Chemosphere 57 (2004) 309–317

www.elsevier.com/locate/chemosphere

Kinetics and mechanism of TNT degradation in TiO2

photocatalysis

Hyun-Seok Son, So-Jin Lee, Il-Hyoung Cho, Kyung-Duk Zoh *

Institute of Health and Environment, Graduate School of Public Health, Seoul National University,

28 YeonKeon-Dong, Jongro-Gu, Seoul 110-799, South Korea

Received 9 September 2003; received in revised form 12 May 2004; accepted 18 May 2004

Abstract

The photocatalytic degradation of TNT in a circular photocatalytic reactor, using a UV lamp as a light source and

TiO2 as a photocatalyst, was investigated. The effects of various parameters such as the initial TNT concentration, and

the initial pH on the TNT degradation rate of TiO2 photocatalysis were examined. In the presence of both UV light

illumination and TiO2 catalyst, TNT was more effectively degraded than with either UV or TiO2 alone. The reaction

rate was found to obey pseudo first-order kinetics represented by the Langmuir–Hinshelwood model. In the miner-

alization study, TNT (30 mg/l) photocatalytic degradation resulted in an approximately 80% TOC decrease after 150

min, and 10% of acetate and 57% of formate were produced as the organic intermediates, and were further degraded.

NO�3 NO

�2 , and NH

þ4 were detected as the nitrogen byproducts from photocatalysis and photolysis, and more than 50%

of the total nitrogen was converted mainly to NO�3 in the photocatalysis. However, NO

�3 did not adsorbed on the TiO2

surface. TNT showed higher photocatalytic degradation efficiency at neutral and basic pH.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: TNT(2,4,6-trinitrotoluene); TiO2; Photocatalysis; Acetate; Formate; NO�3 ; NO

�2 ; NH

þ4

1. Introduction

TNT (2,4,6-trinitrotoluene) was widely used during

World War II and has since been used in detonators,

mines, rocket boosters, and plastic explosives (Yinon,

1990). During its early manufacture, wastewater gener-

ated at the munitions facilities was often dumped into

unlined pits, or disposed of in lagoons. This improper

disposal practice led to contamination by TNT, and,

even now, TNT is often found in the soil, groundwater,

and surface waters at the sites where it was manufac-

*Corresponding author. Tel.: +82-2-740-8891; fax: +82-2-

745-9104.

E-mail address: zohkd@snu.ac.kr (K.-D. Zoh).

0045-6535/$ - see front matter � 2004 Elsevier Ltd. All rights reserv

doi:10.1016/j.chemosphere.2004.05.008

tured (Comfort et al., 1995). TNT is classified as a pri-

ority EPA pollutant (US EPA, 1988), and is found to be

toxic to aquatic and terrestrial organisms (McCormick

et al., 1976; Smock et al., 1976), mutagenic (Kaplan and

Kaplan, 1982), and can pose a risk for human health

through the food chain (Won et al., 1976). Past methods

for disposing of TNT contaminated wastes have in-

cluded dumping at sea, dumping in specific landfill

areas, and incineration in the case of small quantities.

All of these methods may cause serious harm to eco-

systems (Spalding and Futton, 1988).

Several methods of treating TNT contaminated

water have been developed. Among them, carbon

adsorption has often been used for the effective treat-

ment of munitions plant wastewater (Wujick et al.,

1992). However, the spent carbon constitutes a hazard-

ous waste and, if allowed to dry, may also become

explosive. Incineration is another preferred technology,

ed.

310 H.-S. Son et al. / Chemosphere 57 (2004) 309–317

but has the disadvantages of producing NOx during the

process.

The photocatalytic degradation of recalcitrant or-

ganic contaminants in the presence of TiO2 has recently

aroused interest (Carraway et al., 1994; Maugans and

Akgerman, 1997). One of the reasons for this is that

photocatalysis may completely mineralize a variety of

aliphatic and aromatic compounds under suitable con-

ditions. When titanium dioxide (TiO2) is illuminated

with light of band gap energy, electrons in conduction

bond (e�CB) and holes in valence bond (hþVB) are produced

according to

TiO2 þ hm ! hþVB þ e�CB ð1Þ

These charge carries can recombine, or the holes can

be scavenged by oxidizing species (for example, H2O,

OH�), and electron by reducible species (for example,

O2) in the solution

hþVB þH2O! �OHþHþ ð2Þ

hþVB þOH� ! �OH ð3Þ

e�CB þO2 ! �O�2 ð4Þ

The hydroxyl radical is a highly reactive oxidizing re-

agent and can decompose most organic contaminants.

