Abstract
The nitroaromatic explosive 2,4,6-trinitrotoluene is a widespread, toxic groundwater contaminant. The objective of this work was to describe TNT fate in contaminated aquifer sediments. A series of bench scale experiments and model simulations were performed to evaluate the fate of TNT in historically contaminated aquifer sediments. A TNT contaminated site on the National Priorities List, Former Nansemond Ordnance Depot (FNOD), Suffolk, VA, served as the model site for this work. To describe desorption rate in contaminated sediments, two approaches for a first order single-site desorption were evaluated. In Model 1, the driving force for desorption is mathematically related to the sorbed phase concentrations, whereas in Model 2 the rate is based on aqueous phase concentrations. Two data sets were used to evaluate the models: (1) batch draw-and-fill experiments using FNOD sediment and (2) results from a previously published report from the Louisiana Army Ammunition Plant. Both models provided adequate fit, but Model 2 was better behaved and first order parameters fell within a smaller confidence interval. Draw-and-fill experiments were observed to yield first-order mass transfer coefficients well aligned with those derived from column experiments.
The effect of organic amendments on anaerobic TNT degradation rate and microbial community structure in culture enriched from the FNOD site was studied in batch anaerobic microcosms. TNT readily degraded under all experimental conditions. A reductive pathway of TNT degradation was observed across all conditions, however, denaturing gradient gel electrophoresis (DGGE) analysis revealed distinct bacterial community compositions. In all microcosms, Gram-negative γ- or β-Proteobacteria and Gram-positive Negativicutes or Clostridia were observed. According to non-metric multidimensional scaling analysis of DGGE profiles, the microcosm communities were most similar to field site sediment corresponding to the highest TNT concentration, relative to moderately and uncontaminated sediments, suggesting that TNT contamination itself is a major driver of microbial community structure. Candidate degraders were identified and a Pseudomonas sp. was observed to be stimulated under all conditions, which was confirmed to rapidly degrade TNT in pure culture.
Mathematical modeling of the batch microcosm results revealed that TNT degraded 1.7 times faster in lactate amended microcosms than in ethanol amended microcosms, which degraded 3.0 times faster than natural organic matter amended microcosms. Simulation of the TNT degradation pathway included determination of branching coefficients representing whether the first reduction of nitro group occurred in the ortho or para position or whether TNT was removed from the aqueous phase (i.e. bound to dissolved organic matter). Branching coefficients were greater for initial reduction of para (17-27% initial TNT concentration) over ortho (3-9% initial TNT concentration) for all test conditions. However, a greater degradate recovery and a different (lower para/ortho) ratio was observed for ethanol compared to lactate and un-amended conditions. Given the difference in sorption parameters between degradate isomers, these results suggest that differences in pathway branching stimulated by different electron donors are potentially relevant to long term site models. This work provides parameter values and model simulations of desorption relevant to other TNT contaminated sites, qualitative observations of how TNT-reducing bacterial community structure changes in response to electron donor addition, and quantitative comparison of the effect of electron donor addition on biodegradation rate with cultures relevant to field conditions; in addition, this work serves as a feasibility study demonstrating biodegradation as well as biostimulation potential at FNOD.
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