Degradation of explosives by bio-reduced iron bearing soil minerals

Abstract

The reductive degradation of explosives (2,4,6-trinitrotoluene (TNT) and hexahydro-1,3,5-trinitro-1,3,5- triazine (Royal Demolition Explosive, RDX)) can be enhanced by iron bearing soil minerals (IBSMs) reduced by Shewanella putrefaciens CN32 (CN32). Among five different IBSMs (green rust, magnetite, lepidocrocite, hematite, and goethite), the highest degradation kinetic rate constant of TNT and RDX was obtained from magnetite (0.0039 h-1) and lepidocrocite (0.1811 h-1), respectively. The rate constants for each soil mineral were closely related to production of biogenic Fe(II) during the degradation of TNT and RDX, indicating that Fe(II) reduced by CN32 was the key factor controlling the enhanced degradation of explosives in this study. The experimental results showed that bio-reduced IBSMs by CN32 can significantly enhance the degradation kinetics of explosives. Experiments were also conducted for degradation of RDX by lepidocrocite and CN32 in the presence of electron transfer mediator (i.e., riboflavin). No significant degradation of RDX was observed by CN32 and lepidocrocite with CN32 in the absence of riboflavin, while 60 and 100 % of RDX was degraded by CN32 and lepidocrocite with CN32 in the presence of riboflavin. The biogenic Fe(II) reduced from lepidocrocite by CN32 acted as a catalyst to rapidly transfer electron to riboflavin during the RDX degradation. RDX was continuously degraded by mixture of shooting range soil and lepidocrocite in the presence of riboflavin and CN32 during three reaction cycles, while the degradation kinetic was significantly inhibited by CN32 only at second cycle. The experimental results showed that riboflavin can rapidly transfer the electron to RDX due to the catalytic role of biogenic Fe(II). Keywords: 2,4,6-trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), Shewanella putrefaciens CN32, riboflavin, iron bearing soil minerals (IBSMs)