00456535(95)0001 l-9

Chemosphere, Vol. 30, No. 6, pp. 1125-l 136, 1995 Copyright 0 1995 Ekvier Science Ltd

Printed in Great Britain. AU rights reserved 0045-6535/95 $9.5O+O.Ofl

photocatalytic lb9hedhtion of2#4&Trlnl~toluene in Aqueous suspensions of l-ltanlum Dioxide

Zhikai Wang and Charles K&al*

Department of Chemistry

The University of Georgia

Athens, Georgia 30602 USA

(Received in USA 24 September 1994; accepted 5 January 1995)

The photocatalytic destruction of 2,4,6+initrotoluene (TNT) in aqueous suspensions of

titanium dioxide (TiOg, Degussa type P25) was investigated. Under aerobic conditions,

irradiation with the Pyrex-filtered output of a 200-W high pressure mercury-arc lamp resulted

in complete (95 f 5%) mineralization of 50 ppm of TNT within a few hours. Rapid

photodegradation of TNT also occurred under anaerobic conditions but without appreciable

mineralization. In the absence of TiOg, direct photolysis of TNT yielded a complex mixture of

colored organic products in both aerobic and anaerobic environments.

INTRQDUCTION

2,4,6-Trinitrotoluene, TNT, is a conventional explosive used by military forces

worldwide. Past manufacturing and disposal practices have resulted in contamination of soil

and groundwater by this toxic nitroaromatic compound. 1 In recent years the remediation of

TNT-contaminated sites has been recognized as a challenging environmental problem, and

several remediation strategies have been implemented with varying degrees of success.

Removal of TNT from water by adsorption onto activated carbon is a proven technology, but it

suffers from problems associated with the regeneration and ultimate disposal of the spent

solid adsorbent. Biological treatmentgf or direct solar photolysis495 of TNT results in complex

mixtures of products, the identities and toxicities of which are not entirely known. Photolysis

of hydrogen peroxide/ferrous ionA!NT solutions (photo-Fenton chemistry) was reported to

1125

1126

decompose TNT,6 but no information about the nature of the photoproducts was provided.

The goal of any remediation strategy is the conversion of targeted pollutants to non-toxic,

environmentally benign products. In the ideal case, organic compounds undergo complete

mineralization to carbon dioxide and other simple inorganic molecules. Several studies have

demonstrated that a wide range of organics can be mineralized when irradiated in an aerated

suspension of an oxide semiconductor such as titanium dioxide, TiOz.7-9 While these

heterogeneous reactions often proceed by complicated mechanisms, it is generally agreed that

the photogenerated holes and electrons which reach the semiconductor surface initiate the

oxidation and reduction, respectively, of adsorbed organic species. Dioxygen, 02, also plays an

important role as an electron scavenger and source of reactive oxygen species such as

superoxide ion, Ox.

We and otherslo- have begun to examine the feasibility of using semiconductor-

mediated photocatalysis for the remediation of TNT-contaminated water. Reported here are

studies of the photolysis of this nitroaromatic compound in aqueous suspensions of TiO2 under

aerobic and anaerobic conditions. For purposes of comparison, a companion study of the direct

photochemistry of TNT in aqueous solution also is described.

-ALDETAIlS

2,4,6-Trinitrotoluene (commercially available from Chem Service with 1020% water

added) was obtained from the Environmental Research Laboratory, U.S. Environmental

Protection Agency and used as received. A 0.198 g sample was stirred with 509 mL of

deionized water (18.0 * 0.1 megaohm resistivity) for 48 hours in the dark, and the resulting

saturated solution, with a TNT concentration of 128 ppm or 5.60 x 10-4 M, was filtered through

a 0.22 pm nylon disk filter. Less concentrated samples were prepared from this stock solution

by dilution. Typically, the pH of these aqueous solutions of TNT was 6.3 + 0.1. Titanium dioxide

powder, type P25, was obtained from Degussa Corporation and used without further

treatment. This material, with a composition of -70% anatase and -30% rutile, has an average

particle size of 21 nm and a specific area of 50 m2/g. Acetonitrile (Baker HPLC grade) and

Ba(OH)2*8HzO (Fisher reagent grade) were used as received.