In this study, we present the results of the photo-

catalytic degradation of TNT using a TiO2 slurry system

in a circular photoreactor. The impacts of various

parameters on the reaction rate, such as initial concen-

tration of TNT, pH of the solution, and adsorption on

TiO2 were investigated to determine the optimum

treatment conditions. Through the analysis of TOC and

various byproducts from TNT reaction products, the

mechanism of the TNT photocatalytic reaction was also

determined.

Fig. 1. Schematic diagram of the circular photocatalytic reac-

tor.

2. Materials and methods

2.1. Chemicals

Pure TNT was kindly provided by the Agency for

Defense Development of Korea. Before being used, the

TNT was dried for 24 h in desiccators. 100 mg/l of TNT

stock solution was prepared by dissolving 100 mg of

dried TNT into 1 litre of deionized water with moderate

heating due to the low solubility of TNT in water (�120mg/l at 25 �C). The titanium dioxide (TiO2) powder used

as the photocatalyst was commercially available De-

gussa P-25 (Degussa Chemical Co.). This Degussa P-25

TiO2 was composed mostly of anatase and had a BET

surface area of 50 m2/g and an average particle diameter

of 30 nm.

2.2. Photoreactor system

All photocatalytic degradation experiments were

performed in a circulating photoreactor system. Fig. 1

shows the photoreactor system. The reactor system

consisted of a temperature-controlled reservoir, a

metering pump (Cole–Parmer) for the circulation of the

reactor contents, and a photoreaction chamber, all of

which were connected with flexible Teflon tubing. The

reservoir was a glass bottle with a total volume of 1 litre

and whose contents were stirred during the experiment.

The TNT solutions were circulated with a metering

pump at a flow rate of 1 l/min. The reactor reservoir

bottle was stirred to maintain the uniformity of the

reaction solution.

The UV chamber consisted of four UV lamps and

five photoreactor columns. The reactor columns con-

sisted of a cylindrical quartz tube with an outside

diameter of 10 mm and a length of 100 cm, each of

which allowed the transmission of UV light and held a

total volume of 400 ml. The reactor assembly was illu-

minated by four UV lamps with a diameter of 30 mm

and a length of 100 cm. The distance between the col-

umn and the UV lamp was 15 mm and it emitted

approximately 90% of its radiation at 254 nm with a 15

W power input. The light intensity was measured using

a Radiometer (VLX-3W Radiometer 9811-50, Cole

Farmer Instrument) at 254 nm (UV-C region). The

intensity of a single UV lamp measured by the Radio-

meter was 4.1 mW/cm2. The external surface of the

reactor was covered with aluminum foil for UV-safety.

Reactions were performed under atmospheric pres-

sure at 293 K. The reaction solutions were prepared by

dilution of stock solutions and the aqueous phase was

then introduced into the reservoir. The initial pH of the

reaction solution was adjusted to pH 7.0(�0.4) exceptthe pH experiment. The reaction solution was trans-

ferred to the sample collector and solution storage using

a three-way valve. The top of the reservoir bottle re-

mained open to the air in order for the solution to be

subject to a constant pressure of oxygen.

Fig. 2. TNT degradation under the control conditions (TiO2

only and UV only) and photocatalytic condition (experimental

conditions: TNT¼ 30 mg/l, TiO2¼ 0 mg/l in photolysis, 1.0 g/lin photocatalysis, pH¼ 7, UV254nm¼ 8.2 mW/cm2 in photolysis

and photocatalysis conditions).

H.-S. Son et al. / Chemosphere 57 (2004) 309–317 311

2.3. Sample analysis

All liquid samples were filtered through MCE mem-

brane filters having a pore size of 0.2 lm (Advantec

MFS Inc.) to remove the TiO2 particles prior to analysis.

