The Science

Colloids can transport nutrients, contaminants, and organic matter throughout watersheds. Their persistence, reactivity, and heterogeneous compositions render them key contributors to biogeochemical reactions. A multi-institutional team of researchers detected iron (Fe)-rich colloids in anoxic groundwater of a redox-active floodplain of the Slate River, CO. The colloids were characterized by a wide array of advanced techniques and found to be mixed-phase assemblages composed of silicon (Si)-coated and organic matter–enmeshed ferrihydrite nanoparticles. Both Fe(II) and Fe(III) co-existing in the colloids under anoxic conditions illustrates the passivating effects of the Si and organic matter matrix against redox-triggered transformations.

The Impact

The ability of ferrihydrite-based colloids to withstand anoxic conditions that are also rich in dissolved Fe(II) highlights the extent to which organic matter-Si coatings can protect Fe(III) from reductive dissolution. This passivating feature may also explain the existence of Fe(II) and sulfur within the colloidal structure. Ultimately, the persistence of the colloids suggests they may transport throughout anoxic zones and reach oxic surface waters. These findings shed light on the composition and dynamics of natural Fe-rich colloids in floodplain systems, with implications for elemental transport and cycling.

Summary

Geochemical interfaces are ubiquitous in floodplain environments and sustain a multitude of biogeochemical processes, including the formation and release of reactive, mobile colloids. Colloids are known vectors of micronutrient, contaminant, and organic matter transport and are suspected to be important export agents from floodplains to stream water. However, few studies have characterized naturally occurring Fe-rich colloids at the molecular scale.

Now, a multi-institutional team of researchers combined advanced characterization techniques to decipher the composition of Fe-rich colloids at a floodplain field site of the Slate River, CO. Cascade filtering revealed the presence of Fe-rich colloids in the riparian anoxic soil water and their abundance and composition varied with season. Cryo-electron microscopy and transmission electron microscopy (TEM)–energy dispersive X-ray spectroscopy imaging showed mono-dispersed and nano-assemblages of spherical colloids in the 10–50 nm range containing Fe, oxygen, Si, carbon, and calcium. TEM selected-area electron diffraction analysis and Mössbauer spectroscopy indicated a poorly crystalline ferrihydrite-like phase. Fe-extended X-ray absorption fine structure spectroscopy further verified ferrihydrite and Fe(II)- and Fe(III)-organic matter interactions, as well as a small contribution from Fe-sulfur bonding. Results indicate that the colloids are primarily composed of nanosized ferrihydrite spheres that are stabilized by organic matter, Si, and bridging cations (e.g., calcium). These Fe(III)-rich colloids existed in primarily anoxic zones, which is striking. The Si-organic matter coating is postulated to serve as a passivating layer against reduction, but its efficiency likely depends on the biogeochemical and hydrological conditions.

Principal Investigator

Kristin Boye
SLAC National Accelerator Laboratory
kboye@slac.stanford.edu

Program Manager

Paul Bayer
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
paul.bayer@science.doe.gov

Funding

Funding was provided by the U.S. Department of Energy’s (DOE) Biological and Environmental Research (BER) program, through its support of the SLAC Floodplain Hydro-Biogeochemistry Science Focus Area (SFA) under Contract No. DE- AC02–76SF00515. Stanford Synchrotron Radiation Lightsource (SSRL) and SLAC are supported by the DOE Office of Science Basic Energy Sciences program. The technical staff at SSRL supported X-ray absorption measurements. Support was also provided by the Stanford-SLAC Cryo-EM Facilities, which is supported by the National Institutes of Health Common Fund Transformative High-Resolution Cryo-Electron Microscopy program (U24 GM129541). Funding was also received from the Stanford Interdisciplinary Graduate Fellowship program and a project award (10.46936/lser.proj.2021.51929/60000375) from the Environmental Molecular Sciences Laboratory, a DOE Office of Science User Facility sponsored by BER under Contract No. DE-AC05–76RL01830.

References