1127

Continuous photolyses were performed with a 200-W high pressure mercury-arc lamp

situated 40 cm corn the sample. Excitation wavelengths were confined to the region above 290

nm by use of Pyrex reaction vessels. Electronic absorption spectra were recorded on a Varian

DMS 300 spectrophotometer. Solution pH was measured with a Fisher Accumet 25 pH/Ion

meter. Photolyzed solutions were analyzed by high performance liquid chromatography

(HPLC) on an apparatus consisting of an Alltech Associates model 425 pump, a Linear

Instruments UVIS-200 absorbance detector, a Rheodyne sample injector (a 20 PL sample loop

volume was used), and a Hamilton PRP-1 reversed-phase column (25 x 0.41 cm) with an

associated Hamilton PRP-1 guard column. The mobile phase consisted of an 80:20 (v:v) mixture

of acetonitrile/water at a flow rate of 1 mL/min. Prior to use, the mobile phase was passed

through a microfiltration system (0.20 pm pore size) and then sonicated to remove dissolved

gas. Chromatograms were recorded on a Hewlett Packard 3396 Series II integrator. Previously

constructed calibration plots (slightly different plots were obtained in the presence and absence

of TiO2) were used to convert the chromatographic peak height to TNT concentration.

Measurements of total organic carbon (TOC) were performed with a Dohrmann DC-85A TOC

analyzer. Potassium hydrogen phthalate standards (l-50 mg/L) were used for sample

calibration.

For the photocatalytic experiments, a suspension of TiOz was prepared by vigorously

stirring 0.1 g of the semiconductor powder with 1 L of deionized water containing a known

concentration (typically 10 or 50 ppm) of TNT. An aliquot of this mixture was pipetted into a

cylindrical, water-jacketed photolysis cell of 2.0 cm pathlength maintained at 25.0 f 0.5% with

a constant temperature bath. During irradiation, the contents of the cell were continually

purged with water-saturated 02 or N2 gas and stirred with a small magnetic stirring bar. No

effort was made to control solution pH, which typically fell by 2.0-2.5 units for a completely

photolyzed sample. After photolysis, the sample was filtered through a 0.22 Frn nylon disk

filter to remove the larger TiO2 particles. The Iilter was then washed with acetonitrile to

dissolve any substances adhering to the semiconductor surface. The filtrate and washings

were analyzed individually by uv-vis spectrophotometry and HPLC. Similar procedures were

followed in the direct photolysis (TiO2 absent) experiments, except that the photolyzed solution

was not filtered prior to analysis.

1128

Detection and quantitation of photoproduced carbon dioxide were accomplished by

passing the 02 or N2 purge stream from the photolysis cell through two consecutive bubblers,

each containing 15 mL of an aqueous barium hydroxide solution of known concentration.

Formation of a white precipitate of barium carbonate confirmed the presence of CO2 (eq. 1) in

the gaseous effluent. The change in pH of the barium hydroxide solution provided a convenient

measure of the number of moles of CO2 liberated. This value was corrected for the small pH

change that occurs in an u&radiated blank sample purged for an identical length of time.

Ba2+ + 20H- + CO2 ----) BaCO&) + H20 (1)

RESULTS AND DISCUSSION

a Dhct Phdolysis of TNT

Figure 1 illustrates the spectral changes undergone by a Na-purged aqueous solution of

TNT (10 ppm) upon irradiation with Pyrex-filtered light. The decrease in intensity of the 234

nm absorption band is accompanied by the appearance of new features at -320 and -510 nm.

The persistence of an isosbestic point at 265 nm throughout the 120 minute photolysis suggests

that a single photochemical reaction dominates. The originally colorless solution turned pink

after 10 minutes of irradiation and retained this color thereafter.

0.80

0.60

Irradiation Times (minutes) 0, 20,40, 60, 80, 120

500 650

WAVELENGTH (IUD)

Firmre Spectral changes observed upon photolysis (lu290 nm) of TNT in a Na-bubbled aqueous solution.

1129

Periodic analysis of the photolyte by HPLC revealed a steady decrease in the

concentration of TNT as a function of irradiation time. As shown in Figure 2, the compound

had disappeared completely after 120 minutes. At least eight new peaks could be detected in

the chromatogram of the photolyte, in agreement with earlier reports that the direct photolysis

of TNT in water affords a complex mixture of products. 415 Extraction of the photolyte with

diethyl ether yielded a yellow ether phase and a pink aqueous phase. A detailed identification

of the components in each phase was not undertaken.

Photochemical decomposition of TNT in water occurs more rapidly upon switching the

purge gas from N2 to 02 (Figure 2). This behavior contradicts the results of Mabey et al.,5 who

reported that 02 quenches the triplet excited state of TNT and thereby slows its

photodegradation in aquatic environments. While the origin of this discrepancy is unclear, we

note that our experiments were performed at significantly higher (10x) TNT concentrations.

