Environ. Sci. Technol. 1999, 33, 3802-3805
Kinetics of the Alkaline Hydrolysis of 2,4,6-Trinitrotoluene in Aqueous Solution and Highly Contaminated Soils MONIKA EMMRICH* Institute for Hygiene, Environmental and Occupational Medicine, Free University of Berlin, Universita¨tsklinikum Benjamin Franklin, Dekanat, Hindenburgdamm 30, 12 200 Berlin, Germany
During the two World Wars, large amounts of TNT were released into the environment. Until today, high concentrations of TNT can be found in the soil of former ammunition plants. To obtain basic data for a novel treatment process for highly contaminated soils, the homogeneous aqueous hydrolysis of TNT in the pH range from 10 to 12 and the alkaline treatment of two contaminated soils at pH 11 and pH 12 were investigated. The experimental data were described for their respective pH values using a pseudofirst-order model. In the homogeneous experiments, 9597% of the TNT was hydrolyzed. During alkaline hydrolysis, up to two nitrogoups per TNT molecule were released, indicating the irreversible destruction of TNT. Except for the formation of small traces of amino dinitrotoluenes and trinitrobenzenes, no nitroaromatic benzenes or toluenes were detected during GC analysis. For the less contaminated soil, ELBP2, with an initial TNT concentration of 116 mg/ kg, a destruction of 99% was achieved. The highly contaminated soil, HTNT2 (16.1 g of TNT/kg), showed a hydrolyzation level of 90-94%. The results show that the alkaline treatment of highly contaminated soils may prove to be effective as an alternative treatment technology.
Introduction Military sites represent a serious and potentially hazardous contamination problem of growing environmental concern due to the end of the Cold War. Especially at former ammunition production plants, explosive wastes were released into the environment during manufacture, loading, and assembling; were discharged into wastewater; and escaped through accidents. 2,4,6-Trinitrotoluene (TNT) is the most commonly used military explosive because of its stability and low sensitivity to impact friction as well as the relative safety of the methods used in producing it. During the two World Wars, millions of tons of TNT was produced under war conditions. Waste was disposed of improperly during the production process, and TNT was also released through occasional accidents during bombardments and through the improper dismantling of these plants at the end of the war. Although production reached its highest level during World War II, TNT and its congeners are still found in high concentrations in soil and groundwater, indicating significant resistance to degradation. * Corresponding author address: Universita¨tsklinikum Benjamin Franklin, Dekanat, Hindenburgdamm 30, 12 200 Berlin, Germany; phone: 030-8445 36 14; fax: 030-8445 44 90; e-mail: emmrich@ zedat.fu-berlin.de. 3802
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TNT represents an environmental hazard because of its toxicity to humans, fish, algae, and microorganisms (1-3). It is mutagenic to Salmonella typhimurium (4) and can cause cancer. The toxicity and mutagenicity of the compound along with its resistance to degradation in the environment require the development of appropriate treatment technologies. For less contaminated sites, bioremediation of TNT using white rot fungi or bacteria seems a viable treatment option (5, 6). Washing the soil with water causes TNT to migrate from coarse soil particles to fine ones. This technology divides the soil at least into two parts: a less contaminated one, which can be refilled, and a more highly contaminated one, which requires further treatment (7). Direct incineration of the soil is possible for highly contaminated military sites even when pieces of pure TNT are present. This is, however, not only a relatively expensive technology but also a dangerous one because of the risk of explosion. TNT is known to be sensitive to bases (8-10). The presence of three nitro groups as strong electron-withdrawing goups has the effect of reducing the electron density of the aromatic ring, thus favoring the nucleophile attack of bases such as OH- or RO-. While the reactions of TNT with RO- in organic solvents have been well studied (11-15), very little is known about the reactions of TNT with OH- under alkaline aqueous conditions. The pathway will most strongly be influenced by the medium in which the reaction takes place. Bernasconi (11) reported that in the presence of excess base a minor change in the water content of the medium has a significant effect on the chemistry of TNT. He suggested that radicals are formed and that the TNT anion, if formed at all, must be a very transient species. In contrast to the TNT anion, the presence of Meisenheimer complexes has been sufficiently prooven. According to Yinon (16) the color of the “red water”, which is formed during the sodium sulfite purification of TNT, is probably due to these Meisenheimer complexes. Dahn et al. (17) observed substantial amounts of nitrite and, at higher temperatures, also of ammonia during alkaline treatment of TNT, indicating the irreversible destruction of large amounts of TNT. Alkaline hydrolysis promises to be an effective treatment technology especially for remediation of highly contaminated soils, without any risk of explosions. Therefore, the purpose of the basic research described here was to determine the chemical kinetics of alkaline hydrolysis of TNT under aqueous homogeneous conditions. The pH values were varied in the range of 10-12, while the respective reaction rates were calculated using a pseudo-first-order model. On the basis of these findings, the alkaline hydrolysis of TNT was applied to two contaminated soils from former military sites in Germany. These studies were performed at pH values of 11 and 12 to test the applicability of alkaline hydrolysis on inhomogeneous soil sytems.
