Adsorption and UV Photooxidation of Gas-Phase Phenanthrene on

Chapter 9

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Adsorption and UV Photooxidation of Gas-Phase Phenanthrene on Atmospheric Films Jing Chen, Franz S. Ehrenhauser, Kalliat T. Valsaraj*, and Mary J. Wornat Cain Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA 70803

The adsorption and U V photo-oxidation o f gas-phase phenanthrene on atmospheric water films were studied using a flow-tube reactor. B u l k and interface air-water partition constants for phenanthrene were obtained. The interfacial partitioning o f phenanthrene was increased in the presence o f Suwannee River fulvic acid ( S R F A ) and sodium dodecyl sulfate ( S D S ) . Three main photo-oxygenated products o f phenanthrene were identified: 9, 10-phenanthrenequinone, 3,4benzocoumarin, and 9-fluorenone. Photooxidation o f phenanthrene proceeded faster as the film thickness decreased. Effects o f SRFA and D O on the product formation rates were investigated. Based on the dependence o f the product formation rates on the concentration o f SRFA and D O, it was proposed that phenanthrene photodegraded v i a three different pathways: through radical cation intermediates, v i a reaction with singlet oxygen, and via reaction with hydroxyl radical. 2

2

© 2009 American Chemical Society

Valsaraj and Kommalapati; Atmospheric Aerosols ACS Symposium Series; American Chemical Society: Washington, DC, 2009.

127

128 Introduction Polycyclic aromatic hydrocarbons ( P A H s ) are ubiquitous contaminants in the environment (/). They range from the basic 2-ring naphthalene to multi-ring compounds o f very large molecular weight. P A H s are considered persistent organic pollutants since they do not degrade easily in the environment unless external stimuli are given. The extensive π-orbital systems o f P A H s allow them to absorb sunlight in the visible (400 - 700 nm) and the ultraviolet (290-400 nm) range o f the solar spectrum (2). The photo-excitation o f the P A H molecule often leads to the formation o f oxygenated products which are more toxic than the parent compound (3). The parent compounds are hydrophobic and o f low vapor pressure and they also display a tendency to adsorb at soil-water, sedimentwater, and the air-water interfaces in the environment. Atmospheric P A H s are subjected to a number o f fate and transport processes through which their removal, distribution and transformations could occur. These processes can include physical removal by dry and wet deposition (rain, fog, snow), chemical and photochemical reactions in the gas phase and aerosol phase, and dispersion by convection. Whereas, the basic 2-ring compound, naphthalene is mostly present in the gas phase, phenanthrene and other higher molecular weight compounds reside primarily associated with the aerosol phase. The aerosols are composed o f solid particles and liquid water as thin films. P A H s can be distributed between the solid and liquid phases o f the aerosols. Thus the overall fate and transport o f P A H s in aerosols are dependent on the processes occurring in both the solid and liquid aerosol fractions. There have been numerous reports o f the homogeneous reactions o f gasphase P A H s with atmospheric oxidants (/). Similarly, there have been some reports o f the heterogeneous reactions o f adsorbed P A H s with ozone, hydroxyl and nitrate radicals on solid particulate surfaces (soot, and aerosols) in the atmosphere (4-6). In general P A H s adsorbed to natural particles such as soot or fly-ash are more stable than in the pure form or adsorbed on silica gel, alumina or glass surfaces. Most o f these reports are for dry particles in the atmosphere. Air-water interface presents the largest environmental interface. This can be in the form o f bulk phases in contact (air-sea), dispersed phases (air bubbles or water droplets) or thin films o f water (aerosols). Apart from the equilibrium distribution o f a chemical between bulk phases (water and air), very little information is available on the behavior at the air-water interface. When the surface area presented by water is much larger than the bulk volume, heterogeneous chemistry becomes more important than homogenous reactions in either the bulk air or water phases. Thin water films are ubiquitous in the atmospheric environment (e.g., aerosols, fog, ice) and for these the surface processes become more significant. It has been recently demonstrated from our


