Comparison of biotic and abiotic treatment approaches for co-mingled perchlorate, nitrate, and nitramine explosives in groundwater

Biological and abiotic approaches for treating co-mingled perchlorate, nitrate, and nitramine explosives in groundwater were compared in microcosm and column studies. In microcosms, microscale zero-valent iron (mZVI), nanoscale zero-valent iron (nZVI), and nickel catalyzed the reduction of RDX and HMX from initial concentrations of 9 and 1 mg/L, respectively, to below detection (0.02 mg/L), within 2 h. The mZVI and nZVI also degraded nitrate (3 mg/L) to below 0.4 mg/L, but none of the metal catalysts were observed to appreciably reduce perchlorate ( approximately 5 mg/L) in microcosms. Perchlorate losses were observed after approximately 2 months in columns of aquifer solids treated with mZVI, but this decline appears to be the result of biodegradation rather than abiotic reduction. An emulsified vegetable oil substrate was observed to effectively promote the biological reduction of nitrate, RDX and perchlorate in microcosms, and all four target contaminants in the flow-through columns. Nitrate and perchlorate were biodegraded most rapidly, followed by RDX and then HMX, although the rates of biological reduction for the nitramine explosives were appreciably slower than observed for mZVI or nickel. A model was developed to compare contaminant degradation mechanisms and rates between the biotic and abiotic treatments.     

Analysis of the key intermediates of RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) in groundwater: occurrence, stability and preservation

Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) is a widely used explosive that is present in soils at a number of military sites, including training and testing ranges. Because of its relatively weak adsorption to soil, RDX frequently migrates through the unsaturated zone and causes groundwater contamination. In the environment, RDX can transform to produce mono-, di-, and tri-nitroso derivatives (MNX, DNX, and TNX) and the ring cleavage products methylenedinitramine (MEDINA) and 4-nitro-2,4-diazabutanal (NDAB). The present study was undertaken to analyze RDX and its products in groundwater samples taken from various US military sites. The stability of some of the common transformation intermediates of RDX, including the nitroso derivatives, NDAB and MEDINA, under typical conditions in a groundwater aquifer is not well understood, and appropriate preservation methods for these compounds have not been established. Therefore, we studied the inherent stability of these compounds in deionized water and in groundwater, and evaluated various preservation techniques, including adjustment of pH, temperature, and salinity. NDAB and nitroso derivatives were stable under typical ambient environmental conditions, but MEDINA was highly unstable. The addition of sea salts (10% w/v) was found to stabilize MEDINA when the samples were stored at 4 °C. Using appropriate preservation techniques, we detected nitroso derivatives and NDAB, but no MEDINA, at some of the sites investigated. Stabilizing RDX intermediate products in field samples to allow detection is important because the presence of any of these chemicals can indicate past contamination by RDX and provide insight into the occurrence of in situ natural attenuation.     

In‐Situ and Ex‐Situ Bioremediation Options for Treating Perchlorate in Groundwater

Perchlorate has been identified as a water contaminant in 14 states, including California, Nevada, New Mexico, Arizona, Utah, and Texas, and current estimates suggest that the compound may affect the drinking water of as many as 15 million people. Biological treatment represents the most‐favorable technology for the effective and economical removal of perchlorate from water. Biological fluidized bed reactors (FBRs) have been tested successfully at the pilot scale for perchlorate treatment at several sites, and two full‐scale FBR systems are currently treating perchlorate‐contaminated groundwater in California and Texas. A third full‐scale treatment system is scheduled for start‐up in early 2002. The in‐situ treatment of perchlorate through addition of specific electron donors to groundwater also appears to hold promise as a bioremediation technology. Recent studies suggest that perchlorate‐reducing bacteria are widely occurring in nature, including in groundwater aquifers, and that these organisms can be stimulated to degrade perchlorate to below the current analytical reporting limit (< 4 μg/l) in many instances. In this article, in‐situ and ex‐situ options for biological treatment of perchlorate‐contaminated groundwater are discussed and results from laboratory and field experiments are presented. © 2002 Wiley Periodicals, Inc.