The filtrate was used for the determination of the TNT

concentration. The TNT concentrations were measured

by HPLC using a SummitTM HPLC system (Dionex),

and a modified version of the technique described in

EPA method 8330 was used for the TNT analysis (US

EPA, 1994). Detection was performed with a photodi-

ode array detector UVD 340S (Dionex). The detector

was set at 254 nm to measure the TNT. We used an

isocratic mixture of methanol and water (50:50, v/v%) at

a flow rate of 1.0 ml/min. A C-18 Supelco (Supelco)

silica column (25 cm, 4.6 mm i.d., 5 mm particles) and a

security guard column (all-guard Adsorbosphere C18-

5V; Phenomenex Inc.) were used and replaced regularly.

TOC analysis of the liquid samples was performed on

a Shimadzu TOC analyzer (Shimadzu). Acetate, for-

mate, NO�2 and NO

�3 were measured with a DX-120 ion

chromatograph (Dionex) using a Dionex Ion Pac AS14

column with a mixture of 3.5 mM Na2CO3 and 1 mM

NaHCO3 as an eluent. NHþ4 ion was also measured with

ion chromatograph using a Dionex Ion Pac CS12-A

column with a 20 mM of methyl sulfonate as an eluent.

3. Results and Discussion

3.1. Comparison of TNT degradation and TOC between

photolysis and photocatalysis

To confirm the role of TiO2 in the photocatalysis

reaction, three sets of experiments were performed to

compare TNT degradation rates with and without cat-

alysts. One set was performed with TNT (30 mg/l)

exposed to TiO2 (1 g/l) but no UV (TiO2 only condi-

tion). The second set was performed by exposing TNT

(30 mg/l) to UV without TiO2 (photolysis condition).

Then, the third set was performed by exposing TNT to

TiO2 (1 g/l) in the presence of UV illumination (photo-

catalysis condition). The results are presented in Fig. 2.

First, the experiment with TiO2 only showed that the

small amount of TNT (about 10%) was adsorbed on the

TiO2 surface after 150 min. The disappearance of TNT

in the solution due to the adsorption increased for the

first 10 min, then leveled off after that time. Next, the

results of the photolysis and photocatalytic experiments

showed that the photolysis reaction resulted in about

70% decrease in the TNT concentration after 150 min,

whereas the TNT was completely removed after 90 min

in the case of the photocatalytic reaction. In both

experiments, the colorless TNT solution changed to a

pink color as the reaction proceeded, and subsequently

faded to a lighter shade of pink. Several researches have

shown that the photolysis and photocatalysis undergo

the same degradation pathway, and the difference in the

reaction rates between two processes is in the hydroxyl

radical concentration (Schmelling and Gray, 1995;

Goutailler et al., 2001). Therefore, it can be concluded

that TNT degradation in photocatalysis and photolysis

proceed by the oxidation of TNT by hydroxyl radical.

3.2. Mineralization and organic intermediates from pho-

tolysis and photocatalysis

Due to the inaccessibility of labeled 14C-TNT, TOC

measurements were performed as an indirect evidence of

mineralization during the TNT photolysis and photo-

catalysis experiments. The results are shown in Fig. 3. It

was found that more than 80% of the TNT was miner-

alized within 150 min by the photocatalytic reaction, but

only 40% was mineralized by photolysis under identical

conditions. The low conversion of TOC in the photolysis

is believed to be due to the presence of organic

byproducts in the photolysis of TNT. On the other

hand, TOC data in the case of TiO2 photocatalysis

indicates that the majority of the degraded TNT was

converted inorganically.

In order to confirm the mineralization during TNT

photocatalysis, the carbon type organic byproducts

measurement was performed. First, the organic inter-

mediates were measured during the photocatalysis

reaction using ion chromatograph. Fig. 4 showed that

about 10% of carbon was converted to acetate in the

early period of reaction, and approximately 57% of

carbon was converted to formate, and these compounds

were further degraded as the reaction proceeded. Also,

no aromatic derivatives were detected by HPLC at

Fig. 4. Concentration change of acetate and formate during

photocatalytic degradation of TNT (experimental conditions:

TNT¼ 30 mg/l, TiO2¼ 1 g/l, pH¼ 7, light intensity ¼ 8.2 mW/

cm2).

Fig. 3. Comparison of removal efficiency and TOC degradation

between photocatalytic degradation and photolysis of TNT

(experimental conditions: TNT¼ 30 mg/l pH¼ 7, UV254nm¼ 8.2mW/cm2, TiO2¼ 0 mg/l in photolysis, 1.0 g/l in photocatalysis).