129 laboratory via molecular dynamics simulations that the P A H molecules have deep free energy minima at the air-water interface (7). Experimentally it has been shown that the photoreactivity o f P A H molecules at the air-water interface o f a thin water film is enhanced as a result (8). Heterogeneous reactions o f P A H s at the air-water interface with oxidants such as gas-phase ozone, singlet oxygen and hydroxyl radicals have been experimentally demonstrated (8-10). Reactions in thin films such as those on aerosols have been evaluated for only the most volatile naphthalene (8). For a few other P A H s (pyrene and anthracene) reactions in water-ice films and bulk water have been reported (4, 11, 12). In this paper, we selected a semi-volatile compound (phenanthrene), a 3-ring P A H often detected in the ambient atmosphere, and studied its adsorption, photooxidation reaction and fate process at the air-water interface o f water films. Phenanthrene is a persistent organic pollutant ( P O P ) and is among the species monitored in the polar contaminant study (13). The behavior o f phenanthrene is expected to be markedly different from naphthalene, which we reported earlier (#), since they differ in vapor pressure, aqueous solubility and hydrophobicity. Surfactant material such as long-chain alkanoic acids are often observed in aerosols and fog and they have been known to influence the behavior o f gasphase organic species in water films (14, 15). Hence, we also studied the effect o f a natural surfactant film on the adsorption and photoreactivity o f gas-phase phenanthrene in the water film.

Experimental Materials Chromosorb Ρ (60-80 mesh size, acid washed) and porous polymer adsorbent (Orbo 43) were obtained from Supelco (Bellefonte, P A ) . Phenanthrene .(>96%), furfuryl alcohol (99%), 9, 10-phenanthrenequinone (>99%), 9- fluorenone (98%), and deuterium oxide (99.9% atom % D ) were obtained from A l d r i c h . Suwannee River fulvic acid ( S R F A ) was obtained from the International H u m i c Substances Society (Cat. N o . 1 SI OIF). Sodium dodecyl sulfate ( S D S ) (>99.5%) was obtained from G i b c o B R L (Grand Island, N Y ) . Water and acetonitrile used in H P L C were obtained from E M D Chemicals Inc. A l l above reagents were used as received. 3, 4-benzocoumarin was synthesized via Baeyer-Villiger oxidation o f 9-fluorenone according to M e h t a et al. (35) and purified by chromatography on an open silica column. The identity and purity (>99%) were confirmed via H P L C - U V / M S and G C - M S by matching the mass spectral pattern (36).


130 E x p e r i m e n t a l Setup Experiments were performed using a custom-constructed flow tube photoreactor described in detail in our previous work (8, 16). Phenanthrene vapor was generated by passing air through a P A H saturator and was then introduced to the flow tube photoreactor in which gas phase phenanthrene adsorbed onto a thin aqueous film coated on a 3.5 χ 92 cm glass trough. The P A H saturator was made o f six serially connected tubular columns (SS 316, 1/2" O. D . , 0.37 m long). Each saturator column was packed with 15 g o f Chromosorb Ρ coated 10 percent by weight with phenanthrene. T o obtain reproducible carrier gas flow rates in the P A H saturator, a mass flow controller (0-200 mL-min" , A a l b o r g Inc., Orangeburg, N Y ) was used to set the air flow rate to the saturator columns at 75 mL-min" . The gas phase concentration o f phenanthrene obtained in this manner ranged from 0.4 to 0.7 μg-L" at room temperature (23 °C), while the reported value o f the saturated gas phase concentration o f phenanthrene ranges from 2.0 to 6.5 μg·L" (17). 1

