Pilot-scale anaerobic bioslurry remediation of RDX- and HMX-contaminated soils

Bioremediation of RDX- and HMX-contaminated soil slurries (40-44% clay soil) was conducted under optimal conditions in two 8 L drum-style reactors and a 250 L pilot-scale reactor. The RDX and HMX content of the soil were 10,355 and 4,720 mg/kg soil respectively in the first case, and 24,060 and 7,860 mg/kg soil respectively in the later case. The results showed that RDX was completely removed after about 45 days of operation, while the HMX removal, which started once RDX was completely removed, took an additional 35 days. HPLC and LC/MS analysis showed that both RDX and HMX were first biotransformed to their corresponding nitroso-metabolites before they were completely degraded. The average RDX removal rate was two times higher in the run with an initial RDX concentration of 24,060 mg/kg than in the run with an initial RDX concentration of 10,400 mg/kg (513 against 213 mg RDX/kg soil.d). This indicates that the explosive mass transfer from the solid to the liquid phases rather than the biodegradation was the limiting step.