312 H.-S. Son et al. / Chemosphere 57 (2004) 309–317

254 nm after ca. 30 min of photocatalysis reaction,

implying that compounds arising from aromatic ring

opening absorbing at the UV wavelength.

The formate produced from TNT degradation is

known to be mineralized to CO2 as follows (Carraway

et al., 1994):

HCOO� þ hþVB ! HCOO� ð5Þ

HCOO� ! CO2 þHþ þ e� ð6Þ

HCOO� þO2 ! CO2 þHþ þO��2 ð7Þ

The acetate also can further undergo the minerali-

zation during the photocatalysis. From the 14C labeled

experiment of photocatalytic oxidation of ethanol,

Muggli et al. (1998) found that acetate is transformed to

formate, formaldehyde, and finally mineralized to CO2

in photocatalysis as follows:

CH3COOH! CO2 þHCHO! HCOOH! CO2 ð8Þ

The mass balance of carbon during the TNT degra-

dation shows the unbalance between TOC removal

(80%) and the carbon balance (57% as formate). This

unbalance can be explained by the two hypothesis: either

a significant amount of the carbon in the TNT was in-

deed being mineralized to CO2, or the presence of a

reaction byproduct from TNT degradation that is

purgeable in the TOC analysis. Fig. 4 showed that the

formate production during TNT photocatalysis in-

creased up to 57% until 150 min of reaction, but then

decreased to 45% after 150 min. This result indicates

that formate is further oxidized to such as CO2 as shown

in Eqs. (5)–(7).

The acetate is also transformed to formaldehyde

during photocatalytic oxidation as shown in Eq. (8), and

the formate is usually formed via formaldehyde during

the oxidation of carbon compounds. Since the samples

were purged during TOC analysis, the less amount of

carbon would be present than the original carbon

amount due to the volatilization of this purgeable

intermediate. Therefore, the higher TOC removal can be

obtained compared to the amount of formate produced

from TNT photocatalysis.

3.3. Formation of nitrogen byproducts

Since TNT contains three nitro groups (NO2) that

are susceptible to oxidation or reduction, the formation

of nitrogen byproducts is to be expected from both TNT

photolysis and photocatalysis. Therefore, the nitrogen

byproducts during TNT photocatalysis and photolysis

were measured.

The change in concentration of the nitrogen

byproducts produced during the photocatalysis of TNT

is shown in Fig. 5, and the corresponding result for the

photolytic reaction is shown in Fig. 6. The concentra-

tions of TNT and of the nitrogen byproducts are rep-

resented by the percentage of nitrogen. A comparison of

the production patterns of the nitrogen byproducts for

the two reactions indicated that the concentrations of

byproducts increased with decreasing TNT concentra-

tion in both cases. However, the way in which the con-

centration of NO�2 changed, and the amount of nitrogen

byproducts produced differed between the two reactions.

Fig. 5 showed that NO�2 , NO

�3 and NH

þ4 ions were

produced as the nitrogen byproducts in the photocata-

lytic reaction of TNT. As shown in Fig. 5, these species

were produced with the concomitant decrease in TNT

concentration. Fig. 5 also shows that the concentration

of NO�2 increases initially and then decreases, and the

Fig. 6. Concentration change of NO�3 , NO

�2 , NH

þ4 ions pho-

tolytic degradation of TNT (experimental conditions: TNT

¼ 30 mg/l, pH¼ 7, light intensity¼ 8.2 mW/cm2).

Fig. 5. Concentration change of NO�3 , NO

�2 , NH

þ4 ions during

photocatalytic degradation of TNT (experimental conditions:

TNT¼ 30 mg/l, TiO2¼ 1 g/l, pH¼ 7, light intensity ¼ 8.2 mW/

cm2).

H.-S. Son et al. / Chemosphere 57 (2004) 309–317 313

maximum concentration of NO�3 is reached when the

NH�2 concentration decreases to zero. Finally, almost

60% of the total nitrogen initially present in the TNT

was converted to NO�3 , and about 10% of total nitrogen

was converted to NHþ4 .

NO�3 was the most predominant of the three nitrogen

products, indicating that oxidation is the primary mode

for the TNT photocatalysis. This phenomenon can be

explained by that NO�2 is separated from the TNT

molecule during the photocatalytic reaction, and be-

comes rapidly oxidized to NO�3 by the hydroxyl radical.