1

,

,

E q u i l i b r i u m P a r t i t i o n i n g at the A i r - w a t e r Interface Uptake o f phenanthrene onto thin aqueous films was investigated to study the interfacial behavior o f phenanthrene at the air-water interface. The phenanthrene vapor/air mixture was introduced to the flow tube photoreactor through a moveable injector (SS 316, 1/8" O . D.) and adsorption o f phenanthrene occurred on the aqueous film coated on the 3.5 χ 92 c m glass trough. The temperature o f the film was maintained at 296 Κ by a cooling bath. The aqueous sample was collected and the concentration o f phenanthrene was quantified using a high performance liquid chromatograph ( H P L C ) after partition equilibrium was achieved between the gas and liquid phases and the aqueous concentration o f phenanthrene ceased increasing. The time it took to reach equilibrium varied with the thickness o f the aqueous film. T o maintain consistency o f our experiments, the adsorption duration was set as 10 hours, which is the time required to reach equilibrium on the thickest film (1714 μιτι) employed in our experiments. Adsorption o f phenanthrene on pure water films, S R F A aqueous solution films and S D S aqueous solution films was investigated and the thickness o f each kind o f aqueous solution films ranged from 22 to 515 μιη. A s has been mentioned above, the gas phase concentration o f phenanthrene generated by the P A H saturator varied from day to day between 0.4 and 0.7 μg·L" . In order to correct for the aqueous concentration variation brought about by the gas phase concentration change o f phenanthrene, a pure water film o f fixed thickness (1714 μιτι) was coated on a 3.5 χ 5 cm glass trough and placed next to the target film, serving as the control for the adsorption experiments. The 1


131 equilibrium concentration o f phenanthrene in the target film was normalized to the equilibrium concentration in the control film for data analysis.

Measurement of Henry's Law Constant The Henry's constant for phenanthrene was obtained by measuring the concentrations o f phenanthrene in the bulk water and vapor phases in contact after equilibrium. A mini-bubbler (10 m L in capacity) filled with 5 m L deionized water was connected to the P A H saturator and the phenanthrene vapor/air mixture was bubbled through the bubbler at 75 mL-min" . After equilibrium was achieved between the water and vapor phases, a polymer trap was connected downstream to the bubbler to trap the vapor phase phenanthrene over a period o f several hours. Water samples were withdrawn from the top o f the bubbler both before and after collection o f the vapor and analyzed using H P L C . The adsorbed phenanthrene was extracted into acetonitrile and also analyzed in H P L C . The vapor phase concentration o f phenanthrene was estimated based on the gas flow rate, the vapor collection time, and the volume o f acetonitrile used for extraction. Henry's constant was obtained by the direct ratio o f the average aqueous concentration over the vapor collection period and the measured vapor phase concentration. 1

Photooxidation of Phenanthrene on Thin Aqueous Films Photooxidation o f phenanthrene on the thin aqueous film in the photoreactor was started after adsorption o f phenanthrene onto the film was complete. T w o U V lamps delivering U V light with wavelengths ranging between 280 and 315 nm were employed to provide illumination that simulated the U V - B component o f sunlight. The U V light intensity on the surface o f the aqueous film was 1.85 W - m " , around five times that o f the U V - B solar irradiance at the Earth's surface for a midsummer day at 40 °N latitude (38). Photooxidation o f phenanthrene was allowed to occur for a given duration o f time before samples were taken for analysis in H P L C . Phenanthrene vapor was continuously introduced into the reactor throughout the experiments to compensate for the reacted aqueous phase phenanthrene. A s a result, the aqueous concentration o f phenanthrene remained constant. Effect o f film thickness on the photooxidation rate o f phenanthrene was investigated. The film thickness ranged from 22 to 515 μπι. Effect o f S R F A on the photooxidation rate o f phenanthrene in a 515 μιτι aqueous film was also investigated and the concentration o f S R F A ranged from 2 to 250 mg-L" . 2

