Molecular Features of Uranium-Binding Natural Organic Matter in a Riparian Wetland Using Ultrahigh Resolution Mass Spectrometry

Authors

Peter H. Santschi¹(santschi@tamug.edu), Chen Xu¹, Chris M. Yeager², Alex Goranov³, Peng Lin⁴, Patrick G. Hatcher³, Daniel I. Kaplan⁴* (daniel.kaplan@uga.edu)

Institutions

¹Texas A&M University–Galveston, TX; ²Los Alamos National Laboratory, Los Alamos, NM; ³Old Dominion University, Norfolk, VA; ⁴Savannah River Ecology Laboratory, University of Georgia–Aiken, SC

Abstract

Tim’s Branch riparian wetland located in South Carolina, USA has immobilized 94% of the uranium (U) released >50 years ago from a nuclear fuel fabrication facility. Sediment organic matter (OM) has been shown to play an important role in immobilizing U. Yet, U-OM-mineral interactions at the molecular scale have never been investigated at ambient concentrations.

Sediment organic matter (OM) along the stream water pathway were extracted, purified, and concentrated. Molecular characterization was performed using Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS). Out of 9,614 identified formulas, 715 contained U. These U-containing formulas were enriched with iron (Fe), nitrogen (N), and/or sulfur (S) compared to the entire pool of OM. Lignin-like and protein-like molecules accounted for 40% and 19% of the U-containing formulas, respectively. Phosphorus-containing formulas were found to exert an insignificant influence on complexing U. U-containing formulas in the mobile fraction (groundwater extractable) had lower nominal oxidation states of carbon (NOSC); lower N and S concentrations; and less aromatic moieties than the U-containing formulas recovered from the immobile fraction (sodium pyrophosphate extractable). U-containing formulas in the redox interfacial zones (stream banks) compared to those in nearby up-slope zones tended to have smaller molecular weights; lower NOSC; higher contents of COO and/or CONO functional groups; and higher abundance of Fe-containing formulas. Fe was present in 38% of the U-containing formulas but only 20% of the total OM formulas. It is postulated that Fe played an important role in stabilizing the structure of sedimentary OM, especially U-containing compounds. The identification for the first time of hundreds of U-containing formulas demonstrates the that the complexity of the system is much greater than commonly believed and numerically predicting U binding behavior in OM-rich systems should not be limited to measuring metal complexation with well-defined individual analogue organic ligands.