Environ. Sci. Technol. 2008, 42, 3975–3981
Abiotic Transformation of Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) by Green Rusts PHILIP LARESE-CASANOVA AND MICHELLE M. SCHERER* Department of Civil and Environmental Engineering, University of Iowa 4126 Seamans Center, Iowa City, Iowa 52240
Received September 22, 2007. Revised manuscript received February 13, 2008. Accepted February 26, 2008.
The rate and extent of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) transformation was measured in the presence of carbonate and sulfate green rust suspended in solutions containing common groundwater anions. Formaldehyde (HCHO), nitrous oxide gas (N2O(g)), and ammonium (NH4+) were the major end products, accounting for about 70% of the carbon mass balance and about half of the nitrogen mass balance. Results from experiments with both 14C-RDX and LC-MS analysis indicate that the remaining carbon products are soluble and most likely small ( 20% of reacted parent compound) in this study, whereas thin boxed compounds only formed in trace amounts (products found to be sulfate green rust (13). In the chromate study, differences in surface areas could not account for the differences in reaction rates, and the
FIGURE 4. Influence of inorganic anions on the disappearance kinetics of RDX with sulfate green green rust. All reactors contained anion concentrations of 10 mM (except SiO2, 1 mM). Ionic strength was adjusted to 0.1 M with KBr. authors suggest that preferential reaction within interlayer space or at crystal plate edges may be responsible (13, 39, 43). Being a much larger molecule, RDX may not have such a preference and may explain why we observe little influence of the interlayer anion on rates of RDX reduction by green rust. Effect of Anions on Rate of RDX Transformation. Most studies of contaminant reduction by green rusts have been done with freshly prepared green rust suspensions. These suspensions contained only the anions present from the initial Fe salts (e.g., iron chlorides or sulfates) and the buffer used to adjust the pH (e.g., NaHCO3 or KOH). A few studies have also been done with dried green rusts suspended in an organic buffer. Given the affinity of inorganic anions for mineral surfaces, including green rusts (44), it is likely that common groundwater anions, such as phosphate and silicate, bind to green rust surface sites and influence rates of contaminant reaction. To evaluate the effect of anions on contaminant reduction rates, we measured the rates of RDX removal by freshly precipitated green rusts after the addition of 10 mM bromide, sulfate, bicarbonate, phosphate, or 1 mM silicate. We tracked only the formation of the nitrosointermediates, and small amounts of nitroso-intermediates were detected in all of the anion experiments, except those with phosphate present. The presence of the anions affected the transformation rates of RDX by green rusts similarly for both sulfate (Figure 4) and carbonate green rust (Figure S3 of Supporting Information). Phosphate had the most dramatic effect with essentially no RDX removal observed for either green rust. Phosphate is well-known to sorb strongly to iron oxides and green rusts (44, 45) and most likely inhibits RDX removal by blocking access to Fe(II), either by binding to reactive surface sites or by forming secondary surficial Fe(II)-PO4 precipitates, such as vivianite, which was observed in the XRD patterns provided as Figure S4 in Supporting Information. Surface-bound phosphate has been shown to inhibit the dissolution and rearrangement of carbonate green rust to siderite (FeCO3) and magnetite (45, 46), and both phosphate and silicate were shown to inhibit the transformation of ferric oxyhydroxycarbonate (a solid state oxidation product of carbonate green rust) to goethite (44). Note that we did not observe any substitution of phosphate into the green rust interlayer as was observed by Hansen and Poulsen for sulfate green rust (based on slight changes in interlayer d-spacing as measured by XRD) (47). Rates of removal in the presence of chloride, bromide, and silicate were all about the same, with a slightly slower
rate observed in the presence of silicate. Carbonate species had an odd effect on the kinetics in the form of a short lag period followed by zero-order kinetics with the lag period more pronounced for sulfate than carbonate green rust. Sulfate produced a similar effect with carbonate green rust but only slowed the kinetics (without a lag period) with sulfate green rust. We speculate that the slowed kinetics could be due to either (i) limited site availability by surface complexation of carbonate and sulfate or (ii) the exchange of interlayer anions between solution and green rust. Hansen and co-workers (19) reported interlayer anion exchange between chloride green rust and sulfate resulted in slower removal rates of nitrate. Interlayer anion exchange for aqueous anions has been shown to readily occur among synthetic sulfate, carbonate, and chloride green rusts (19, 48, 49) and XRD patterns shown in Figure S4 of the Supporting Information do indicate that interlayer exchange between sulfate and carbonate did indeed occur during the course of our experiments. The interlayer anion exchange process, however, does not explain the presence of the lag phase, as we observed a similar lag period when we suppressed the interlayer exchange by adding both sulfate and carbonate to the solution. XRD patterns also did not reveal the formation of any iron carbonate, sulfate, or silicate minerals that could passivate the green rust surface. The processes producing the lag phase are not clear, but we suspect anion surface complexation could be a factor. With this work, we demonstrate rapid transformation of RDX to formaldehyde and the inorganic nitrogen species N2O and NH4+ by green rust minerals. Transient appearance of the reduced RDX products, MNX, DNX, and TNX indicate that the transformation is initiated by reduction by Fe(II) present in the green rusts. These findings, along with previous reports of RDX reduction by magnetite in the presence of Fe(II) and organically complexed Fe(II) (4, 6) species provide compelling evidence that reduction of RDX by Fe(II) species may be an important abiotic natural attenuation pathway, particularly in subsurface environments where microbial Fe respiration is occurring. The work also suggests that addition of green rusts, or more likely addition of soil amendments that promote green rust formation (such as Fe(0)), may be a viable in situ treatment option for RDX-contaminated soil and water, such as was recently demonstrated for metolachlor removal (50, 51). Some caution, however, is warranted for application of green rust in groundwaters with high phosphate content, such as in agricultural soils.
Acknowledgments We thank Craig Just, Collin Just, Benoit Van Aken, and Richard Valentine (University of Iowa) for their valuable guidance with analytical methods and for synthesizing RDX, MNX, DNX, and TNX. We gratefully acknowledge financial support for this work from the National Science Foundation through a Graduate Research Fellowship to PL-C.
Supporting Information Available Experimental procedures, a summary of literature data available on rates of RDX transformation, additional kinetic data for RDX transformation by green rusts, and XRD patterns of green rusts. This material is available free of charge via the Internet at http://pubs.acs.org.
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