.691 *
1,3,5-Trichlorobenzene 586 0.324 *
Alachlor 482 0.257 *
m-Dichlorobenzene 458 0.63 *
m-Dichlorobenzene 458 0.63 *
Hexachlorobenzene 450 0.6
†
Pentachlorophenol 443 0.339 *
Pentachlorophenol 436 0.34 *
Oxamyl 416 0.793 *
Anthracene 376 0.7
†
p-Chlorotoluene 376 0.34 *
4-Nitrobiphenyl 370 0.27
†
m-Xylene 343 0.614 *
Styrene 334 0.479 *
Fluorene 330 0.28
†

DDT 322 0.5
†
2-Acetylaminofluorene 318 0.12
†
o-Chlorotoluene 316 0.378 *
α-BHC 303 0.43
†
Anethole 300 0.42
†
3,3-Dichlorobenzidine 300 0.2
†
Lindane 299 0.433 *
Atrazine 289 0.291 *
γ-BHC (lindane) 285 0.43 *
2,4-Dinitrotoluene 284 0.157 *
2-Chloronaphthalene 280 0.46
†
Carbofuran 275 0.408 *
Phenylmercuric acetate 270 0.44
†
o-Dichlorobenzene 263 0.378 *
Hexachlorobutadiene 258 0.45
†
p-Nonylphenol 250 0.37
†
4-Dimethylaminoazobenzene 249 0.24
†
Cyanazine 244 0.126 *
PCB-1221 242 0.7
†

Acifluorofen 236 0.198 *
Metolachlor 233 0.125 *
13.20 CHAPTER THIRTEEN
TABLE 13.1 Freundlich Adsorption Isotherm Parameters for Organic Compounds
(Continued)
Compound K (mg/g)(L/mg)
1/n
1/n Reference
DDE 232 0.37
†
Acridine yellow 230 0.12
†
p-Xylene 226 0.418 *
Benzidine dihydrochloride 220 0.37
†
β-BHC 220 0.49
†
n-Butylphthalate 220 0.45
†
n-Nitrosodiphenylamine 220 0.37
†
Dibromochloropropane (DBCP) 220 0.501 *
Silvex 215 0.38 *
Phenanthrene 215 0.44
†
Bromobenzene 213 0.364 *
Dimethylphenylcarbinol 210 0.34
†
Dinoseb 209 0.279 *
4-Aminobiphenyl 200 0.26

†
β-Naphthol 200 0.26
†
p-Xylene 200 0.42 *
Glyphosate 199 0.119 *
α-Endosulfan 194 0.5
†
Chlordane 190 0.33 *
Acenaphthene 190 0.36
†
4,4′-Methylene-bis-(2-chloroaniline) 190 0.64
†
Metribuzin 185 0.193 *
Benzol |k| fluoranthene 181 0.57
†
Acridine orange 180 0.29
†
α-Naphthol 180 0.32
†
o-Xylene 174 0.47 *
4,6-Dinitro-o-cresol 169 0.27
†
Ethyl benzene 163 0.415 *
α-Naphthylamine 160 0.34
†
1,1,1-Trichloropropanone 159 0.11 *
2,4-Dichlorophenol 157 0.15
†
1,2,4-Trichlorobenzene 157 0.31
†

2,4,6-Trichlorophenol 155 0.4
†
β-Naphthylamine 150 0.3
†
Simazine 150 0.227 *
2,4-Dinitrotoluene 146 0.31
†
2,6-Dinitrotoluene 145 0.32
†
4-Bromophenyl phenyl ether 144 0.68
†
Tetrachlorethene 143 0.516 *
p-Nitroaniline 140 0.27
†
1,1-Diphenylhydrazine 135 0.16
†
Aldicarb 133 0.402 *
Naphthalene 132 0.42
†
1-Chloro-2-nitrobenzene 130 0.46
†
p-Chlorometacresol 124 0.16
†
1,4-Dichlorobenzene 121 0.47
†
Benzothiazole 120 0.27
†
Diphenylamine 120 0.31
†
Guanine 120 0.4

