Fate and transport rates can vary considerably among energetic compounds and are dictated by differences in chemical structure. Biodegradation of 2,4,6-trinitrotoluene (TNT), a nitroaromatic, is relatively rapid but incomplete, and amino degradates are frequently observed. Hexanitrohexaazaisowurtzitane (CL-20 or HNIW) is rapidly transported and undergoes moderate rates of abiotic and biotic degradation. On the other hand, hexahydro-1,3,5,7-trinitro-1,3,5-triazine (RDX) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), nitramines, are slow to degrade in aerobic environments and are highly mobile in the vadose and saturated zones. Collectively, the lack of extensive biodegradation combined with moderate to high transport rates results in continued persistence in the subsurface, and with time, eventual contamination of groundwater by both parent and biotic intermediates. This situation is particularly problematic at firing ranges where continued use of energetic compounds has occurred for decades. New technologies are urgently needed that can accelerate natural attenuation processes and remediate these contaminated sites. To achieve full risk reduction, however, it is desirable that energetics become mineralized rather than simply biotransformed.

The objective of this project was to quantify processes and determine the effectiveness of abiotic/biotic mineralization of energetics (RDX, HMX, TNT) in aquifer sediments by combinations of biostimulation (carbon, trace nutrient additions) and chemical reduction of sediment to create a reducing environment.

In this project, energetic compound microbial reactions, coupled abiotic/biotic effects, and upscaling in flowing systems were investigated over four tasks in increasingly complex systems, from maximizing microbial growth (Task 1), modifying the abiotic reduction process (Task 2), assessing coupled abiotic and biotic processes (Task 3), and upscaling to flowing systems (Task 4).

Initially it was hypothesized that a balance of chemical reduction of sediment and biostimulation would increase the RDX, HMX, and TNT mineralization rate significantly (by a combination of abiotic and biotic processes) so that this abiotic/biotic treatment might be more efficient for remediation than biotic treatment alone in some cases. Because both abiotic and biotic processes are involved in energetic mineralization in sediments, it was further hypothesized that consideration for both abiotic reduction and microbial growth was needed to optimize the sediment system for the most rapid mineralization rate.

Results showed that there are separate optimal abiotic/biostimulation aquifer sediment treatments for RDX/HMX and for TNT. Optimal sediment treatment for RDX and HMX (which have chemical similarities and similar degradation pathways) was mainly chemical reduction of sediment, which increased the RDX/HMX mineralization rate 100 to 150 times (relative to untreated sediment), with a secondary treatment of carbon or trace nutrients, which increased the RDX/HMX mineralization rate an additional 3 to 4 times.  In contrast, the optimal aquifer sediment treatment for TNT involved mainly biostimulation (glucose addition), which stimulated a TNT/glucose cometabolic degradation pathway (6.8 times more rapid than untreated sediment), with secondary treatment by chemical reduction (13 times additional rate increase). TNT was transformed to triaminotoluene, which irreversibly sorbs in reduced systems but is rapidly degraded in oxic systems. Although the TNT degradation pathway was biologically dominated, the iron-reducing conditions created by abiotic reduction of sediment promoted more rapid abiotic degradation of amino-intermediates than biodegradation of these intermediates. Chemical reduction of sediment alone is not an effective treatment for TNT (intermediates that irreversibly sorb are not produced), even though the TNT degradation rate (to 2- or 4-aminodinitrotoluene) increases.

This project quantified the biochemical reductive processes occurring in the redox barrier and determined the importance of these transformations on total mineralization rates. Different optimal subsurface treatments for RDX/HMX and for TNT is likely manageable for field scale remediation, as few sites have groundwater contamination of all three energetics in the same location. RDX and HMX are common groundwater contaminants due to slow aerobic degradation in soils and vadose zone sediments, and minimal sorption. In contrast, TNT is a common soil/shallow sediment contaminant with limited TNT subsurface migration because of aerobic and anaerobic degradation in soils and significantly greater sorption.