The effect of organic matter (OM) quality on the redox stability of OM-Fe association in freshwater sediments

Abstract. Redox sensitive iron (Fe) oxyhydroxides participate in the biogeochemical cycling of nutrients and trace metals. Their co-precipitation with organic matter (OM) leads to environmentally relevant OM-Fe associations. The stability of OM in such associations is still uncertain. It has been proposed that OM either increases the stability of the complex against reductive dissolution or enhance the dissolution (both abiotic and biotically) of Fe oxyhydroxides. The OM character, in terms of specific functional groups binding to Fe, could be a critical factor determining the fate and stability of OM-Fe. Here, sediment samples from a vertical profile of a mesotrophic freshwater reservoir were treated using a sequential extraction scheme designed to distinguish Fe oxyhydroxides of different redox reactivity based on dissolution kinetics. To assess the reactivity and stability of the complexes, special attention was payed to the determination of redox active vs. stable fractions of Fe and the corresponding dissolved organic matter (DOM) components sequentially extracted. The quality of the extracted DOM was evaluated using UV-VIS and fluorescence spectroscopy together with the PARAFAC model. A selectivity model was used to ascertain the quality of OM preferentially associated with the different redox stable Fe oxyhydroxides. Accordingly, we found that humic-like substances render the OM-Fe associations redox labile, while non-humic substances enhance the stability of the associations. These findings improve the understanding required for predicting the fate of OM-Fe associations in freshwater sediments with different organic matter sources and characters.