Exploring the Utility of Compound-Specific Isotope Analysis for Assessing Ferrous Iron-Mediated Reduction of RDX in the Subsurface

Abstract

Subsurface contamination with the explosive hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) at ordnance production and testing sites is a problem because of the persistence, mobility, and toxicity of RDX and the formation of toxic products under anoxic conditions. While the utility of compound specific isotope analysis for inferring natural attenuation pathways from stable isotope ratios has been demonstrated, the stable isotope fractionation for RDX reduction by ironbearing minerals remains unknown. Here, the N isotope fractionation of RDX during reduction by Fe(II) associated with Fe minerals and natural sediments was evaluated and applied to the assessment of mineral-catalyzed RDX reduction in a contaminant plume and in sediment columns treated by in-situ chemical reduction. Laboratory studies revealed that RDX was reduced to nitroso compounds without denitration and the concomitant ring cleavage. Fe(II)/iron oxide mineral-catalyzed reactions exhibited N isotope enrichment factors, εN, between -6.3±0.3‰ to -8.2±0.2‰ corresponding to an apparent 15 N kinetic isotope effect of 1.04-1.05.The observed variations of the δ 15 N of ~15‰ in RDX from groundwater samples suggested an extent of reductive transformation of 85% at an ammunition plant. Conversely, we observed masking of N isotope fractionation after RDX reduction in laboratory flow-through systems, which was presumably due to a limited accessibility to reactive Fe(II).

Electron Transfer Energy and Hydrogen Atom Transfer Energy-Based Linear Free Energy Relationships for Predicting the Rate Constants of Munition Constituent Reduction by Hydroquinones

“…The fact that a single linear equation (eq 6) can describe the intrinsic reactivity of NACs/MCs of different molecular structures and charges toward multiple hydro-quinone species supports that a shared mechanism exists for all the nitro compounds. Along with isotope fractionation results, 15,27,33,73,74 the findings from this study suggest the Gibbs free energy for the first ET is proportional to that for the rate-limiting step, i.e., dehydration of the N,N-dihydroxylamine intermediate.…”

“…Most MCs contain one or multiple nitro groups, making them susceptible to reduction in anoxic environments. These reactions can occur with natural reductants such as organic matter (OM), , iron oxides in the presence of aqueous Fe 2+ , − and Fe 2+ -bearing minerals. − While the reduction of TNT and other nitroaromatic compounds (NACs) has been studied, ,,,− little is known about the reactivity of the MCs that are currently in use and/or under development. Many of these MCs are non-aromatic [e.g., nitroguanidine (NQ) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX)] or negatively charged at pH 7 [e.g., NTO and 3,4-dinitropyrazole (DNP)] and therefore may react differently than NACs.…”

Abiotic reduction of 3-nitro-1,2,4-triazol-5-one (NTO) and other munitions constituents by wood-derived biochar through its rechargeable electron storage capacity

“…Specifically, MNX and NO 2 − were formed through reduction, since an addition of two electrons is required for MNX formation and reductive denitration of RDX to form NO 2 − has been well-documented. 51,74 RDX degradation was further confirmed by the accumulation of NO 2 − following repeated additions of RDX in reactors containing Rogue RED , as detailed in Section S6 (Fig. S10†).…”

“…The same method was used to quantify daughter products of DNAN 48 and RDX: 49,50 2ANAN, 4ANAN, and DAAN were measured at 4.2 min (254 nm), 3.2 min (234 nm) and 2.3 min (210 nm), respectively, and MNX, DNX, and TNX were detected at 2.9, 2.4, and 1.9 min, respectively, based on absorbance at 234 nm. Nitrite, a potential RDX reduction product, 51 was measured using Hach NitriVer® 3 Nitrite Reagent (Loveland, CO) and a Vernier LabQuest 2 UV-vis spectrophotometer.…”

Organic Matter Inhibits Redox Activity and Impacts Heterogeneous Growth of Iron (Oxyhydr)oxides on Nano-Hematite

“…Ultrafine assemblages of iron oxides and oxyhydroxides play critical roles in natural geochemical processes within groundwaters, including transformation of environmental contaminants, − biogeochemical nutrient cycling, − and mobility of trace elements. − As mineral phases form, grow, and transform, both the accessible and the reactive surface area of an assemblage can change dramatically. Thus, understanding the reactions that occur at the water–mineral interface is important for assessing evolving reactivity of minerals and predicting contaminant fate and transport. , …”