Treatment of Waste from Explosives Industries

CHAPTER

23

Treatment of Waste from Explosives Industries Introduction A chemical explosive may be defined as a compound or mixture of compounds that reacts very rapidly to produce relatively large amounts of gas and heat. The rate of detonation is very high. Exotherrnic oxidation-reduction reactions provide the energy released during detonation. It is the nearly instantaneous formation of gases plus their rapid expansion due to pressure and heat that results in the destructive force or useful work. Large amounts of explosives are used annually, more for constructive commercial purposes than for military, combat, or terror purposes. The discovery of explosives must be considered as one of the greatest milestones in the development of modern society. Whether it is for mining, excavation of tunnels, construction of roads and pipelines, or rock quarrying, explosives are needed. Explosives contain oxidizers and fuel. Molecular explosives contain both of these within the same molecule (2,4,6-trinitrotoluene, pentaerythritol tetranitrate, and nitroglycerin), while in composite explosives the two portions come from different molecules (ammonium nitrate and liquid fuel oil). Explosives are categorized as three groups, based on their sensitivity to detonation, as follows: 9 Primary explosivesmmost sensitive (get readily initiated) ~ Secondary explosives~less sensitive (less hazardous) 9 Tertiary explosives--least sensitive Some of the commonly used explosives are listed in Table 23-1. Of all the known explosives, the most widely known are the ones having a -N=O group. This includes nitro groups (both aromatic and aliphatic), nitrate esters, nitrate salts, nitramines, and nitrosamines. Prominent

233

234 Biotreatment of Industrial Effluents TABLE 23-1 Commonly Used Explosives

Compound name

Symbol

Composition

m

Hg(CNO)2

Primary explosives Mercury fulminate Lead azide

Pb(N3)2

Silver azide

AgN3

Mannitol hexanitrate

MHN

C6H8(ONO2)6

Diazodinitro phenol

DDNP

C6H2N405

Nitroglycerin

NG

C3H5(ONO2)3

Pentaerythritol tetranitrate

PETN

C(CH2ONO2)4

Trinitrotoluene

TNT

CH3C6H2(NO2)3

Ethyleneglycol dinitrate

EGDN

C2H4(ONO2)2

Cyclotrimethylene trinitramine

RDX

C3H6Ng(NO2)3

Cyclotetramethylene tetranitramine

HMX

C4H8N4(NO2)4

Nitroguanidine

NQ

CH4N3NO2

Nitromethane

NM

Nitrocellulose

NC

CH3NO2 Variable

Ethylenedinitrate

EDDN

Prilled ammonium nitrate-fuel oil

ANFO

Secondary explosives

Water gels

C2H10N406 94/6--AN/FO Variable mixtures of oxidizers, fuels, and water

Tertiary explosives Mononitro toluene

MNT

CH3C6H4NO 2

Ammonium perchlorate

AP

NH4C104

Ammonium nitrate

AN

NH 4NO 3

examples are nitromethane, 2,4,6-trinitrotoluene (TNT), nitroglycerin (NG), pentaerythritol tetranitrate (PETN), ethylenediamine dinitrate (EDDN), hexahydro-l,3,5-trinitro-l,3,5-triazine (RDX), cyclotetramethylene tetranitramine (HMX), and a m m o n i u m nitrate (Fig. 23-1 ). The synthesis and use

of these explosives contaminates the environment with high amounts of nitrate compounds. The industrial effluent from these industries has low pH value and is usually high in nitrates.

Treatment of Waste from Explosives Industries

CH3 O2N

235

fONO2 NO2

O2NO~~'"~

ONO2

O2NO" PETN

NO2 TNT

~O2

/NN

ONO2 O2NO~ONO2

O2N/N~N~No2 RDX

NG

~ O2 O2N/ N / ~ N"~

L

N /NNNo2 I NO2

HMX FIGURE 23-1. Commonly used explosives.

T o x i c i t y and O c c u r r e n c e The toxicity of nitroorganics, inorganic nitrates, and nitrites is widely known. Some of the common symptoms are irritation of digestive tract, methemoglobinemia, disturbed heart function, kidney trouble, dysfunction of the vascular system, and severe jaundice (Kanekar et al., 2003). The nitroaromatic explosives are toxic, but their environmental transformation products, including arylamines, arylhydroxylamines, and condensed products such as azoxy- and azo- compounds, are equally or more toxic as the parent nitroaromatic. TNT is on the list of U.S. Environmental Protection Agency priority pollutants. RDX is a class C possible human carcinogen and has adverse effects on the central nervous system in mammals. Aromatic nitro compounds are resistant to chemical or biological oxidation and to hydrolysis because of the electron withdrawing nitro groups (Rodgers et al., 2001). The hydrophilic lipophilic balance (HLB)of these compounds favors lipid solubility, thereby reducing their mobility in the environment. Thus, because of the lipophilic character and deactivated