1000

\

80-- _\ ‘,

0

6o 0

0 bubbled with 02

0 bubbled with Np

0 20 40 60 80 100 120

IRRADIATION TIME (minutes)

m Time profiles of the photodecomposition of TNT (10 ppm) in aqueous solution under aerobic and anaerobic conditions.

As seen in Figure 3, photolysis of an On-purged solution of TNT (10 ppm) gives rise to an

isosbestic point at 261 nm that persists until essentially all of the parent compound is

consumed (-30 minutes). Thereafter, one or more secondary photochemical processes become

significant. Interestingly, the absorbance at 320 nm continues to increase as irradiation

1130

1.00

I Irradiation Times(minutes)

0.80 J/ 0, 10,20, 30, 60

0.60

0.40

0.20

0.00 . 200 GO 500 650

WAVELBN0TH(nm)

Figure Spectral changes observed upon photolysis (lu29O nm) of TNT in 02-bubbled

aqueous solution.

proceeds while that at 510 nm reaches a constant value. This behavior explains the observation

that the photolyzed solution at first appears pink and then changes to yellow. Extraction of the

photolyte with diethyl ether again yields a yellow ether phase and a pink aqueous phase.

Neither colored solution underwent detectable spectral changes when stored in the dark at

room temperature for two weeks.

Photochemical conversion of TNT to inorganic carbon in an On-purged solution was

monitored by total organic carbon analysis. Figure 4 displays TNT concentration and TOC

content of the solution as a function of irradiation time. Complete disappearance of TNT

occurred within two hours, whereas TOC underwent no perceptible change during the same

period. These results reveal that the direct photolysis of TNT in the absence TiO2 generates

organic products which resist mineralization to CO2. In contrast, addition of TiOz to the yellow

photolyte and further irradiation resulted in decolorization and a continuous decrease in TOC

(Figure 4). As shown in the next section, this ability of illuminated TiO2 to mineralize

refractory compounds forms the basis of an effective remediation strategy for TNT-

contaminated water.

1131

80

\

TOC

0

IRRADIATION TIME (hours)

lm?iamA Time profile of the photodecomposition of TNT (50 ppm) in OS-bubbled aqueous solution and the accompanying change in total organic carbon (TOC). At the time indicated by the asterisk, TiO2 was added to the sample.

Irradiating an Og-bubbled heterogeneous mixture of TiOz (0.1 g/L) and aqueous TNT (10

ppm) causes the spectral changes depicted in Figure 5. The 264-m-n absorption of TNT steadily

decreases in intensity and, significantly, no new features are evident in the near ultraviolet or

visible wavelength region. Likewise, no new bands are observed in the spectrum of the

acetonitrile used to wash the TiO2 after it was separated from the photolyzed solution. These

results establish that the photodegradation of TNT in the presence of TiOz proceeds via a

pathway that differs from the one followed in direct photolysis.

Disappearance of TNT in the irradiated TiO2 slurry obeyed first-order kinetics

(correlation coefficient = 0.998) over several half lives. As seen in Figure 6, >98% of the organic

substrate had decomposed after 69 minutes of photolysis. Most importantly, loss of TNT was

accompanied by a decrease in the TOC of the sample, albeit at a much slower rate. This

behavior suggests that TiOz-catalyzed photolysis of TNT in On-purged water initially generates

organic intermediates which subsequently undergo mineralization to CO2 and other inorganic

1132

1.20

Irradiation Times(minutea) 1.00

0,2.5,5, 10,30

8 0.80

9

8 0.60

v1 : 0.40

0.20

0.00 200 250 300 330 400 450

WAVELENGTH

Fieure Spectral changes observed upon photolysis (10290 nm) of an 02-bubbled aqueous slurry of TiO2 containing TNT (10 ppm). Spectra were measured after removing TiO2 by filtration.

compounds. The amount of CO2 produced was determined by bubbling the effluent gas from

the photolysis cell through a barium hydroxide solution and measuring the resulting change

A

\ TOC A

\

A.A\A_ 01 ??4 : 1 1 I

0.0 1 .o 2.0 3.0 4.0 5.0

IRRADIATION TIME (hours)

Fieure Time profile of the photodecomposition of TNT (50 ppm) in an Oa-bubbled aqueous slurry of TiO2 and the accompanying change in total organic carbon (TOC).

