Formation and mobility of FeS colloids in natural waters

Export of highly reactive FeS-compounds from reducing to more oxidizing environments has down-stream consequences for electron transfer and biogeochemical reactivity. Our field investigations indicate that Fe-bearing colloids are generated under sulfidic conditions in montane riparian floodplains and are exported to adjacent stream water. Detailed knowledge of formation conditions, composition/structure, and stability of these colloids is critical for developing conceptual models to predict Fe-remobilization. However, our current understanding of the generation and biogeochemical behavior of Fe-S-colloids is insufficient to develop such models. In this study, we investigated the FeS colloid generation during sulfidation of Fe(III)-hydroxide nanoparticles and physico-chemical parameters controlling colloidal stability, aggregation and mobility. We observed that reductive dissolution of ferrihydrite by aqueous sulfide generates nano-scale FeS clusters. We found that their subsequent aggregation, promoting settling of FeS aggregates into the solid fraction, is directly correlated with sulfide/Fe ratio. At sulfide/Fe ratios 0.5, FeS (nano)clusters and larger colloids remained in suspension for 14 days up to few months. These suspended FeS clusters/colloids have high specific surface area and surface charge, and can sorb and transport contaminant/ions in groundwater. At sulfide/Fe ratios >0.5, sulfidation reaction rates were rapid and FeS cluster aggregation was accelerated. Moreover, the presence of organic compounds increased the duration of FeS colloid suspension. Finally, we found that in low-sulfate groundwater systems (lakes, floodplains, peatlands), sulfidation of ferrihydrite generates FeS colloids that remain suspended over long time periods. More recently, we showed that the presence of oxidant impurities (crystalline Fe(III) or dissolved oxygen) promote destabilization of aggregates and could favor remobilization of FeS phases. These findings strongly support the hypothesis that FeS could form stable nano-colloids in natural groundwater and moreover, that they could be important for mobilizing Fe and sorbed (micro-)nutrient exports from floodplains.