Complexes with aquatic organic matter suppress hydrolysis and precipitation of Fe(III)

The fate of iron (Fe) in freshwaters is largely influenced by chemical interactions with natural organic matter (NOM). However, the knowledge about the molecular structure of the Fe species formed in association with aquatic NOM is still limited. In this work we have studied the interactions between Fe(III) and aquatic organic matter from the Suwannee River at a range of pH values and Fe concentrations (6489–55,659 μg g− 1 dry weight, pH 3.0–6.8) by means of extended X-ray absorption fine structure (EXAFS) and infrared (IR) spectroscopy. Our results showed that Fe(III) formed stable complexes with aquatic NOM, and that carboxylates were the predominant functional group involved in these interactions. At low Fe concentrations (6489–11,953 μg g− 1; pH 3.0–6.7) mononuclear Fe(III)–NOM complexes dominated the speciation while at higher pH values and Fe concentrations (~ 23,000 μg g− 1 pH 5.0–6.2 and 55,659 μg g− 1 pH 3.1–6.7) we detected mixtures of mononuclear Fe(III)–NOM complexes and a polymeric Fe(III) (hydr)oxide phase. The structure of the latter was indicated to be ferrihydrite-like. It was evident that the strong complexation between Fe and aquatic NOM suppressed the hydrolysis and polymerization of Fe(III), and it is likely that presence of NOM favored the formation of presumably small ferrihydrite particles with an inherent large reactivity due to high surface area per unit mass. Furthermore, our IR data showed that the formation of hydrolyzed Fe(III) was accompanied with structural changes of the NOM material, which implied that these interactions potentially also could affect the reactivity of NOM. Thus, our aggregative results highlight the importance of Fe(III)–NOM interactions for the geochemistry of Fe and NOM as well as for other elements (e.g. P and As), that is known to be strongly associated with Fe(III), in freshwater systems.