†
1,3-Dichlorobenzene 118 0.45
†
ADSORPTION OF ORGANIC COMPOUNDS 13.21
TABLE 13.1 Freundlich Adsorption Isotherm Parameters for Organic Compounds
(Continued)
Compound K (mg/g)(L/mg)
1/n
1/n Reference
Acenaphthylene 115 0.37
†
Methoxychlor 115 0.36 *
4-Chlorophenyl phenyl ether 111 0.26
†
Diethyl phthalate 110 0.27
†
Chlorobenzene 101 0.348 *
Chlorobenzene 101 0.348 *
2-Nitrophenol 99 0.34
†
Dimethyl phthalate 97 0.41
†
Hexachloroethane 97 0.38
†
Toluene 97 0.429 *
Bromoform 92 0.655 *
Dicamba 91 0.147 *
Chloropicrin 88 0.155 *
Pichloram 81 0.18 *
2,4-Dimethylphenol 78 0.44

†
4-Nitrophenol 76 0.25
†
Acetophenone 74 0.44
†
1,2,3,4-Tetrahydronaphthalene 74 0.81
†
1,2,3-Trichloropropane 74 0.613 *
Ethylene thiourea 73 0.669 *
Adenine 71 0.38
†
Dibenzo |a,h| anthracene 69 0.75
†
1,1,1,2-Tetrachlorethane 69 0.604 *
Nitrobenzene 68 0.43
†
2,4-D 67 0.27 *
Isophorone 63 0.271 *
Methyl isobutyl ketone 61 0.279 *
3,4-Benzofluoranthene 57 0.37
†
Trichloroethene 56 0.482 *
2,4,5-Trichlorophenoxy acetic acid 55 0.21 *
Trichloroacetic acid 52 0.216 *
2-Chlorophenol 51 0.41
†
o-Anisidine 50 0.34
†
Benzene 50 0.533 *
Dibromochloromethane 47 0.636 *

5-Bromouracil 44 0.47
†
Dichloroacetic acid 40 0.462 *
1,1-Dichloropropene 35 0.374 *
Methomyl 35 0.29 *
Benzo |a| pyrene 34 0.44
†
2,4-Dinitrophenol 33 0.61
†
1,1,2-Trichloroethane 33 0.652 *
Isophorone 32 0.39
†
1,3-Dichloropropane 28 0.497 *
Thymine 27 0.51
†
Chloral hydrate 27 0.051 *
5-Chlorouracil 25 0.58
†
N-Nitrosodi-n-propylamine 24 0.26
†
Bis(2-Chloroisopropyl) ether 24 0.57
†
Carbon tetrachloride 23 0.594 *
13.22 CHAPTER THIRTEEN
Single-stage contactors are often used for small groundwater systems, but if more
than one contactor is required, lower activated carbon usage rates can be achieved by
arranging the contactors either inseries or in parallel as shown inFigure 13.13,possibly
yielding a lower-cost system. GAC in a single-stage contactor must be removed about
the time the MTZ begins to exit the column (Figure 13.11).At this point only a portion
of the activated carbon is saturated at the influent concentration, so the activated car-

