Noel and 8 co-authors reported in 2020 that they have used synchrotron-based X-ray absorption spectroscopy (structure of Fe–S clusters), transmission electron microscopy (solid-phase crystallinity), Fourier-transform ion-cyclotron-resonance mass spectrometry (identity and composition of natural organic carbon compounds), inductively coupled plasma optical emission spectrometry (total aqueous Fe), and the revised ferrozine method (aqueous Fe(II) and F(III) concentrations) to determine the stability and nature of colloids generated by sulfidation of ferrihydrite nanoparticles in the absence and presence of organic compounds.
The authors observed that reductive dissolution of ferrihydrite by aqueous sulfide generates nm-scale FeS clusters. Their subsequent aggregation, which promotes settling of FeS aggregates into the solid fraction, was directly correlated with sulfide/Fe ratio. At sulfide/Fe ratios ≤0.5, FeS clusters and larger colloids remained in suspension for at least 14 days (and up to several months). 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 time of suspension of FeS colloids, whereas increased ionic strength inhibited the generation of FeS colloids.
The authors presented a general conceptual model to predict when and where FeS colloids can form and enhance or inhibit the mobility of contaminants and nutrients associated with them. Our study indicates that in low-salinity fresh groundwater systems poor in sulfate (i.e. low sulfidation; lakes, floodplains, peatlands etc.), the ferrihydrite sulfidation reaction generates aqueous FeS clusters and larger colloids that remain suspended over long time periods, thus mobilizing a substantial fraction of the total aqueous Fe and S.
Environmental Significance
Colloids mediate the mobility of nutrients, metals and radionuclides in sediments that experience strong wet-dry cycling and thus impact groundwater quality. Sulfidation of Fe(III)-(oxyhydr)oxides nanoparticles has been proposed to generate sulfidic colloids. However, their nature and the parameters controlling their formation are not well understood. Such information would improve understanding of their impact on water quality.
The authors concluded that in fresh groundwater systems poor in sulfate (i.e. low sulfidation; lakes, floodplains, peatlands), sulfidation of ferrihydrite generates FeS colloids that remain suspended over long time periods, thus mobilizing a substantial fraction of the Fe and sulfide budgets. These results provide a conceptual model for predicting under what conditions FeS colloids form and enhance or inhibit the mobility of contaminants( metals) and nutrients associated with them.
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