Photochemistry of pyrene on unactivated and activated silica surfaces

Article abstract of DOI:10.1021/es9905391

Photolysis of pyrene at the solid/air interface of unactivated and activated silica gel proceeds slowly to give mainly oxidized pyrene products. We have identified 1-hydroxypyrene, 1,6-pyrenedione, and 1,8-pyrenedione among the main reaction products. The remaining minor products show molecular weights and spectral properties consistent with oxygenated pyrenes. Furthermore, small amounts of 1,1′-bipyrene dimer are also formed at higher surface coverages (2 × 10⁵ mol/g). When photolysis is carried out at 5 × 10^-5 mol/g pyrene, photodegradation rate drops sharply and pyrene loss becomes insignificant. No significant change in the product distribution is observed when the photolysis is carried out on unactivated or activated silica. Photodegradation rate is slightly faster on activated silica compared to unactivated silica. Mechanistic studies indicate that the precursor to photoproduct formation is pyrene cation radical which is postulated to be formed by electron transfer from pyrene excited state to oxygen (type I) or by photoionization of pyrene. The cation radical reacts with physisorbed water on silica to give the observed oxidation products.

Full text of DOI:10.1021/es9905391

Environ. Sci. Technol. 2000, 34, 415-421
these molecules and the threat they pose when present in
respirable ambient particles (2, 7, 8).
Photochemistry of Pyrene on
Unactivated and Activated Silica
Surfaces
C E L S O A . R E YE S , † M YR I A M M E D I N A , †
C A R L O S C R E S P O - H E R N A N D E Z , †
M A YR A Z . C E D E N O , † R A F A E L A R C E , †
O S V A L D O R O S A R I O , †
D A N I E L M . S T E F F E N S O N , ‡
I L I A N . I V A N O V , ‡
M I C H A E L E . S I G M A N , ‡ A N D
R E Z A D A B E S T A N I * , ‡
Marine and lacustrine sediments are among the ultimate
environmental sinks for PAHs (9). Small PAHs have been
shown to reside mainly in the gas phase when present in the
atmosphere (10). Insulators such as silica, alumina, sili-
coaluminates, and calcium carbonate are believed to con-
stitute up to 20-30% of inorganic particulates present in the
atmosphere (11). The use of silica as a model substrate in
this work is the continuation of our effort in the area of PAH
photochemistry under controlled conditions, to gain a better
understanding of PAH photodegradation mechanism that
are relevant to environmental processes occurring on similar
materials.
Department of Chemistry, University of Puerto Rico,
P.O. Box 23346, Rio Piedras, Puerto Rico 00931-3346, and
Chemical and Analytical Sciences Division, Oak Ridge
National Laboratory, P.O. Box 2008, Ms-6100,
Oak Ridge, Tennessee 37831-6100
Photodegradation studies of pyrene and other PAHs on
particulate matter have focused on the effect of surface’s
chemical and physical properties such as color, carbon
content, surface area, particle porosity and size, and surface
pH on the rate of phototransformation (12-15). Photolytic
half-lives for pyrene and 17 other PAHs adsorbed on different
substrates such as alumina, silica, fly ash, and carbon black
have been determined (14).
Photolysis of pyrene at the solid/air interface of unactivated
and activated silica gel proceeds slowly to give mainly
oxidized pyrene products. We have identified 1-hydroxypyrene,
1,6-pyrenedione, and 1,8-pyrenedione among the main
reaction products. The remaining minor products show
molecular weights and spectral properties consistent with
oxygenated pyrenes. Furthermore, small amounts of 1,1′-
bipyrene dimer are also formed at higher surface coverages
(2 × 10-5 mol/g). When photolysis is carried out at 5 ×
10-5 mol/g pyrene, photodegradation rate drops sharply and
pyrene loss becomes insignificant. No significant change
in the product distribution is observed when the photolysis
is carried out on unactivated or activated silica. Photo-
degradation rate is slightly faster on activated silica compared
to unactivated silica. Mechanistic studies indicate that
the precursor to photoproduct formation is pyrene cation
radical which is postulated to be formed by electron transfer
from pyrene excited state to oxygen (type I) or by
photoionization of pyrene. The cation radical reacts with
physisorbed water on silica to give the observed oxidation
products.
