Reaction of polycyclic aromatic hydrocarbons adsorbed on silica in aqueous chlorine

Article abstract of DOI:10.1021/es062005n

The reaction of polycyclic aromatic hydrocarbons (PAHs) previously adsorbed on silica gel or diatomaceous earth with sodium hypochlorite was carried out to elucidate their reactivity to aqueous chlorine. It was demonstrated that the PAHs adsorbed on silica reacted more rapidly than the PAHs themselves in water, leading to the formation of many chlorinated and oxidized derivatives. A similar reaction in the presence of potassium bromide was found to preferentially produce corresponding brominated derivatives. These reactions seem to proceed through PAHs adsorbed on the silica surface and halogenating agents, the electrophilicity of which may be raised by the catalytic effect of the silanol group of the silica surface. These findings from the environmental viewpoint suggest that the reaction of hydrophobic compounds adsorbed on sediment cannot be neglected.

Full text of DOI:10.1021/es062005n

Environ. Sci. Technol. 2007, 41, 2190-2195
It is well-known that the disinfection process using
Reaction of Polycyclic Aromatic
Hydrocarbons Adsorbed on Silica in
Aqueous Chlorine
aqueous chlorine leads to the formation of various chlorinated
or otherwise oxidized byproducts. From the viewpoint of
toxicity of halogenated PAHs, 1-chloropyrene was reported
to be higher mutagenic than pyrene (23). Although reactions
of PAHs with aqueous chlorine in the laboratory have been
reported in several papers (24-27), such reactions seem not
to proceed smoothly in water without a cosolvent due to
their hydrophobic property. This paper describes that pyrene,
fluorene, and fluoranthene, which are frequently detected
in aquatic environments, adsorbed on silica easily react with
aqueous chlorine in water to produce various chlorinated or
brominated derivatives. Silica was used as a model adsorbent
because of its prevalence in the environment and well
characterized properties.
H I D E Y U K I N A K A M U R A , †
Y U Z O T O M O N A G A , † K A N A M I Y A T A , †
M I T S U O U C H I D A , ‡ A N D
Y O S H I Y A S U T E R A O * , †
Institute of Environmental Sciences, COE Program in the 21st
Century, and School of Pharmaceutical Sciences, University of
Shizuoka, 52-1 Yada Suruga-ku, Shizuoka 422-8526, Japan
Materials and Methods
The reaction of polycyclic aromatic hydrocarbons (PAHs)
previously adsorbed on silica gel or diatomaceous
earth with sodium hypochlorite was carried out to elucidate
their reactivity to aqueous chlorine. It was demonstrated
that the PAHs adsorbed on silica reacted more rapidly than
the PAHs themselves in water, leading to the formation
of many chlorinated and oxidized derivatives. A similar reaction
in the presence of potassium bromide was found to
preferentially produce corresponding brominated derivatives.
These reactions seem to proceed through PAHs adsorbed
on the silica surface and halogenating agents, the
electrophilicity of which may be raised by the catalytic
effect of the silanol group of the silica surface. These findings
from the environmental viewpoint suggest that the
reaction of hydrophobic compounds adsorbed on sediment
cannot be neglected.
Chemicals. Pyrene, fluorene, 9-fluorenone, fluoranthene, and
1-bromopyrene [5] were purchased from Sigma-Aldrich
Chem. Co. Ltd. (Tokyo, Japan). Sodium sulfite, sodium
hypochlorite solution (>5% available chlorine), and sodium
hypobromite solution (>9% available bromine) were pur-
chased from Wako Pure Chemicals Industries Ltd. (Osaka,
Japan). The available chlorine or bromine concentration in
the solution was measured by iodometric titration. Ethyl
acetate, methanol, and hexane used were purchased from
Kanto Co. Ltd. (Tokyo, Japan). Deionized water was used for
all the experimental procedures. Silica gel 60N (spherical,
neutral) was obtained from Kanto Co. Ltd. (Tokyo, Japan).
Diatomaceous earth (chemical grade) and all the other
chemicals used were purchased from Wako Pure Chemicals
Industries Ltd. (Osaka, Japan).
