SLAC Floodplain Hydro-Biogeochemistry Science Focus Area: Mechanisms Controlling Colloid Formation and Impact on Water Quality in Alluvial Sediments
Vincent Noël1* (email@example.com), Eleanor Spielman-Sun1, Maya Engel1, Brandy Stewart1, Tristan Babey2, and Kristin Boye1
1SLAC National Accelerator Laboratory, Menlo Park, CA; 2School of Earth, Energy, and Environmental Sciences, Stanford University, Stanford, CA
Recurring seasonal wetting and drying of soils and sediments drives redox processes at solid–water interfaces, promoting shifts in aqueous phase parameters (e.g., pH, ionic strength, and ionic composition) and chemical, organic, and mineral transformation of the solid phase, which could generate colloids (1 nm to 1 µm), and/or influence colloidal stability. Because they are typically associated with organic matter, micronutrients, and contaminants, colloids may serve as transport vectors throughout redox-affected terrestrial and aquatic systems, impacting biogeochemical reactivity downstream as well as the products exported to the ground- and surface waters. As an example, aqueous metal sulfide clusters, generated in sulfidic conditions, are remarkably resistant to oxidation and have been found to contribute to contaminant transport in rivers. Despite evidence that redox cycles play a significant role in generation and transport of colloids, the mechanisms and the nature and reactivity of generated colloids are poorly understood.
To resolve this knowledge gap, the research team is developing an approach consisting of detecting different particle distribution in size, chemical composition, molecular weight, and zeta potential using asymmetric field flow fractionation combined with inductively coupled plasma mass spectrometry, UV-, fluorescence-, multi-angle light scattering-, and zetasizer-dynamic light scattering detectors. The different particle distributions are thus separated and collected in redox-preserved conditions for deeper molecular-scale characterization (e.g., single particle inductively coupled plasma time of flight mass spectrometry; transmission electron microscopy; scanning transmission X-ray microscopy; X-ray absorption spectroscopy; nanoscale secondary ion mass spectrometry; Mössbauer). The research team has examined the influence of redox changes on the generation and transport of colloids through a transect from bedrock to floodplains. To date, bedrock shale oxidation lab-simulation investigations have revealed that oxidative dissolution of pyrite at neutral pH generates 50–100 nm iron (Fe) colloids, promoting the mobilization of nutrients and contaminants (e.g., nickel and chromium) through pore and fracture networks.
The team further examined the influence of sulfidation on formation of Fe colloids in floodplains, combining results from lab-simulation sulfidation of ferrihydrite aggregates and natural colloidal fraction from a redox active floodplain at Slate River, CO. While low sulfidation increased colloidal stability of ferrihydrite, higher sulfidation (sulfur/Fe < 0.5) generated nanoscale Fe(II) sulfide colloids, stabilized by oxidative impurities. However, in natural samples, ferrihydrite nanoparticles persisted under sulfidic conditions, which were attributed to the passivation by silicon and organic matter. These results contribute knowledge needed to predict redox-generated colloid-facilitated nutrient and contaminant transport in response to disturbances, such as weather events and management strategies.