Photocatalytic degradation of explosives contaminated water

S.-J. Lee, H.-S. Son, H.-K. Lee and K.-D. Zoh Department of Environmental Health, Graduate School of Public Health, Seoul National University, 28 Yeungeon-Dong, Chongno-Gu, Seoul, 110-799, Korea Abstract This study was undertaken to examine the degradation of TNT, RDX and HMX in a circular photocatalytic reactor with TiO2 as a photocatalyst. We examined the impact of parameters such as the initial concentration, initial pH of solution on rates of photocatalized transformation, and the mineralization. The results showed that photocatalysis is an effective process for the degradation of TNT, RDX and HMX. They could be completely degraded in 150 min with 1.0 g/L TiO2 at pH 7. An increase in the photocatalytic degradation of HMX was noticed with decreasing initial HMX. The rates of RDX and HMX degradation were greater in neutral pH than in acidic and alkaline conditions. In case of TNT degradation, the rate of degradation was the fastest at pH 11. Approximately 82% TOC decrease in the TNT degradation was achieved after 150 min, whereas TOC decrease in RDX and HMX was 24% and 59%, respectively. Nitrate, nitrite, and ammonium ions were detected as the nitrogen byproducts from the photocatalysis, and more than 50% of the total nitrogen was recovered as nitrate ion in every explosives. Keywords Ammonium; HMX; nitrate; nitrite; photocatalysis, RDX; TiO2 (titanium dioxide); TNT

Introduction

Highly explosive compounds such as TNT (2,4,6-trinitrotoluene), RDX (hexahydro-1,3,5trinitro-1,3,5-triazine), and HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) have been used in detonators, mines, rocket boosters, and plastic explosives. During their early manufacture, wastewaters generated at the munitions facilities were often dumped into unlined pits, and disposed of in lagoons. This improper disposal practice led to the contamination of these compounds, until now they are often found in the soil, groundwater, and surface waters at the sites where they are manufactured (Urbansky, 1965). The toxicity of the explosive compounds and their degradation products has led to concern about their fate in the environment and potential for human exposure. TNT and RDX are found to be toxic to algae (Yinon, 1990), and chronic exposure to TNT by humans causes harmful health effects (Peters et al., 1991). TNT and RDX are classified as possible human carcinogens (class C) by the EPA (McLellan et al., 1988). Several methods of treating TNT, RDX and HMX contaminated soils and groundwater have been developed. Carbon adsorption was often used to treat munitions plant wastewater (Wujcik et al., 1997). Incineration is another preferred technology for remediating munitions-contaminated soils. However, this method has disadvantage due to the production of NOx and cyanide during the process. Bioremediation is also being developed as an alternative technology (Ronen et al., 1998; Young et al., 1997). The low solubility of the compounds, however, limits their availability to microorganisms for biodegradation. Furthermore, the degradation rate is often slow and may be incomplete (Coleman et al., 1998). Composting is another effective method for biodegrading explosives, but several concerns are associated with its use at large scale. The photocatalytic degradation of organic environment pollutants in the presence of TiO2 has become interesting over the last few decades (Maugans and Akgerman, 1997; Carraway, 1994). Photocatalysis offers an important potential in the treatment of water and

Water Science and Technology Vol 46 No 11–12 pp 139–145 © IWA Publishing 2002

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wastewaters because the process is relatively rapid, capable of simultaneously degrading a wide range of contaminants, and potentially cost competitive with existing technologies. TiO2-mediated photocatalytic degradation of TNT solutions has also been reported (Schmelling and Gray, 1995; Wang and Kutal, 1995). In this research, we present the results of photocatalytic degradation of TNT, RDX, and HMX in a circular reaction system. We examined the effect of initial concentration and pH on the degradation of TNT, RDX, and HMX, and intermediate transformation products were also determined. Materials and methods Chemicals

The pure TNT, RDX and HMX used for experimental work were obtained from Agency for Defense Development of Korea. These starting materials were used without further purification. Aqueous stock solutions, containing 100 ppm TNT, 40 ppm RDX and 5 ppm HMX were prepared by dissolving the pure compound with heating and stirring for 24 hr. Titanium dioxide (TiO2) was Degussa P-25, which was mostly anatase and had a Bet surface area of 50 m2/g and an average particle diameter of 30 nm. Reactor system