Mamantov et al. (15) have reported that pyrene was least
photoreactive when adsorbed on the carbonaceous sub-
fraction of coal stack ash but showed high reactivity when
adsorbed on mineral subfraction, silica or alumina. An early
report on the photodegradation of pyrene adsorbed on garden
soil identified five of the eight photoproducts as 1,1′-bipyrene,
1,6- and 1,8-pyrenedione, and 1,6- and 1,8-dihydroxypyrene
(16).
The formation of excited singlet and triplet states of pyrene
on silica (17-24) and alumina (25-27) have been reported.
The nature and dynamics of these states have been used as
probes to provide information about the surface properties
and adsorption sites. Using diffuse reflectance laser flash
photolysis technique, the formation and kinetics of pyrene
excited triplet state and cation radical on alumina have been
studied (26). More recently, Thomas and Mao (28) investi-
gated the photochemical behavior of pyrene adsorbed on
active sites of activated γ-alumina and silica-alumina (T >
350 °C) and reported that the main photoproducts of this
reaction were mono- and dihydroxypyrene. They also
reported that pyrene adsorbed on these surfaces forms
pyrene-adsorbate charge-transfer complexes and pyrene
radical cation via a photoinduced process as revealed by
EPR and diffuse reflectance spectroscopy (28). The study
further states that photolysis of pyrene adsorbed on non-
activated surfaces proceeds via a similar radical process to
give only one detectable product.
Introduction
Analyses of airborne particulate matter have shown signifi-
cant mutagenic and/ or carcinogenic activity. This is ascribed
not only to the primary pollutants but also to their degrada-
tion products (1-5). Although only at trace levels, the organic
fraction of a particulate represents a real danger to humans
because its assimilation and possibilities of transformation
increases once in the body. Polycyclic aromatic hydrocarbons
(PAHs), an important class of pollutants present in air, are
released into the atmosphere by the incomplete combustion
of fossil fuels. Many of these PAHs have been classified as
priority pollutants by the EPA (6). Thus, the study of PAHs
and their possible phototransformations in the atmosphere
is important owing to the carcinogenic nature of some of
Previous reports on photochemistry of a number of PAHs
adsorbed on silica, alumina, and other surfaces have focused
on determining the photoproducts and mechanism of their
formation (12-15, 29-38). In this study, we report the
photochemical reaction of pyrene adsorbed on activated and
unactivated silica under (a) simulated laboratory irradiation
conditions and (b) solar irradiation in order to mimic the
atmospheric particulate photolysis conditions. Photodeg-
radation of pyrene with time was followed directly on the
adsorbent by front face emission and diffuse reflectance
absorption spectroscopy. Mechanistic studies indicate that
the intermediate leading to photoproducts is pyrene radical
cation which may be generated by electron transfer from
excited triplet state of pyrene to oxygen (type I) or by
photoionization. No evidence of singlet oxygen involvement
in the photooxidation process is observed.
* Corresponding author phone: (423)576-7325; fax: (423)574-7596;
e-mail: dabestanir@ornl.gov.
† University of Puerto Rico.
‡ Oak Ridge National Laboratory.
9
10.1021/es9905391 CCC: $19.00
Published on Web 12/22/1999
2000 Am erican Chem ical Society
VOL. 34, NO. 3, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 4 1 5

followed by sonication in an ice bath for 10 min. The solution
was then centrifuged for 5 min to remove any suspended
particles and preconcentrated to 1-2 mL under a soft flow
of nitrogen for HPLC analysis.
Materials and Methods
Chem icals. Pyrene (Supelco, 99.9%) was recrystallized from
ethanol before use. All solvents used were HPLC grade. Silica-
60 (EM Reagents) particle size 0.04-0.06 mm and surface
area of 550-675 m2 g-1 as reported by the manufacturer was
used as received (unactivated). The physical characteristics
of activated silica (J. T. Baker, Phillipsburg, PA; reagent grade,
60-200 mesh) used in this work have been reported
previously (34). The N2 BET surface area for the activated
silica was determined to be 274 m2 g-1 with an average pore
radius of 60 Å. The silica was activated by heating for a
minimum of 24 h at 200 °C in air.