The conversion of NO�2 to NO

�3 by the hydroxyl radical

was observed by others (Low et al., 1991; Zoh and

Stenstrom, 2002).

The formation of NHþ4 ions during TNT photoca-

talysis indicates a concurrent reduction reaction during

photocatalysis of TNT. The reduction of nitro group to

amine by electron (e�CB) was proposed by Piccinini et al.

(1997). The resulting amine group can be detached from

the main compound via the ring opening and carbon–

carbon breakage by hydroxyl radical. The small quan-

tity of NHþ4 ion produced also decreased at the end of

this experiment in the photocatalysis (Fig. 5). This

indicates that NHþ4 is being oxidized to NO

�3 . A similar

oxidation of NHþ4 to NO

�3 was also observed by Low

et al. (1991) during the photocatalytic oxidation of n-

pentylamine.

Total nitrogen produced, as NO�2 , NO

�3 and NHþ

4

were about 62% of the initial nitrogen after about 160

min of TNT photocatalysis reaction. The lack of nitro-

gen mass balance on complete degradation of TNT may

result from the formation of two classes of compounds,

nitrogen-containing organic intermediates, and molecu-

lar nitrogen, resulting in a loss of nitrogen from the

system.

In contrast to the photocatalysis reaction, the total

amount of nitrogen byproducts produced in the pho-

tolysis of TNT was lower than that produced by pho-

tocatalysis. The NO�3 produced by the photolysis

reaction, while being the major byproduct, represented

only 25% of the total nitrogen initially present in the

TNT, as shown in Fig. 6. Another difference between

these two processes is that, while the NO�2 produced in

the photocatalysis reaction represented only 3–4% of the

total nitrogen, and the concentration of NO�2 did not

decrease but steadily increased up to approximately 10%

until equilibrium is reached in the case of the photolysis

reaction. This phenomenon can be explained by the fact

that the hydroxyl radical concentration in photolysis is

much lower than that in photocatalysis as mentioned,

and therefore, NO�2 and NH

þ4 could not be easily oxi-

dized into NO�3 , and were instead accumulated in the

solution.

3.4. Effect of initial TNT concentration

Since the pollutant concentration is a very important

parameter in the treatment, the effect of initial TNT

concentration on the photocatalytic degradation rate

was investigated over the concentration range of 10 to

100 mg/l of TNT, and the experimental results are pre-

sented in Fig. 7. Fig. 7(a) shows that the degradation

rate decreases as the initial TNT concentration in-

creases. This result indicates that the TNT degradation

kinetics is not simple first-order but pseudo first-order.

The Langmuir–Hinshelwood kinetic expression has

been used for heterogeneous photocatalysis to describe

the relationship between the initial degradation rate and

the initial concentration (Matthews, 1988; Lu et al.,

1993; Mills and Hoffmann, 1993; Chen and Raym,

1998). In this model, the reaction rate for second-order

surface decomposition of TNT is written as follows:

Table 1

Effect of initial concentration of TNT on the TNT degradation

rate (experimental conditions: TNT¼ 30 mg/l, TiO2¼ 1 g/l, UVdensity ¼ 8.2 mW/cm2)

Initial TNT

(mg/l)

kobs(min�1)

1/kobs(min)

t1=2(min)

R2

10 0.0989 10.1 7.07 0.979

20 0.0644 15.5 10.76 0.980

30 0.0405 24.7 17.11 0.986

50 0.0269 37.1 25.77 0.997

100 0.0165 60.7 42.01 0.981

Fig. 8. Plot of the initial TNT concentration versus the re-

ciprocal of the observed first-order rate constant ð1=kobsÞ.Fig. 7. Effect of initial TNT concentration on the removal

efficiency: (a) the percent removal of TNT at the different initial

TNT concentration and (b) the plot of lnðC0=cÞ versus irradi-ation time.

