Biodegradation of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) under nitrate-reducing conditions

Hexahydro-1,3,5-trinitro-1,3,5-triazine is a widely used military explosive that is more commonly known as Royal Demolition Explosive (RDX). Because o f concerns with the potential toxicity of RDX, increasingly stringent regul ations are anticipated for wastewater discharges from munitions manufacturi ng facilities. At the Holston Army Ammunition Plant (HAAP), where RDX is cu rrently produced in the U.S., the treatment sequence includes an anoxic fil ter prior to aerobic filters and activated sludge reactors. The intent of t he anoxic filter is to remove the high levels of nitrate that are often pre sent in munitions wastewater. as well as RDX and other nitrated energetic c ompounds. However, RDX removal across the filter is typically not adequate to meet the anticipated regulatory target, and RDX tends to persist under a erobic conditions. The objective of this study was to evaluate the reason f or poor RDX removal in the anoxic filter. Microcosms were set-up with HAAP wastewater and biofilm scraped from the anoxic filter. Nitrate-reducing con ditions were readily established, with organics present in the wastewater a s well as acetate serving as electron donors. However, as long as nitrate w as present, no decrease in RDX (15 mu M) occurred. As soon as nitrate was d epleted and some primary substrate was still available, RDX was rapidly bio transformed to metabolites that included mononitroso, dinitroso, and trinit roso derivatives.: The disappearance of nitrate, followed by biotransformat ion of RDX, coincided with a decrease in redox potential to below -200 mV. These results were confirmed with an ethanol-grown nitrate-reducing enrichm ent culture that was inoculated with biofilm from the HAAP anoxic filter. T he presence of RDX had no apparent effect on nitrate reduction, whereas RDX biotransformation was completely inhibited until all of the nitrate was co nsumed. Nitrate removal was achieved by repeated additions of ethanol. Thus , in order to ensure efficient biotransformation of RDX, nitrate removal mu st be complete, which requires an electron donor supply that exceeds the ac ceptor capacity of the nitrate. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.