1


132 M e a s u r e m e n t o f Singlet O x y g e n Singlet oxygen has been suggested to be the dominant reaction intermediate in the photooxidation o f P A H s induced by U V light (18, 19). The steady state concentration o f singlet oxygen in an illuminated water film with adsorbed phenanthrene was quantified by measuring the loss o f low concentrations o f furfuryl alcohol in illuminated 100% H 0 and 50/50 H 0 / D 0 films with adsorbed phenanthrene. The concentration change o f furfuryl alcohol during illumination was determined using H P L C . The initial concentration o f furfuryl alcohol used for the measurement was 1.2*10" M and the system studied was a 515 μιτι water film containing 3.5* ΙΟ" M adsorbed phenanthrene. 2

2

2

6

6

P h o t o o x i d a t i o n o f P h e n a n t h r e n e on A q u e o u s F i l m s C o n t a i n i n g D 0 2

The decay rate o f singlet oxygen in dilute solution is controlled by solvent quenching and there is a considerable water-related H / D isotope effect on the lifetime o f singlet oxygen. T o investigate the role o f singlet oxygen played in the photooxidation o f phenanthrene, photooxidation rates o f phenanthrene on 515 μιη 100% H 0 , 50/50 H 0 / D 0 , and 100% D 0 films were measured. 2

2

2

2

Sample Analysis Quantification o f phenanthrene and photooxidation products in the aqueous samples was done using the same H P L C as was described in detail in our previous work (16). Identification o f compounds was achieved by matching retention times o f standard solutions within +/- 0.1 min and by matching the U V spectrum o f the standards and the sample. The injection volume was 25 μ L and the column thermostat was set to 40 ° C . The mobile phase started with a 20/80 acetonitrile/water mixture and ramped to 80/20 acetonitrile/water within 12 m i n , then held at this concentration for 3 min, and finally returned to 20/80 acetonitrile/water in 3 min at a constant flow rate o f 0.5 ml/min. The detection wavelength was set to 250 nm with 100 nm bandwidth and 4 nm slit.

Results and Discussion E q u i l i b r i u m U p t a k e o f Gas-phase Phenanthrene on W a t e r F i l m s Let us consider a film o f water o f thickness δ in the flow reactor that is exposed to a gaseous stream o f phenanthrene at a constant concentration. The


133 overall equilibrium uptake o f phenanthrene in the water film is due to two processes, v i z . , adsorption at the air-water interface and dissolution within the bulk liquid. Thus, the overall equilibrium concentration o f phenanthrene in the water film,

is given by

r

T

- Γ

ι

C

R

Where K

W

A

w

Q

K

q

A

m

0

WA

is the dimensionless bulk water-air equilibrium partition constant

(Henry's constant) for phenanthrene, Κ water interface, and C

w 0

σ Α

(μιη) is the partition constant at the air-

is the concentration in the bulk water phase for which

the surface area is negligible in comparison to the bulk volume. The above equation clearly demonstrates that as the film thickness, δ is small, and the surface

partition constant,

Κ

σ Α

is large, the contribution from the

surface

adsorption becomes larger. Figure 1 shows the variation in the total aqueous concentration in the film as a function o f the inverse o f the water film thickness. A linear relationship was observed indicating the validity o f the assumption that with decreasing thickness (increasing l / δ ) the surface adsorption becomes the

film

predominant

uptake mechanism. The y-intercept and the slope o f the plot give the values o f the bulk phase uptake ( C ) and K - C w o / K w A respectively. The bulk water-air w 0

equilibrium partition constant ( K

aA

W A

) was determined separately by passing the

phenanthrene vapor/air mixture through a bubbler and measuring the equilibrium concentrations

o f phenanthrene in the

determined was K A W

literature (Table I).

=

liquid

and gas

phases. The

value

1019, which agreed with the value reported in other

From the y-intercept and the slope o f Figure 1, one can,

therefore calculate the value o f the interface partition (adsorption) constant, Κ The value o f Κ

σ Α

.