314 H.-S. Son et al. / Chemosphere 57 (2004) 309–317

rate ¼ � d½TNTdt

¼ kcKTNT½TNT

1þ KTNT½TNT0ð9Þ

where ½TNT is the TNT concentration at time t, kc is thesecond-order rate constant, KTNT is the equilibrium

adsorption constants of TNT onto TiO2, and ½TNT0 isthe initial concentration of TNT. According to Eq. (9),

the photocatalytic degradation of TNT in the presence

of TiO2 exhibits pseudo first-order kinetics with respect

to TNT concentration as in

� d½TNTdt

¼ kobs½TNT

¼ kcKTNT

1þ KTNT½TNT0½TNT ð10Þ

where kobs is the observed pseudo first-order rate con-stant for the photocatalytic oxidation of TNT. There-

fore, the integration of Eq. (10) results in

ln½TNT0½TNT

� �¼ kobst ð11Þ

Based on Eq. (11), the straight-line relationship of

lnð½TNT0=½TNTÞ versus irradiation time was observedas indicated in Fig. 7(b) and Table 1.

Next, the relationship between kobs and ½TNT0 fromEq. (10) can be expressed with Eq. (12)

1

kobs¼ 1

kcKTNTþ ½TNT0

kcð12Þ

Eq. (12) shows that the linear expression also can be

obtained by plotting the reciprocal of degradation rate

(l=kobs) as a function of the initial TNT concentration.Based on this equation, the values of kobs at differentinitial TNT concentration were fitted, and plotted in

Fig. 8. By means of a least square best fitting procedure,

the values of the adsorption equilibrium constant

(KTNT), and the second-order rate constant (kc) wereobtained, and this value found to be KTNT ¼ 0:093 l/mgand kc ¼ 1:78 mg/lmin (R2 ¼ 0:987), respectively.

3.5. Effect of pH

In order to investigate the effect of pH, the TNT

degradation rate and the pH change were measured at

H.-S. Son et al. / Chemosphere 57 (2004) 309–317 315

different initial pH (pH 3, 7, 11). The results are shown

in Fig. 9 and Table 2. Table 2 shows that TNT degra-

dation is least effective at acidic pH and showed a higher

degradation rate at neutral and basic pH. The higher

degradation rate at neutral pH can be explained by the

point of zero charge (pzc) of the TiO2. The pzc value isfound at pH 6.25 (Poulios and Tsachpinis, 1999). The

TiO2 surface is positively charged in acid media (pH

< 6:25), whereas it is negatively charged under alkalineconditions (pH > 6:25). Since TiO2 exhibits an ampho-

teric character with a zero charge in the pH range

around 6.2–6.4, the adsorption of TNT can be favorable

at pH near the pzc of TiO2, and therefore higher TNT

degradation efficiency was expected at neutral pH. Also,

the higher degradation rate at basic pH can be explained

by the quantity of OH� ions on the surface of TiO2

increased as the pH increases, and these ions can com-

bine with hþVB to produce more OH radicals, as shown in

Eq. (3).

Fig. 9 also showed that the pH decreased during the

TNT degradation at different pH. On UV illumination,

a decrease in TNT concentration is concomitant with a

decrease in slurry pH. The pH drop with contaminant

degradation can be explained by two reactions: decrease

in concentration of hydroxide ion (OH�) from produc-

tion of OH radicals, and production of low molecular-

weight organic acids in the oxidation process such as

Fig. 9. The effect of initial pH on the TNT degradation and the

pH change during the reaction.

Table 2

The pseudo first-order rate constant kobs, half-life t1=2, andcorrelation coefficients for photocatalytic degradation of TNT

at different pH (experimental conditions: TNT¼ 30 mg/l,

TiO2¼ 1 g/l, UV density¼ 8.2 mW/cm2)

pH kobs (min�1) t1=2 (min) R2

3.0 0.0173 27.6 0.982

7.0 0.0422 20.1 0.983

11.0 0.0451 21.5 0.990

acetic acid and formic acid during TNT degradation in

this study. Due to these reasons, the TiO2 is rapidly

hydrated in aqueous solution according to the following

equation Peterson et al., 1991:

½2ðTiO2Þ � 2TiðIVÞ �OþH2O

$ ½2ðTiO2Þ � 2TiðIVÞ �OH ð13Þ

where TiO2 represents titanium dioxide in bulk solid

phase and Ti(IV) is a surface titanium. The hydrated

TiO2 surface is amphoteric and accordingly, has a pH-

dependent speciation. The equilibrium among the sur-

face species can be depicted by

½TiðIVÞ �OHþ2 $�H

þ½TiðIVÞ �OH $�OH

�

½TiðIVÞ �O� þH2O ð14Þ

With regard to the economical and practical aspects of

treatment, the result of the pH effect implies that pH

adjustment steps will be not required in the treatment of

TNT contaminated water using TiO2 photocatalysis.

