| In recent years, the low-cost supply of xylene resulted in its replacement for toluene in the processes of trinitrotoluene (TNT) synthesis. Consequently, dinitroxylene isomers (DNX) are found as by-products and often detected as co-contaminants persisting at TNT and dinitrotoluene isomers (DNT) contaminated sites. Of all the DNX isomers, 4,6-DNX appears to be the predominant. As an emerging contaminant, little is known about the toxicity and fate of DNX. However, based on similarities in molecular structure and chemical properties to TNT and DNT, it is reasonable to assume that DNX isomers are highly explosive and toxic, and thus, pose risks to human health and the environment. Therefore, the objectives of this research were to investigate the degradability of DNX and to establish an effective remediation strategy for DNX contaminated sites.The first experiment was conducted to determine the reduction of 4,6-DNX by granular iron using standard flow-through column procedures. 4,6-DNX was rapidly disappeared in the 30% iron column with the concentration profiles were consistent with pseudo-first-order kinetics. The average first-order rate constant (with standard deviation, SD) was 1.28±0.04 min-1, giving a half-life of 0.54±0.02 min and a corresponding surface area-normalized rate constant (kSA) of (6.59±0.20)×10-4 L/m2/min. 1,3-dimethyl-4-amino-6-nitro-benzene (ANX) was identified as an intermediate as well as other transient intermediates believed to be nitroso- and hydroxylamino- products. 1,3-dimethyl-4,6-diamino-benzene (4,6-DAX) was identified as the end product with almost 100% mass balance.Subsequently, biodegradation of 4,6-DNX was assessed under anaerobic conditions using anaerobic enrichment cultures previously shown to degrade nitrogen-containing explosives. Either nitrate or sulphate was supplied as an electron acceptor and with/without acetate as a primary substrate. At an initial concentration of approximately 10 mg/L, 4,6-DNX removal was observed, with no lag period, under both nitrate-reducing and sulphate-reducing conditions. Among these, the rate of 4,6-DNX degradation was slightly influenced by the source of bacteria and the growth conditions. As with granular iron, 4,6-DNX was biotransformed to 4,6-DAX via ANX as an intermediate product through nitro reduction process.This study also presented the biotransformation of 4,6-DNX using contaminated soil from Barksdale site. Slow degradation of 4,6-DNX was observed in the two contaminated soils containing different levels of NACs (Barksdale-1 and Barksdale-2), suggesting that the recalcitrant nature of 4,6-DNX in the contaminated soil can be attributed to several factors. The temporary inhibition of 4,6-DNX biotransformation in Barksdale-1 soil was due to the enzyme induction or enrichment of the degrading species over time, caused by either the high level of 4,6-DNX concentration or high concentrations of other DNX isomers in the soil sample. In contrast, no degradation of 4,6-DNX was observed in the active sets inoculated with Barksdale-2 soil over the 126-days incubation period. However, both 2,4- and 2,6-DNT originated from soil sample were degraded with relatively short lag period even at high initial concentrations of 40 mg/L each. Therefore, it is reasonable to believe that the co-occurrence of DNT isomers in the mixture with 4,6-DNX may have negative influence on the degradation of 4,6-DNX by indigenous microbial community in contaminated soils.No evidence supports biodegradation of 4,6-DAX by enrichment cultures under nitrate-reducing and sulphate-reducing conditions when using 4,6-DAX as the initial contaminant, though evidences indicated that the cultures were active. Mineralization of 4,6-DAX was accomplished by subsequent aerobic treatment using soil samples taken from explosives contaminated sites (Barksdale and Colorado soil). In the Barksdale soil batch experiment, at initial concentrations of 9.82, 10.93 and 14.70 mg/L, the indigenous microbial population degraded 4,6-DAX rapidly, showing the rate constant of 0.48±0.04 day-1 and half-lives of 1.44±0.11 day during the three single-spike periods. In the Colorado soil, at initial concentrations of 9.45– 13.26 mg/L, 4,6-DAX was rapidly degraded with the rate constants of 0.19 and 0.10 day-1 and half-lives ranging between 3.7 and 6.9 d for each spike period, respectively. The degradation of 4,6-DAX by indigenous microbial population in both soils does not require the presence of 2,4-DAT as a primary substrate. Based on the carbon and nitrogen mass balances, it is concluded that a minimum of 17.35% and 21.70% of 4,6-DAX was mineralized in Barksdale soil and Colorado soil experiments, respectively.In conclusion, this study demonstrates that a sequential anaerobic-aerobic system is a viable treatment strategy for remediating DNX contaminated sites. It also provides insight for treatment of other nitroaromatic compounds.
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