1133

in pH (recall eq. 1). Within our limits of error (* 5%), we find that TNT undergoes

stoichiometric conversion to CO2 (eq. 2). Presumably, mineralization converts the nitrogen in

TNT+ 702 hv ) 7Ca + nitrogen-containing (2) TiO, inorganic compounds

TNT to NOi and/or NH: . Recent studies of the TiOa-photocatalytic destruction of

nitroaromatic compounds identified both species among the products and demonstrated that

their relative concentrations depended upon the parent compound, its concentration, and the

degree of conversion.13114

c. Mechanistic considerations

Irradiating TiO2 with light of energy equal to or greater than the band gap excites an

electron from the valence band to the conduction band, thereby creating a separated electron-

hole (e--h+ ) pair (eq. 3).7-s These photogenerated charge carriers can recombine within the

bulk semiconductor or migrate to the surface where they also can recombine or undergo redox

processes with adsorbed species. In aqueous media, the holes can oxidize water molecules (eq.

4) or hydroxide ions (eq. 5) to form hydroxyl radicals. Conduction band electrons can be

TiO2 x e- + h+ (3)

Ti02-OH2 + h+ ---) Ti02-OH’ + H+ (4)

TiOz-OH- + h+ - Ti02-OH’ (5)

02 + e- --) 0: (6)

scavenged by 02 to produce the superoxide ion, 0: (eq. 6), which can undergo further reactions

such as reduction to O”,-, protonation to HO; , or disproportionation to 02 and Oi-.

Considerable evidence supports the assignment of the hydroxyl radical, either bound to

the semiconductor surface or diffusing freely in solution, as the active oxidizing species in the

TiOa-catalyzed photooxidation of organic substrates. 7-o In the case of TNT, *OH attack on the

methyl group and/or the aromatic ring initiates a sequence of reactions that ultimately results

in complete mineralization. Dioxygen can enhance the quantum efficiency of this process by

scavenging photogenerated electrons (eq. 6) and thereby hinder unproductive e--h+

recombination. Moreover, 02 appears to be the logical source of the reactive oxygen species

1134

required for the stoichiometric production of CO2 (eq. 2).

Given the importance of dioxygen in the photomineralization process,15 we were

surprised to find that the disappearance of TNT also occurs rapidly in an illuminated TiO2

slurry under anaerobic conditions (Figure 7). In an attempt to understand this behavior, we

photolyzed a N2-bubbled suspension of TiO2 and aqueous TNT (56 ppm) for 6 hours and

analyzed the effluent gas. Less than 6% of the TOC was converted to CO2. By comparison, an

OS-bubbled sample irradiated for 4 hours underwent 60% conversion to CO2. These results

demonstrate that the rapid loss of TNT under anaerobic conditions does not result from

mineralization. As an alternative explanation, we suggest that conduction band electrons are

captured by the electronegative nitro groups of TNT. This process should be particularly

favorable in the absence of competitive electron scavenging by 02. Precedent for a reductive

degradation pathway can be found in a variety of biological systems, where the initial steps in

0 bubbled with 02

0 bubbled with Np

IRRADIATION TIME (minutes)

Firmre Time profiles of the photodecomposition of TNT (10 ppm) in an aqueous slurry of TiO2 under aerobic and anaerobic conditions.

the metabolism of TNT effect the sequential reduction of the nitro groups, via nitroso and

hydroxylamine intermediates, to the amine. 1s Further studies are needed to determine

whether a similar pathway exists in the presence of illuminated TiO2.

1135

CONCLUDING REMARKS

Direct photolysis of TNT in water under an Og or Ng atmosphere yields a complex

mixture of colored organic products. In contrast, complete (95 f 5%) mineralization of TNT

occurs in an irradiated aqueous slurry of TiOg under aerobic conditions. While the detailed

mechanism of TiOg-catalyzed photomineralization has yet to be elucidated, a pathway

involving hydroxyl radicals as the active oxidizing species seems likely. Dioxygen can function

as a scavenger of conduction band electrons and as a source of reactive oxygen species such as

superoxide ion. Under anaerobic conditions, TiOz-catalyzed photolysis results in the rapid

degradation of TNT but without appreciable mineralization. Reduction of the nitro groups of

TNT by conduction band electrons is suggested to be responsible for this behavior. Our results

indicate that TiOg photocatalysis deserves further study as a strategy for the destruction of

TNT.

Acknowledgments. We thank the U.S. Environmental Protection Agency for financial support

and Drs. Richard G. Zepp, Eric J. Weber, William L. Miller, and Michael S. Elovitz for

technical assistance and advice.

1.

2.

3.

4.

5.

6.

7.

8.

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