Deciphering controls on metal migration within floodplains: The critical role of redox environments on metal-organic complexes
EMSL Project ID
51383
Abstract
Dissolved organic matter (DOM) is a major but poorly understood control over the mobility of micronutrient and contaminant metals in surface and subsurface systems. Variations in the chemical composition of organic ligands resulting from different redox environments of watersheds has yet to be appreciated. We posit that critical different in the functionality and reactivity of dissolved organic matter will result within distinct hydrologic environments, and concomitant redox conditions, of watersheds. For many metals, organic-ligand complexes dominate chemical speciation in natural waters because they have higher conditional stability constants and solubilities compared to most of the major inorganic metal species that would otherwise limit their dissolved concentrations. The organic ligands control metal concentrations as well as their bioavailability and reactivity, and thus they are an essential within biogeochemical models of metal transport impacting the quality of both surface and groundwater. Chemical speciation models used within reactive transport codes explicitly represent major functional components of DOM (carboxylic and phenolic groups) to account for competition effects among metal ions that can occupy organic matter binding sites, providing a unified framework to consider metal speciation in complex systems with multiple elements. Presently, however, only organic matter derived from aerobic environments is used to assess reactivity toward metal ions; organic matter derived from anaerobic or fluctuating redox environments are not considered.
The overarching goal of our project is to determine the effect of redox conditions resulting from differing hydrologic environments on formation and transport of soluble metal-organic complexes. Presently, there is no systematic understanding of how redox conditions control organic ligand composition, representing a significant hurdle towards accurately incorporating metal speciation into predictive watershed scale models of metal mobilization and transport in surface and subsurface systems. Accordingly, we propose a multifaceted study that will elucidate variations in DOM functional groups arising from different redox conditions across a series of field sites. Our work will address the following questions: Do redox conditions affect the composition and concentration of metal-binding functional groups in dissolved organic matter? How do differences in composition and abundance of functional groups affect metal binding across redox environments? How does adsorption of organic ligands to mineral phases vary across redox interfaces?
Addressing these questions requires a combination of advanced analytical approaches for determining both the DOM ligand sites and metal-ligand complexes. Our work will pair field measurements with laboratory experiments to evaluate the role of redox fluctuations on the composition of dissolved metal-ligand complexes. We propose to use LC-FT-ICRMS as well as GCMS at EMSL to characterize organic ligand composition, as well as LC-ICPMS to evaluate competitive metal binding and to fractionate organic metal binding ligands for further spectroscopic characterization by synchrotron-based FTIR, C, N, and S XAS spectroscopy at other user facilities. Careful synthesis of the data obtained from these techniques will yield a thorough characterization of the chemical composition of DOM, including ligand functional groups. Understanding these processes is necessary for modelling and predicting (hydro)biogeochemical cycles of carbon and trace elements.
Project Details
Project type
Large-Scale EMSL Research
Start Date
2020-10-01
End Date
2022-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members