Sustained and Complete Hexahydro-1,3,5-Trinitro-1,3,5-Triazine (RDX) Degradation in Zero-Valent Iron Simulated Barriers under Different Microbial Conditions

Abstract

Flow-through columns packed with "aged" zero-valent iron (ZVI) between layers of soil and sand were constructed to mimic a one-dimensional permeable reactive iron barrier (PRB). The columns were continuously fed RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine, ca. 18 mg l(-1)) for over one year. Two columns were bioaugmented with dissimilatory iron reducing bacteria (DIRB) Shewanella algae BrY or Geobacter metallireducens GS-15 to investigate their potential to enhance the reactivity of aged iron by reductive dissolution of passivating iron oxides or via production of biogenic reactive minerals. A third column was not bioaugmented to evaluate colonization by indigenous soil microorganisms. [14C]-RDX was completely removed in all columns at the start of the iron layer, and concentration profiles showed rapid and sustainable RDX removal over one year; however, a phylogenetic profile conducted after one year using DGGE analysis of recovered DNA did not detect S. algae BrY or G. metallireducens in their respective columns. Bacterial DNA was recovered from within the ZVI. Several unidentified 14C-labeled byproducts were present in the effluent of all columns. Dissolved 14C removal and the detection of dissolved inorganic 14C in these columns (but not in the sterile control) suggest microbial-mediated mineralization of RDX and sorption/precipitation of degradation products. Enhanced RDX mineralization in bioaugmented columns was temporary relative to the indigenously colonized column. However, shorter acclimation periods associated with bioaugmented PRBs may be desirable for rapid RDX mineralization, thereby preventing breakthrough of potentially undesirable byproducts. Overall, these results show that high RDX removal efficiency by ZVI-PRBs is achievable and sustainable and that the efficacy and start-up of ZVI-PRBs might be enhanced by bioaugmentation.

Horizontal gene transfer (HGT) as a mechanism of disseminating RDX-degrading activity among Actinomycete bacteria

“…2004; Fuller et al. 2005; Shrout et al. 2005), and a number of micro‐organisms have been characterized that can degrade or transform RDX (Coleman et al.…”

“…Of the nitroaromatic and nitramine explosives contaminants on these sites, hexahydro-1,3,5trinitro-1,3,5,-triazine (RDX) is known to migrate easily through soil into surface water and groundwater (Sheremata et al 2001;Clausen et al 2004;Davis et al 2004). Bioremediation of RDX in soils and groundwater has been demonstrated (Davis et al 2004;Fuller et al 2005;Shrout et al 2005), and a number of micro-organisms have been characterized that can degrade or transform RDX ammonium, formaldehyde and carbon dioxide (Fournier et al 2002;Jackson et al 2007).…”

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

“…The catalytic activity of Fe +2 coordinated to oxides is believed to be the result of complexation of Fe +2 with surface hydroxyl groups and the formation of inner-sphere bonds, which increases the electron density of the adsorbed Fe +2 . Temporal monitoring of zero-valent iron surfaces during laboratory batch studies and field measurements of PRB have identified a wide range of iron minerals such as: akagenite, goethite, lepidocrocite, green rust, magnetite, ferrous sulfide, and maghemite [356,370,377]. Given that PRBs are emplaced at sites with widely differing groundwater chemistry, the suite of minerals that precipitate, as well as the rate of formation, can vary widely [372].…”

“…Although more than 150 PRBs have been installed worldwide [354], the majority of PRBs have been targeted for chlorinated compounds and only recently has research been aimed at using PRBs for environmental contaminants with multiple nitro groups (e.g., TNT, RDX, HMX) [60,339,[354][355][356][357][358][359][360]. Laboratory-based research on the destruction of explosives has provided support for using zero-valent iron in PRBs for field-scale treatment.…”

Functional Characterization of pGKT2, a 182-Kilobase Plasmid Containing the xplAB Genes, Which Are Involved in the Degradation of Hexahydro-1,3,5-Trinitro-1,3,5-Triazine by Gordonia sp. Strain KTR9

“…Mobile explosive contaminants, like RDX, threaten overall sustainability of Department of Defense (DoD) ranges by migrating away from the source areas and contaminating surface and subsurface soils at relatively low but potentially toxic levels. The potential for bioremediation of RDX in groundwater and soils via the addition of nutrients (19,23) or via combined abiotic/biotic processes (49) has been established. While in situ bioremediation is an attractive remedial action for these sites, the capability to predict the efficacy of in situ bioremediation of RDX is generally lacking.…”