Steve D. Comfort

Removal of TNT and RDX from water and soil using iron metal

Contaminated water and soil at active or abandoned munitions plants is a serious problem since these compounds pose risks to human health and can be toxic to aquatic and terrestrial life. Our objective was to determine if zero-valent iron (Fe(0)) could be used to promote remediation of water and soil contaminated with 2,4,6-trinitrotoluene (TNT) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). As little as 1% Fe(0) (w/v) removed 70 mg TNT litre(-1) from aqueous solution within 8 h and removed 32 mg RDX litre(-1) within 96 h. Treating slurries (1:5 soil:water) of highly contaminated soil (5200 mg TNT and 6400 mg RDX kg(-1) soil) from the former Nebraska Ordnance Plant (NOP) with 10% Fe(0) (w/w soil) reduced CH(3)CN-extractable TNT and RDX concentrations below USEPA remediation goals (17.2 mg TNT and 5.8 mg RDX kg(-1)). Sequential treatment of a TNT-contaminated solution (70 mg TNT litre(-1) spiked with (14)C-TNT) with Fe(0) (5% w/v) followed by H(2)O(2) (1% v/v) completely destroyed TNT and removed about 94% of the (14)C from solution, 48% of which was mineralized to (14)CO(2) within 8 h. Fe(0)-treated TNT also was more susceptible to biological mineralization. Our observations indicate that Fe(0) alone, Fe(0) followed by H(2)O(2), or Fe(0) in combination with biotic treatment can be used for effective remediation of munitions-contaminated water and soil.

Nitrotoluene destruction by UV-catalyzed fenton oxidation

The capacity of UV-catalyzed Fenton oxidation to destroy mono-, di- and trinitrotoluenes (MNT, DNT, TNT) in aqueous solution was evaluated. Mononitrotoluenes were rapidly destroyed by Fenton oxidation in the dark at pH 3.0 or by UV/Fenton oxidation without pH adjustment. UV/Fenton oxidation at pH 3.0 was most effective for DNT and TNT destruction. The number and position of nitro substitutions influenced oxidation rate: 2-nitrotoluene > 4-nitrotoluene > 2,4-dinitrotoluene > 2,6-dinitrotoluene > 2,4,6-trinitrotoluene. UV/Fenton oxidation mineralized more than 95% of the TNT in aqueous extracts of contaminated soil. While dissolved humic and fulvic acids differentially influenced destruction rate, total TNT destroyed after 4 h was not greatly affected. Our results demonstrated that UV-catalyzed Fenton oxidation effectively destroyed nitrotoluenes in water and aqueous extracts of contaminated soil.

Soils contaminated with explosives: Environmental fate and evaluation of state-of- the-art remediation processes (IUPAC Technical Report)*

An explosion occurs when a large amount of energy is suddenly released. This energy may come from an over-pressurized steam boiler, from the products of a chemical reaction involving explosive materials, or from a nuclear reaction that is uncontrolled. In order for an explosion to occur, there must be a local accumulation of energy at the site of the explosion, which is suddenly released. This release of energy can be dissipated as blast waves, propulsion of debris, or by the emission of thermal and ionizing radiation. Modern explosives or energetic materials are nitrogen-containing organic compounds with the potential for self-oxidation to small gaseous molecules (N2, H2O, and CO2). Explosives are classified as primary or secondary based on their susceptibility of initiation. Primary explosives are highly susceptible to initiation and are often used to ignite secondary explosives, such as TNT (2,4,6-trinitrotoluene), RDX (1,3,5-trinitroperhydro-1,3,5-triazine), HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazocane), and tetryl (N-methyl-N-2,4,6-tetranitro-aniline).

Remediating TNT-contaminated soil by soil washing and Fenton oxidation

Abstract Past disposal practices at munitions production facilities have generated numerous hectares of TNT-contaminated soil. We previously showed that Fenton oxidation could destroy TNT in soil and water. Our objective was to determine the potential of combining Fenton oxidation with soil washing to remediate TNT-contaminated soil and to test whether plants could be established on washed soil. Aqueous soil washing effectively reduced CH3CN-extractable TNT concentrations but large volumes of water were required to meet remediation goals. This volume was reduced when the wash water temperature was increased to 45°C. Complete destruction of TNT in wash solutions was achieved by Fenton oxidation with greater than 40% mineralized. Combining soil washing with phytoremediation will require plant establishment on the washed soil. We observed no significant reduction in tall fescue (Festuca arundinacea Schreb.) germination or early seedling development in contaminated soil following two wash cycles (soil:H2O, 1:5). This corresponded to reducing CH3CN-extractable soil concentrations from 499 mg TNT kg−1 to approximately 72 mg TNT kg−1. Our results indicate Fenton oxidation can be combined with soil washing for effective abiotic remediation of TNT-contaminated soils. Washed soils may be planted to tall fescue, demonstrating the potential of combining soil washing, Fenton oxidation and phytoremediation into an integrated treatment train for TNT-contaminated sites.

