Plutonium Partitioning Behavior to Humic Acids from Widely Varying Soils Is Related to Carboxyl-Containing Organic Compounds

In order to examine the influence of the HA molecular composition on the partitioning of Pu, ten different kinds of humic acids (HAs) of contrasting chemical composition, collected and extracted from different soil types around the world were equilibrated with groundwater at low Pu concentrations (10^–14 M). Under mildly acidic conditions (pH ∼ 5.5), 29 ± 24% of the HAs were released as colloidal organic matter (>3 kDa to <0.45 μm), yet this HA fraction accounted for a vast majority of the bound Pu, 76 ± 13% on average. In comparison, the particulate HA fraction bound only 8 ± 4% on average of the added Pu. The truly dissolved Pu fraction was typically <1%. Pu binding was strongly and positively correlated with the concentrations of organic nitrogen in both particulate (>0.45 μm) and colloidal phases in terms of activity percentage and partitioning coefficient values (log<i>K</i>_d). Based on molecular characterization of the HAs by solid state ¹³C nuclear magnetic resonance (NMR) and elemental analysis, Pu binding was correlated to the concentration of carboxylate functionalities and nitrogen groups in the particulate and colloidal phases. The much greater tendency of Pu to bind to colloidal HAs than to particulate HA has implications on whether NOM acts as a Pu source or sink during natural or man-induced episodic flooding

Temporal Variation of Iodine Concentration and Speciation (¹²⁷I and ¹²⁹I) in Wetland Groundwater from the Savannah River Site, USA

¹²⁹I derived from a former radionuclide disposal basin located on the Savannah River Site (SRS) has concentrated in a wetland 600 m downstream. To evaluate temporal environmental influences on iodine speciation and mobility in this subtropical wetland environment, groundwater was collected over a three-year period (2010–2012) from a single location. Total ¹²⁷I and ¹²⁹I showed significant temporal variations, ranging from 68–196 nM for ¹²⁷I and <5–133 pCi/L for ¹²⁹I. These iodine isotopes were significantly correlated with groundwater acidity and nitrate, two parameters elevated within the contaminant plume. Additionally, ¹²⁹I levels were significantly correlated with those of ¹²⁷I, suggesting that biogeochemical controls on ¹²⁷I and ¹²⁹I are similar within the SRS aquifer/wetland system. Iodine speciation demonstrates temporal variations as well, reflecting effects from surface recharges followed by acidification of groundwater and subsequent formation of anaerobic conditions. Our results reveal a complex system where few single ancillary parameters changed in a systematic manner with iodine speciation. Instead, changes in groundwater chemistry and microbial activity, driven by surface hydrological events, interact to control iodine speciation and mobility. Future radiological risk models should consider the flux of ¹²⁹I in response to temporal changes in wetland hydrologic and chemical conditions

Plutonium Immobilization and Remobilization by Soil Mineral and Organic Matter in the Far-Field of the Savannah River Site, U.S.

To study the effects of natural organic matter (NOM) on Pu sorption, Pu­(IV) and (V) were amended at environmentally relevant concentrations (10^–14 M) to two soils of contrasting particulate NOM concentrations collected from the F-Area of the Savannah River Site. More Pu­(IV) than (V) was bound to soil colloidal organic matter (COM). A de-ashed humic acid (i.e., metals being removed) scavenged more Pu­(IV,V) into its colloidal fraction than the original HA incorporated into its colloidal fraction, and an inverse trend was thus observed for the particulate-fraction-bound Pu for these two types of HAs. However, the overall Pu binding capacity of HA (particulate + colloidal-Pu) decreased after de-ashing. The presence of NOM in the F-Area soil did not enhance Pu fixation to the organic-rich soil when compared to the organic-poor soil or the mineral phase from the same soil source, due to the formation of COM-bound Pu. Most importantly, Pu uptake by organic-rich soil decreased with increasing pH because more NOM in the colloidal size desorbed from the particulate fraction in the elevated pH systems, resulting in greater amounts of Pu associated with the COM fraction. This is in contrast to previous observations with low-NOM sediments or minerals, which showed increased Pu uptake with increasing pH levels. This demonstrates that despite Pu immobilization by NOM, COM can convert Pu into a more mobile form

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

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

EPS Assembly analysis.

<p>EPS Assembly analysis.</p

Assembly kinetics of EPS monitored with DLS.

<p>(A) Assembly kinetics of EPS of <i>Amphora sp.</i> (B) Assembly kinetics of EPS of <i>Ankistrodesmus angustus</i> (C) Assembly kinetics of EPS of <i>Phaeodactylum tricornutum</i> EPS assembly in Ca²⁺-free ASW (black) was monitored to investigate assembly kinetics with decreased divalent ion availability. Different concentrations of ENs (polystyrene nanoparticles): 0 (red), 10 (green) and 100 ppb (blue), were added to investigate the effect of ENs on EPS microgel formation.</p

Chemical analysis of EPS.

<p>*Zhange et al., (2008) <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0021865#pone.0021865-Zhang1" target="_blank">[40]</a>.</p

Fluorescence images of EPS and ENs-induced EPS microgels.

<p>Nile Red was used to determine the microgel morphology. Green fluorescent signals indicated the fluorescent ENs. From the overlay images, results showed that the ENs incorporated within EPS matrixes. Scale bar is 10 µm.</p

ESEM images of EPS microgel.

<p>(A) <i>Amphora sp.</i> (Scale Bar = 4 µm) (B) <i>Ankistrodesmus angustus</i> (Scale Bar = 5 µm) (C) <i>Phaeodactylum tricornutum</i> (Scale Bar = 5 µm).</p