Roles of graphite in the reduction of azo -aromatic compounds with elemental iron

Elemental iron, typically in the form of cast iron filings, has been used as a reactive material in permeable reactive barriers (PRB) for groundwater remediation for more than a decade. Elemental iron has also been shown to be potentially useful for water and wastewater treatment. Graphite is a major non-iron component in cast iron. Recent studies by Oh et al. and Jafarpour et al. suggested that graphite inclusions in cast iron might serve as adsorption sites as well as reaction sites for nitrogenous compounds, including nitro-aromatic compounds, heterocyclic nitramines, and nitrate esters. Oh et al. also suggested that electron and atomic hydrogen might be involved in graphite-mediated reduction of these compounds in cast iron-water systems.;In this study, we investigated the potential role of graphite in the reduction of azo-aromatic compounds with elemental iron, with particular focus on elucidating the reductant(s) involved in graphite-mediated reactions with elemental iron. We hypothesized that azo-aromatic compounds could be reduced on graphite surface, similar to the other nitrogenous compounds. We also hypothesized that graphite could conduct both electron and atomic hydrogen from corroding iron to nitrogenous compounds and thus facilitate their reduction reactions.;The corrosion behavior of iron electrode in the presence of azobenzene in 0.1 M Na2SO4 electrolyte solution was investigated using a three-electrode reactor through measurement of open-circuit potentials and corrosion currents. The presence of azobenzene increased both the open-circuit potential and corrosion current of the iron electrode, indicating that azobenzene was acting as an oxidizer of iron.;Graphite-mediated reduction of azo-aromatic compounds with elemental iron was investigated using dialysis cells, where azo compounds and elemental iron were placed in two compartments physically separated by graphite foil. Both the non-polar azobenzene and the water-soluble orange G were reduced to aniline, indicating significant hydrophobic sorption was not necessary for graphite-mediated reactions. In sharp contrast to azobenzene, which adsorbed to graphite foil rapidly, reduction of orange G followed zero-order kinetics and commenced only after a long initial lag (on the order of an hour).;Results of this study suggest that exposed graphite in cast iron can conduct both electron and atomic hydrogen and may serve as reaction sites for the reduction of nitrogenous compounds. This implies that graphite inclusion in cast iron may not slow down the reduction rate even for hydrophobic nitrogenous compounds due to its dual roles as both reaction and adsorption sites. Therefore, carbon content may be a factor to consider in selecting cast iron for treatment purposes. In addition, the dialysis cell system in this study may be useful for assessing reactivity of other aqueous oxidants towards graphite-bound atomic hydrogen. This provides an alternative to the traditional methods of atomic hydrogen generation on catalysts for the hydrogenation of organic compounds. (Abstract shortened by UMI.).