4

σ Α

obtained was 3.3 χ ΙΟ μηι and compared well with the

estimate from correlation as shown in Table I. U s i n g this value one estimates that at equilibrium 6 0 % o f phenanthrene w i l l be present on the surface o f an aqueous film 22 μιη thick, whereas for a 515 μιτι film only 6% o f the total mass o f phenanthrene w i l l be on the surface. 1

Figure 1 also shows that for an aqueous film that contains 207 mg.L" o f S D S in the aqueous phase, which is equivalent to a monolayer o f S D S , the partition constant, Κ

5

σ Α

increases to 1.2 χ ΙΟ μιη. The corresponding bulk water-

air partition constant with aqueous phase S D S was 1019 showing no variation in the bulk phase equilibrium. Figure 1 also shows that for two different levels o f 1

S R F A in the aqueous phase, 51.5 and 280 mg.L" , which correspond to 15% and 50% surface coverage o f S R F A (76), the values o f Κ

σ Α

obtained were 3.1 χ 10

4

and 8.1 χ ΙΟ μηι respectively. The corresponding K

W

A

4

values for the two

aqueous concentrations o f S R F A were 1019 and 1175 respectively indicating


134 little variation in the bulk phase equilibrium. Thus, it is clear that the presence o f surface active materials in the aqueous phase at substantial concentrations effectively increase the overall equilibrium partitioning to the air-water interface and uptake by the water film. The degree o f increase in the equilibrium partitioning was determined by the surface coverage o f the surfactants and the hydrophobic interactions between the surfactants and phenanthrene.

0.00

0.01

0.02

1/δ/μΠΪ

0.04

0.03 1

Figure 1. Uptake of phenanthrene from the gas phase on aqueous films with varying thicknesses.

T a b l e I. B u l k a n d Interface A i r - w a t e r P a r t i t i o n C o n s t a n t s o f P h e n a n t h r e n e (T=296 K ) Partition constant K Κ

W A

σ Α

Water This work 1019

/[-] /[μηι]

3.3 χ 10

4

Reference

SDS 207 mg.L

51.5 mg.L'

SRFA 280 mg.L

955 (17)

1019

1019

1175

3.5χ10

4 σ

SRFA 1

1.2 χ 10

5

1

3.1 χ 10

4

1

8.1 x l O 2

4

" Data obtained from correlation; log ( K / m ) = +0.940 log (K /[-]) - 8.607; r = 0.987 (20). KQA is the octanol-air partition constant for the compound, log K =7.602 for phenanthrene at 298 Κ (21). oA

0A

OA


135 Photochemical Reactions of Phenanthrene on Thin Aqueous Films Photochemical reactions o f phenanthrene under simulated sunlight conditions were studied after adsorption o f phenanthrene onto the film was complete. Figure 2 shows the typical H P L C trace o f an aqueous film containing adsorbed phenanthrene that was exposed to U V radiation for 12 hours. The chromatogram shows several peaks apart from phenanthrene, among which three compounds with a relatively high abundance were identified and confirmed with pure standards. The products identified were: 9, 10-phenanthrenequinone, 9fluorenone and 3, 4-benzocoumarin. Note that in the case o f the thin film experiments reported in this work we see the three main products in a l l o f our samples. Figure 3 shows the accumulation o f the three main products in a 5 1 5 μ π ι water film as the reaction proceeds. Quantification o f the products was done on HPLC.

6

8

10

12

18

Retention time / min Figure 2. HPLC trace of a 515 μηι aqueous film sample after 12 hours exposure to UV light. The compounds identified are: 9, 10-phenanthrene quinone (PHEQ), 3, 4-benzocoumarin (BzC), 9-fluorenone (FLU), and phenanthrene (PHE).


136

Time / min Figure 3. Accumulation of the three main photooxidation products of phenanthrene in a 515μm water film.