Separation of photoproducts was achieved by HPLC using
a C-18 reverse phase column (Supelco, 10 mm diameter) on
a Waters (501 and 600 A pumps, 994 diode array and 470
fluorescence detectors) or Shimadzu LC-6A system. Optimum
product separation for 25 µL injections was obtained using
a gradient mode starting with methanol/ water (70:30, v/ v)
at a flow rate of 1 mL/ min and changing the ratio linearly
to 85:15 in 15 min during the run. The extracts and the
collected chromatographic fractions were analyzed by GC-
FID and GC-MS (Hewlett-Packard 5890 and 5995A) using a
SPB-5 (Supelco 30 mm × 0.032 mm id) GC column. The
injected sample volume was 3 µL with a 3 min solvent delay
at an initial oven temperature of 100 °C during the first 3 min
which was ramped to 250 °C at a rate of 15 °C/ min.
Photolysis of pyrene on activated silica was carried out
in quartz tubes in a Rayonet photoreactor using 300 nm light
bulbs (2.68 × 1015 photons cm-2 s-1 measured irradiance) in
a horizontal orientation as described previously (29-38).
Extraction of the unreacted pyrene and photoproducts from
the surface for HPLC analysis (HP 1090, C-18 reverse phase
column, 2.1 mm diameter, 0.21 mL/ min flow rate and 35:65
by volume acetonitrile:water isocratic changing to 100%
acetonitrile between 20 and 30 min into the run) was
performed in a similar fashion reported earlier (32).
Steady state diffuse reflectance spectra of samples pre-
pared on activated silica were recorded on a Cary-4 (Varian)
spectrophotometer equipped with an integrating sphere. A
Cary 1E (Varian) spectrophotometer was used to follow the
progress of the reaction by taking the absorption spectra of
extracts from samples of pyrene on unactivated silica
irradiated at different time intervals. Emission and excitation
spectra of pyrene adsorbed on unactivated silica before and
after irradiation were obtained in front face mode on an
SLM/ Aminco 4800S spectrofluorimeter. Emission spectra for
pyrene adsorbed on activated silica were recorded on a Spex
Fluorolog II spectrofluorimeter and are reported fully cor-
rected for silica background.
Synthesis of 1,6- and 1,8-Dihydroxypyrene. 1,6- and 1,8-
pyrenediones were first synthesized by a method described
previously (39, 40). Photoreduction of both diones to the
corresponding diols was carried out in 2-propanol (41). A
degassed solution of 1,8-pyrenedione (0.2 g) in 50 mL of
2-propanol was irradiated with visible light (λex> 400 nm,
Oriel 200-W Xe-Hg lamp using cutoff filter) for 1 h. Removal
of solvent afforded 1,8-dihydroxypyrene (90% yield). The air
sensitive 1,8-dihydroxypyrene could not be purified and as
a result was used without further purification. Photoreduction
of 1,6-pyrenedione (0.2 g) in degassed 2-propanol (50 mL)
yielded a mixture of 1,6-dihydroxypyrene (50% yield) and
unreacted dione. Because 1,6-dihydroxypyrene was very
oxygen sensitive, it could not be purified and thus was used
in its impure form.
Methods. Adsorption of pyrene onto unactivated silica
was achieved by adding the silica to a hexane solution of
pyrene and allowing it to equilibrate for 30 min. Removal of
the solvent carried out by any of the following methodss
rotary evaporation, nitrogen flow, or desiccation (avoiding
the use of heat)sleads to efficient loss of solvent. Pyrene
surface coverage was determined by extracting pyrene from
the silica surface (a known weight of silica in a known volume
of solvent) and injecting a measured volume of the extracted
pyrene solution into HPLC. Using a pyrene calibration curve
(determined by HPLC from known solutions), the concen-
tration of pyrene in extracted solution and thus on silica
surface was calculated.
Adsorption of pyrene onto activated silica (several different
surface coverages were employed as described in the text)
from a cyclohexane solution was achieved in a similar manner
described before (29-38).
Time-resolved fluorescence studies of pyrene on activated
silica were carried out in a diffuse reflective mode using 355
nm excitation beam of a Minilite II Nd:YAG laser (Continuum)
with a 5 ns pulse width and energies less than 9 mJ/ pulse.
