Impact of Dissolved Organic Matter Chemistry on the Fate of Iron(II) in Oxic Environments.

The nature and environmental implications of the interactions between Fe and natural organic matter (NOM) remain important questions. At circumneutral pH and oxic conditions, iron exists primarily in the hydrolyzed ferric state, which tends to polymerize and precipitate, making it largely unavailable for cellular uptake. Interactions with dissolved organic matter (DOM) have been shown to increase the solubility of Fe, but the exact molecular nature of these interactions and how they impact the redox state of iron are still areas of investigation. Several studies have noted the presence of NOM-bound Fe(II) under conditions favorable for iron oxidation, yet the molecular mechanisms by which NOM inhibits or enhances Fe(II) oxidation are not clearly understood.

We studied the interactions of Fe(II) with various types of NOM, including Suwannee River NOM, humic acid and fulvic acid, Leonardite humic acid and DOM extracts from Rifle, CO, at pH 5–7. Oxidation of Fe(II) in anaerobic solutions of NOM indicated the presence of oxidizing functional groups. Consequently, we reduced the NOM solutions with H₂(g) in the presence of a Pt catalyst. X-ray absorption spectroscopy (XAS) was used to investigate the molecular structure of Fe complexes formed upon reaction of Fe(II) with reduced and unreduced NOM. X-ray absorption near edge spectroscopy (XANES) and electron paramagnetic resonance (EPR) spectroscopy analyses indicated that under anaerobic conditions the reduced NOM formed stable complexes with iron(II).

Analyses of extended X-ray absorption fine structure (EXAFS) data for the reference NOM types demonstrated that Fe(II) formed mononuclear complexes with NOM functional groups containing oxygen. The EXAFS data for Fe(II) complexes with NOM could be well reproduced by a linear combination of data from Fe(II) reference complexes plus a small amount of Fe(III) complexed by the same NOM material.

Preliminary tests showed that complexed Fe(II) exhibited faster initial kinetics of oxidation by O₂ but a fraction of iron(II) was preserved over substantially longer timescales than for organic-free solutions. We hypothesize that both Fe(II) complexation and NOM redox state are key factors controlling the fate of iron in NOM solutions subjected to varying redox conditions.