Syntheses of Chlorinated and Brominated Derivatives
of Pyrene as Reference Materials. 1-Chloropyrene [1], 1,6-
dichloropyrene [2], 1,8-dichloropyrene [3], 1,3,6-trichloro-
pyrene [4], 1,6-dibromopyrene [6], 1,8-dibromopyrene [7],
and 1,3,6-tribromo-pyrene [8] were synthesized by the
reaction of pyrene with sodium hypochlorite in methanol
solution in the presence or absence of potassium bromide.
The structures of chlorinated or brominated derivatives of
pyrene shown in Figure 1 were determined by FAB-MS and
NMR spectroscopy. The detailed synthetic procedures and
the spectral data for chlorinated or brominated derivatives
of pyrene are provided in the Supporting Information.
Reaction of PAHs Adsorbed on Silica with Sodium
Hypochlorite. A solution of appropriate PAH (25 µmol) in
acetone (0.2 mL) was added to silica gel or diatomaceous
earth (0.5 g), the mixture was stirred for 30 min, and then
acetone was completely removed under reduced pressure.
Deionized water (10 mL) was added to the silica-adsorbed
PAH, where it had not been detected in the aqueous layer
after the suspension had been stirred for 1 h. An aqueous
solution of sodium hypochlorite was added to the suspension
prepared at pH 5 (by adding 2% hydrochloric acid) or pH 9
(by adding 2% sodium hydroxide), and the mixture (available
chlorine, 1 ppm) was stirred at room temperature without
serious fluctuation of the pH value. The reactions were started
in six vessels at the same time and stopped by adding an
aqueous solution of sodium sulfite (1 mM, 0.3 mL) after 1,
2, 3, 6, 12, and 24 h. After the mixture was acidified with 2%
hydrochloric acid, ethyl acetate was added to the mixture,
and the mixture was stirred for 30 min. After the solids had
been removed by filtration, the organic layer of the filtrate
was separated. The aqueous layer and the solids on the filter
were extracted with ethyl acetate until no peaks due to the
products in the extract could be detected by GC-MS analysis.
Introduction
A number of hydrophobic compounds are released into
aquatic environments. The association of these organic
compounds with sediments plays an important role in the
distribution and dynamics of organic pollutants in the aquatic
environments. Of these compounds polycyclic aromatic
hydrocarbons (PAHs) have been found at high concentrations
in sediments (1-4). Our interest is in the reactivity of adsorbed
PAHs to aqueous chlorine, which is used in the disinfection
processes of many water supply systems and sewage treat-
ment plants.
PAHs are ubiquitous environmental pollutants. Many
PAHs are suspected or known potent mutagens and car-
cinogens that may pose serious human and environmental
risks (5, 6), and they have also been recognized as aryl
hydrocarbon receptor (AhR) agonists (7-13). Therefore, many
papers have recently been published highlighting their effects
on health due to their endocrine-disrupting activities (14-
18). PAHs are predominantly produced by anthropogenic
activities such as the burning of fossil fuels or other
combustion sources. The occurrence of PAHs in raw water
is due to atmospheric fallout, urban runoff, municipal
effluents, industrial effluents, and oil spillage or leakage (1-
4, 19-22).
* Corresponding author phone: +81-54-264-5788; fax: +81-54-
264-5788; e-mail: terao@u-shizuoka-ken.ac.jp.
† Institute of Environmental Sciences, COE Program in the 21st
Century, University of Shizuoka.
‡ School of Pharmaceutical Sciences, University of Shizuoka.
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2190 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 7, 2007
10.1021/es062005n CCC: $37.00
2007 American Chemical Society
Published on Web 03/01/2007

FIGURE 1. Structures of pyrene, fluorene, 9-fluorenone, fluoranthene, and their halogenated derivatives.
After the combined organic solution was dried over sodium
sulfate, the ethyl acetate was removed under reduced
pressure. The residue obtained was subjected to GC-MS
analysis.