Diffusely reflected emission was refocused with a lens onto
the entrance slit of a polychromator (EG&G PARC-1226). The
light from the exit port of the polychromator was then directed
to the front entrance port of a gated intensified charge coupled
detector (InstaSpec IV, Andor Technology). Data acquisition
was performed at g5 ns time intervals after the excitation
using manufacturer software (Andor Technology) which was
integrated with a DG355 Digital Delay Generator (Stanford
Research System, Inc.) to control the gate width and delay
times through a GPIB board. Emission traces recorded at
different time intervals were exported as ASCI files to Origin
4.0 software program (Microcal) and fitted using standard
procedures to obtain the kinetic information.
Photolysis of pyrene adsorbed on unactivated silica was
carried out using a Xe-Hg 1000W Oriel light source (Oriel
Corp.) equipped with a 10 cm long cylindrical tube for water
circulation to absorb the IR radiation. A Corning 7-51 band-
pass filter was used to isolate the 320-420 nm wavelength
region that overlaps well with the absorption bands of pyrene.
The samples were irradiated in a rotatory cell (12), where the
powder is tumbled during irradiation. The loss of pyrene
with time upon photolysis studies were carried out in a 1
mm flat optical cell. The cells were placed on an optical bench
at a distance of 35 cm from the lamp housing. An irradiance
of 66 Wm-2 was measured at this distance using K3Fe2(C2O4)
as chemical actinometer (42). A similar value was obtained
with a radiometer (IL1700 International light) using a SD033
#2585 detector.
Transient absorption spectra for pyrene adsorbed on
unactivated silica were recorded after excitation with 266
nm light from a Quantel YG-660A/ ND-YAG (Continuum,
Santa Clara, CA) laser (4-10 mJ/ pulse of 8 ns duration). The
signal was recorded and analyzed as described previously
(36, 38).
For sunlight radiation studies, the samples were placed
on Petri dishes, sealed with Teflon tape, and then exposed
to solar radiation on the roof of the College of Natural Sciences
building in Rio Piedras, Puerto Rico. For background cor-
rection, a similar sample was covered with aluminum foil to
protect it from light, and a second sample containing an
equal weight of silica without adsorbed pyrene was exposed
to sunlight for the same period of time. After irradiation, the
unreacted pyrene and photoproducts were extracted from
the silica surface by adding the powder to 20 mL of methanol
Results and Discussions
Spectroscopic Evidence for Photodegradation of Pyrene
on Unactivated Silica. The loss of adsorbed pyrene on
unactivated silica (1 × 10-6 mol/ g) due to photodegradation
process was monitored by emission spectroscopy. The
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FIGURE 3. Absorption spectra of photoproducts in methanol obtained
from an irradiated sample of pyrene adsorbed on unactivated silica
(1 × 10-6 mol/g) under an oxygen atmosphere at different irradiation
times (s 0 h, ... 1 h, -2- 2 h).
FIGURE 1. Decrease in the emission intensity of pyrene band at 393
nm with time for the photolyzed samples of pyrene adsorbed on
unactivated silica (1 × 10-6 mol/g) and purged with argon (-b-) and
oxygen (‚‚2‚‚).
FIGURE 2. Fluorescence spectra of photoproducts in methanol
obtained from an irradiated sample of pyrene adsorbed on
unactivated silica (1 × 10-6 mol/g) under an oxygen atmosphere
at different irradiation times (-[- 0 h, ... 1 h, s 2 h).
decrease in the emission intensity of pyrene bands suggests
that the molecule is undergoing photodegradation as ir-
radiation proceeds. Accordingly, the sample developed a
yellow color with time indicative of pyrene phototransfor-
mation. No color change was observed in samples that were
kept in the dark or when pure silica was irradiated under
similar conditions. The photodegradation rate of pyrene
adsorbed on unactivated silica decreased by 30% when
photolysis was carried out on silica containing residual
solvent (left on silica as a result of incomplete removal of
solvent from the surface during the adsorption process). The
decrease in photodegradation rate in the presence of residual
solvent is not too surprising since the solvent molecules could
replace loosely held pyrene molecules from the surface to
cover the sites. This in turn could result in an increase in
collisional deactivation rate of excited pyrenes (e.g. excimer
formation) and lead to less photoproduct formation. Pyrene
diffusion on silica and other surfaces has been reported to
increase in the presence of coadsorbed solvent leading to
dynamic excimer formation (43). The photodegradation rates
for air saturated or oxygen purged samples of pyrene adsorbed
on unactivated silica were similar. However, under an
atmosphere of argon, the photodegradation rate dropped by
a factor of 2 (Figure 1).