Materials and Methods Homogeneous Alkaline Treatment. Experiments were performed at 20 °C. A stock solution of TNT was prepared by solving 100 mg of TNT (obtained from WIWEB Swisttal, Germany) in 1000 mL of deionized water. Alkaline treatment at pH 12 consisted of placing 77.1 mL from this solution into a reaction vessel (a 150-mL Erlenmeyer flask) to which 22.9 mL of a saturated and filtered solution of Ca(OH)2 was added. Alkaline treatment at pH 11 and pH 10 involved using 97.7 and 100 mL of the stock solution to which 2.29 and 0.23 mL of the saturated solution of Ca(OH)2 were added, respectively. Another reaction vessel with 100 mL of the stock solution 10.1021/es9903227 CCC: $18.00
1999 American Chemical Society Published on Web 09/30/1999
was put on a magnetic stirrer, served as reference, and was kept at the same conditions as the reaction mixtures. To reach the same electrolyte concentration as the reaction mixture at pH 12, appropiate amounts of 3 M KCl were added to the mixtures at pH 11, 10, and 7. The mixtures were stirred immediately following the addition of Ca(OH)2. Alkaline Treatment of Soil. Soil samples contaminated with TNT and other nitroaromatic compounds were collected from two former ammunition plants in Germany. Before treatment, the soils were passed through a 2-mm sieve and air-dried. To achieve alkaline hydrolysis, 5 g of the soil samples was transferred into reaction vessels (100-mL Erlenmeyer flasks) and then stirred with 50 mL of a solution of Ca(OH)2 at pH 11 and pH 12, respectively. Soil samples (5 g) were used as references treated with 50 mL of distilled water. Appropiate amounts of KCl were added in order to reach the same electrolyte concentration as the mixtures at pH 12. To achieve a complete mass balance of the residual TNT after alkaline treatment, identical test arrangements are necessary for each pH and reaction time. During alkaline treatment, the pH decreased as TNT reacted with the base. Regulation of the pH was performed by a programmable 8052 AH basic microcontroller and was kept constant within a range of (0.1 pH unit by controlled addition of a saturated and filtered solution of Ca(OH)2. Extraction of Homogeneous Samples. Samples of 5 mL were taken at predefined times. Extraction was carried out by shaking the samples three times for 3 min with 3 mL of ethyl acetate (SupraSolv, Merck). The combined organic phases were dried over anhydrous sodium sulfate, evaporated to dryness, and redissolved with 1 mL of methanol. Extraction of Soil Samples. At predefined times, the respective soil samples were neutralized with HCl to halt the reaction. Subsequent centrifugation was carried out at 2500 rpm for 10 min. From the supernatant, 50 mL was used for triplicate extraction by shaking with 25 mL of ethyl acetate for 3 min each. The combined organic phases were dried with anhydrous sodium sulfate, evaporated to dryness, and redissolved with 1 mL of methanol. The remaining soil was dried with Na2SO4 and extracted in a Soxhlet apparature with 80 mL of tert-butylmethyl ether (SupraSolv, Merck) for 3 h. The extract was then dried over anhydrous sodium sulfate and evaporated to dryness. The samples were redissolved with tert-butylmethyl ether. Gas Chromatographic Conditions (GC). Nitroaromatic compounds were analyzed using a gas chromatograph (GC) equipped with an ECD (Hewlett-Packard 5890 series II, Amsterdam, The Netherlands) and a cold injection system KAS 3 (Gerstel, Mu ¨ lheim a.d. Ruhr, Germany). Separations were performed on an HT8 column (SGE, Weiterstadt, Germany) with helium as the carrier gas. The following temperature program was used: 2 min at 40 °C, 40-130 ° C with 10 °C/min, 1 min at 130 ° C, and then 130-250 °C with 3 °C/min.
