Organic Nature of Colloidal Actinides Transported in Surface Water Environments

Abstract

Elevated levels of (239,240)Pu and 241Am have been present in surficial soils of the Rocky Flats Environmental Technology Site (RFETS), CO, since the 1960s, when soils were locally contaminated in the 1960s by leaking drums stored on the 903 Pad. Further dispersion of contaminated soil particles was by wind and water. From 1998 until 2001, we examined actinide ((239,240)Pu and 241Am) concentrations and phase speciation in the surface environment at RFETS through field studies and laboratory experiments. Measurements of total (239,240)Pu and 241Am concentrations in storm runoff and pond discharge samples, collected during spring and summer times in 1998-2000, demonstrate that most of the (239,240)Pu and 241Am transported from contaminated soils to streams occurred in the particulate (> or = 0.45 microm; 40-90%) and colloidal (approximately 2 nm or 3 kDa to 0.45 microm; 10-60%) phases. Controlled laboratory investigations of soil resuspension, which simulated storm and erosion events, confirmed that most of the Pu in the 0.45 microm filter-passing phase was in the colloidal phase (> or = 80%) and that remobilization of colloid-bound Pu during soil erosion events can be greatly enhanced by humic and fulvic acids present in these soils. Most importantly, isoelectric focusing experiments of radiolabeled colloidal matter extracted from RFETS soils revealed that colloidal Pu is in the four-valent state and is mostly associated with a negatively charged organic macromolecule with a pH(IEP) of 3.1 and a molecular weight of 10-15 kDa, rather than with the more abundant inorganic (iron oxide and clay) colloids. This finding has important ramifications for possible remediation, erosion controls, and land-management strategies.

Influence of Addition Order and Contact Time on Thorium(IV) Retention by Hematite in the Presence of Humic Acids

“…The total reversibility could only be obtained lowering pH to 4 within 24 h, where UO 2+ 2 is the major specie. Under our conditions, we can estimate, referring to Neck and Kim (30), that Th(OH) + 3 and Th(OH) 4 (aq) are the major thorium (IV) species in solution in the interval 6 ≤ pH ≤ 9. As in the case of U(VI), the presence of strongly hydrolysed Th(IV) species can be responsible of the irreversible process.…”

Science-based cleanup of Rocky Flats

“…A growing number of field studies, however, document the movement of low concentrations of low-solubility radionuclides in surface and ground waters. 6,9,10 Those small concentrations can be transported in surface water and soils by particles of sizes typically ranging from a nanometer to several microns. 11,12 Because the particles remain suspended in ground water, they can move in the natural watershed and settle into the series of ponds around RFETS.…”

“…The bar graph shows a summary of ultrafiltration results from Peter Santschi and colleagues 10 that compares normal discharge (blue) at a particular monitoring station with data recorded shortly after a rainstorm (red). The group's measurements demonstrated that a large majority of the plutonium material that passed the 0.5-μm filter was not dissolved, but colloidal, because it could be filtered out using smaller 0.1-μm filters.…”

Prognosticating the humic complexation for redox sensitive actinides through analogy, using the charge neutralisation model

“…Furthermore, thorium (IV) and plutonium are known to have the same behaviour in presence of natural organic matter [92]. Nevertheless, readers should bear in mind that these constants are only estimates and should be used with caution.…”