FIGURE 4. Representative chromatogram of photoproducts for a
sample of pyrene adsorbed on unactivated silica (1 × 10-6 mol/g)
that was irradiated for 3 h (a) absorption and (b) fluorescence
(excitation wavelength 280 nm, emission at 405 nm during the first
25 min, excitation wavelength 334 nm, and emission wavelength
393 nm during the next 10 min).
and 455 nm due to photoproducts. Changes in the absorption
spectra of pyrene with irradiation time are shown in Figure
3. The new red-shifted bands between 350 and 500 nm, which
appear as irradiation proceeds, are attributed to the observed
photoproducts. Similar spectral changes were also observed
when samples of pyrene adsorbed on unactivated silica were
exposed to sunlight (not shown).
HPLC analysis of extracted photoproducts shows a total
of 16 components (Figure 4). Assuming similar absorptivities
at the monitoring wavelength, there are three major products
(F6, F7, and F8) and the remaining 13 account for minor
products. Except for fractions F1-F4, all the other fractions
show structured absorption and emission spectra consistent
Figure 2 shows the changes in emission spectra of
photoproduct mixtures (in methanol) extracted from the
surface at different time intervals after irradiation. The
decrease in the emission intensity of pyrene bands is
accompanied by the appearance of new bands at 405, 430,
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with that of pyrene (parent compound) implying that these
products are substituted pyrenes. The main difference
between the absorption and emission spectra of these
products and the parent compound pyrene is a bathochromic
displacement and the presence of a broad visible band with
maxima around 450-470 nm.
Photoproduct identification by GC-MS using co-injection
of authentic samples identified three of the products as 1,6-
pyrenedione (F6, major), 1,8-pyrenedione (F7, major), and
1-hydroxypyrene (minor). The fragmentation patterns of the
MS spectra show consistent loss of fragments corresponding
to m/ z (mass-to-charge ratio) values of 28 (-CO) and 29
(-COH) in good agreement with the oxygenated nature of
these pyrene derivatives. Although the molecular mass
corresponding to F8 (236) is consistent with pyrenedihy-
drodiols, its MS and absorption spectrum are different from
those reported for 4,5-pyrenedihydrodiol (44-46). It is
conceivable that other pyrenedihydrodiols or dihydrodihy-
droxypyrene (e.g. reduced forms of 1,6- and 1,8-pyrenedi-
ones) could form.
The GC/ MS analysis of F4 revealed presence of two
isomers with m/ z equal to 208. Although 1,6- and 1,8-
dioxapyrenes (47, 48) have a molecular formula of C14H8O2
and a mass of 208, the observed MS fragmentation pattern
and absorption spectrum for F4 do not allow its assignment
as either of the two species. Fraction F5 shows an absorption
spectrum resembling substituted phenanthrene and a MS
fragmentation pattern indicating loss of -COH and -CO2H.
Although its molecular weight suggests a molecular formula
of C15H12O3, based on the available data we cannot assign a
definitive structure to this fraction. Fraction F10 (m/ z ) 220)
exhibits a MS and absorption spectrum similar to that of
4-oxapyrene-5-one (C15H8O2, a strong mutagen found in
diesel fuel) first identified by Pitts et al. (49). The MS
fragmentation pattern for F4, F5, and F10 contains ions with
m/ z values of 45 (-COOH), 29 (-COH), and 28 (-CO),
respectively, suggesting presence of oxygen functionality in
these fractions. We cannot assign molecular formulas to the
remaining minor products based on our available absorption
and mass spectral data.
FIGURE 5. Time-resolved fluorescence spectra of degassed 2 ×
10-6 mol/g (a) and 5 × 10-5 mol/g (b) pyrene adsorbed on activated
silica. A 5 ns pulse of an Nd:YAG laser (355 nm) was used for
excitation. Curves E, J, and R shown in the top figure (a) are the
emission profiles obtained at 20, 40, and 60 ns after the laser pulse.
