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ABIOTIC REDOX TRANSFORMATION OF ORGANIC COMPOUNDS AT THE CLAY-WATER INTERFACE
Citation:
Yan, L. AND G W. Bailey. ABIOTIC REDOX TRANSFORMATION OF ORGANIC COMPOUNDS AT THE CLAY-WATER INTERFACE. Presented at Bouyoucos Conference on Environmental Chemistry at the Clay-Water Interface, Honolulu, HI, March 6-9, 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:
The interactions of clay, water and organic compounds considerably modify the structural and physico-chemical properties of all components and create a unique domain for biological and chemical species in environments. Previous research indicates that the nature and properties of this interfacial region control both abiotic and biotic reactions. Under anoxic conditiorLT nitroaromatics and aromatic azos are reduced primarily by abiotic processes. These compounds, therefore, should serve as suitable probes to follow interfacial redox reactions and help define the reaction mechanism.
Structural or exchangeable transition metals cations in clays act as a carrier in either forward or reverse electron transfer in both abiotically and microbially mediated redox reactions. The intervalence Fe2-+ Fe 3+ charge transfer (Fe2+-to-02- -to-Fe 3+electron transfer) process is proposed as the mechanism for the solid state electron transfer to the basal surfaces and/or edges of Febearing phyllosilicates. To evaluate this hypothesib chemically reduced nontronite was reacted with nitrobenzene and the reduction product(s) were followed using HPLC. Nitrosobenzene and hydroxyarnine are reaction intermediates; aniline is the main reduction product and was found to be the major product in our study.
Two pathways are proposed forthe interfacial electron transfer: (1) interfacial coordination complex formation and (2) hydroxyl radical (-OH) formation. Nitrobenzene forms a charge transfer complex via sorption with smectite accepting electrons from the clay lattice; the excess electrons in the crystal layer structural Fe of reduced phyllosilicate migrate to the surface/edges and are transported to organic compounds in the interfacial region. Further research is needed to define the efficiency and rate of electron transfer at the clay-water interface before predictive relationship are possible.