Hydroxamate siderophore-promoted reactions between iron(II) and nitroaromatic groundwater contaminants

Abstract

Recent studies show that ferrous iron (Fe II), which is often abundant in anaerobic soil and groundwater, is capable of abiotically reducing many subsurface contaminants. However, studies also demonstrate that Fe II redox reactivity in geochemical systems is heavily dependent upon metal speciation. This contribution examines the influence of hydroxamate ligands, including the trihydroxamate siderophore desferrioxamine B (DFOB), on Fe II reactions with nitroaromatic groundwater contaminants (NACs). Experimental results demonstrate that ring-substituted NACs are reduced to the corresponding aniline products in aqueous solutions containing Fe II complexes with DFOB and two monohydroxamate ligands (acetohydroxamic acid and salicylhydroxamic acid). Reaction rates are heavily dependent upon solution conditions and the identities of both the Fe II–complexing hydroxamate ligand and the target NAC. Trends in the observed pseudo-first-order rate constants for reduction of 4-chloronitrobenzene ( k obs, s −1) are quantitatively linked to the formation of Fe II species with standard one-electron reduction potentials, E H 0 (Fe III/Fe II), below −0.3 V. Linear free energy relationships correlate reaction rates with the E H 0 (Fe III/Fe II) values of different electron-donating Fe II complexes and with the apparent one-electron reduction potentials of different electron-accepting NACs, E H 1 ′ (ArNO 2). Experiments describing a redox auto-decomposition mechanism for Fe II–DFOB complexes that occurs at neutral pH and has implications for the stability of hydroxamate siderophores in anaerobic environments are also presented. Results from this study indicate that hydroxamates and other Fe III-stabilizing organic ligands can form highly redox-active Fe II complexes that may contribute to the natural attenuation and remediation of subsurface contaminants.