bon usage rate mayberelatively high.Alternatively,columns may bearrangedin series
so that the MTZ is entirely contained within the downstream columns after the lead
column has been saturated with the influent concentration.When the activated carbon
is replaced in the lead column,the flow is redirected so that it goes through the freshest
activated carbon last.Thus, the activated carbon“moves”countercurrent to the flow of
ADSORPTION OF ORGANIC COMPOUNDS 13.23
TABLE 13.1 Freundlich Adsorption Isotherm Parameters for Organic Compounds
(Continued)
Compound K (mg/g)(L/mg)
1/n
1/n Reference
Dalapon 23 0.224 *
1,2-Dibromoethane 23 0.471 *
1,2-Dibromoethene (EDB) 22 0.46 *
Endothall 22 0.329 *
Bromodichloromethane 22 0.655 *
Phenol 21 0.54
†
1,2-Dichloropropane 19 0.597 *
Methyl ethyl ketone 19 0.295 *
1,1-Dichloroethene 16 0.515 *
trans-1,2-Dichloroethene 14 0.452 *
1,1,1-Trichloroethane 13 0.531 *
Diquat 12 0.242 *
cis-1,2-Dichloroethene 12 0.587 *
Bis(2-Chloroethoxy) methane 11 0.65
†
Uracil 11 0.63
†
Benzo |g,h,i| perylene 11 0.37

†
1,1,2,2-Tetrachloroethane 11 0.37
†
Chloroform 9.4 0.669 *
Dibromomethane 9 0.701 *
1,2-Dichloropropene 8.2 0.46
†
1,1-Dichloroethane 8 0.706 *
Cyclohexanone 6.2 0.75
†
tert-Butyl-methyl ether 6 0.479 *
Trichlorofluoromethane 5.6 0.24
†
5-Fluorouracil 5.5 1
†
1,2-Dichloroethane 5 5.33 *
2-Chlorothethyl vinyl ether 3.9 0.8
†
Methylene chloride 1.6 0.801 *
Acrylonitrile 1.4 0.51
†
Acrolein 1.2 0.65
†
Cytosine 1.1 1.6
†
Ethylenediaminetetraacetic acid 0.86 1.5
†
Benzoic acid 0.76 1.8
†
Chloroethane 0.59 0.95

†
N-Dimethylnitrosamine 6.8 × 10
−5
6.6
†
* Speth and Miltner, 1998.
†
Dobbs and Cohen, 1980/Faust and Aly, 1983.
water, and lower activated carbon usage
rates are achieved than with single-stage
contactors. Series configuration is best
utilized when the effluentcriterionis very
low compared to the influent concentra-
tion (Wiesner, Rook, and Fiessinger,
1987). The increased cost of plumbing
counters the cost benefit of reduced acti-
vated carbon usage rate, however, espe-
cially when more than two columns must
be used in series.
When parallel-flow activated carbon
adsorbers are operated in staggered
mode, they can also be used to decrease
the activated carbon usage rate from
that which is possible with a single-stage
contactor (Westrick and Cohen, 1976;
Roberts and Summers, 1982). Because
the effluent from each of the units is
blended, each unit can be operated until
it is producing a water with an effluent
concentration in excess of the treated

water goal. Only the composite flow
must meet the effluent quality goal.
Other flow arrangements can be used to produce lower activated carbon usage
rates. A parallel-series arrangement of gravity filters is used in North Holland
(Schultink, 1982), and Sontheimer and Hubele (1987) report that a similar arrange-
ment using pressure filters with two layers of GAC was employed at Pforzheim,
West Germany. Each of the two layers can be backwashed and replaced indepen-
dently, and the order of flow through the layers can be reversed.A 35 percent lower
activated carbon usage rate for this system for removing halogenated hydrocarbons
from groundwater was reported compared to a single-stage system.
GAC contactors can also be classified by their position in the treatment train.The
filter-adsorber employs GAC to remove particles as well as dissolved organic com-
pounds.These contactors may be constructed simply by removing all or a portion of
the granular media from a rapid filter and replacing it with GAC. Alternatively, a
new filter box and underdrain system for the GAC may be designed and con-
structed. Graese, Snoeyink, and Lee (1987a) discuss these types of filters in detail.
The postfilter adsorber is preceded by a granular media filter,and thus has as its only
objective the removal of dissolved organic compounds. Backwashing of these adsor-
bers is unnecessary for particle removal, but if extensive biological growth occurs,
backwashing may be required as often as once per week, especially if immediately
preceded by ozonation (Fiessinger, 1983; Sontheimer, 1983).
PERFORMANCE OF GAC SYSTEMS
Factors Affecting Organic Compound Removal Efficiency
Adsorbate and GAC properties both have important effects on adsorption that have
been discussed in earlier sections. Additional factors that must be considered in the
design of full-scale systems are presented here.
13.24 CHAPTER THIRTEEN
FIGURE 13.13 Adsorber systems.
GAC Particle Size. The effect of particle size on the rate of approach to equilib-
rium in isotherm determination was discussed previously. It has a similar effect on