Results and Discussion Under alkaline conditions, the rate of TNT hydrolysis depends not only on the concentration of the base but also on the concentration of TNT itself. Therefore, a second-order rate equation is assumed to the alkaline hydrolysis of TNT in water:
dCTNT(t) ) -k2CTNTCOHdt
(1)
where CTNT is the TNT concentration, COH- is the hydroxide ion concentration, and k2 is the second-order rate constant. The kinetic experiments were performed with COH- far exceeding the concentration of TNT, and the base was kept
FIGURE 1. Alkaline hydrolysis of TNT at pH 10, 11, and 12, and the respective reaction rate constants k1 vs hydroxide ion concentration.
TABLE 1. Pseudo-First-Order Rate Constant k1 of Alkaline Hydrolysis of TNT for Different OH- Concentrations pH 12 pH 11 pH 10
COH- (mmol/L)
C0 (mmol/L)
k1 (h-1)
r2
10.0 1.0 0.1
0.34 0.40 0.46
0.361 0.026 0.000
0.882 0.982
constant at pH 12 and pH 11. In this case, eq 1 can be reduced to a pseudo-first-order rate equation, where k1 is the pseudofirst-order rate constant:
dCTNT(t) ) -k1CTNT dt
(2)
For CTNT ) C0 at t ) 0, the solution of the differential equation is achieved by integration from t ) 0 to t:
ln
CTNT(t) ) -k1t ) -k2COH-t C0
(3)
Homogeneous Alkaline Treatment. Figure 1 shows the linearized plot of the TNT concentration versus time at pH 10, 11, and 12 and the pseudo-first-order rate constants as a function of hydroxide concentration. After linearization of the experimental data, the least-squares method was used for calculation of C0 and the pseudo-first-order rate constant k1 (Table 1). Under the experimental conditions, alkaline hydrolysis occurs only if the base concentration exceeds that of TNT. At pH 10, the base has a concentration of 0.1 mmol/L and is below that of TNT with a concentration of 0.46 mmol/ L. When performing the experiments, the alkaline water takes on a slight violet tint, but a decrease in TNT concentration could not be measured. According to Caldin and Long (18), the violet color indicates the formation of the TNT anion. VOL. 33, NO. 21, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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The stock solution at pH 7, which served as reference, and was kept under the same conditions showed no coloring. At pH 11, the OH- concentration is 2.5 times higher than the TNT concentration. The experimental data correlated closely with the pseudo-first-order model with a good correlation coefficient of 0.982. During treatment, an extensive removal of TNT was achieved. But according to the low-rate constant of 0.026 h-1, 120 h is necessary to achieve a 95% reduction of the initial TNT. With further increasing base concentration, TNT hydrolysis becomes more rapid according to the higher reaction rate constant of 0.361 h-1 at pH 12. The TNT concentration decreases within 6 h by more than 90%, and after 24 h only 3% of the initial TNT could be detected. The pseudo-first-order rate constants show only small deviations from OH- concentration. Hence the pseudo-firstorder model simulation is considered applicable for alkaline homogeneous hydrolysis of TNT. During alkaline hydrolysis of TNT, the separation of several nitro groups can be observed. By using 0.22 mmol of TNT at pH 13, the formation of 0.18 mmol of NO2- and 0.24 mmol of NO3- was measured. By complete separation of all nitro groups of the TNT molecule, one would expect the formation of 0.66 mmol of NO2- and/or NO3-. Therefore the release of 0.42 mmol of NO2- and NO3- indicates the release of two of the three nitro groups from the TNT molecule. At pH 12, the formation of 0.35 mmol of NO2- and NO3- was measured, suggesting the release of 1.5 nitro groups per TNT molecule. Only negligible amounts of NH4+ were detected. With the extraction method used for sample preparation, not only the detection of TNT is possible but also the detection of nitroaromatic benzenes and toluenes. Only small amounts of the two amino dinitrotoluene congeners were analyzed at pH 11 