All photocatalytic degradation experiments were performed in a circulating photoreactor system. The reactor system consisted of a reservoir, a metering pump (Master Flex) for circulation of reactor contents, and photoreaction chamber (Figure 1.). The three parts were connected with flexible Teflon tubing. The reservoir was a glass bottle with a total volume of 1 litre and a magnetic stirrer was placed in the bottom of the bottle. The UV chamber consisted of four UV lamps and five photoreactor columns. All reactor columns were Pyrex glass with outside diameter of 10 mm and 100 cm long, connected with Teflon tubing. Five UV lamps with a diameter of 30 mm and 100 cm long illuminated the reactor assembly. It emits approximately 90% of its radiation at 254 nm with a 40 W power input. Reactions were performed at 293 K. The reaction solutions were prepared by dilution of stock solutions and the aqueous phase was then introduced into the reservoir. In order to determine the influence of pH, the solutions were adjusted with 1N-NaOH and 1N-HCl. The solutions were circulated with a metering pump at a rate of 1 L/min. The aliquots (10 ml) were sampled from reaction mixture at regular intervals, filtered into sample bottles, capped and refrigerated prior to analysis Sample analysis

All liquid samples were filtered through MCE membrane filters having a pore size of 0.2 µm (Advantec MFS Inc., Pleasanton, CA) to remove the TiO2 suspension. A modification 3-way valve Sampling outlet

100 cm

UV chamber

Reservoir

Metering pump Magnetic stirrer 140

Figure 1 Schematic diagram of the photocatalytic reactor

Photoreactor column UV

15 cm

Cross section of UV chamber

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of the technique in EPA method 8330 (US EPA, 1994) was used. The TNT, RDX and HMX concentration were analyzed by a HPLC system (Dionex, Sunnyvale, CA). Detection was performed with a diode array detector UVD 340S. The detector was set at 254 nm to detect TNT, RDX and HMX. We used an isocratic mixture of methanol and water (50:50, v/v %) at a flow rate of 1.0 mL/min. Aliquots of 20 µL were injected into a C-18 Supelco (Supelco, Supelco Park, PA) silica column (25 cm × 4.6 mm, 5-µm particles). Total organic carbon (TOC) in liquid samples was performed on a TOC analyzer (Shimadzu, Japan), and degradation products such as nitrate, nitrite, and ammonium ion were estimated using Standard Methods (1992). Results and discussion

It is important to carry out blank experiments to ascertain whether degradation of objective chemicals was caused by the photocatalytic reaction of TiO2. We compared the photolytic and the photocatalytic degradation reaction for TNT by examining concentration of TNT as a function of time as shown in Figure 2. The results showed that a moderate decrease in the concentration of TNT was observed in the presence of TiO2 without irradiation of UV. It may result from the oxidation of TiO2 by oxygen dissolved in water or the adsorption of TNT on the surface of TiO2. The reduction of TNT concentration in the photolysis condition (UV only) may arise from the photodegradation of TNT by the strong UV wavelength. Comparing the uncatalyzed degradations and TiO2-catalyzed degradation with the irradiations of UV, the rate enhancement was observed in the latter case. The TNT almost fully disappeared within 90 min in the photocatalytic condition, while 20% of TNT remained at 150 min in the photolytic condition. This can be considered as the evidence of the photocatalytic degradation of TNT. Next, the effect of initial explosive concentration on the photocatalytic degradation rate was investigated for HMX, and is shown in Figure 3. The photocatalytic degradation of HMX can be described using the first-order law kinetic model defined as –dC/dt = kappC0

(1)

and thus ln (C0/C) = kappt

(2)

where C0 is the initial concentration of HMX, kapp the pseudo first-order reaction constant, t the irradiation time, and C the concentration of HMX at time t.