the rate of adsorption in columns. If the rate of adsorption is controlled by intra-
particle diffusion, the time to reach equilibrium with a given solution concentration
in a column approximates that for a batch test. With all other factors constant,
decreasing particle size will decrease the time required to achieve equilibrium and
will decrease the length of the MTZ in a column. Thus, to improve adsorption effi-
ciency and to minimize the size of column required, the particle size selected for a
contactor should be as small as possible.
The rate of head loss buildup caused by particle removal may limit the size of
GAC that can be used in adsorbers.The smaller the GAC, the higher the initial head
loss and rate of head loss buildup; thus, cost of energy and availability of head have
an important influence on the GAC size selected for a design.Additionally, if a filter-
adsorber is constructed by replacing media in an existing rapid filter, turbidity
removal efficiency generally increases as the GAC size decreases. If the medium is
too small, however, the rate of head loss buildup because of particle accumulation
becomes excessive, and the net water production will be too small for cost-effective
operation.The filters also become difficult to clean by backwashing.
The commercial sizes of GAC are typically characterized by a relatively large
uniformity coefficient of up to 1.9. This large coefficient causes the bed to restrat-
ify more easily after backwashing. This large uniformity coefficient also requires
that greater percent expansion of the adsorber be used during backwash in order
to expand the bottom media (Graese, Snoeyink, and Lee, 1987a). Some GAC fil-
ters use GAC with a small uniformity coefficient (∼1.3) in deep beds to improve
depth removal of turbidity and to increase net water production (Graese,
Snoeyink, and Lee, 1987b). Mixing of the media in these filters undoubtedly is
more than in filters with large uniformity coefficients, so they should not be used
in applications where desorption from the mixed GAC will require early GAC
replacement.
Common practice is to use 12 × 40 US Standard mesh (1.68 × 0.42 mm) or similar
activated carbon in postfilter adsorbers, because backwashing is rarely required due
to head loss buildup.This carbon size also is commonly used in filter-adsorbers when

it is the only filter medium and the filter depth is less than about 75 cm (30 in.).
Deeper beds that will be used to remove turbidity commonly employ 8 × 30 US
Standard mesh (2.38 × 0.60 mm) or larger activated carbon to promote longer filter
runs. The option of using custom-sized GAC to obtain a better media design for a
particular application also is available.
Contact Time, Bed Depth, and Hydraulic Loading Rate. The most important
GAC adsorber design parameter that affects performance is the contact time, most
commonly described by the EBCT.For a given situation,a critical depth of GAC and
a corresponding minimum EBCT (Equation 13.8) exist that must be exceeded to
contain the MTZ and minimize or eliminate immediate breakthrough.As the EBCT
increases, the bed life or service time (expressed in bed volumes of product water to
breakthrough) will increase until a maximum value is reached. Correspondingly, the
activated carbon usage rate will decrease to a minimum value. For example, Figure
13.14 shows that the operating time or service time of a column will increase with
increasing depth, although the increase is not always linear with depth.These curves
are commonly called bed depth–service time curves and may be used to determine
the critical depth as shown in Figure 13.14. Figure 13.14 also shows that the percent-
age of activated carbon in a column exhausted at MTZ breakthrough increases as
depth or EBCT increases.The mass of organic matter adsorbed per unit mass of acti-
vated carbon increases as percent exhaustion increases and, correspondingly, the
ADSORPTION OF ORGANIC COMPOUNDS 13.25