Question

An active army depot looking into full scale biotechnology to treat explosives contaminated soil. Reported concentrations of trinitrotoluene (TNT) and 1,3,5-trinitro-1,3,5-triazine (RDX) are 88,000 mg/kg and 5,250 mg/kg in the blended soil, respectively.

As an environmental engineer tasked with this site, Select the best bio-remediation option to deal with this problem and explain your reasoning, with advantages and disadvantages.

Answer 1

Biological treatment, or bioremediation, is a technology that uses micro-organisms to degrade organic contaminants into less hazardous compounds.

TriNitro Toluene (TNT) degrades under aerobic conditions into monoamine, diaminos, hydroxylamine-DNT, and tetranitro-azoxynitrotoluenes by using Acinetobacter, Corynebacterium, Pseudomonas genus bacterias. Similarly Researched and Developed Explosive (RDX) and High Melting Explosive (HMX) degrade into carbon dioxide and water under anaerobic conditions Rhodococcus bacterias, (HMX are usually treated with Methylobacterium bacterias). Explosive content degradation may occur under aerobic or anaerobic conditions where the explosive compound serves as a carbon and nitrogen source.

Following Methods are Usually followed for bioremediation.

Landfarming: Landfarming has been used extensively to treat soils contaminated with petroleum hydrocarbons, pentachlorophenol (PCP), and polycyclic aromatic hydrocarbons (PAHs), and potentially could be used to treat low to medium concentrations of explosives as well. In land farming, soils are excavated to treatment plots and periodically tilled to mix in nutrients, moisture, and bacteria.

Limitation

• A large amount of space is required.
• Conditions affecting biological degradation of contaminants (e.g., temperature, rain fall) are largely uncontrolled, which increases the length of time to complete remediation.
• Inorganic contaminants will not be biodegraded.
• Volatile contaminants, such as solvents, must be pretreated because they would evaporate into the atmosphere, causing air pollution.
• Dust control is an important consideration, especially during tilling and other material handling operations.
• Presence of metal ions may be toxic to the microbes and possibly leach from the contaminated soil into the ground.
• Runoff collection facilities must be constructed and monitored.
• Topography, erosion, climate, soil stratigraphy, and permeability of the soil at the site must be evaluated to determine the optimum design of facility.
• Waste constitutes may be subject to Land-ban regulation and thus may not be applied to soil for treatment by land treatment (e.g., some petroleum sludges).
• The depth of treatment is limited to the depth of achievable tilling (normally 18 inches).

Phytoremediation: This technology is developed to effectively clean up contaminated soil with residues of explosives like TNT, RDX, HMX, and DNT. One potential treatment alternative is phytoremediation using constructed wetlands. Phytoremediation is a process which uses plants to degrade, not uptake, explosives. Once this process is proven in constructed wetlands, it could be applied in natural wetlands to remediate explosives-contaminated ground water. Constructed wetlands have already proven to be effective for treating acid mine drainage and municipal waste waters. Wetlands phytoremediation is a technology that is relatively self-sustaining and cost-effective to maintain. In addition, this technology, unlike GAC, does not produce secondary waste streams. A plant nitroreductase enzyme shown to degrade TNT, RDX, and HMX in concert with other plant enzymes. An immunoassay test has been developed that identifies nitroreductase activity in a wide variety of aquatic and terrestrial plants.

Limitation

• The depth of the treatment zone is determined by plants used in phytoremediation. In most cases, it is limited to shallow soils.
• High concentrations of hazardous materials can be toxic to plants.
• It involves the same mass transfer limitations as other biotreatments.
• It may be seasonal, depending on location.
• It can transfer contamination across media, e.g., from soil to air.
• It is not effective for strongly sorbed (e.g., PCBs) and weakly sorbed contaminants.
• The toxicity and bioavailability of biodegradation products is not always known.
• Products may be mobilized into ground water or bioaccumulated in animals.
• It is still in the demonstration stage.
• It is unfamiliar to regulators.

White Rot Fungus Treatment: White rot fungus, Phanerochaete chrysosporium, has been evaluated more extensively than any other fungal species for remediating explosives-contaminated soil. Although white rot has been reported in laboratory-scale settings using pure cultures a number of factor increase the difficulty of using this technology for full-scale remediation. These factors include competition from native bacterial populations, toxicity inhibition, chemical sorption, and the inability to meet risk-based cleanup levels.

In bench-scale studies of mixed fungal and bacterial systems, most of the reported degradation of TNT is attributable to native bacterial populations. High TNT concentrations in soil also can inhibit growth of white rot fungus. Some reports indicate that TNT losses reported in white rot fungus studies can be attributed to adsorption of TNT onto the fungus and soil amendments, such as corn cobs and straw.

Limitation

• Cleanup goals may not be attained if the soil matrix prohibits contaminant-microorganism contact.
• The circulation of water-based solutions through the soil may increase contaminant mobility and necessitate treatment of underlying ground water.
• Preferential colonization by microbes may occur causing clogging of nutrient and water injection wells.
• Preferential flow paths may severely decrease contact between injected fluids and contaminants throughout the contaminated zones. The system should not be used for clay, highly layered, or heterogeneous subsurface environments because of oxygen (or other electron acceptor) transfer limitations.
• High concentrations of heavy metals, highly chlorinated organics, long chain hydrocarbons, or inorganic salts are likely to be toxic to microorganisms.
• Bioremediation slows at low temperatures.
• Concentrations of hydrogen peroxide greater than 100 to 200 ppm in groundwater inhibit the activity of microorganisms.
• A surface treatment system, such as air stripping or carbon adsorption, may be required to treat extracted groundwater prior to re-injection or disposal.

In Situ Biological Treatment: In situ treatments can be less expensive than other technologies and produce low contaminant concentrations. The available data suggest, however, that in situ treatment of explosives might create more mobile intermediates during biodegradation. In addition, biodegradation of explosive contaminants typically involves metabolism with an added nutrient source, which is difficult to deliver in an in situ environment. Mixing often affects the rate and performance of explosives degradation.