A Colloid is Born: Understanding iron-rich colloid genesis and transformation for improved watershed biogeochemical models
EMSL Project ID
51929
Abstract
Interfaces between subsurface layers of floodplains are ‘hot spots’ for thriving biogeochemical activity and exchange, owing to distinct compositional and permeability/porosity differences between the juxtaposed layers, as well as fluctuating hydrologic conditions. One interfacial boundary of particular importance is that between shallow fine-grained/diffusion-controlled soils and underlying coarse-grained/high-conductivity basal gravel sediments. Observations from our floodplain field site at Slate River (CO) imply that reduced Fe-rich colloids form near the interface within the overlying soils and are released into the more conductive gravel bed. We posit that oxidative transformations of these colloids will facilitate the transport of Fe, as well as any colloid-associated elements such as organic matter (OM), micronutrients, and contaminants, throughout the floodplain and into the river. Thus, colloid dynamics largely contribute to and are affected by the biogeochemical function of these floodplain interfaces. Because current reactive transport models lack colloid representation, there is an urgent need to obtain new data of biogeochemical colloid chemistry/transport that can inform model development. Therefore, our ultimate goal is to enhance our understanding of the biogeochemical processes taking place at floodplain subsurface interfaces, by developing reactive transport models that encompass all critical elements and their reaction pathways. We will study colloid reactivity and composition to improve our mechanistic knowledge of interfacial Fe-rich colloid genesis/function, and to integrate colloid biogeochemical behaviour into reactive transport models.
Our research is guided by the following hypotheses (Figure 1):
1. Fe-rich colloids are composed of FeS, ferrihydrite (Fh) and clays, that are bound together by organic molecules and can carry sorbed metal micronutrients (Mn, Cu) and contaminants (Zn).
2. Oscillating redox conditions, which occur along riparian flow paths, drive colloid transformation, demonstrated in changes in the FeS/Fh ratio within the colloids and association with metals and OM.
Our working hypotheses will be tested using the following research questions (cf., Table 1):
1. What is the chemical nature and bonding environment of Fe within the colloids? Specifically, what is the Fe oxidation state (i.e., FeS/Fh ratio)? Are the Fe minerals associated with other elements (OM, clays, and metals)? How is Fe distributed within the colloid?
2. What is the mechanism of colloid transformation? Specifically, how does the FeS/Fh ratio in the colloid particles change following redox oscillations? What are the resulting changes in colloid physical and chemical properties (composition, morphology, and size)?
EMSL resources will support investigation of the nano-scale dimensions of the colloids. We will analyse and compare reduced Fe-rich colloids to those that have undergone oxidative transformation. The size, morphology, and composition of the colloids will be analysed using S/TEM, coupled with the EDS and EELS features to map individual colloids, identify the spatial distribution of elements and examine whether specific Fe(II)/(III) domains exist. Detailed exploration of the oxidation states and mineralogy of Fe will be employed using Mössbauer spectroscopy. Furthermore, we will explore the identity of organic molecules associated with the colloids using FT-ICR-MS. Lastly, nano-scale elemental imaging will be performed using NanoSIMS to corroborate our findings. The information gathered from this suite of analyses will be combined with our in-house synchrotron-based x-ray methodologies to advance process-level understanding of colloid dynamics and bring us closer to our goal of their proper representation in biogeochemical predictive models.
Project Details
Project type
Large-Scale EMSL Research
Start Date
2021-10-01
End Date
2023-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Co-Investigator(s)
Team Members