236 Biotreatment of Industrial Effluents aromatic ring, these compounds accumulate in the environment. Activities associated with manufacturing, training, waste disposal, and closure of military bases have resulted in severe soil and groundwater contamination with explosives (Fournier et al., 2002). These wastewaters are contaminated with explosives as well as the raw materials used for the production of explosives. The nitro aromatic compound TNT is introduced into soil and water ecosystems mainly by military activities like the manufacture, loading, and disposal of explosives and propellants. This problem of contamination may increase in the future because of the demilitarization and disposal of unwanted weapon systems. The disposal of obsolete explosives is a problem for the military and associated industries because of the polluting effect of explosives in the environment (Wyman et al., 1979). Bioremediation Past methods of disposing of munitions wastes have included dumping in deep sea, dumping at specified landfill areas, and incineration when quantities were small. All of these cause serious harm to the ecosystem. For example, incineration causes air pollution, and disposal on land leads to soil and groundwater pollution. Other than these, methods such as resin adsorption, surfactant complexing, and liquid-liquid extraction have been used. These methods only transfer the explosive from soil or water into another medium, which then needs further treatment. Chemical methods of oxidation do not yield the necessary products, and the unreacted toxic intermediates still remain. Thus, the biofriendly treatment is bioremediation. Microorganisms are known for their versatile metabolic activity and have evolved diverse pathways that allow them to mineralize specific nitro compounds. Despite this, relatively few microorganisms have been described as being able to use nitro aromatic compounds as nitrogen, carbon, and energy sources because nitro groups deactivate the aromatic ring to electrophilic attack by oxygenase or other enzymes. Be that as it may, biological degradation is one of the primary routes by which nitro aromatic compounds are broken down in the environment. There has been considerable interest in the past 30 years in the microbial transformation of these compounds. Both aerobic and anaerobic degradation of nitro aromatics has been reported. Aerobic microorganisms use diverse biochemical reactions to initiate the degradation of nitro aromatic compounds. Reactions that attack the nitro substituent can be grouped into two general categories: oxidative or reductive (Rieger and Knackmuss, 1995). With mono- or di-nitro substituted aromatic compounds, the preferred route for their initial degradation is hydr~xylation carried out by mono- or di-oxygenases. These reactions normally result in replacement of the nitro group by a hydroxy group with nitrite release. When the number of nitro substituents on the aromatic ring

Treatment of Waste from Explosives Industries 237 is greater than two, the predominant initial reactions become reductive. These reactions reduce the nitro (NO2)substituent first to nitroso (NO), then to hydroxylamino (NHOH), followed by an amino (NH2)derivative prior to further processing with the release of ammonium ion. In some Rhodococcus (Lenke and Knackmuss, 1992) and Mycobacterium (Vorbeck et al., 1994) strains, the aromatic ring, rather than the nitro group, may be reduced first to generate a hydride-Meisenheimer complex. On protonation and rearomatization, the nitro group is replaced by a proton and nitrite is released. Most aerobic microorganisms reduce TNT to the corresponding amino derivatives via the formation of nitroso and hydroxylamine intermediates. However, condensation of the latter compounds yields highly recalcitrant azoxytetranitro toluenes. Certain strains of Pseudomonas use TNT as the nitrogen source through the removal of nitrogen as nitrite (Esteve-Nunez et al., 2001 ). Phanerochaete chrysosporium mineralizes TNT under lignolytic conditions. Because the manufacturing processes for RDX and HMX are the same, each is present as an impurity in the other. Because of the copresence of RDX and HMX in contaminated waters or at contaminated sites, degradation of both in each other's presence becomes important. P. chrysosporium degraded this mixture to carbon dioxide and nitrous oxide (Hawari et al., 2000). In a study of RDX degradation by Rhodococcus sp., nitrite formation was observed with RDX disappearance. Ecological observations suggest that sulfate-reducing and methanogenic bacteria might metabolize nitroaromatic compounds under anaerobic conditions if appropriate electron donors and electron acceptors are present in the environment. The successful demonstration of the degradation of RDX by sewage sludge under anaerobic conditions (McCormick et al., 1978) further indicated the usefulness of anaerobes in explosive waste treatment. Under anaerobic conditions, the sulfate-reducing bacteria Desulfovibrio sp. (B strain)metabolized TNT. Of all the metabolites produced, the formation of toluene was significant (Boopathy and Kulpa, 1992). Most Desulfovibrio sp. have nitrite reductase enzymes that reduce nitrate to ammonia. Figure 23-2 elaborates a general pathway for the transformation of TNT that involves the initial reduction of aromatic nitro groups to aromatic amines. Boopathy and Kulpa (1994) isolated a methanogen, Methanococcus sp., that transformed TNT to 2,4-diaminonitro toluene. The observations of sulfate reducers and methanogenic bacteria by many workers suggest that these organisms could be exploited for bioremediation of explosives under anaerobic conditions by supplying proper electron donors and electron acceptors. The first step in the metabolism of nitoaromatics is reduction. This step is followed by reductive deamination, which removes all of the nitro groups present in the ring, leaving the ring intact and forming toluene and ammonia as end products. The toluene can be further degraded by toluene-degrading organisms. As discussed earlier, aerobic transformations of TNT have shown the production of dead-end products like amino derivatives or azoxy compounds.

