Properties Use Environmental Problems and Kinetics

The most prominent compound of this group is 2,4,6-trinitrotoluene (TNT). It has been produced in large amounts since 1900 and is used particularly as an agent of warfare. The soil of nearly all production sites is polluted by TNT and the products of its slow, natural biodegradation. Cleaning of these sites will be a task for many years to come. Because of its relatively low solubility in water (130 mg L-1), TNT causes mainly soil pollution problems.

In contrast, several other nitroaromatics are characterized by a higher solubility in water (Table 9.5). 2,4-Dinitrotoluene (2,4-DNT) and 4-nitrophenol (4-NP), for example, are intermediates in the production of TNT. Therefore, they are more likely to be found in wastewater; and our knowledge about their biodegradability will be discussed briefly below.

Rippen (1991) reported an annual worldwide production of 60000 t a-1 and a German production of 10000 t a-1 TNT. Usually, 4-NP is produced in batch reactors on the basis of purchase orders. After each production run, the reactor, pipes and stirring vessels are washed with water and the remaining raw materials, prod-

Table 9.5 Some nitroaromatic compounds, their properties and uses. For symbols used, see Table 9.1.

Compound

(mg L-1)

H

Uses

EPA No.

Nitrobenzene, csh5no2

2000, 20°C

0.0029

Intermediate in the production of aniline

Intermediate in the production of dyes Rubber, pharmaceutical products, photochemicals

4-Nitrophenol, C6H4OHNO2

13 700, 20°C

0.021 • 10-6

Intermediate in the production of pesticides, azo and sulfur dyes, as well as chemicals for photo industry

58

2,4-Dinitrophenol, C6H3OHN2O4

200, 12.5°C

0.65 • 10-6

Pesticide, fungicide

Intermediate in the production of dyes, explosives and chemicals for photo industry

20-25°C

2.4-200 • 10-6

Intermediate in the production of polyurethane and of 2,4,6-dinitrotoluene

toluene

C7H5N3O6

130, 20°C

Intermediate in the production of dyes and pharmaceuticals

ucts and by-products enter the wash water, which is eventually discharged to the next wastewater treatment plant. Because of the multiple dilution processes, the concentration of 4-NP in water is normally relatively low and specialized cultures of bacteria cannot establish themselves in activated sludge plants. Therefore, 4-NP must be removed from the concentrated industrial effluents near the source by biological or physico-chemical processes. Its human toxicity is based on damage caused to the function of liver, kidney and central nervous system (Thiem and Booth 1991). The respiration rate of activated sludge is inhibited by 50% at a 4-NP concentration of 110 mg L-1 (Pagga et al. 1982).

With 660 0001 a-1 worldwide and 97 000 t a-1 in Germany, the annual production of 2,4-DNT is much higher than that of 4-NP (Rippen 1991). 2,4-DNT is mainly used as an intermediate in the production of TNT and polyurethane. Rippen (1991) reported concentrations of 9.7 mg L-1 in TNT production effluents and about 14 mg L-1 in the wastewater of special production processes for organic chemicals. Biodegradation experiments with anaerobic bacteria have been unsuccessful (Hu and Shieh 1987; Battersby and Wilson 1989).

Table 9.6 shows published stoichiometric coefficients for the description of aerobic catabolism and anabolism. It follows from these coefficients that 67.5% of the carbon is used for catabolism (formation of CO2) and 24% of the nitrogen for anabolism, resulting in 76% being transformed to NO2 (YNO2/S-N = 0.76 mol NO2-N (mol S-N)-1).

The low value of pmax = 0.072 d-1 (Heinze 1997) for 4-NP degradating bacteria results in a long generation time of tG = 9.6 d, but this value obtained with a mixed culture is considerably longer than that obtained by Schmidt et al. (1987) with Pseudomonas sp.

Table 9.6 Stoichiometric and kinetic coefficients for aerobic degradation of 4-NP (Heinze et al. 1995; Heinze 1997).

Compound, reference

Bacteria

4-NP

Jensen and Lautrup-Larsen (1967)

Jakobezyk et al. (1984)

Schmidt et al. (1987)

Ou and Sharma (1989)

Heinze (1997)

2,4-DNT

Bausum et al. (1992) Heinze (1997)

Pseudomonas sp.

Mixed culture Pseudomonas sp. Pseudomonas sp. Mixed culture 0.82

Mixed culture Mixed culture

0.71

0.41

0.62

0.52

0.76

0.76

0.80

0.675

0.64

Bausum et al. (1992) carried out measurements with 2,4-DNT labelled by 14C. Offgas measurements of 14CO2 made it possible to determine YCO2/S = 0.64 mol CO2 (mol C)-1 (Table 9.6). The same authors used first-order kinetics to describe the influence of 2,4-DNT concentration, but their results for the reaction rate showed a relatively high degree of scattering. Further kinetic results were published by Heinze et al. (1995) and Heinze (1997; Table 9.6). It is remarkable that the maximum growth rate pmax = 2.4 d-1 is higher by a factor of 33 than for the mineralization of 4-NP. Additionally, the higher YX/S and the lower YO2/S show that a higher amount of carbon was used in anabolism, although two NO2- ions have to be separated from one 2,4-DNT molecule!

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