Biodegradation of the energetic compound TNT through a multiple-stage treatment approach.
Biodegradation of the energetic compound 2,4,6-trinitrotoluene (TNT) and its intermediate 2,4,6-triaminotoluene (TAT) was investigated in this study. From previous investigations, a relationship between the biological utilization of ethanol as co-substrate for the reduction of TNT under anaerobic conditions was proposed using an anaerobic fluidized-bed reactor (AFBR). In this study, the theoretical co-substrate requirement for reduction of TNT to TAT was further investigated through the systematic lowering of the ethanol loading to the reactor. Near complete reduction to TAT was observed up to a critical ethanol loading point, as well as the production of methane from the limited excess available ethanol. Once ethanol deficient loading conditions were established, the increased presence of incompletely reduced degradation intermediates, such as 2,4-diamino-6-nitrotoluene, and even TNT, was observed. The cessation of methanogenesis confirmed that no excess ethanol was available. Degradation of the TAT intermediate in the reactor effluent was investigated using two second-stage reactors under oxidizing conditions. The first was an aerobic activated sludge reactor, and the second was a denitrifying fluidized-bed reactor (DenFBR). The aerobic reactor was successful in lowering the chemical oxygen demand (COD), but complete removal of TAT was not accomplished. Because of TAT polymerization and auto-oxidation under aerobic conditions, it was difficult to confirm to what extent of TAT removal was biological. In the DenFBR, incompletely reduced TNT intermediates were not successfully degraded, but strong evidence existed for the degradation of TAT. This is the first known report of second stage degradation of TAT under denitrifying conditions.