Among the key environmental factors influencing the fate and transport of Pu in the environment is natural organic matter (NOM). While this has been well-known for decades, there still remains great uncertainty in predicting NOM-Pu interactions due to lack of understanding of Pu interactions with the specific organic moieties within NOM. A series of studies at multiple field sites were undertaken to describe NOM-Pu interactions, with particular attention directed at the practical problem of identifying conditions when NOM is an aqueous ligand promoting Pu mobility versus when it is a soil-bound ligand reducing mobility. At the field scale, Pu concentrations in wetland surface sediments collected 700m downstream of a former nuclear processing facility in the Savannah River Site (SRS), South Carolina were ∼2.5 times greater than those measured in aquifer sediments recovered only 30m upstream from the facility. Sediment Pu concentrations were correlated to total organic C and total N concentrations and even more strongly to hydroxamate siderophore (a hydroxamic acid chelate) concentrations. The importance of hydroxamates within NOM on Pu binding was demonstrated in soils from SRS and Rocky Flats, Colorado. Isoelectric focusing experiments isolated most of the Pu in a single NOM fraction. Hydroxamates in that fraction were then identified using ESI-FTICR-MS and hydroxamate standards. From a survey of 14 well-characterized humic acid (HA) collected from different soils around the world, it was shown that even though colloidal NOM (&gt;3kDa to &lt;0.45μm) accounted for a small fraction of the total HA, it had a much greater tendency to bind Pu than particulate HA. Finally, 55% of the bomb-derived Pu detected in a sediment core from Nagasaki, Japan was enriched in NOM (&gt;3kDa) associated with Fe-oxides and 31% was associated with a NaOH-extractable OM fraction, suggesting that NOM may ultimately be the long-term sink of Pu in surface soils.

Soil organic matter and plutonium interactions

256th National Meeting (2018)

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