Influence of chromium on the biodegradation of hexahydro-1,3,5- trinitro-1,3,5-triazine (RDX) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX)

Abstract

Hexavalent chromium [Cr(VI)], frequently encountered due to metal plating operations, is normally considered toxic to microorganisms. RDX and HMX are the most widely used explosives since World War II and now contaminate many munitions and other related sites. The DOE facility in Amarillo, TX is contaminated with both high explosives and Cr (VI). The interaction of these co-contaminants is the focus of this research. Previous studies focussed on the degradation of RDX and HMX (Hallgarth, 1996,. Jankowski, 1997), whereas these studies were performed to evaluate the influence of Cr (VI) on the biodegradation of RDX and HMX. RDX biodegradation experiments were performed under nitrogen limiting conditions to examine the influence of Cr on the growth of microorganisms and their degradation of RDX. In experiments with HMX, carbon and nitrogen were supplemented periodically. Nutrients were added to maintain culture viability throughout experiments. Under these conditions, RDX concentrations were reduced by 8% and 17% in reactors with 1.0 and 2.5 mg/L Cr (VI), respectively; whereas 21% of the RDX was reduced in reactors with no Cr(VI). RDX biotransformation in reactors with 5.0 mg/L Cr (VI) was below 2%. HMX biotransformation percentages in reactors containing 0.0, 1.0, 2.5, 5.0 mg/L were 38%, 33%, 27%, 29% respectively. Less than 27% of HMX was transformed in reactors that contained 10 mg/L Cr (VI). Kinetic rate experiments were performed in the presence of Cr (VI) to characterize the response of cultures at different concentrations of carbon. The kinetic parameters for RDX degrading cultures were [] = 0.50 hr⁻¹, K[] = 58.60 mg/L and K[] = 5.00 mg/L. In the case of HMX degrading cultures the kinetic parameters were [] = 0.06 hr⁻¹, K[] = 96.99 mg/L and K[]=6.06 mg/L. Concentrations of Cr (VI) lower than 2.5 mg/L will inhibit RDX and HMX biodegradation without altering the biomass concentration whereas a Cr (VI) concentration of 5.0 mg/L and above inhibits the degradation as well as the growth of the cultures.