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Figure 2 Photolysis and photocatalytic degradation of TNT (30 ppm)

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Figure 3 Effect of initial explosive concentration on the photocatalytic degradation of HMX

The interesting fact is that the line slope was decreased with an increase of the initial HMX concentration. The concentrated HMX solution took longer time than the diluted solution to accomplish the similar C/C0 level. This indicates that the initial concentration of HMX in solution has a significant influence on the degradation rate. Next, we investigated the impact of pH changes in the degradation of explosives. pH can be a strong factor controlling the adsorption of contaminant and the reactivity of TiO2 because pH can influence the surface charge of TiO2 and also shift the potential of some redox reactions. Figure 4 shows photocatalytic degradation of TNT in TiO2 slurries at three different pH. As shown in this figure, the degradation of TNT is more rapid in a neutral or an alkaline medium than in an acidic condition. In general, the photocatalysis rate of TNT is known to increase with the increase in pH (Schmelling et al., 1997). We also investigated the pH effect on the degradation of RDX and HMX, and measured kapp value. Table 1 shows the results. It is noted that the rate of RDX and HMX showed the maximum at neutral pH. Consequently, t1/2 at pH 3 is larger than t1/2 at pH 7 and pH 11 for every explosive, indicating that acidic condition causes delay of photocatalytic degradation. This result implicates that proton ion in an acidic medium could inhibit the photocatalysis by affecting the concentration of OH radicals since photocatalysis involves formation of OH radical. Figure 5 compares the degradations of TNT, RDX and HMX as a function of irradiation time. The degradation of HMX was so fast that no HMX was detected by HPLC after 30 min, while the TNT was still detected after 60 min. Such a difference in degradation rate 7 pH 3 pH 7 pH 11 Fitted Line

6

ln (C0/C)

5 4 3 2 1 0 0

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Figure 4 Effect of initial pH on the photocatalytic degradation of TNT (30 ppm) with 1 g/L of TiO2

Table 1 Apparent first-order rate constant kapp, half-life t1/2, and correlation coefficients R2 for degradation of TNT (30 ppm), RDX (20 ppm), and HMX (5 ppm) at different pH

TNT

HMX

t1/2 (min)

R2

0.0173 0.0422 0.0451 0.0367 0.0506 0.0460 0.0773 0.2067 0.1127

27.6 20.1 21.5 18.7 14.7 14.4 8.6 4.8 5.9

0.9822 0.9827 0.9900 0.9964 0.9948 0.9986 0.9779 0.9674 0.9902

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RDX

pH 3 pH 7 pH 11 pH 3 pH 7 pH 11 pH 3 pH 7 pH 11

kapp (min–1)

100 TNT (30 ppm) RDX (20 ppm) HMX (5 ppm)

C/C0 x 100

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0 0

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Figure 5 Photocatalytic degradation of TNT, RDX and HMX with 1 g/L of TiO2

TOC (%)

between explosives resulted from the different initial concentrations of explosives rather than the difference between molecular structures of explosives. Next step is to investigate possible mineralization and byproducts produced from photocatalysis. For the mineralization study, we measured TOC remaining in the photoreacting solution after 30, 60, 150 min. Figure 6 shows the results. After 150 min, TOC in TNT solution shows the lowest value (about 20%), indicating that more than 80% of TNT was mineralized by photocatalysis, i.e., considerably oxidized to inorganic carbons, CO2 gas, H2O, or some UV invisible species. On the other hand, RDX that disappeared rapidly within 60 min as shown in Figure 5 shows only little decrease in TOC by about 24% after 150 min. However, the significant mineralization of HMX (about 60%) was observed after 150 min. From TOC measurement, we can conclude that the photocatalytic treatment can give more perfect degradation to TNT or HMX rather than RDX.

100

0 min

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30 min

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20 0

TNT

RDX

Figure 6 TOC of TNT, RDX, and HMX with 1 g/L of TiO2

H MX 143

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0 40

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Percent of total nitrogen

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+

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Figure 7 Percent of total nitrogen in photodegradation of TNT (30 ppm), RDX (20 ppm) and HMX (5 ppm)

It is known that nitrate, nitrite, and ammonium ions are formed as the photocatalytic transformation products of nitro-aromatic and ring compounds such as TNT, RDX, and HMX (Schmelling et al., 1997). In our study, the concentrations of NO3–, NO2–, and NH3 were measured during the degradation of each explosive, and the results are shown in Figure 7. The figures show the concentration of each byproduct, presented as the percentage of the total moles of nitrogen. In the case of TNT degradation, the concentration of NO3– increased until 90 min and then was saturated at about 48%. However, the concentrations of nitrite and ammonium ion species did not exceed 10% of the initial nitrogen present in TNT, indicating that nitrate ion is the predominant byproduct in the photocatalytic degradation. In fact, OH radical under photocatalytic condition is known to oxidize NO2– into NO3– (Low et al., 1991). It is also interesting that the concentration of NO2– is precisely related with the degradation rate of TNT as observed by HPLC. The maximum concentration of NO2– was achieved at 30 min in the TNT degradation. After 30 min, the concentration of NO2– began to decrease and reached almost zero in 90 min. In the TNT degradation profile, TNT concentration was almost zero at 90 min (Figure 5). This is the evidence that NO2– is directly formed as a byproduct of photocatalytic degradation of TNT and this species is then further oxidized into NO3–. Total nitrogen byproducts concentration profile of RDX and HMX degradation exhibited similar pattern with TNT case. Such a fast production of nitrogen byproducts is more remarkable in HMX case. These results are matched by the fact that the degradations of HMX and RDX are faster than that of TNT (Figure 5). Conclusions