3.6. Effect of TNT and nitrogen byproduct adsorption on

the TiO2 surface

From the control experiments, we found that about

10% of the initial TNT (30 mg/l) was adsorbed onto the

TiO2 particles under conditions of darkness. In order to

confirm that the concentration decrease of TNT under

conditions of darkness was indeed due to the adsorption

of TNT onto the TiO2 particles, and not due to chemical

reactions, the experiment was performed such that, for

the first 60 min, it was conducted with only TiO2 and,

for the next 60 min, it was conducted with both TiO2

and UV irradiation, so that the pattern of byproducts

production could be compared. Samples were collected

throughout the experiment, and the TNT and nitrogen

byproduct concentrations were determined. The results

are shown in Fig. 10. It can be seen that the NO�2 and

NO�3 byproducts were not formed under conditions of

darkness, however, the concentration of NO�2 and NO

�3

increased steeply, starting at the point where the UV

irradiation began. Furthermore, no color change was

observed during the adsorption reaction of TNT, but the

solution turned pink when the UV irradiation was

started. In the case of the photocatalytic reaction, the

adsorption of the reactants by the catalyst may play a

significant role in their removal, in addition to that

resulting from the TiO2 photocatalytic reactions.

As seen in Fig. 9, the pH decreased from 7 to 5.7

during TNT degradation in photocatalysis at initial pH

7. Since the TiO2 surface is positively charged as the

reaction proceeds (pH < 6:25), the adsorption of NO�3 ,

major anion byproduct, on the positively charged TiO2

surface is expected.

Fig. 10. Concentration change of TNT and nitrogen byprod-

ucts (NO�3 and NH�

2 ) during adsorption and photocatalysis

(experimental conditions: TNT¼ 30 mg/l, TiO2¼ 1 g/l, pH¼ 7,light intensity¼ 0–8.2 mW/cm2).

316 H.-S. Son et al. / Chemosphere 57 (2004) 309–317

Several researchers found that the efficiency of pho-

tocatalysis decreased the presence of anions due to the

interaction between anions and positively charged TiO2

surface (Abdullah et al., 1990; D’Oliverira et al., 1993).

Therefore, the experiment was performed to investigate

the effect of adsorption by NO�3 , major nitrogen anionic

byproduct, on the TiO2 surface. The solutions with

different concentration of NO�3 (10, 20, and 40 mg/l)

were added to TiO2 (10 g/l) solution in the darkness,

then the NO�3 concentration in the water phase was

measured after 150 min. The initial solution pH con-

taining TiO2 slurry was around 5.0 due to the TiO2

hydration. The result showed that less than 2% of NO�3

was adsorbed even after 150 min in all NO�3 concen-

trations. This result indicates that the adsorption effect

of NO�3 on TiO2 surface is minimal and does not affect

on the TNT degradation in photocatalysis.

4. Conclusions

This study was performed to determine the possibil-

ity of applying the TiO2 photocatalytic reaction to the

treatment of TNT contaminated water. The conclusions

which can be made from this study are summarized

below:

1. The percentage decreases in TNT concentration,

resulting from the photolysis and photocatalytic reac-

tions conducted for 150 min, were 72% and 100%,

respectively. After this same time period, the de-

creases in TOC concentrations were 40% and 80%,

respectively.

2. About 10% of TNT was adsorbed by the photocata-

lyst in the absence of UV irradiation.

3. TNT degradation followed a pseudo first-order rate

expression according to the Langmuir–Hinshelwood

kinetic model.

4. TNT showed higher photocatalytic degradation effi-

ciency at neutral and basic pH.

5. Determination of the nitrogen byproduct concentra-

tions revealed that NO�3 constitutes the major

byproduct of both TNT photocatalysis and photoly-

sis. It was confirmed that NO�2 increases initially but

is subsequently converted to NO�3 in the case of pho-

tocatalysis. In contrast, NO�2 increased steadily in

photolysis. The produced NO�3 however did not ad-

sorbed on the TiO2 surface.

6. Formate and acetate were qualitatively identified as

the organic intermediates, and these were further

mineralized.

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