REDOX MECHANISMS CONTROLLING COLLOID FORMATION AND BEHAVIOR: IMPACT ON WATER QUALITY IN ALLUVIAL SEDIMENTS

Seasonal wet-dry redox cycling at solid–water interfaces promote shifts in aqueous phase parameters (pH, ionic strength, and ionic composition) and chemical, organic and mineral transformation of the solid phase, which could generate colloids (1nm–1µm), 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 the products exported to the 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, we developed an approach consisting of detecting different particle distribution in physico-chemical composition using Asymmetric Field Flow Fractionation combined with ICP-MS, UV-, fluorescence-, MALS- and zetasizer-DLS detectors. The different particle distributions are then separated and collected in redox-preserved conditions for deeper molecular-scale characterization (TEM, STXM, XAS, NanoSIMS, ...).

We have thus examined the impact 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 50-100nm Fe-colloids, promoting the mobilization of nutrients and contaminants (e.g., Ni and Cr) through pore and fracture networks. Further, we have examined the influence of reducing conditions in floodplains, combining results from lab-simulation sulfidation of ferrihydrite aggregates and natural colloidal fraction from a redox active floodplain at Slate River (Crested Butte, CO, USA). While low sulfidation increases colloidal stability of ferrihydrite, higher sulfidation (S/Fe<0.5) generates nano-scale FeS colloids. However, in the natural samples, ferrihydrite nanoparticles persisted under sulfidic conditions, which could be due to the passivation by OM as we confirmed through lab-simulation. Finally, incorporating colloidal transport highlighted through our study significantly improved model accuracy.