Impact of Natural Organic Matter on Plutonium Vadose Zone Migration from an NH₄Pu(V)O₂CO₃(s) Source

Abstract

We investigated the influence of natural organic matter (NOM) on the behavior of Pu(V) in the vadose zone through a combination of field lysimeter and laboratory studies. Well-defined solid sources of NH 4 Pu(V)O 2 CO 3 (s) were placed in two 5-L lysimeters containing NOM-amended collected from Savannah River Site or unamended vadose zone soils and exposed to three years of natural South Carolina, USA meteorological conditions. The lysimeter soil cores were removed from the field, used in desorption experiments, and characterized by wet chemistry methods and X-ray absorption spectroscopy

Plutonium Redox Transformation in the Presence of Iron, Organic Matter, and Hydroxyl Radicals: Kinetics and Mechanistic Insights

“…Indeed, HA can significantly accelerate Pu oxidation to Pu­(V/VI). Our results suggest that the observations and conceptual models of enhanced Pu migration reported in the studies by Molz et al and Maloubier et al may indeed be explained by the transient Pu oxidation that occurs during the infiltration of H 2 O 2 -containing water into sediments. , However, high concentrations of NOM may also decrease Pu migration by facilitating the reduction of Pu­(V/VI), as observed by Maloubier et al Indeed, the subsurface is a highly complex and heterogeneous environment that can exhibit dynamic geochemical conditions compared to systems presented in this study. Ongoing research will extend the framework developed here by evaluating the role of numerous variables (e.g., extended pH range, H 2 O 2 concentrations, and NOM compositions).…”

“…For surface water and groundwater interactions, enhanced Pu migration has been observed at the Savannah River site due to Pu­(IV) oxidation to Pu­(V) from infiltrating and oxidizing rainwater . However, a quite recent study showed that the presence of NOM significantly limited Pu migration through reduction of Pu­(V) and increased Pu sorption to soils . These contrasting observations suggest that our understanding of the impact of NOM on Pu migration in surface and groundwater is incomplete.…”

“…Environmental redox processes play a critical role in controlling Pu mobility as the reduced forms (III/IV) are generally 2–3 orders of magnitude less mobile and have higher affinity for mineral surfaces and organic matter than the oxidized forms (V/VI) in most environments. ,, However, formation of intrinsic Pu­(IV) oxide/hydroxide and pseudo Pu colloids could facilitate the subsurface transport of Pu under certain conditions . The presence of these colloids further complicates the fate of Pu as it can lead to mobilization of the reduced forms of Pu via colloid-facilitated transport. ,, …”

“…NOM is commonly found associated with Fe­(III) oxides in surface water and groundwater, − and their interactions can control the speciation and transport of Pu depending on colloidal properties of NOM–Fe. ,, For example, under oxic conditions, citric acid, humic acid (HA), and fulvic acid (FA) were found to increase Pu adsorption onto Fe­(III) oxides at low pH by forming ternary Pu­(IV)–NOM–Fe­(III) oxide complexes. , Furthermore, these NOM–Fe colloids have been shown to serve as geochemical nanocarriers, either facilitating or reducing the overall transport of Pu in surface water depending on their particle transport properties. ,− Chen et al studied the mobility of Pu in soil-groundwater suspensions and found that Pu­(IV) formed mobile colloids with NOM, while Pu­(V) sorbed more readily to prevailing solid clay mineral surfaces . During seasonal mixing events and associated transient redox conditions, transport of NOM–Fe­(II) colloids from groundwater to surface water can result in the formation of NOM–Fe­(III) oxide colloids and generation of hydroxyl radicals. , …”

“…21 The presence of these colloids further complicates the fate of Pu as it can lead to mobilization of the reduced forms of Pu via colloid-facilitated transport. 10,22,23 NOM is ubiquitous in aquatic environments and profoundly impacts the biogeochemical cycling of metals and radionuclides. 24−27 NOM comprises complex heterogeneous organic macromolecules, a fraction of which has been traditionally classified as humic substances.…”