Germination and seedling development of switchgrass and smooth bromegrass exposed to 2,4,6-trinitrotoluene

It is estimated that explosives contaminate approximately 0.82 million cubic metres of soil at former military installations throughout the US; major contaminants often include 2,4,6-trinitrotoluene (TNT) and its degradation products. At some sites, phytoremediation may be a viable option to incineration or other costly remediation treatments. Grasses may be particularly suited for remediation because of their growth habit and adaptability to a wide range of soil and climate conditions. We characterized the effects of TNT on germination and early seedling development of switchgrass and smooth bromegrass to evaluate their potential use on contaminated sites. Switchgrass and smooth bromegrass seeds were germinated in nutrient-free agar containing 0 to 60 mg TNT litre(-1). Smooth bromegrass germination decreased as TNT concentration increased, while switchgrass germination was unaffected by TNT. Concentrations up to 15 mg TNT litre(-1) did not affect switchgrass root growth rate, but bromegrass root growth was reduced at TNT concentrations above 7.5 mg litre(-1). At 7.5 mg TNT litre(-1), however, shoot growth rate was reduced in both species. Examination at 20-fold magnification revealed switchgrass radicles were unaffected by TNT, while smooth bromegrass radicles appeared slightly swollen. Results indicate switchgrass is more tolerant of TNT than smooth bromegrass, but the establishment of both species may be limited to soil containing less than 50 mg kg(-1) of extractable TNT.

TNT and 4-amino-2,6-dinitrotoluene influence on germination and early seedling development of tall fescue

Cost-effective and environmentally acceptable methods are needed to remediate munitions-contaminated soil. Some perennial grass species are tolerant of soil contaminants and may promote remediation because of their high water use and extensive fibrous root systems. The effects of 2,4,6-trinitrotoluene (TNT) and its reduction product, 4-amino-2,6-dinitrotoluene (4ADNT), on germination and early seedling development of tall fescue (Festuca arundinacea Schreb.) were determined. Tall fescue seeds were germinated in nutrient-free agar containing 0-60 mg TNT litre(-1) or 0-15 mg 4ADNT litre(-1). Germination decreased linearly as TNT concentration increased but was not significantly affected by 4ADNT at these concentrations. Concentrations less than 30 mg TNT litre(-1) or 7.5 mg 4ADNT litre(-1) had little effect on seedling growth and development. Higher TNT or 4ADNT concentrations substantially delayed seedling development, caused abnormal radicle tissue development, and reduced secondary root and shoot growth. Seedling respiration rates decreased linearly with increasing TNT concentration. Experiments indicate that tall fescue may be grown in soils that maintain soil solution concentrations of 30 mg TNT litre(-1) or less.

Rapid spectrophotometric determination of 2,4,6-trinitrotoluene in a Pseudomonas enzyme assay

Although TNT (2,4,6-trinitrotoluene) and its degradation products can be quantified by HPLC, this method is not suitable for simultaneous analyses of the numerous samples typically encountered in enzyme studies. To solve this problem, we developed a simple and rapid spectrophotometric assay for TNT and tested the procedure using partially purified nitroreductase(s) from a Pseudomonas aeruginosa isolate, which transformed TNT in the culture medium. In highly alkaline solution, TNT (pK(a)=11.99) exhibits significant absorbance at 447 nm, while major metabolites, 2-amino-4, 6-dinitrotoluene (2ADNT), 4-amino-2,6-dinitrotoluene (4ADNT), and 2,6-diamino-4-nitrotoluene (2,6DANT) display no absorbance at this wavelength. Assay mixtures of TNT, Tris-HCl buffer, a reductant, and the enzyme(s) were analyzed by measuring absorbance 4 min after adjusting the pH to 12.2. TNT transformation to colorless metabolites was linear with respect to protein and substrate concentrations. Using the assay, we determined that TNT nitroreductase(s) from the isolate required an electron donor and preferred NADH to NADPH. TNT transformation increased when NAD was recycled to NADH using glucose-6-phosphate (GP) and glucose-6-phosphate dehydrogenase (GPDH). Enzymatic transformation of TNT was completely inhibited by Cu(2+) (5 mM) and was partially inhibited by other divalent metallic cations. Because the assay is sensitive to ammonium sulfate, dithiothreitol, ascorbic acid, and sodium phosphate, extracts should be assayed in the absence of these components.

