Colloids formed from Iron (Fe) and Sulfur (S) can transport nutrients and metals through floodplain groundwater into aquifers

The Science

Nanometer- to micrometer- sized mineral particles that are often associated with organic carbon and referred to as colloids remain suspended in water and can play a major role in mediating the mobility of nutrients, metals, and radionuclides in groundwater. Yet, the factors controlling colloid occurrence and stability are poorly understood. Sulfidic colloids in the nanometer- size range seem to be naturally generated by reaction of common soil iron [Fe(III)] oxyhydroxides with dissolved bisulfide in groundwater. A multi-institutional team of scientists confirmed that this reaction can form stable iron monosulfide nanoclusters that likely are precursors to the colloids. A conceptual model for predicting the conditions under which the colloids would form by sulfidation of the ferrihydrite was devised based on laboratory experiments. Colloid formation and stability were observed to be controlled by the rate of sulfidation, ionic strength of the groundwater, and abundance of organic compounds.

The Impact

In low-salt groundwater systems, common in many floodplains, sulfidation of ferrihydrite will generate iron(II) sulfide (FeS) nanoclusters that will remain suspended and can be transported by groundwater. Relative to the dissolved groundwater solute load, these colloids contain large amounts of Fe and can pick up metal micronutrients such as manganese and contaminants such as zinc, transporting them to surface waters or to reactive zones in the aquifer where they may be utilized by microorganisms or accumulate as contaminant loads. These findings highlight the potential for sulfidic conditions to mobilize trace metals and promote their biogeochemical cycling and transport. This conclusion is contrary to the conventional view that sulfidic conditions generally stabilize metals through precipitation reactions. Moreover, these findings highlight the need to revise biogeochemical reactive transport models to account for colloid mobility and reactivity.

Summary

Synchrotron-based Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy, transmission electron microscopy, Fourier-transform ion-cyclotron-resonance mass spectrometry, and aqueous measurements were used to determine the stability and molecular structure of nanoclusters generated by sulfidation of ferrihydrite, and to identity the composition of natural organic carbon compounds associated with them. Led by scientists at SLAC National Accelerator Laboratory, a multi-institutional team of scientists confirmed that sulfidation of ferrihydrite generates stable nanometer-scale aqueous FeS clusters. Their tendency to condense into nanoparticles, aggregate, and settle, was directly related to the sulfide/Fe ratio. At sulfide/Fe ratios ≤ 0.5, FeS nanoclusters and larger nanoparticles remained in suspension for up to several months. At sulfide/Fe ratios > 0.5, sulfidation reaction rates were rapid and FeS nanocluster aggregation was accelerated. The presence of organic compounds increased the time of suspension of FeS nanoclusters, whereas increased ionic strength inhibited the generation of FeS nanoclusters.

FeS nanoclusters are responsible for electron transfer in many biogeochemical pathways. Thus, suspended FeS nanoclusters could function as electron shuttles, influencing geochemical processes and heterotrophic microbial activity in aquifers. Further, FeS nanocluster transport may play important roles in controlling the compositions of groundwater and stream water.

Principal Investigator(s)

John Bargar
SLAC National Accelerator Laboratory
[email protected]

Funding

Funding was provided by the DOE Office of Biological and Environmental Research (BER), within the U.S. Department of Energy (DOE) Office of Science, Subsurface Biogeochemistry Research (SBR) activity to the SLAC Scientific Focus Area (SFA) program under contract DE-AC02-76SF00515 to SLAC. Research was performed at the Stanford Synchrotron Radiation Lightsource (SSRL), a national user facility supported by the U.S. DOE, Office of Basic Energy Sciences. A portion of the research was performed using the Environmental Molecular Sciences Laboratory (EMSL; (grid.436923.9), an Office of Science User Facility sponsored by the U.S. DOE, Office of Biological and Environmental Research BER.

References

Noël, V., Kumar, N., Boye, K. et al. “FeS Colloids – Formation and Mobilization Pathways in Natural Waters.” Environmental Science Nano., 7, 2102–2116 (2020). DOI:10.10389/C9EN01427F