ABIOTIC AND BIOLOGICAL TRANSFORMATION OF HEXAHYDRO-1,3,5-TRINITRO-1,3,5-TRIAZINE (RDX) IN AQUIFER SEDIMENT

RDX, a nitramine explosive compound, is a potential human carcinogen, and its contamination of subsurface environments is a significant threat to public health. This study investigated abiotic and biological degradation of RDX within contaminated aquifer sediment.

Batch incubations of contaminated aquifer material were performed with and without pre-aeration to distinguish biological RDX reduction from abiotic RDX reduction. Acetate or lactate was added as an electron donor to promote the biological reduction of RDX.

Without pre-aeration of the sediment, RDX was reduced by coupled abiotic and biological processes. Abiotic RDX reduction in the sediment was attributed to chemical reductants in the native aquifer sediment, primarily Fe(II) associated with mineral surfaces. To assess biological process alone, chemical reductants in the native sediment were removed by pre-aeration. RDX (50µM) was completely reduced and transformed to ring cleavage products when 2mM of acetate or lactate as the electron donor were provided. Biomass increased 3 and 1.6 times in the presence of acetate and lactate, respectively, suggesting that electron donors added to contaminated aquifer sediments can stimulate RDX biotransformation. RDX reduction occurred concurrently with iron reduction when acetate was provided, while both iron and sulfate reduction were dominant when lactate was provided. This indicates that acetate and lactate promote the development of different types of microbial communities and different terminal electron accepting processes, which in turn control the dynamics of RDX transformation. Although significant amounts of iron and/or sulfate were reduced, reactive minerals (i.e., magnetite, green rust, and sulfide minerals) were not observed, suggesting RDX reduction in the aquifer sediment is likely due to surface associated Fe(II).

These results suggest that both biotic and abiotic processes can play an important role in RDX reduction under in situ conditions. Investigation of the dominant microorganisms associated with RDX, Fe(III), and/or sulfate reduction using total microbial community analysis (i.e., clone library construction and high-throughput sequencing) is ongoing.