Influence of iron speciation on redox cycling and reactivity with persistent organic contaminants

Although a number of past studies have been aimed at characterizing iron's redox properties in aqueous systems and its contribution to natural attenuation processes of groundwater contaminants, many questions remain. It is especially important to understand the molecular properties that control the reactivity of both FeII and FeIII with oxidizing and reducing agents, respectively. Unfortunately, most studies to date have focused on iron reactions in heterogeneous systems where molecular-level understanding of the reacting Fe species is limited.;In this study, FeII/FeIII complexes with low molecular weight organic ligands were used as surrogate models for studying iron-mediated redox processes. Initially, the redox reactivity of Fe II-organic ligand complexes with nitro-organic explosives was investigated. Ligand-screening experiments demonstrate that organic ligands containing catechol, thiol, and hydroxamate functional groups form FeII-complexes capable of rapidly reducing both N-heterocyclic nitramine explosives and monosubstituted nitrobenzenes. Detailed kinetic investigations show that the reactivity of FeII-organic complexes is significantly dependent on solution conditions (e.g., pH and [FeII]/[organic ligand] ratios). Observed reaction rate constants for contaminant reduction measured in batch reactions (kobs, s-1 ) increase with ligand concentration and pH when FeII concentration is fixed. Correlation analysis reveals that a single Fe II species typically dominates overall FeII reactivity with target compounds (FeL26- FeHL 0, and FeL3- for tiron, DFOB, and acetohydroxamic acid, respectively; L=ligand). These species share a common characteristic in that they possess the lowest standard reduction potentials [EH0(FeIII/FeII)] among possible FeII complexes with each ligand. For nitroaromatic contaminants, linear free energy relationships (LFERs) are observed between species-specific second-order rate constants (ki; M-1 s-1) and reduction potentials of the FeIII/FeII redox couple, E H0(FeIII/FeII), and the nitroaromatic compound, EH1' (ArNO2). Kinetic studies indicate that some FeII-organic complexes lose their apparent reactivity with contaminants over time, For FeII-hydroxamate complexes, loss of FeII reactivity results from FeII oxidation coupled with reduction of hydroxamate Lewis base groups. The reduction and redox cycling of aqueous FeIII complexes with the model siderophore ligand DFOB (desferrioxamine B) in solutions containing a biogenic reducing agent, flavin mononucleotide (FMN), was also investigated. Results of kinetic studies show that FeIII-DFOB complexes are reduced to the corresponding FeII complexes by the fully reduced hydroquinone form of FMN (FMNHQ) over a wide pH range. Reaction rates are strongly dependent on pH and FMNHQ concentration. The observed rate constants for the forward FeIII reduction rate (kf,obs, min-1) increase with increasing FMNHQ concentration and decreasing pH, the latter trend being opposite to the trend for FeII-DFOB reacting with nitroaromatic contaminants. At higher pH conditions, incomplete Fe III reduction is also observed because two reverse processes re-oxidize FeII in the experimental system, autodecomposition of Fe II-DFOB complexes (FeII oxidation coupled with hydroxamate ligand reduction) and reaction of FeII-DFOB with the fully oxidized flavin mononucleotide product (FMNOX). Although no significant net reduction of FeIII-DFOB can be measured at pH 7, formation of ligand autodecomposition products is observed, indirectly indicating that FeIII-DFOB reduction is occurring followed by autodecomposition of some portion of the resulting FeII-DFOB complexes. Steady state [FeII]/[FeIII] ratios observed at different pH conditions are consistent with both kinetic and equilibrium models developed in this study. Quantitative comparison between kinetic trends and changing Fe speciation reveals that reduced and oxidized FMN species react predominantly with diprotonated FeIII- and FeII-DFOB complexes, respectively, where one of the hydroxamate groups is protonated and open coordination positions are available on the central Fe ions. This finding suggests that formation of ternary complex (FMN-Fe-DFOB) formation may be facilitating inner-sphere electron transfers between the flavin and metal center.