Interaction with phosphate alters the environmental behavior of iron minerals: double eutrophication trouble?

Iron (oxyhydr)oxides (FeOx) such as ferrihydrite (Fh) and lepidocrocite (Lp) control the mobility of trace elements, contaminants and nutrients such as phosphorus (P) in aquatic systems. Conversely, the sorption of P can alter the structure and reactivity of FeOx. As such, elevated P concentrations in eutrophic, coastal aquatic systems may have far-reaching but currently poorly understood consequences for coupled Fe-nutrient cycling. Here, we present laboratory and field experiments to elucidate the effects of P incorporation on (i) FeOx structure and reactivity and (2) environmental FeOx transformations (crystallization, sulfidation). The structure of the FeOx, synthesized in the absence or presence of P (‘pure’ or ‘P-bearing’ respectively), was probed with synchrotron-based methods (X-ray absorption spectroscopy, high-energy X-ray scattering). Laboratory-based acidic and reductive dissolution experiments (abiotic and microbial) with pure and P-bearing FeOx were combined with novel in-situ field experimentation. The field experiments, which were conducted in freshwater and marine aquatic systems, involved gel-based diffusive samplers loaded with pure and P-bearing FeOx (Fh and Lp) to obtain detailed insight into FeOx chemistry and structure without interference from the sediment matrix. Results from FeOx synthesis experiments showed differences in the impact of P incorporation between FeOx. Ferrihydrite underwent only minor structural changes because of P sorption, yet these changes significantly destabilized the mineral, as evidenced by enhanced rates of reduction and dissolution. Incorporation of P during Lp formation resulted in FeOx precipitate that was significantly less structured than pure Lp. Field experiments in Fe(II)-rich freshwater sediment conducted with Fh showed relatively slow crystallization rates for Fh compared to published laboratory studies. This likely was the result of FeOx surface passivation by adsorption of pore-water P. In H₂S-rich sediment, the degree of sulfidation was higher for P-bearing Fh compared to pure Fh, while the opposite was observed for pure and P-bearing Lp. These findings may be related to differences in electron transfer characteristics and surface reactions with sulfide between Fh and Lp. The novel field experiments provide detailed insight into natural FeOx dynamics in relation to environmental conditions. Decreased stability of FeOx formed in the presence of high nutrient concentrations, leading to less efficient retention of these nutrients, may represent an important feedback mechanism in eutrophication.