Environmental Engineering Reference
In-Depth Information
Nevertheless, Freundlich parameters are often used in models and many
data obtained for activated carbons are available in the literature [32-37]. Only
a few data for NSO-heterocycles are known, and isotherms were determined
especially for NSO-heterocycles on the activated carbon Epibon Y12×40.
To assess the adsorption of N-HETs in PRBs, the influence of the pH value
was investigated for 2-methylquinoline (pK
a
= 5.86) as a model compound.
Knowledge of the surface properties of activated carbons is essential in the
explanation and comparison of adsorption parameters. Isotherms differ accord-
ing to the type of activated carbons. A comparison of adsorption parameters
was performed for activated carbons usually employed for the remediation of
organic compounds in PRBs. Furthermore, experiments were performed with
artificial mixtures of six compounds, evaluated by the the ideal adsorbed solu-
tion (IAS)-model [38-40] with basic algorithms from Reference 31.
11.3.2.1 Adsorption Isotherms of Single Compounds in Water
Equilibrium concentrations (
c
), and adsorbed loadings (
q
), were obtained for
a number of compounds using Epibon Y12×40 (Donau Carbon) as activated
carbon. A plot of log (
q
) versus log (
c
) was performed and Freundlich param-
eters are presented in Table 11.3.
The dependence of
q
=
f
(
c
) is always nonlinear, and Freundlich exponents
(
n
), are in the range of 0.1-0.4. For N-HETs,
K
is always lower compared to the
parent PAH (e.g., quinoline and isoquinoline vs. naphthalene or phenanthri-
dine vs. phenanthrene). A few examples available in the literature suggested
an order
K
(S-HET) >
K
(O-HET) >
K
(N-HET) for the same parent structure,
verified in this study for all compounds shown in Table 11.3.
Freundlich parameters are valid only in a limited concentration range. Unlike
benzene, the strong adsorbing compounds acenaphthene, 9-methylacridine,
phenanthrene, and dibenzothiophene (Figure 11.5) can serve as examples
with adsorption isotherms evaluated with, for example, the combined
Langmuir-Freundlich model.
11.3.2.2 Adsorption Isotherm of 2-Methylquinoline: Dependence on pH
The pH-dependent adsorption of 2-methylquinoline was investigated in the
pH range of 1.9-8. The load of 2-methylquinoline (shown in Figure 11.6), was
calculated from 12 experimental adsorption isotherms on Epibon Y12×40
(see also Table 11.3) using the Freundlich model for
c
0
= 10 mg/L and the
fraction of the neutral molecule (calculated from pK
a
= 5.86). The load of the
neutral molecule (e.g., at pH ≫ pK
a
) is approximately a factor of 4 higher com-
pared to the load of the cation (e.g., at pH ≪ pK
a
). Both curves for the frac-
tions of the neutral compound in the solution, and the experimental loads
proceed nearly parallel and are shifted to each other by approximately 1.5
pH-units. The two-component mixture consists of the pH-dependent neu-
tral and cationic form of 2-methylquinoline. Using the data from Table 11.3,
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