A n overall reaction scheme,

P H E —^—» PI — ^ — » P2 , was proposed to

interpret the kinetic data. A s a result, the concentration change o f the products during the reaction can be described as

Cp.O^-^-Cpheofl-e^')

(2)

where C is the concentration o f the product, C o is the concentration o f phenanthrene, kj is the product formation rate constant, and k is the product reaction rate constant. Note that the concentration o f phenanthrene, C h o, was kept constant throughout the reaction because o f the continuous supply o f phenanthrene from the gas phase. Moreover, since the water-air equilibrium partition constants are high for the oxygenated product compounds, e.g., K = 3 . 6 1 0 for 9fluorenone at 2 9 8 K (37), we can neglect their gas phase concentrations within the reactor. Separate control experiments with standards o f products dissolved in water also showed that the evaporative loss o f products from the water film in the reactor was negligible. A detailed deduction o f Eqn. 2 was given in our previous work (8). In the case where PI is a stable product, further degradation o f PI can be neglected and thereby Eqn. 2 can be simplified to P 1

Phe

2

P

e

χ

W

C (t) - k,C P 1

P h e 0

4

A

t


(3)

137 A s shown in Figure 3, the concentrations o f the products increase linearly with time and the kinetic data fits well to E q n . 3. The formation rate constants for the products were obtained by fitting the kinetic data to E q n . 3.

Effect of Film Thickness on Reaction Rates Figure 4 shows that the formation rate constants for the three main products from phenanthrene greatly increased as the film thickness decreased. Table II lists the product formation rate constants measured in water films with both the largest (515 μηι) and the smallest thickness (22 μηι) employed in our experiments. Increases o f 4 7 % (9, 10-phenanthrenequinone), 1495% (3, 4benzocoumarin), and 1264% (9-fluorenone) were observed as the film thickness decreased from 515 μπι to 22 μπη. Heterogeneous reactions at the gas-aqueous interface have been shown to proceed faster than homogeneous reactions in the bulk water phase (8, 22, 23). The contribution o f surface reaction increases as the surface area per unit volume ( l / δ ) increases, therefore the measured overall formation rate constants for the products increased with decreasing film thickness. Similar effect o f film thickness on the photooxidation rate o f naphthalene was shown in our previous work (8). However, the highest increase in the product formation rate constant for naphthalene was 154% as the film thickness decreased to 22 μπι, compared to 1495% for phenanthrene. The reported values o f Κ and K for naphthalene were 21 μιη and 86 respectively (8). U s i n g these values it can be calculated that at equilibrium only 1% o f naphthalene is present on the surface o f an aqueous film 22 μηι thick, whereas 6 0 % o f phenanthrene is present on the same aqueous film. G i v e n a fixed film thickness, the proportion o f surface reaction for phenanthrene is higher than for σ Α

W

A

naphthalene, therefore, the measured overall rates for product formation from phenanthrene show a higher increase.

Effect of S R F A on Reaction Rates It is shown in literature reports that fulvic acid is the most important component o f dissolved organic matters in natural waters. It is also shown v i a molecular characterization that Suwannee River fulvic acid ( S R F A ) is a good surrogate model to represent polycarboxylic acids in fog waters (14). In this work, we chose S R F A to study the effect o f dissolved surfactants on the photooxidation o f phenanthrene in thin water films. Figure 5 shows the effect o f S R F A on the observed formation rate constants for 9, 10-phenanthrenequinone, 3, 4-benzocoumarin, and 9-fluorenone in a 515 μπι aqueous film. Interestingly, the effects o f S R F A on the observed formation rate constants for the three main products were totally different. The formation rate constant o f 9, 10-


138

0.0016

τ

0.0014 0.0012 0.0010 C

0.0008

ε ^·

0.0006 Η 0.0004 Α 0.0002 -I

ψ

•

°

ο

• ο ο

0.0000

0.00

•

0.04

0.03

0.02

0.01

1/δ / μΓΠ

PHEQ BzC FLU

0.05

-1

Figure 4. Effect offilm thickness on the observedformation rate constants of 9,10-phenanthrenequinone, 3, 4-benzocoumarin, and 9-fluorenone.

