Colloid Generation, Stability, and Transport in Redox-Dynamic Mountain Watersheds and Impact on Water Quality in Alluvial Sediments
Authors
Eleanor Spielman-Sun1, Maya Engel1, Brandy Stewart1*, Tristan Babey2, Sam Pierce2, Lizzie Paulus1, Sharon Bone1, Kristin Boye1, Vincent Noël1* (noel@slac.stanford.edu), Eoin Brodie3 (elbrodie@lbl.gov)
Institutions
1SLAC National Accelerator Laboratory, Menlo Park, CA; 2School of Earth, Energy, and Environmental Sciences, Stanford University, Stanford, CA; 3Lawrence Berkeley National Laboratory, Berkeley, CA
URLs
Abstract
Mountainous watersheds are experiencing more frequent interacting press and pulse disturbances that create new hydrological regimes with consequences for biogeochemical transformations at critical interfaces. For example, episodic wet-dry cycling at solid–water interfaces promote shifts in aqueous phase parameters (pH, oxygen, ionic strength, and ionic composition) and chemical, organic and mineral transformation of the solid phase, generating colloids (1 nm to 1 µm), and/or influencing colloidal stability. Being 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 and the products exported to ground-/surface waters. Despite evidence that redox cycles play a significant role in generation and transport of colloids, the mechanisms, chemical composition, reactivity, and stability of generated colloids are poorly understood.
To resolve this knowledge gap, researchers developed an approach to detect physico-chemical composition of different particles using Asymmetric Field Flow Fractionation combined with ICP-MS, UV-, fluorescence-, MALS- and zetasizer-DLS detectors. Separation of colloids and collection in redox- preserved conditions then enables deeper molecular-scale characterization (e.g., TEM, STXM, XAS, NanoSIMS). Previously, researchers have examined the impact of redox changes on the generation and transport of colloids through a transect from bedrock to floodplains through a series of laboratory and field experiments. This work revealed that oxidative dissolution of pyrite from bedrock at neutral pH generates 50 to 100 nm Fe-colloids, promoting the mobilization of nutrients and contaminants (e.g., Ni and Cr). In floodplains, these results show that low sulfidation increases colloid stability of ferrihydrite, whereas higher sulfidation (S/Fe<0.5) generates nano-scale FeS colloids. In natural samples, ferrihydrite colloids remained stable under sulfidic conditions, which could be due to the passivation by organics as the team confirmed through lab-simulations. Finally, incorporating colloidal transport highlighted in the column experiments significantly improved accuracy of model predicting floodplain biogeochemical processes.