In the past decade, several reports of organic matter-associated Fe(II) found in oxic conditions have suggested that dissolved organic matter (DOM) can preserve Fe(II). We used X-ray absorption spectroscopy (XAS) to determine the effect of DOM complexation on Fe(II) redox stability. Our results indicated that Fe(II) added to anoxic solutions of DOM oxidized to Fe(III) and complexed in the same way as Fe(III) added to DOM. However, Fe(II) was observed to bind primarily to citrate-like and catechol-like functional groups in chemically reduced DOM.  Although studies of pure organic compounds have shown that complexation with citrate enhances Fe(II) oxidation, our experiments revealed that association with reduced DOM significantly slowed iron oxidation. These results suggest that reduced DOM may preserve Fe(II) by functioning both as redox buffer and complexant, which may help explain the presence of Fe(II) in oxic circumneutral waters.<br/>            In another study, we explored the impact of temperature on adsorption of DOM to iron oxides. Using a dynamic flow system with a fixed bed column of ferrihydrite-coated sand, we found that humic acids were in general retained less than fulvic acids. Response to temperature varied from no effect to a marked increase in the quantity adsorbed depending on the DOM type. Modeling of DOM breakthrough curves suggests that the DOM adsorbed to the ferrihydrite acts as a sorbent that is much more influential than the oxides alone.<br/>            Analyses to determine adsorptive fractionation are ongoing, and we are developing our capabilities for analyzing Fourier transform ion cyclotron resonance mass spectra (FTICRMS) data in similar systems. We recently used this technique to evaluate the chemical fractionation of DOM on iron-enriched clays. From these results, we determined that sorption was concentration and OM-composition dependent. Our future work will apply the data analysis techniques we have developed to iron oxide – organic matter systems.

Mechanistic characterization of iron-organic matter interactions

255th National Meeting (2018)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.