238

Biotreatment of Industrial Effluents

CH3 NO2

NO2

TNT

CH3 NO2

CH3 NO2

NO2

NH2

NO,

NH2 6Hf-,~"~ CH3 NO,2

NH2

NH2

CH3 NH2

CH3 NH2

NH2

Toluene

FIGURE 23-2. Degradation of T N T by Desulfovibrio sp.

Therefore, the applicability of aerobes in bioremediation of sites contaminated with nitroaromatics is doubtful at present. However, the use of anaerobes like sulfate-reducing bacteria may prove useful in decontaminating sites polluted by nitro compounds.

References Boopathy, R., and C. F. Kulpa. 1992. Trinitrotoulene (TNT) as a sole nitrogen source for a sulfate-reducing bacterium Desulfovibrio sp. (B strain) isolated from an anaerobic digester. Curr. Microbiol. 25:235-241.

T r e a t m e n t of Waste from Explosives Industries

239

Boopathy, R. and C. F. Kulpa. 1994. Biotransformation of 2,4,6-trinitrotoulene (TNT) by a Methanococcus sp. (B strain) isolated from a lake sediment. Can. J. Microbiol. 40:273-278. Esteve-Nunez, A, A. Caballerno and J. L. Ramos. 2001. Biological degradation of 2,4,6trinitrotoulene. Microbiol. Mol. Biol. Rev. 65(3):335-352. Fournier, D, A. Halasz, J. Spain, p. Fiurasek, and J. Hawari. 2002. Determination of key metabolites during biodegradation of hexadhydro-l,3,5-trinito-l,3,5-triazine with Rhodococcus sp. Strain DN22. Appl. Environ. Microbiol. 68:166-172. Hawari, J., S. Beaudet, A. Halasz, S. Thiboutot. and G. Ampleman. 2000. Microbial degradation of explosives: biotransformation versus mineralization. Appl. Microbiol. Biotechnol. 54(5):605-618. Lenke, H., and H. J. Knackmuss. 1992. Initial hydrogenation during catabolism of picric acid by Rhodococcus erythropolis HL24-2. J. Bacteriol. 58(9):2933-2937. McCormick, N. G., J. H. Cornell, and A. M. Kaplan. 1978. Identification of biotransformation products from ,4-dinitrotoluene. Appl. Environ. Microbiol. 35(5):945-948. Rieger P. G., and H. J. Knackmuss. 1995. Basic knowledge and perspectives on biodegradation of 2,4,6-trinitrotoluene and related nitroaromatic compounds in contaminated soil. In Biodegradation of nitroaromatic compounds, J. C. Spain (ed.), pp. 1-18. New York: Plenum Press. Vorbeck, C., H. Lenke, P. Fischer, and H. J. Knackmuss. 1994. Identification of hybridMeisenheimer complex as a metabolite of 2,4,6-trinitrotoluene by a Mycobacterium strain. J. Bacteriology. 176: 932-934. Wyman, J. F, H. E. Guard, W. D. Won. and J. H. Quay. 1979. Conversion of 2,4,6-trinitrophenol to a mutagen by Pseudomonas aeruginosa. Appl. Environ. Microbiol. 37:222-226.

Bibliography Kanekar, P., P. Dautpure, and S. Sarnaik. 2003. Biodegradation of nitro-explosives. Indian. J. Expt. Biol. 41(September): 991-1001. Rodgers, J. D., and N. J. Bunce. 2001. Treament methods for the remediation of nitroaromatic explosives. Water Res. 35(9):2101-2111.