The production and use of TNT, RDX, and HMX in the world has led to the contamination of soil and water at numerous sites. Remediation is therefore needed to reduce the threat they pose to human health and the environment. In our study, TNT, RDX and HMX were effectively degraded in circular photocatalytic reactor. With TiO2 catalysis, TNT, RDX, and HMX were completely degraded fast, and the degradation products such NH4+, NO2–, NO3– were obtained, and some mineralization was achieved. From the results of this study, the TiO2 photocatalytic system can be one of the treatment process for ex-situ remediation of explosives contaminated groundwater and wastewater at the contaminated sites. References Carraway, E.R., Hoffman, A.J. and Hoffman, M.R. (1994). Photocatalytic oxidation of organic acids on quntum-sized semiconductor colloids, Environ. Sci. Technol., 28, 786–793. Coleman, N.V., Nelson, D.R. and Duxbury, T. (1998). Aerobic biodegradation of hexahydro-1,3,5-trinitro1,3,5-triazine (RDX) as a nitrogen source by a rhodococcus sp., strain DN22. Soil Biol. Biochem., 30(8–9), 1159–1167. 144

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Low, G., McEvoy, S.R. and Matthews, R.W. (1991). Formation of nitrate and ammonium ions in titanium dioxide mediated photocatalytic degradation of organic compounds containing nitrogen atoms. Environ. Sci. Technol., 25, 460–467. Maugans, C.B. and Akgerman, A. (1997). Catalytic wet oxidation of phenol over a Pt/TiO2 catalyst, Wat. Res., 31(12), 3116–3124. McLellan, W., Hartley, W.R. and Brower, M. (1988a). Health advisory for hexahydro-1,3,5-tetranitro1,3,5-triazine; Technical Report No. PB90-273533; Office of Drinking Water, U.S. Environmental Protection Agency: Washington, DC. Peters, G.T., Burton, D.T., Paulson, R.L. and Turley, S.D. (1991). The acute and chronic toxicity of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) to 3 freshwater invertebrates. Environ. Toxicol. Chem., 10, 1073–1081. Ronen, Z., Brenner, A. and Abeliovich, A. (1998). Biodegradation of RDX-contaminated wastes in a nitrogen-deficient environment, Wat. Sci. Tech., 38(4–5), 219–224. Schmelling, D.C. and Gray, K.A. (1995). Photocatalytic transformation and mineralization of 2,4,6trinitrotoluene (TNT) in TiO2 slurries. Wat. Res., 29, 2651–2662. Schmelling, D.C., Gray, K.A. and Kamat, P.V. (1997). The influence of solution matrix on the photocatalytic degradation of TNT in TiO2 slurries, Wat. Res., 31(6), 1439–1447. Urbansky, T. (1965). Chemistry and Technology of Explosives, vol. 2, p. 32–61, Pergamon Press, Oxford, England. Wang, Z. and Kutal, C. (1995). Photocatalytic mineralization of 2,4,6-trinitrotoluene in aqueous suspensions of titanium dioxide. Chemosphere, Vol. 30, 1125–1136. Wujcik, W.J., Lowe, W.L., Marks, P.J. and Sisk, W.E. (1992). Granular activated carbon pilot treatment studies for explosives removal from contaminated groundwater, Env. Progress, 11(3), 178–189. Yinon, J. (1990). Toxicity and metabolism of explosives. P. 81–122, CRC Press, Boca Raton, Fla. Young, D.M., Kitts, C.L., Unkefer, P.J. and Ogden, K.L. (1997). Biological breakdown of RDX in slurry reactors proceeds with multiple kinetically distinguishable paths. Biotechnol. and Bioengr., 56(3), 258–267.

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