Curves D, I, and P shown in the bottom figure (b) are the emission
profiles obtained at 20, 40, and 60 ns after the laser pulse.
Tim e-Resolved Fluorescence Studies. Our time-resolved
fluorescence data indicated that the emission of samples
became predominantly excimer-like as the surface loading
was increased from 2 × 10-6 to 5 × 10-5 mol/ g (Figure 5).
Time-resolved fluorescence studies of the degassed samples
provided additional information that substantiated the
presence of monomers as the main species at low surface
coverage (2 × 10-6 mol/ g, Figure 5a) and ground-state pairs
and/ or aggregates at higher surface coverage (5 × 10-5 mol/
g, Figure 5b). For both surface coverages, the fluorescence
decay curves fitted best to a biexponential fit at the
wavelengths of monomer (λ ) 384, 404, 427 nm) or excimer
(λ ) 482 nm) emission. The lifetimes of these species are of
the order of 160 ( 10 and 60 ( 5 ns in good agreement with
those reported by Bauer et al. (20) on various silica surfaces.
Photoproducts of Pyrene Photolysis on Activated Silica.
Analysis of photoproducts on activated silica revealed that
the yield of 1,6- and 1,8-pyrenediones decreased significantly
relative to unactivated silica. Furthermore, no better than
60% mass balance could be obtained despite our repeated
attempts to account for 100% loss of pyrene. Since activation
of silica through heat treatment initially induces the loss of
physisorbed water followed by subsequent dehydration of
silanol groups (20), we decided to assess the role of
physisorbed water in the observed phenomenon. Identical
samples of pyrene (2 × 10-6 mol/ g) adsorbed on unactivated
silica, activated silica, and silica containing g one monolayer
of physisorbed water were prepared, photolyzed in parallel,
and analyzed for pyrene loss and photoproducts formation.
Although the photolysis rate was slower for unactivated silica
and silica containing physisorbed water relative to activated
silica, the yield of both 1,6- and 1,8-pyrenedione were higher
for unactivated and wet silica samples. The 1-hydroxypyrene,
Photoproduct yields were compared for samples irradiated
up to 30% conversion (pyrene loss) under argon, air, and
oxygen. The yield of fraction F1 was unaffected while those
of F10, F11, and F12 dropped to almost zero in the absence
of oxygen. A 4-fold increase in the yield of pyrenediones
(fraction F6 and F7) and a 2-fold increase in fraction F3 was
observed when oxygen purged samples were photolyzed.
Fraction F12 disappeared in the presence of oxygen, while
the yield of F11 dropped by about 50%. In general, except
for F1, F11, and F12, the yield of all the other fractions
increased in the presence of oxygen suggesting that they are
formed via photooxidation processes.
Photolysis of Pyrene Adsorbed on Activated Silica. When
photolysis (λex ) 300 nm) of pyrene was carried out on
activated silica (2 × 10-6 mol/ g), the same number of
photoproducts was observed. In addition, a small amount
of 1,1′-bipyrene (identified by matching its absorption,
emission, and HPLC retention time with an authentic sample)
was also observed. A decrease in the rate of pyrene loss was
observed as the surface coverage increased from 2 × 10-6 to
5 × 10-5 mol/ g. The photodegradation rate was extremely
slow at 5 × 10-5 mol/ g. This drop in the photodegradaion
rate of pyrene can be attributed to the presence of ground-
state pairs and/ or aggregates (formed as surface loading is
increased) which can act as a light sink to absorb the incident
light without inducing any photochemistry. We have observed
a similar behavior with other PAHs reported previously (32-
38). This notion was further supported by our steady state
and time-resolved fluorescence studies (vida infra).
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SCHEME 1
on the other hand, did not show any significant change in
its yield in the presence of physisorbed water. When
photolysis of pyrene adsorbed on activated silica was carried
out in air or oxygen, no significant change in the photo-
degradation rate was observed. However, the yield of 1,6-
and 1,8-pyrenedione in oxygen purged sample were increased
by a factor of 5 and 3, respectively, while that of 1-hydroxy-
pyrene decreased by a factor of 2. Similar increases were also
observed for other products.