The reaction of adsorbed pyrene with sodium hypochlorite
in the presence of excess potassium bromide (125 µmol) and
the reaction with sodium hypobromite were carried out with
a method similar to that described above.
using the SPB-35 column. GC-MS analysis was at a flow rate
of 1.5 mL/min and an injection temperature of 280 °C. The
column temperature was initially maintained at 50 °C for 1.5
min, then was programmed from 50 to 280 °C at a rate of 2
°C /min, with a final hold time of 24 min. The column
temperature for GC analysis was programmed from 150 to
250 °C at a rate of 10 °C /min, with a final hold time of 20
min.
Isolation of Chlorinated and/or Oxidized Fluorene and
Fluoranthene. To isolate the chlorinated and/or oxidized
products, large scale reactions were carried out as follows.
The reaction of fluorene or fluoranthene (250µmol) adsorbed
on silica gel (5 g) in deionized water (100 mL) was undertaken
with a method similar to that described above. The extract
was subjected to preparative HPLC using an ODS-A column
(50 × 250 mm, YMC Co. Ltd., Kyoto, Japan; 80% acetonitrile).
The structures of the isolated products were determined by
MS and NMR spectroscopy. The spectral data for chlorinated
and/or oxidized derivatives of fluorene and fluoranthene are
provided in the Supporting Information.
GC-MS analysis using the DB-5 column was performed
on the products of fluorene, 9-fluorenone, and fluoranthene
at a flow rate of 1 mL/min and an injection temperature of
280 °C. The column temperature was initially held at 50 °C
for 1.5 min, then was programmed to 150 °C at a rate of 20
°C/min, from 150 to 310 °C at a rate of 8 °C/min, with a final
hold time of 15 min.
Ethyl acetate extracts were prepared for analysis by
diluting them (1/500) with ethyl acetate. Aliquots (1 µL) were
then injected on the GC or GC-MS. The yields were shown
as a percentage of the mole of chlorinated pyrene, which
was provided by calibration curves with a correlation
coefficient (r2) ranging from 0.979 to 0.999, to the mole of
starting pyrene (25 µmol).
Analytical Procedure for PAHs. The gas chromatography
(GC) and the gas chromatography/mass spectrometry (GC-
MS) measurements were done on a SHIMADZU 14-A gas
chromatograph (GC) using flame ionization detector (FID)
and a Hewlett-Packard HP-6890 gas chromatograph/HP-5972
mass selective detection (MSD) system for GC-MS. The MSD
system was run in the electron impact ionization mode, and
the electron energy was 70 eV. Nitrogen was used as the
carrier gas for GC and helium for GC-MS. The capillary
columns used were either a SUPELCO SPB-35 (35% diphenyl
and 65% dimethylsiloxane, coating film thickness 0.25 µm,
30 m × 0.32 mm i.d., Sigma-Aldrich Chem. Co. Ltd.) or a
DB-5 (5% phenylmethylpolysiloxane, coating film thickness
0.25 µm, 30 m × 0.32 mm i.d., J & W Scientific).
Results
Reaction of Pyrene Adsorbed on Silica. We carried out the
reaction of PAHs previously adsorbed on silica, where silica
gel and diatomaceous earth were adopted as model adsor-
bents, with aqueous chlorine.
The chromatogram (GC-FID) of the raw products, which
were obtained by reacting of pyrene adsorbed on silica gel
in water at pH 5 and 9 for 24 h, are shown in Figure 2. The
peaks due to monochloro-, dichloro-, and trichloropyrene
were observed with a small peak of pyrene at pH 5 and 24
h contact time. Each peak was identified by comparing it
with authentic samples, which were synthesized by the
The qualitative and quantitative analysis of the chlorinated
or brominated pyrene was carried out by GC and GC-MS
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VOL. 41, NO. 7, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 2191

FIGURE 2. Chromatograms (GC-FID) of raw products obtained by reaction of pyrene adsorbed on silica gel or diatomaceous earth (D.E.)
in water with sodium hypochlorite (at pH 5 or 9 for 24 h) 1: 1-chloropyrene, 2: 1,6-dichloropyrene, 3: 1,8-dichloropyrene, 4: 1,3,6-
trichloropyrene.
reaction of pyrene with sodium hypochlorite in methanol.
