Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) reduction is concurrently mediated by direct electron transfer from hydroquinones and resulting biogenic Fe(II) formed during electron shuttle-amended biodegradation

This study investigated multiple electron transfer pathways for hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) biodegradation in the presence of bioavailable Fe(III) and electron shuttling compounds. In order to identify the dominant electron transfer pathway for RDX biodegradation, three sets of experiments were performed including aquifer material incubations, kinetics experiments, and cell suspensions. Incubations with aquifer sediment reduced RDX most rapidly in the presence of electron shuttling compounds such as anthraquinone-2,6-disulfonate (AQDS) and purified humic substances. In addition, RDX was reduced before the onset of significant accumulation of Fe(II), suggesting that reduced shuttles transferred electrons to Fe(III) rapidly, with the resulting Fe(II) reducing RDX. This hypothesis was also supported by the kinetic experiments; the rate of electron transfer from anthrahydroquinone-2,6- disulfonate (AH2QDS) to Fe(III) was approximately 10 ⁵ times faster than the rate of AH2QDS electron transfer to RDX. However, an alternate hypothesis considered was direct reduction of RDX by the hydroquinone prior to the onset of Fe(III) reduction. Pure culture studies with a model Fe(III)/electron shuttle reducer (G. metallireducens) were performed to determine which pathway was most dominant. The resting cell suspension experiments demonstrated that there are four possible electron transfer pathways for RDX biodegradation; however, the rates of the electron shuttle-mediated pathways were consistently the fastest. When the Fe(II)-mediated electron transfer pathway was inhibited with the Fe(II) ligand Ferrozine, the rate and extent of RDX degradation decreased, but reduction continued. This suggests that multiple electron transfer pathways [reduction by hydroquinones and Fe(II)] overlapped in the presence of Fe(III), but inhibiting the iron pathway did not limit degradation. This demonstrates that RDX is concurrently reduced by electron shuttles and Fe(II) during electron-shuttle mediated biodegradation.