These results clearly indicate that (a) water plays a
significant role in the formation of pyrenediones and (b) the
precursors to these diones are air sensitive and readily
oxidized (vida infra). Although dihydroxypyrenes have been
reported to form during photolysis of pyrene on activated
γ-alumina and silica-alumina (28), we observed no signifi-
cant amount of either 1,6- or 1,8-dihydroxypyrene in the
reaction mixture. Lack of significant accumulation of either
diol on silica, compared to γ-alumina and silica-alumina
(28), suggests that these species are unstable and undergo
facile air oxidation to the corresponding diones. This is not
too surprising in view of the fact that catechols are known
to readily air oxidize to the more stable quinones.
diols are found in the reaction products isolated from
photolyzed pyrene on silica, it can be concluded that 1,6-
and 1,8-dihydroxypyrene may be the precursor to 1,6- and
1,8-pyrenediones. These results are in good agreement with
the proposed mechanism of pyrene photolysis in water (50).
The major products observed during photolysis of pyrene in
water were 1,6- and 1,8-pyrenediones and 1-hydroxypyrene
with only trace amounts of the corresponding diols.
A type I (electron transfer) mechanism where pyrene
cation radical is produced by electron transfer from its excited
state to oxygen was postulated to explain photodegradation
of pyrene in water (50). It appears that a similar mechanism
may be operating on silica as well. Initial formation of pyrene
cation radical and its subsequent reaction with physisorbed
water can lead to 1-hydroxypyrene. The 1-hydroxypyrene
can undergo secondary photolysis by a similar mechanism
to form 1,6- and 1,8- dihydroxypyrene as well as other pyrene
oxidation products. Unstable 1,6- and 1,8-dihydroxypyrene
can air oxidize (or possibly undergo photolytic transforma-
tion) to form the corresponding diones.
When an aerated sample of 1-hydroxypyrene adsorbed
on silica (4.3 × 10-7 mol/ g) was photolyzed at 300 nm, about
80% of it was lost during the same irradiation period as the
time used for photolyzing a pyrene/ silica sample. Conversion
of 1-hydroxypyrene to the corresponding diones in aerated
water upon photolysis has also been reported in our previous
work (50). These findings further support the notion that
initial formation of 1-hydroxypyrene and its subsequent
photodegradation on silica may account for the observed
photochemistry of pyrene.
To examine this possibility, we followed the change in
absorption spectra of both 1,6- and 1,8-dihydroxypyrene (in
air saturated acetonitrile) with time in the dark. Absorption
spectral results revealed (not shown) that both diols were
very unstable and readily oxidized to the corresponding
diones. When the solutions were purged with oxygen,
oxidation proceeded much more rapidly. Based on these
findings and the fact that no significant amounts of these
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VOL. 34, NO. 3, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 4 1 9

photolysis to dihydroxypyrenes (which can air oxidize to
diones) and other oxidation products of pyrene. Scheme 1
depicts a plausible mechanism for the formation of products
upon photolysis of pyrene on silica.
In summary, our photochemical studies of pyrene on
unactivated and activated silica have revealed that (a)
photodegradation proceeds efficiently at low surface cover-
ages to give mainly 1,6- and 1,8-pyrenediones, small amounts
of 1-hydroxypyrene, and some other oxidation products, (b)
photodegradation rate drops significantly at higher surface
coverages as a result of ground-state pair and/ or aggregation
believed to be responsible for the observed formation of
1,1′-bipyrene dimer, (c) photooxidation of pyrene proceeds
predominantly by type I (electron transfer) mechanism and
does not involve singlet molecular oxygen (type II) as an
intermediate, and (d) photodegradation rate drops in the
presence of physisorbed water.
Acknowledgments
This research was sponsored by EPA-U.S.A., Grant R-8233-
28-01, U.S. DOE/ EPSCoR Grant DE-FGO2-94ER-75764 to the
University of Puerto Rico and by the Division of Chemical
Sciences, Office of Basic Energy Sciences, U.S. Department
of Energy under contract DE-AC05-96OR22464 with Oak
Ridge National Laboratory, managed by Lockheed Martin
Energy Research Corp.
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9
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Received for review May 11, 1999. Revised manuscript re-
ceived September 27, 1999. Accepted November 11, 1999.
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