Anaerobic degradation of 2,4,6-trinitrotoluene in granular activated carbon fluidized bed and batch reactors

Abstract

In this study, an anaerobic fluidized bed reactor (AFBR) was used to treat a synthetically produced pink water waste stream containing trinitrotoluene (TNT). The synthesized waste consisted of 95 mg/l-TNT, the main contaminant in pink water, which was to be co-metabolized with 560-mg/l ethanol. Granular activated carbon was used as the attachment medium for biological growth. TNT was reduced to a variety of compounds, mainly 2,4,6-triaminotoluene (2,4,6-TAT), 2,4-diamino-6-nitrotoluene (2,4-DA-6-NT), 2,6-diamino-4-nitrotoluene (2,6-DA-4-NT), 2-amino-4,6-dinitrotoluene (2-A-4,6-DNT), and 4-amino-2,6-dinitrotoluene (4-A-2,6-DNT). These conversions resulted through the oxidation of ethanol to carbon dioxide under anoxic conditions, or reduction to methane under methanogenic conditions. The anaerobic reactor was charged with 1.0 kg of 16 x 20 U.S. Mesh Granular Activated Carbon (GAC) and was pre-loaded with 200 g of TNT prior to the addition of the mixed seed culture. During the first three weeks of operation, ethanol was completely degraded and no methane was produced. Effluent inorganic carbon revealed stoichiometric conversion of the feed ethanol to dissolved inorganic carbon with accumulation of carbon dioxide in the headspace of the reactor. GAC extraction showed incremental reduction of the nitro groups to amino groups, with 2,4,6-TAT as the final product. After three weeks, the oxygen from the nitro groups was depleted and methane production commenced. The reproducibility of this phenomenon was confirmed by repeating the experiment in the same manner using an identical AFBR. Furthermore, serum bottle tests were conducted using TNT loading ratios of 0.2, 0.4, 0.8, 1.0 g-TNT/g-GAC as well as experiments in the absence of GAC. Similar behavior to that of the columns was observed, with degradation rates varying according to the particular condition. GAC greatly enhanced the degradation rates and the higher TNT loading resulted in slower degradation rates of ethanol.