Royal Demolition Explosive (RDX) is one of the most recalcitrant and toxic contaminants in the subsurface. This project sought to develop a new and efficient method to remediate RDX-contaminated aquifers. This method is based on combining a novel chemical process (reductive treatment with zero valent iron [Fe0]) with a promising bioremediation approach (in situ reactive zones). This integrated Fe0-microbial system is more than a mere juxtaposition of two technologies, because Fe0 and some microorganisms interact synergistically to degrade RDX.

Building on SERDP SEED project ER-1175, the objective of this project was to delineate the applicability and limitations of an integrated Fe0-microbial approach to managing RDX plumes. To advance the understanding of relevant biogeochemical processes, investigations focused on demonstrating bacterial colonization of granular iron, assessing the role of reduced iron in the removal process, and evaluating the robustness and long-term performance of bioaugmented iron columns simulating permeable reactive barriers (PRB).

Iron-Based Bioremediation with Polishing by Natural Attenuation

Processes mediated by the Fe0 barrier will promote chemical and biological reactions responsible for enhanced RDX degradation. Chemical reduction by the barrier will occur by direct contact with either Fe0 or some reactive iron oxides. Fe0 corrosion will rapidly induce anoxic conditions that favor RDX biodegradation. The production of cathodic (water-derived) hydrogen (H₂) by Fe0 corrosion will increase the availability of an excellent electron donor to support microbial reduction of RDX and the further degradation of some dead-end products that could accumulate during abiotic reduction by Fe0. Iron-reducing bacteria also could participate in the cleanup process by enhancing the reactivity of Fe0 barriers by reductive dissolution of iron oxides (i.e., depassivation) and possibly by mediating the formation of highly reactive surface-associated Fe(II) (e.g., green rust and magnetite).

Batch and microcosm experiments showed extensive RDX mineralization [to carbon dioxide (CO₂) and nitrous oxide (N₂O)] in biologically active treatments with Fe0, but not in sterile Fe0–amended systems. Column studies showed high and sustainable RDX removal efficiency in biologically active iron columns and no clogging problems or interference by HMX, TNT, DNTs. Key biogeochemical interactions that enhanced process performance were identified, namely (1) biostimulation of anaerobic microorganisms by H₂ gas production during Fe0 corrosion, (2) production of reactive Fe(II) species by dissimilatory iron reducing bacteria (DIRB) that respire (inert) iron oxides, (3) abiotic RDX degradation by both structural and adsorbed Fe(II) with formaldehyde (HCHO) and N₂O as significant end products, and (4) primary productivity by homoacetogens in iron columns, which utilize RDX possibly as a nitrogen source and transform CO₂ to acetate (the production of acetate might comensalistically support heterotrophic activity). Investigators also showed that RDX byproducts that could break through a PRB are rapidly mineralized by indigenous organisms.

Altogether, this project suggests that iron barriers might be an effective tool to intercept and destroy RDX plumes and that the treatment efficiency can be improved by the concurrent or sequential participation of some microorganisms. While the development of a cost-effective and sustainable remediation approach has great intrinsic merit, this project also has significant extrinsic merit related to enhancing the understanding of biogeochemical interactions in contaminated aquifers and the role of mineral surfaces in natural attenuation. (Project Completed - 2004)