The Science

Nanometer to micrometer sized mineral particles (often associated with organic carbon and often referred to as colloids) that remain suspended in water can play major roles in mediating the mobility of nutrients, metals and radionuclides in groundwater. Yet, the factors controlling their occurrence and stability are poorly understood. The reaction of common soil Fe(III) oxyhydroxides with dissolved HS- has been proposed as a pathway by which sulfidic nanoparticles can be naturally generated in groundwater. This study confirms that this process can form stable iron monosulfide nanoclusters. (Clusters are defined here as precursors of nanoparticles) The rate of sulfidation, ionic strength of the groundwater, and abundance of organic compounds, were found to control the stability of FeS nanocluster suspensions generated from ferrihydrite sulfidation. This research provides a conceptual model for predicting the conditions under which sulfidation of ferrihydrite will generate FeS nanoclusters.

The Impact

In low-salinity, low-sulfate groundwater systems, common in many floodplains, sulfidation of ferrihydrite will generate FeS nanoclusters that will remain suspended and can be transported by groundwater. These materials can sorb metal micronutrients (e.g., Mn) and contaminants (e.g., Zn), allowing them to be mobilized to surface waters or to reactive zones in the aquifer where they may be utilized by microorganisms or accumulate as contaminant loads. These observations highlight the potential for sulfidic conditions to mobilize trace metals and promote their biogeochemical cycling. These conclusions place a large asterisk on the conventional view that sulfidic conditions generally stabilize metals through precipitation reactions.

Summary

Synchrotron-based 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. This research shows that sulfidation of ferrihydrite generates nm-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. Moreover, FeS nanoclusters can directly bind nutrients and contaminants via sorption reactions and contribute to their transport in (sub)surface waters. This statement is corroborated by numerous previous studies proposing that contaminant mobility in groundwater can be directly associated with FeS mobility in the aqueous fraction.

Principal Investigator

John Bargar
SLAC National Accelerator Laboratory; Stanford Synchrotron Radiation Lightsource
Bargar@slac.stanford.edu

Program Manager

Jennifer Arrigo
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
jennifer.arrigo@science.doe.gov

Funding

Funding was provided by the DOE Office of Biological and Environmental Research’s (BER) Environmental System Science (ESS) program (formerly, Subsurface Biogeochemistry Research) to the SLAC SFA program under contract DE-AC02-76SF00515. Research was performed at Stanford Synchrotron Radiation Lightsource (SSRL), a national user facility supported by the U.S. DOE’s Office of Basic Energy Sciences (BES). A portion of the research was performed using the Environmental Molecular Sciences Laboratory (grid.436923.9), a science user facility sponsored by DOE BER.

References