Evidence for Hydroxamate Siderophores and Other N-Containing Organic Compounds Controlling ^239,240Pu Immobilization and Remobilization in a Wetland Sediment

Abstract

Pu concentrations in wetland surface sediments collected downstream of a former nuclear processing facility in F-Area of the Savannah River Site (SRS), USA, were ∼2.5 times greater than those measured in the associated upland aquifer sediments; similarly, the Pu concentration solid/water ratios were orders of magnitude greater in the wetland than in the low-organic matter content aquifer soils. Sediment Pu concentrations were correlated to total organic carbon and total nitrogen contents and even more strongly to hydroxamate siderophore (HS) concentrations. The HS were detected in the particulate or colloidal phases of the sediments but not in the low molecular weight fractions (<1000 Da). Macromolecules which scavenged the majority of the potentially mobile Pu were further separated from the bulk mobile organic matter fraction ("water extract") via an isoelectric focusing experiment (IEF). An electrospray ionization Fourier-transform ion cyclotron resonance ultrahigh resolution mass spectrometry (ESI FTICR-MS) spectral comparison of the IEF extract and a siderophore standard (desferrioxamine; DFO) suggested the presence of HS functionalities in the IEF extract. This study suggests that while HS are a very minor component in the sediment particulate/colloidal fractions, their concentrations greatly exceed those of ambient Pu, and HS may play an especially important role in Pu immobilization/remobilization in wetland sediments.

Role of natural organic matter on iodine and 239,240Pu distribution and mobility in environmental samples from the northwestern Fukushima Prefecture, Japan

“…4b, c). Similar correlation trends were also found in wetland soil samples collected from Savannah River Site (SRS), which was contaminated by Pu isotopes due to nuclear material production and disposal activities (Xu, C. et al, 2015). With the help of multi-NMR techniques and other methods, Xu et al (2008) reports that mobile Pu, at sub-pM concentrations, in the surface soil at Rocky Flats Environmental Technology Site (RFETS), was complexed by hydroxamate siderophore functionalities (R-CO-NR'-OH) present in colloidal macromolecules composed of cutin-derived soil degradation products as the backbone cross-linked to hydrophilic moieties (e.g., polysaccharides).…”

“…Measurement accuracy was certified by determination of total iodine in a NIST Standard Reference Material (SRM ® 2709) San Joaquin Soil (TI reported as 5 µg/g), and the difference between the experimental and reported values was well within 3%. 239,240 Pu and 240 Pu/ 239 Pu isotopic ratios A 0.5-g (for α spectrometry) or 10-g (for ICP-MS) soil or litter sample was ashed in a ceramic crucible at 600°C for >16 h. Then 242 Pu was added as a yield tracer to the sample and the sample was acid-digested according to Xu, C. et al (2015). Plutonium isotopes were separated from other radionuclides (e.g., 238 U and 241 Am) with UTEVA column (Cat.…”

“…With the help of multi-NMR techniques and other methods, Xu et al (2008) reports that mobile Pu, at sub-pM concentrations, in the surface soil at Rocky Flats Environmental Technology Site (RFETS), was complexed by hydroxamate siderophore functionalities (R-CO-NR'-OH) present in colloidal macromolecules composed of cutin-derived soil degradation products as the backbone cross-linked to hydrophilic moieties (e.g., polysaccharides). Recent studies by the same author Xu, C. et al (2015) (Ahmed and Holmström, 2014;Essén et al, 2006;Powell et al, 1980;Powell et al, 1983). However, only a few studies so far have reported their presence as highmolecular-weight compounds (> 1000 Da) in the colloidal or particulate fractions of SOM (Xu, C. et al, 2015;Xu et al, 2008).…”

Substructural Components of Organic Colloids from a Pu-Polluted Soil with Implications for Pu Mobilization

“…Even where molecules suggestive of hydroxamate siderophores have been identified as potential Pu carriers in samples with elevated nitrogen, they represent only a small fraction of the total N content which would allow other Ncontaining compounds to contribute to the strong correlation between organic N and Pu concentrations. 7 However, since Pu is at concentrations less than 10 −14 M, and potential ligands are many orders of magnitude more abundant, it is very challenging to isolate the actual bound ligand(s). Unknown metal:ligand stoichiometry, charge state, and kinetics of ligand exchange vs droplet formation in ESI-FTICRMS also limit identification of true natural bound ligands in the gas phase that would be active in solution phase.…”

“…4 The majority of the mobile colloidal Pu was contained in a subfraction that was separated via an isoelectric focusing (IEF) electrophoresis experiment and found to concentrate around the IEF range of 4.1−5.6 (defined as "SRS IEF colloid"). 7 Elevated pH values, as a result of the basin closure and remediation strategies, led to Pu remobilization by complexation with colloidal organic matter and subsequent desorption from soil particles in the downstream wetland sediment. Moreover, wetland sediment Pu concentrations from this site were shown to increase with increasing nitrogen and hydroxamate content, which were proposed to originate partly from siderophores responsible for complexing Pu.…”