Environmental Engineering Reference
In-Depth Information
for isomers. Hence, naphthalenes substituted in 1-position (e.g., methylated,
ethylated, or hydroxylated derivatives) were found in higher concentrations
compared to the corresponding 2-substituted naphthalenes. Furthermore and
in contrast to concentrations of 2-phenylpyridine, 3- and 4-phenylpyridine
were usually found below detection limits.
Adsorption isotherms on activated carbons (using especially Epibon
Y12×40 from Donau Carbon) were obtained from batch experiments for both
single compounds and mixtures. In addition, Freundlich parameters for
both experimental methods were compared with column runs. Freundlich
parameters were in satisfactory agreement using different experimental
methods with a 20%-30% variation. However, adsorption parameters for
many compounds found in contaminated groundwater (e.g., methylated
naphthalenes and methylated NSO-heterocycles) have not been published
in the literature. From the few values available, qualitatively, the adsorption
of S-heterocycles seems to be comparable with the corresponding parent
hydrocarbons, whereas the adsorption of N-HETs was generally found to be
lower. Adsorption parameters of O-heterocycles could be qualitatively clas-
sified between S- and N-HETs (for the same parent structures). These rules of
thumb are in agreement with analytical results. O- and S-heterocycles were
analyzed with comparable detection limits for GC-MS, whereas LC-MS/MS
was found to be a method with the lowest detection limit for the much more
polar N-HETs.
The pH-dependent adsorption of 2-methylquinoline as an example of an
N-HET was determined. As assumed and in agreement with results from
comparable amines and carboxylic acids, the adsorption of the neutral com-
pound (e.g., at pH ≫ pK
a
) was found to be higher compared to the corre-
sponding cation at pH ≪ pK
a
. However, even for 2-methylquinoline with
pK
a
= 5.86, a maximum adsorption was found for pH > 6. Hence, a pH-
dependence is not considered to be important for the N-HETs investigated in
a theoretical discussion of the efficiency of PRBs.
In addition to the efficiency of PRBs for organic compounds using adsorption
parameters for activated carbon, NSO-heterocycles were also investigated in
the study. Only a few adsorption isotherms are available. However, an overall
assessment and a comparison of adsorption parameters of NSO-heterocycles
with their parent hydrocarbons is possible. Benzene and phenol were the
compounds with the lowest adsorption capacities and with the fastest break-
through in column runs. However, although not investigated, pyridine and
methylpyridines were assumed to be N-HETs with even lower Freundlich
parameters and lower adsorption capacities. Hence, pyridine and derivatives
should be included in future investigations. A ranking was conducted based
on column runs and benzofuran was found to be the heterocycle with the
lowest adsorption capacity. The adsorption capacity of benzothiophene was
higher (compared to benzofuran) and comparable to indane.
From the results obtained, general conclusions can be drawn with respect
to the design and operation of reactive barriers using activated carbon. In the
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