In the reaction of pyrene itself in water, a large peak due to
nonreacted pyrene was found along with very small peaks
due to monochloro- and dichloropyrene. The reactivity of
pyrene adsorbed on diatomaceous earth was slightly lower
than that on silica gel. The difference in reactivity between
pH values may be attributed to that the more reactive HOCl
prevails at pH 5, and OCl- at pH 9. These results suggest that
the adsorption of hydrophobic PAHs on silica greatly
increased the reactivity of aqueous chlorine. To clarify these
adsorption effects, we conducted more detailed studies of
the production and the reactions of several other PAHs.
trichloropyrene, tetrachloropyrene, and hydroxylchloropy-
rene were observed in the TIC in addition to the four products
described above. Time-course for reaction of pyrene adsorbed
on diatomaceous earth was analogous to that on silica gel
at pH 5 (data not shown).
Reactions of Fluorene and Fluoranthene. Our findings
on the reaction behavior of pyrene prompted us to study the
reactions of PAHs adsorbed on silica with aqueous chlorine
in detail. We carried out the reactions of fluorene and
fluoranthene that have frequently been detected in the
environment. The TIC of the raw products obtained by the
reactions of fluorene and fluoranthene adsorbed on silica
gel at pH 5 after 24 h are shown in Figure 4.
The reaction of fluorene adsorbed on silica gel at pH 5
produced many kinds of chlorinated derivatives. The main
products were 2-chlorofluorene [9], 2,5-dichlorofluorene [11],
and 2,7-dichlorofluorene [12]. Chlorinated derivatives of
9-fluorenone were also produced. They seem to be produced
by the oxidation of the corresponding chlorinated fluorenes
that are formed first, because the adsorbed 9-fluorenone did
not react with aqueous chlorine under similar conditions.
4-Chlorofluorene [10], trichlorofluorene [13, 18], and fluorene
dimer were tentatively identified by their MS spectra as minor
products.
Many chlorinated derivatives were also produced by the
reaction of fluoranthene adsorbed on silica gel at pH 5. The
main products were 3-chlorofluoranthene [19], 3,8- dichlo-
rofluoranthene [21], and 3,4,8-trichlorofluoranthene [24],
which show the ease of the chlorine substitution at the 3-,
4-, and 8-position.
Time-Course for Reaction of Pyrene Adsorbed on Silica.
The time-course of reaction of the adsorbed pyrene showing
formation of its chlorinated derivatives at pH 9 is displayed
in Figure 3. In the reaction at pH 9, pyrene adsorbed on silica
gel was consumed gradually and decreased to 17% after 24
h. At the same time, 1-chloropyrene [1], 1,6-dichloropyrene
[2], 1,8-dichloropyrene [3], and 1,3,6-trichloropyrene [4]
gradually increased to 26%, 29%, 19%, and 1% yield,
respectively. The reaction behavior of pyrene adsorbed on
diatomaceous earth was slightly different from that on silica
gel; pyrene rapidly decreased to 27% and product 1 increased
to 36% after 3 h. Products 2, 3, and 4 were produced in 15%,
9%, and 1%, respectively, and the yields of these products
were similar even after 24 h.
Pyrene adsorbed on silica gel was consumed rapidly at an
initial stage of the reaction at pH 5 (data not shown). The
production of chlorinated derivatives, not only product 1,
but also 2 and 3, started at the same time. The adsorbed
pyrene decreased to 5% after 1 h and to below 1% after 24
h resulting in 1 (4%), 2 (22%), 3 (12%), and 4 (12%).
