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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
[...]... 13.11 Adsorption column breakthrough curve mass of GAC in column mass CUR ᎏ = ᎏᎏᎏᎏ volume volume treated to breakthrough VB (13.9) Breakthrough curves are strongly affected by the presence of nonadsorbable compounds, the biodegradation of compounds in a biologically active column, slow adsorption of a fraction of the molecules present, and the critical depth of the column relative to the length of the... volume of interparticle voids ADSORPTION OF ORGANIC COMPOUNDS 13.19 enter pores that are unavailable to larger adsorbates As a result, all of the BET surface 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. .. THMs and total organic halide (TOX), and assimilable organic carbon (AOC), a measure of the biodegradable fraction of NOM, by biofiltration of ozonated and settled Ohio River water All filters had a total depth ADSORPTION OF ORGANIC COMPOUNDS 13.29 TABLE 13.2 Process Parameters for Activated Carbon Following Ozone Parameter Value Ozone dosage Biological degradation Oxygen demand for DOC oxidation EBCT... Within the MTZ, the degree of saturation with adsorbate varies from 100 percent (q = [qe]0) to zero The length of the MTZ, LMTZ, depends upon the rate of adsorption and the solution flow rate Anything that causes a higher rate of adsorption, 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... concentration increased as (1) the diffusion coefficient of the ADSORPTION OF ORGANIC COMPOUNDS 13.13 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... carbonization, or pyrolysis, is usually performed in the absence of air at temperatures less than 700°C, while activation is carried out with oxidizing gases such as ADSORPTION OF ORGANIC COMPOUNDS 13.17 FIGURE 13.12 Effect of biodegradation and the presence of nonadsorbable compounds on the shapes of breakthrough curves steam and CO2 at temperatures of 800 to 900°C Chemical activation combines carbonization... the desorption 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... 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 ADSORPTION OF ORGANIC COMPOUNDS 13.25 GAC Particle Size The effect of particle size on the rate of approach to equilibrium in isotherm determination was discussed previously It has a similar effect on the rate of adsorption in columns... (1987) found a small amount of biological oxidation if the water was not preozonated, but application of 1.1 mg O3 /mg DOC resulted in removal of 35 to 40 percent of the influent DOC by biological oxidation Biodegradable compounds may be removed by microbes, without prior 13.28 CHAPTER THIRTEEN FIGURE 13.15 DOC removal by adsorption and biodegradation during GAC filtration of an ozonated humic acid solution... solution (Source: Sontheimer and Hubele, 1987.) adsorption to the GAC, if a biofilm capable of degrading such compounds is developed before they are applied Adsorbable biodegradable compounds may be adsorbed first if the biofilm is not developed when the compounds enter the column, and then desorbed and degraded as the biofilm develops The use of GAC in biofilters has recently been reviewed by Servais . 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:. distribution of lengths, however.
ADSORPTION OF ORGANIC COMPOUNDS 13.17
FIGURE 13.12 Effect of biodegradation and the presence of nonadsorbable compounds
on
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