Environ. Sci. Technol. 2007, 41, 1257-1264
Role of Organically Complexed Iron(II) Species in the Reductive Transformation of RDX in Anoxic Environments DONGWOOK KIM AND TIMOTHY J. STRATHMANN* Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, 205 North Mathews, Urbana, Illinois, 61801
Organically complexed iron species can play a significant role in many subsurface redox processes, including reactions that contribute to the transformation and degradation of soil and aquatic contaminants. Experimental results demonstrate that complexation of FeII by catechol- and thiolcontaining organic ligands leads to formation of highly reactive species that reduce RDX (hexahydro-1,3,5-trinitro-1,3,5triazine) and related N-heterocyclic nitramine explosive compounds to formaldehyde and inorganic nitrogen byproducts. Under comparable conditions, relative reaction rates follow HMX 5 orders of magnitude when pH and ligand concentration are varied in FeII-tiron solutions (see bold lines in Figure 3). Naka et al. (20) reported similar findings for the reduction of nitroaromatic compounds in solutions containing FeII and tiron. For 4-chloronitrobenzene, kFeL26- ) 3.81((0.85) × 104 M-1 s-1, roughly 50 times greater than the value obtained for RDX. The relative values of kFeL26- determined for the two classes of contaminants follow the same reactivity pattern observed in FeII-amended magnetite (Fe3O4(s)) suspensions (16, 28), suggesting that RDX will be more persistent than nitroaromatic co-contaminants in settings where abiotic reactions with FeII represent the predominant natural attenuation process. As described by Naka and co-workers, the high reactivity of the 1:2 FeII-tiron complex and comparatively low reactivity of other FeII species can be explained in terms of the standard one-electron reduction potentials (EH0) of the FeIII/FeII half reactions associated with individual FeII-ligand complexes. The value of EH0 of the 1:2 FeII-tiron complex is -0.509 V vs NHE, much more reducing than the EH0 values of FeII complexes that exhibit no reactivity with RDX (e.g., +0.08 V for FeII-EDTA, +0.353 for FeII-citrate, +0.045 for 1:1 FeIItiron complex; complete list provided in Supporting Infor-
mation of ref 20). As reported for 4-chloronitrobenzene, there appears to be a threshold value of EH0 required before FeII complexes will react with RDX; the lack of any observed reactivity in FeII-salicylate solutions suggests that this value is lower than -0.276 V (the estimated EH0 value for the 1:2 FeII-salicylate complex). Recall that for DMSA, kobs increases with increasing DMSA concentration up to 5 mM and then decreases as ligand concentration is further increased. As discussed earlier, this trend may be a byproduct of enhanced autocatalytic degradation of the FeII-DMSA complexes at higher ligand concentrations. An alternative interpretation of the kinetic trend is that increasing DMSA concentration shifts the distribution of FeII-DMSA species toward a higher order complex (e.g., from a 1:1 complex to a 1:2 complex) that is less reactive with RDX. However, unambiguous assignment of the responsible factors will require further research to improve understanding of both FeII-DMSA speciation and the factors controlling the instability of DMSA solutions. RDX Transformation Products and Stoichiometry. As discussed earlier, none of the nitroso derivatives of RDX are detected as intermediates or endproducts when RDX (or MNX and DNX) is reduced by FeII-organic complexes. Rather, results in Figure 5 show stoichiometric formation of 3 mol equiv of formaldehyde occurs co-incident with RDX loss, as demonstrated by the close agreement with product formation trends predicted with a simple first-order model. The formaldehyde yield accounts for >95% of the carbon mass balance; all other unidentified organic endproducts are quantitatively minor contributors. Formaldehyde is a commonly reported endproduct of different microbial and abiotic RDX reduction mechanisms (11, 12, 15, 16). The lack of any lag time between RDX decay and formaldehyde formation also demonstrates that further transformation of transient organic intermediates (e.g., methylenedinitramine) is much faster than the initial rate-determining reduction of RDX. The carbon mass balance provided in Figure 5 also implies that the major nitrogen endproducts will be inorganic. Over the time period corresponding to RDX degradation, a poor nitrogen mass balance is provided by previously reported inorganic nitrogen byproducts of RDX reduction (N2O, NH4+, NO2-, and NO3-). Of the compounds monitored, only NH4+ accounts for a significant fraction of the nitrogen mass balance (33% after a reaction time of 30 min); all others account for
