Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) degradation in biologically-active iron columns

Flow-through columns were used to evaluate the efficacy of permeable reactive iron barriers to treat groundwater contamination by RDX. Three columns were packed with iron filings (Fe⁰) between soil and sand layers, and were fed continuously with unlabeled plus ¹⁴C-labeled RDX to characterize its removal efficiency under different microbial conditions. One column was poison-sterilized to isolate chemical degradation processes, another was not poisoned to allow colonization of the Fe⁰ layer by indigenous microorganisms, and a third column was amended with anaerobic sludge to evaluate the benefits of enhancing biodegradation through bioaugmentation. Extensive RDX removal (> 99%) occurred through the Fe⁰ layer of all columns for more than one year, although ¹⁴C-label analysis indicated the presence of soluble byproducts such as methylenedinitramine. RDX byproducts accumulated to a lesser extent in biologically active columns, possibly due to enhanced mineralization by the cumulative action of microbial and chemical degradation processes. Denaturing gradient gel electrophoresis (DGGE) profiles and nucleotide sequencing revealed a predominance of Acetobacterium sp. in the iron layer of all columns after 95 days. Such homoacetogenic bacteria probably feed on hydrogen produced during Fe⁰ corrosion and participate on the RDX degradation process. This notion was supported by batch experiments with a mixed homoacetogenic culture isolated from the bioaugmented column, which degraded RDX and produced acetate when H₂ was present. Overall, this work suggests that Fe⁰ barriers can effectively intercept RDX plumes, and that treatment efficiency can be enhanced by biogeochemical interactions though bioaugmentation.