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CHARACTERIZING THE ABIOTIC REDUCTANTS FOR NITROAROMATIC COMPOUNDS AS A FUNCTION OF REDOX ZONATION IN ANOXIC SEDIMENTS
Citation:
Hoferkamp, L. A. AND E J. Weber. CHARACTERIZING THE ABIOTIC REDUCTANTS FOR NITROAROMATIC COMPOUNDS AS A FUNCTION OF REDOX ZONATION IN ANOXIC SEDIMENTS. Presented at Goldschmidt 2000 Conference for Geochemistry, Oxford, UK, September 3-8, 2000.
Impact/Purpose:
Elucidate and model the underlying processes (physical, chemical, enzymatic, biological, and geochemical) that describe the species-specific transformation and transport of organic contaminants and nutrients in environmental and biological systems. Develop and integrate chemical behavior parameterization models (e.g., SPARC), chemical-process models, and ecosystem-characterization models into reactive-transport models.
Description:
Reductive transformation is the dominant reaction pathway for the degradation of nitroaromatic compounds in anaerobic environments (Larson and Weber, 1994). Proposed reductants cover a spectrum ranging from reduced rninerals and organic matter to microbial enzyme systems. Transformation studies carried out in an anaerobic aquifer have implicated specific reductants such as surface-bound Fe(II) and dissolved sulfide species as the primary reducing agents for a series of nitroaromatics (Rugge el al, 1998). The involvement of these various agents in electron transfer can be expected to vary with the redox zonation of a given sediment system. The development of redox zones is associated with sequential utilization of the available electron acceptors by the microbial consortium present in groundwater-infiltrated sediments. At the sediment-water interface dissolved oxygen is the most thermodynamically accessible terminal electron acceptor followed by nitrate, manganese(IV) and iron(III) bearing minerals, sulfate and finally carbon dioxide. The successive reduction of these sediment components results in a series of zones usually associated with depth and identified by variations in concentration of the relevant redox indicators (NO3-, Mn(II), Fe(II), S04 2- and CH4, respectively). With the objective of characterizing the role of chemical reductants as a function of redox zonation, studies were carried out using a representative nitroaromatic compound, p-cyanonitrobenzene (p-CNB) and a river sediment obtained from the Oconee River (OR) in Athens, GA, USA. The mineralogy of this sediment is dominated by quartz, feldspar, kaolinite and iron phases. Electron microprobe examination of representative samples indicated a fine layer of iron oxide on quartz grains. Earlier column studies with OR sediment demonstrated that the reduction kinetics of P-CNB varied as a function of redox zonation as assessed by solution phase concentrations of the redox-active species (Simon et al, 2000). Preliminary experiments in anoxic batch systems designed to monitor the concentration of redox active species were carried out with OR sediment and filtered OR water, with and without 10 MM acetate. These batch studies demonstrated the establishment of a typical series of redox zones over the course of several months. Significant levels of solution-phase FE(II) were detected at 28 days (672 h) in un-amended and 11 days (264 h) in acetate-amended batch systems. Batch studies were also carried out in which additions of P-CNB to un-amended and acetate- amended systems were made initially and changes in the concentrations of P-CNB and its transformation product, cyanoaniline (p-CNA) were monitored as redox zonation progressed. While reduction of the probe compound did occur prior to the establishment of detectable levels of solution-phase ferrous ion in both the amended and un-amended batch systems, a significant increase in the rate of reduction after ~11 days in the batch system containing acetate coincided with a prominent increase in Fe2+ (aq) concentration . These results are consistent with previous studies implicating surface-bound Fe(II) ion as a reductant for xenobiotic species in laboratory columns and model systems.