Behaviour and Treatment of Nitroaromatics in Groundwater
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The purpose of this study was to determine the chemical and/or biological factors that cause 2,4-dinitrotoluene (2,4-DNT), 2,6-dinitrotoluene (2,6-DNT) and nitrobenzene (NB) to transform to their respective aromatic amines in the Borden aquifer, and to investigate the biodegradation of 2,4-diaminotoluene (2,4-DAT) and 2,6-diaminoluene (2,6-DAT) under aerobic conditions. In situ microcosms (ISM) and laboratory microcosm experiments were used in the investigation. In addition, a sequential treatment system was tested in which columns containing granular iron were followed by either an anaerobic or aerobic soil column. Both 2,4- and 2,6-DNT were used to determine if competitive effects exist between the two. The ISM isolates a volume of the aquifer material and allows for in situ solute loading and sampling in order to characterize chemical or biological reactions. Four ISMs were installed below the water table at CFB Borden. Each ISM was injected with 10 mg/L of either 2,4-DNT, 2,6-DNT, NB, or 2,4-DNT + 2,6-DNT, in two repetitions. In all cases, chloride was also injected as a conservative tracer to monitor for dilution. The results indicated transformation of nitroaromatics via nitro-reduction to their intermediate products, mainly as 2,4-DAT, 2,6-DAT, and aniline. Within 20 days, a loss of up to 92% of 2,4-DNT was observed with the formation of 2,4-DAT. Minor amounts of 2-amino-4-nitrotoluene (2-A-4-NT) and 4-amino-2-nitrotoluene (4-A-2-NT) were also observed. Similarly, up to a 96% loss of 2,6-DNT was seen after 29 days, with degradation products including 2-amino-6-nitrotoluene (2-A-6-NT) and 2,6-DAT. When 2,4- and 2,6-DNT were present in combination, 99% loss of both compounds at similar rates was observed over 20 days following the injections, with degradation products including aminonitrotoluenes and diaminotoluenes. Finally, when nitrobenzene was injected, degradation of up to 99% was observed by day 29, with the formation of aniline as the primary product. To determine the cause of the nitro-reduction, laboratory microcosm experiments were conducted using soil from within the chamber of the ISM’s. Duplicate microcosms were prepared with Borden groundwater and spiked with 2,4- and 2,6-DNT in an anaerobic glovebox. Microcosms were incubated and sampled periodically for approximately 3 months. Several different conditions, including: groundwater and soil, autoclaved groundwater and soil, soil taken at ground surface and groundwater, and autoclaved silica sand and groundwater were created for microcosm experiments to determine whether abiotic or biotic factors caused the reduction of 2,4- and 2,6-DNT. Microcosms which duplicated field conditions in the laboratory had average half-lives of 4.2 days and 5.1 days for 2,4- and 2,6-DNT, respectively, compared to the field result with average half-lives between 3.9 days (2,4-DNT) and 3.5 days (2,6-DNT). Subsequently, a nutrient medium was added to each repetition. The behaviour of DNT degradation did not change significantly, suggesting minimal involvement of biological processes. Furthermore soil analysis showed relatively high concentrations of extractable iron and the presence of magnetite, which are species capable of reducing nitroaromatics. Therefore, it is concluded that nitro-reduction in Borden soil is likely a result of abiotic surface mediated processes. The competitive behaviour of 2,4- and 2,6-DNT was studied in a sequential treatment system which consisted of an anaerobic iron column, followed by either an anaerobic or aerobic soil column. Results showed the same rate of transformation from 2,4- and 2,6-DNT within the iron column, with 100% conversion to 2,4- and 2,6-DAT, respectively. Within the anaerobic and aerobic soil columns, the DATs were highly persistent. When a nutrient solution was added only to the aerobic soil column with DNTs as the initial compounds, results showed a reduction of 2,4-DNT of 17%, with an increase in 2,6-DNT of 22%. The increase in 2,6-DNT may have been a result of differing influent concentrations at earlier pore volumes. When stock solutions in the aerobic column were altered to only include DATs, a reduction of 2,4- and 2,6-DAT was observed at 17% and 18%, respectively. It would appear that an acclimated bacterial community able to transform DNT and DAT was present in the aerobic Borden sand column. Degradation of 2,4- and 2,6-DAT was dependant on the degree of nutrients supplied to indigenous bacterial communities under aerobic conditions.