Environ. Sci. Technol. 2001, 35, 874-877
Kinetics of the Alkaline Hydrolysis of Important Nitroaromatic Co-contaminants of 2,4,6-Trinitrotoluene in Highly Contaminated Soils MONIKA EMMRICH† Institute for Hygiene, Environmental and Occupational Medicine, Free University of Berlin, Berlin, Germany
Until this day, large amounts of TNT and related nitroaromatic compounds are found in soils. To obtain basic data for alkaline hydrolysis of these compounds as a novel remediation technology for contaminated soils, we investigated two soils (HTNT2, ELBP2) from two former ammunition plants in Germany. Hydrolysis was performed at pH 11 and pH 12 by addition of Ca(OH)2. During treatment at pH 12 the TNT content dropped to almost zero, and the content of the aminodinitrotoluenes (2A-4,6DNT, 4A-2,6DNT) and the 2,4dinitrotoluene (2,4-DNT) decreased by about 75% (only HTNT2) and 63%, respectively. The experimental data were described using a pseudo-first-order kinetic. Furthermore, an increase of 2,6-DNT and trinitrobenzene (TNB) as well as in one case also of TNT was initially noted in addition to hydrolysis, leading temporarily to an increase of their total amounts of up to 147%, 986%, and 122%, respectively. The results demonstrate that alkaline hydrolysis is difficult when nitroaromatics except TNT represent the major contaminants. However, regarding 2,6-DNT and TNB higher reduction rates than calculated were actually achieved by alkaline hydrolysis. In the case that TNT is the only contaminant or if it is accompanied by certain lower concentrated nitroaromatics alkaline hydrolysis is a valuable remediation technology, especially for soils that are highly contaminated.
Introduction Disposal of explosives and their degradation products from ammunition plants and from disposal sites represents a serious and potentially hazardous contamination problem. 2,4,6-Trinitrotoluene (TNT) is the most widely used military explosive. It is toxic to humans, fish, algae, and microorganisms. Moreover, it has been shown to cause mutagenic effects and is considered a potential cancerogen (1-5). TNT can still be found in former ammunition plants, even though it was mainly produced before and during World War II. At a specific contaminated site various degrees of TNT contamination are often found. As a consequence of accidents and waste disposal one can find extreme highly contaminated areas, with TNT concentrations above 10% or even whole pieces of TNT (6). These soils represent a serious problem † Corresponding author address: Universita ¨ tsklinikum Benjamin Franklin, Dekanat, Free University of Berlin, Hindenburgdamm 30, 12 200 Berlin, Germany; phone: 030-8445 36 14; fax: 030-8445 44 90; e-mail:
[email protected] 874
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not only due to their environmental hazards but also because there is a serious risk of triggering explosions during remediation activities. Direct incineration of highly contaminated soils is expensive and dangerous. Common soil washing technologies with water not only produce a less contaminated soil fraction but also a higher contaminated one that requires further treatment (7). Since TNT is known to be sensitive to bases (8, 9), alkaline hydrolysis of TNT represents a promising technology for remediation of highly contaminated soils. At pH 11 and pH 12 TNT hydrolytic levels of 90-94% were achieved for a soil with an initial TNT concentration of 16.1 g/kg soil. A 99% decomposition was achieved for a lesser contaminated soil containing 116 mg of TNT/kg. In addition, up to two nitrogroups were released during alkaline treatment, indicating an irreversible decomposition of TNT (10). According to Scha¨fer (11) TNT is the most frequently occurring nitroaromatic contaminant in former ammunition plants. However, it may be accompanied by further nitroaromatic compounds. In general, TNT is usually produced by progressive nitration of toluene in a multistage process. The dinitrotoluenes 2,4-DNT and 2,6-DNT result from incomplete nitration and are the main byproducts of TNT synthesis. These substances were released into the environment during TNT production. They are toxic to humans, carcinogenic in animal experiments, are mutagenic to Salmonella typhimurium, and toxic to many aquatic organisms (12, 13). Furthermore, the aminodinitrotoluenes 2A4,6DNT and 4A-2,6DNT are two important transformation products of TNT, resulting from microbial metabolization. In soils they are often detected as co-contaminants of TNT. Only little is known about their environmental chemistry and toxicology. They are mutagenic to Salmonella typhimurium and are suspected to be cancerogenic (1, 12-14). Hence, not only TNT but also related nitroaromatic compounds (such as dinitrotoluenes and aminodinitrotoluenes) are substances of environmental concern that require appropriate treatment technologies. Alkaline hydrolysis has been shown to be effective for soils highly contaminated with TNT. However, in soils that not only contain TNT but also its co-contaminants the hydrolytic degree of the latter greatly influences decontamination efficiency of the soils. To obtain basic data of their hydrolytic rates, alkaline hydrolysis was investigated using two soils containing TNT as well as its major co-contaminants. Alkaline treatment was performed at pH 11 and 12 by addition of a Ca(OH)2 solution. The respective reaction rates of the nitroaromatic compounds were calculated using a pseudofirst-order model.