T a b l e II. O b s e r v e d P r o d u c t F o r m a t i o n Rate Constants in W a t e r F i l m s (T=296 K ) 1

Compound PHEQ BzC FLU

k\ / min (515 μm film) 9.5 χ 10"

4

5

4.2 χ 1(T 1.1 x 10'

4

1

kj / min (22 μm film) 1.4 χ 10"

3

6.7 χ 10" 1.5 χ 10'

4

3

Percentage increase 47% 1495% 1264%



Έ

'c

0

100

150

200 1

250

SRFA concentration / mg.L"

50

PHEQ

300

2.0e-5

4.0e-5

6.0e-5

8.0e-5

c 10e-4 Ε

^-

7

1.2e-4

1.4e-4

1.6e-4

0

100

150

200

1

250

BzC FLU

SRFA concentration / mg.L'

50

• ο

Figure 5. Effect of SRFA on the observedformation rate constants of 9, 10-phenanthrenequinone, 3, 4-benzocoumarin, and 9-fluorenone in a 515 μm aqueous film.

0.0002 -

0.0003 -

0.0004 -

0.0005 -

0.0006 -

0.0007 -

0.0008 -

0.0009 -

0.0010

300

140 phenanthrenequinone

decreased monotonically as the concentration o f S R F A

increased, whereas that o f 3, 4-benzocoumarin decreased in the beginning and then increased. Contrary to 3, 4-benzocoumarin, the formation rate constant o f 9fluorenone increased first and then decreased.

Literature reports on the effect o f

S R F A on P A H photodegradation in bulk water have appeared quite conflicting. Fasnacht and Blough reported that photoreactivities o f P A H s in bulk water solutions were not affected by S R F A (24). However, other reports showed that whereas the photodegradation o f benzo[a]pyrene and benzo[a]anthracene

were

slowed by humic-like substances in water,

(25).

that o f naphthalene increased

These seemingly conflicting results are probably attributed to the

different

reaction mechanisms that different P A H s undergo. In the case o f phenanthrene photooxidation in our work, the different effects o f S R F A on the formation o f the three main products suggest that phenanthrene was photooxidized products

via

different

pathways.

Effect

of

SRFA

on

to the

phenanthrene

photooxidation is discussed further in the following section.

Photooxidation Pathways of Phenanthrene Three main pathways v i a which P A H s photodegrade have been proposed in the literature: through radical cation intermediates, via reaction with singlet !

oxygen ( 0 ) , and v i a reaction with hydroxyl radical ("OH) (24, 26). Figure 6 2

shows the three possible pathways o f P A H photooxidation in oxygen-containing water. It has been proposed in the literature that singlet oxygen is the dominant reaction intermediate in the direct photooxidation o f P A H s induced by U V light (18, 19). In our work, we used furfuryl alcohol ( F F A ) , an efficient '(^-selective trapping agent, to quantify the steady state concentration o f singlet oxygen in the illuminated 515 μπι water film with adsorbed phenanthrene. The steady state concentration o f singlet oxygen was determined by (27)

t

l0 ] 2

k

=

s s

exp,D 0-ke 2

k

r

k

K

Xp>

2

2

(

"2Q

H OXH O + 2

H0

K

4

)

11 D OXD O 2

2

Detailed description o f E q n . 4 and values o f the constants can be found in the references (27, 28). The apparent first-order kinetic rate constants for the loss o f F F A in 100% H 0 and 50/50 H 0 / D 0 films with adsorbed phenanthene 2

k

( ex ,H o P

2

a

n

d

k

2

ex ,D o) P

w

e

r

2

e

2

1

determined to be 2.0>

Adsorption and UV Photooxidation of Gas-Phase Phenanthrene on

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