and pH 12. At pH 11, traces of 1,3,5-trinitrobenzene were additionally measured. They are only transient intermediates reaching their maximum concentration shortly after starting the alkaline treatment. During further treatment, their concentration decreases and little or no trace of intermediates could be detected after reaction was complete. Alkaline Treatment of Soil. The results of the homogeneous alkaline treatment shows an extensive destruction of TNT within some hours or days depending on the pH, indicating that the alkaline treatment of soils might prove to be effective for remediation of contaminants. To determine if alkaline treatment is feasible for remediation of TNT, the two soils HTNT2 and ELBP2 were treated at pH 11 and pH 12 under the same conditions used for homogeneous experiments. HTNT2 is a highly contaminated soil with a TNT concentration of 16 110 mg/kg, while ELBP2 shows a lower level of contamination of 116 mg/kg. Because of the stability of TNT in the homogemous experiments, pH 10 is not further investigated. The soil samples treated only with water were used as references. Because of the buffering capacity of the soils, the pH values decreased to pH 4.4 and pH 5.6 for HTNT2 and ELBP2, respectively. During treatment, desorption of TNT occurred to some extent and dissolved in the water. By this process, only the distribution of the TNT between soil and water was changed, but no hydrolysis of TNT occurred, and the mass balance confirmed a total recovery close to 100%. Conversely, the reduction of TNT at pH 11 and pH 12 is obvious. In Figure 2, the linearized plots of the TNT concentration are depicted over time for HTNT2 and ELPB2. The respective reaction rate constants were shown in Table 2. At pH 12, TNT hydrolysis is most effective within the first 24 h. For HTNT2 the concentration decreases by 94%, corresponding to an absolute amount of 15.2 g/kg. During further treatment, only a small additional reduction is achieved, and the residual TNT concentration remains constant at about 950 mg/kg. 3804
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FIGURE 2. Alkaline hydrolysis of TNT at pH 11 and 12 in the soils HTNT2 and ELBP2.
TABLE 2. Pseudo-First-Order Rate Constant k1 of Alkaline Hydrolysis of TNT for HTNT2 and ELBP2 for pH 11 and pH 12 pH 12 pH 11
k1 r2 k1 r2
(h-1) (h-1)
HTNT2
ELBP2
0.253 0.941 0.045 0.981
0.340 0.994 0.031 0.989
For ELBP2 a pseudo-first-order reaction rate of 0.34 was calculated, which is similar to the reaction rate of the homogeneous experiments at pH 12. Within 24 h, a reduction of 99% was achieved with a residual concentration of about 1.5 mg/kg. During further treatment, the TNT concentration falls below 0.5 mg/kg. In principle, this shows that low residual TNT concentrations can be achieved by alkaline treatment of soils. Not only the high initial TNT concentration of HTNT2 but also the batch process with only one treatment step might cause reaction products to enrich in alkaline mixture, preventing a complete hydrolyzation of TNT. Possibly the application of an optimized treatment scheme with several succeeding treatment steps or a continuous process might be successful to realize lower residual TNT concentrations in highly contaminated soils such as HTNT2. At pH 11, a good reduction of TNT was also achieved, but this process is of course much slower than at pH 12. For HTNT2, 90% of the initial TNT concentration is hydrolyzed within 48 h. During further treatment, TNT reaches a final concentration of 1208 mg/kg, corresponding to a total reduction of 93%. This is only slightly higher than the residual concentration at pH 12, but the degradation after 48 h occurs very slowly. For ELBP2, the TNT concentration decreases continuously during the treatment process. A total reduction of 99% according to a residual concentration of 0.5 mg/kg could be achieved with a reaction rate constant of 0.031 h-1 for the experimental data, which is about 1/10th of the reaction rate constant at pH 12.