Materials and Methods Alkaline Treatment of Soil. Soil samples 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) 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 and treated with 50 mL of distilled water. Appropriate 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 nitroaromatics after alkaline treatment, identical test arrangements are necessary for each pH and reaction time. During alkaline treatment the pH decreased as the nitroaromatic compounds react with the base. An adjustment 10.1021/es0014990 CCC: $20.00
2001 American Chemical Society Published on Web 01/25/2001
TABLE 1. Concentration [mg/kg] of TNT and Related Nitroaromatic Compounds in the Untreated HTNT2 and ELBP2 Soils and Total Amounts [mg] of These Compounds in the Solid and Aqueous Phases after Alkaline Treatment at pH 12 and pH 11a HTNT2
ELBP2
substance
untreated
pH 12
pH 11
TNT 2,4-DNT 2,6-DNT 2A-4,6DNT 4A-2,6DNT TNB
16109.8 289.2 70.5 66.7 80.3 24.2
834.6 1054.4 98.6 208.6 31.0 41.7 14.3 27.3 22.4 41.2 15.2 65.9
untreated pH 12 pH 11 115.7 142.7 58.0 0.7 0.5 1.1
2.4 56.2 34.4 4.4 15.1 nd
2.2 98.5 43.5 1.0 2.0 5.2
a The total amounts are mean values of the amounts measured at the 7th and 14th day (nd: not detected).
of pH was performed with a programmable 8052 AH Basic microcontroller and kept constant within a range of (0.1 pH units by controlled addition of a saturated and filtered Ca(OH)2 solution. Extraction of Samples. At predefined intervals the respective soil slurries were neutralized with HCl to stop the reaction. Subsequent centrifugation was carried out at 2500 rpm for 10 min. Fifty milliliters of the supernatant was used for triplicate extraction by shaking with 25 mL of ethyl acetate for 3 min, respectively. The combined organic phases were dried with anhydrous sodium sulfate, evaporated to about 0.1 mL, and redissolved with 1 mL of methanol. The remaining soil was dried with Na2SO4 and extracted in a Soxhlet device with 80 mL of tert-butyl methyl ether (SupraSolv, Merck) for 3 h. The extract was then dried over anhydrous sodium sulfate and evaporated to about 0.1 mL. The samples were redissolved with tert-butyl methyl ether. The original untreated soils were extracted in the same manner as the centrifugated soil. Gas Chromatography (GC). Nitroaromatic compounds were analyzed using a GC equipped with an ECD (HewlettPackard 5890 series II, Amsterdam, 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) and helium as 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 130250 °C with 3 °C/min. Evaluation of Results. The concentrations of the nitroaromatic compounds measured in the supernatant were multiplied by the volume of the aqueous phase to calculate their quantity in the liquid phase. To determine the quantities of the compounds in the solid phase the concentrations measured in the soil were multiplied by the amount of the soil (5 g). The total quantity of every compound in the reaction vessel is calculated by adding the quantities of the compound in both the liquid and the solid phases. These quantities relate to the 5 g of soil used in the experiments. Therefore they were multiplied with a factor of 200 to achieve the corresponding quantities for 1 kg of soil (Table 1). The quantities of the nitroaromatic compounds measured in the untreated soil were used as reference values (100%) to calculate recovery rates. The recovery rates are defined as ratios of the calculated quantities following alkaline treatment to the untreated soil. The quantities in the solid and the liquid phases were used for the total recovery rates (Figure 1), and the quantities in the solid or the liquid phase were used for recovery rates of the solid and the liquid phases, respectively (Figure 2).
Results and Discussion The HTNT2 and ELBP2 soils were collected from two former ammunition plants. Both HTNT2 and ELBP2 contained TNT,
FIGURE 1. Total recovery rates of TNT over time at pH 12 using ELBP2 and of TNB using HTNT2.
FIGURE 2. Recovery rates of 2,4-DNT over time, shown for solid and liquid phases using ELBP2 and HTNT2. Hydrolysis was conducted at pH 12. (The small picture in Figure 2a shows the course of the recovery rates during the first day on a larger scale.) 2,4-DNT, and 2,6-DNT as major contaminants. In addition, HTNT2 contained 2A-4,6DNT, 4A-2,6DNT, and 1,3,5-trinitrobenzene (TNB) (Table 1). In the ELBP2 soil similar concentrations (58 to 143 mg/kg) were measured for TNT, 2,4-DNT, und 2,6-DNT, whereas in the HTNT2 soil the TNT content was 16.1 g/kg, i.e., the TNT concentration was 55to 665-fold higher than the concentration of the other nitroaromatic compounds. Soil samples treated only with water were used as references. During treatment a partial desorption of the compounds took place. Hence, only the distribution of the compounds between soil and water changed; no hydrolysis VOL. 35, NO. 5, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Reaction Rates (k) Using a Pseudo-First-Order Model and Corresponding Correlation Coefficients (r2)] HTNT2 substance TNT 2,4-DNT 2A-4,6DNT 4A-2,6DNT
k r2 k r2 k r2 k r2
h-1 h-1 h-1 h-1
ELBP2
pH 12
pH 11
pH 12
pH 11
0.237 0.964 0.082 0.999 0.112 0.999 0.106 0.948
0.038 0.970 0.004 0.901 0.005 0.958 0.005 0.952
0.108 0.988 0.017 0.984