Furthermore, peaks with molecular ions corresponding to
Bromination of Pyrene with Sodium Hypochlorite in
the Presence of Potassium Bromide. Many investigators
have reported that bromination takes place in preference to
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FIGURE 3. Time-courses for reaction of pyrene (a) suspended in
water, adsorbed on (b) silica gel and (c) diatomaceous earth at pH
9; b, pyrene; ] 1-chloropyrene; 4 1,6-dichloropyrene; O 1,8-
dichloropyrene; 0, 1,3,6-trichloropyrene.
chlorination in the presence of Br- (26, 28-30). We inves-
tigated such bromination of adsorbed PAHs. As was expected,
the bromination of pyrene adsorbed on silica gel or diato-
maceous earth continues to preferentially yield mono-, di-,
and tribromopyrene in the presence of potassium bromide
(Figure 5), where none of the chlorinated derivative was
detected after 24 h.
When this reaction was compared with that using sodium
hypobromite, the reaction in the KBr/NaOCl system was
observed to be much faster (Figure 6). It is thought that the
reaction species in the KBr/NaOCl system is not NaOBr but
BrCl which was reported to be very active bromination species
(31, 32).
FIGURE 4. TIC (GC-MS) of byproducts obtained from reaction with
hypochlorous acid (pH 5 for 24 h). (a) Reaction of fluorene suspended
in water, (b) reaction of fluorene adsorbed on silica gel, (c) reaction
of fluoranthene suspended in water, and (d) reaction of fluoranthrene
adsorbed on silica gel. Peak numbers correspond to the compound
numbers in Figure 2.
TABLE 1. Recovery of PAHs (%) in the Reaction with Sodium
Hypochlorite for 24 h
silica gel
diatomaceous earth
water
Discussion
compound
pH 5 pH 9
pH 5
pH 9
pH 5 pH 9
It was reported (25, 26) that 2-chlorofluorene, 2,7-dichlo-
rofluorene, and 9-fluorenone were produced by the reaction
of fluorene with aqueous chlorine, and only 3-chlorofluo-
ranthene was detected from fluoranthene. However, in the
reaction of silica-adsorbed PAHs we could isolate many
chlorinated derivatives as described above. Table 1 illustrates
the recovery (%) of used PAHs in the reactions after 24 h.
Although the reaction rate estimated roughly depended on
the structure of PAHs, especially on pH value, it is obvious
that silica-adsorbed PAHs react more rapidly than those in
water.
Ringwald et al. (33) reported that they estimated the
adsorption of gaseous benzene or toluene on silica to occur
via π-system-hydrogen bonding with silanol groups on the
silica surface on the basis of Raman and FTIR spectroscopy.
A silanol group on silica, on the other hand, has acted as a
catalyst in organic reactions (34-37). Therefore, we consider
that the electrophilicity of chlorine of hypochlorous acid
pyrene
0
0
5
17
71
81
18
34
27
29
68
58
77
91
95
101
89
fluorene
fluoranthene
103
increases by hydrogen bonding with silanol on the surface
and the interaction between silanol group and π-electron
system of PAHs increases the chance of contact with PAHs
molecules (Figure 7). The photocatalyzed reaction can be
neglected, because we obtained the same result as shown in
Figure 2 when a reaction of silica gel-adsorbed pyrene was
carried out in a dark place. The difference in reactivity
between silica gel and diatomaceous earth can be mainly
affected by the structure of silicon dioxide.
The concentrations of PAHs in sediments are generally
higher than those in the surrounding water because of their
low solubility in water (1-4). Although the chlorination of
PAHs in aqueous chlorine has often been reported, little
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due to their high hydrophobicity. Byproducts adsorbed on
sediment, including those generated with an aqueous
chlorine or other reactive chemicals, should be investigated
from the viewpoint of aquatic environmental pollution.
Acknowledgments
This work was supported in part by COE21 (Center of
Excellence in the 21st Century) program.
Supporting Information Available
Detailed synthetic procedure for standard chlorinated or
brominated pyrene, spectral measurements by FAB-MS and
NMR, and spectral data for chlorinated, brominated, and/or
oxidized derivatives of pyrene, fluorene, and fluoranthene.
This material is available free of charge via the Internet at
http://pubs.acs.org.
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Received for review August 21, 2006. Revised manuscript
received December 14, 2006. Accepted February 2, 2007.
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