Developing slow-release persulfate candles to treat BTEX contaminated groundwater

The development of slow-release chemical oxidants for sub-surface remediation is a relatively new technology. Our objective was to develop slow-release persulfate-paraffin candles to treat BTEX-contaminated groundwater. Laboratory-scale candles were prepared by heating and mixing Na(2)S(2)O(8) with paraffin in a 2.25 to 1 ratio (w/w), and then pouring the heated mixture into circular molds that were 2.38 cm long and either 0.71 or 1.27 cm in diameter. Activator candles were prepared with FeSO(4) or zerovalent iron (ZVI) and wax. By treating benzoic acid and BTEX compounds with slow-release persulfate and ZVI candles, we observed rapid transformation of all contaminants. By using (14)C-labeled benzoic acid and benzene, we also confirmed mineralization (conversion to CO2) upon exposure to the candles. As the candles aged and were repeatedly exposed to fresh solutions, contaminant transformation rates slowed and removal rates became more linear (zero-order); this change in transformation kinetics mimicked the observed dissolution rates of the candles. By stacking persulfate and ZVI candles on top of each other in a saturated sand tank (14×14×2.5 cm) and spatially sampling around the candles with time, the dissolution patterns of the candles and zone of influence were determined. Results showed that as the candles dissolved and persulfate and iron diffused out into the sand matrix, benzoic acid or benzene concentrations (C(o)=1 mM) decreased by >90% within 7 d. These results support the use of slow-release persulfate and ZVI candles as a means of treating BTEX compounds in contaminated groundwater.

Improving the sweeping efficiency of permanganate into low permeable zones to treat TCE: experimental results and model development.

The residual buildup and treatment of dissolved contaminants in low permeable zones (LPZs) is a particularly challenging issue for injection-based remedial treatments. Our objective was to improve the sweeping efficiency of permanganate into LPZs to treat dissolved-phase TCE. This was accomplished by conducting transport experiments that quantified the ability of xanthan-MnO4(-) solutions to penetrate and cover (i.e., sweep) an LPZ that was surrounded by transmissive sands. By incorporating the non-Newtonian fluid xanthan with MnO4(-), penetration of MnO4(-) into the LPZ improved dramatically and sweeping efficiency reached 100% in fewer pore volumes. To quantify how xanthan improved TCE removal, we spiked the LPZ and surrounding sands with (14)C-lableled TCE and used a multistep flooding procedure that quantified the mass of (14)C-TCE oxidized and bypassed during treatment. Results showed that TCE mass removal was 1.4 times greater in experiments where xanthan was employed. Combining xanthan with MnO4(-) also reduced the mass of TCE in the LPZ that was potentially available for rebound. By coupling a multiple species reactive transport model with the Brinkman equation for non-Newtonian flow, the simulated amount of (14)C-TCE oxidized during transport matched experimental results. These observations support the use of xanthan as a means of enhancing MnO4(-) delivery into LPZs for the treatment of dissolved-phase TCE.

Using slow-release permanganate candles to remediate PAH-contaminated water

Surface waters impacted by urban runoff in metropolitan areas are becoming increasingly contaminated with polycyclic aromatic hydrocarbons (PAHs). Slow-release oxidant candles (paraffin-KMnO(4)) are a relatively new technology being used to treat contaminated groundwater and could potentially be used to treat urban runoff. Given that these candles only release permanganate when submerged, the ephemeral nature of runoff events would influence when the permanganate is released for treating PAHs. Our objective was to determine if slow-release permanganate candles could be used to degrade and mineralize PAHs. Batch experiments quantified PAH degradation rates in the presence of the oxidant candles. Results showed most of the 16 PAHs tested were degraded within 2-4 h. Using (14)C-labled phenanthrene and benzo(a)pyrene, we demonstrated that the wax matrix of the candle initially adsorbs the PAH, but then releases the PAH back into solution as transformed, more water soluble products. While permanganate was unable to mineralize the PAHs (i.e., convert to CO(2)), we found that the permanganate-treated PAHs were much more biodegradable in soil microcosms. To test the concept of using candles to treat PAHs in multiple runoff events, we used a flow-through system where urban runoff water was pumped over a miniature candle in repetitive wet-dry, 24-h cycles. Results showed that the candle was robust in removing PAHs by repeatedly releasing permanganate and degrading the PAHs. These results provide proof-of-concept that permanganate candles could potentially provide a low-cost, low-maintenance approach to remediating PAH-contaminated water.