On the basis of these findings, it can be concluded that alkaline hydrolysis represents a promising technology for remediation of TNT in soils. In the homogeneous experiments 95-97% of the initial TNT was decomposed, and for ELBP2 even a destruction of 99% was achieved. Only the highly contaminated soil HTNT2 showed a slightly less hydrolyzation degree of 90-94%, depending on pH and treatment time. Furthermore, liberation of nitrite indicates the irreversible destruction of TNT as an explosive during alkaline hydrolysis. This offers the treatment especially of highly contaminated soils without creating safety problems and running the risk of explosions. Because of the necessary pH of at least 11, this technology can hardly be applied as in situ decontamination technology in reducing TNT at large military sites nor in treating huge amounts of contaminated soil in treatment plants. On the other hand, the concentrated and highly contaminated fine corne fractions of soil washing facilities seems to be well suited for alkaline treatment. These fractions must otherwise be treated by disposal in landfills or by incineration under extreme safety precautions. Even if pieces of pure TNT are present in the contaminated soil, the alkaline treatment is still an alternative treatment technology free of safety problems. The purpose of the basic research carried out here was to determine the homogeneous alkaline hydrolysis of TNT and their principal applicability on contaminated soils. Therefore, simple batch experiments were conducted for the investigations presented in this paper, and no optimization technologies were applied. Hence further studies on the alkaline hydrolysis of TNT (e.g., the application of higher temperatures, a continuous process, or several subsequent treatment steps) might prove to be effective for remediation of highly contaminated soils within shorter time.
Acknowledgments I thank Heike Neumann and Roswitha Kneiseler for their excellent technical assistance and Dr. M. Kaiser (WIWEB, Germany) for providing purified TNT.
Literature Cited (1) Koss, G.; Lommel, A.; Ollroge, I.; Tesseraux, I.; Haas, R.; Kappos, A. D. Bundesgesundheitsblatt 1989, 32 (12), 527-536. (2) Bringmann, G.; Ku ¨ hn, R. Vom Wasser 1978, 50, 45-60. (3) Klausmeier, R. E.; Osmon, J. L.; Walls, D. R. Dev. Ind. Microbiol. 1973, 15, 309-317. (4) Won, W. D.; Disalvo, L. H. Appl. Environ. Microbiol. 1976, 31, 576-580. (5) Rieger, P. G.; Knackmuss, H.-J. In Biodegradation of Nitroaromatic Compounds; Spain, J. C., Ed.; Plenum Press: New York, 1995; pp 1-18. (6) Stahl, J. D.; Aust, S. D. In Biodegradation of Nitroaromatic Compounds; Spain, J. C., Ed.; Plenum Press: New York, 1995; pp 117-133. (7) Der Rat von Sachversta¨ndigen fu ¨ r Umweltfragen, Ed. Altlasten II. Sondergutachten Februar 1995; Metzler-Poeschel: Stuttgart, 1995. (8) Will, W. Ber. Dtsch. Chem. Ges. 1914, 1, 704-717. (9) Giua, M. Z. Ges. Schiess. Sprengst. 1922, 17, 137-139. (10) Naoum, P. Schiess-und Sprengstoffe; Techn. Fortschrittsberichte 16; Thoedor Steinkopf: Dresden, 1927. (11) Bernasconi, C. F. J. Org. Chem. 1971 36 (12), 1671-1679. (12) Brooke, D. N.; Crampton, M. R. J. Chem. Res. 1980, 100, 44014414. (13) Buncel, E.; Norris, A. R.; Russell, K. E.; Tucker, R. J. Am. Chem. Soc. 1972, 94 (5), 1646-1649. (14) Buncel, E.; Norris, A. R.; Russell, K.; Wilson, H. Can. J. Chem. 1974, 52, 2306-2315. (15) Buncel, E.; Norris, A. R.; Sheridan, P.; Wilson, H. Can. J. Chem. 1974, 52, 1750-1759. (16) Yinon, J. Toxicity and Metabolism of Explosives; CRC Press: Boca Raton, FL, 1990. (17) Dahn, A.; Koehler, P.; Heinze, L.; Saupe, A.; Raschke, R. Milita¨rische Altlasten 1995; Erich Schmidt Verlag: Berlin, 1995; pp 211-241. (18) Caldin, E. F.; Long, G. Proc. R. Soc., London 1955, 228, 263-285.
Received for review March 22, 1999. Revised manuscript received August 9, 1999. Accepted August 16, 1999. ES9903227
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