Mobility Of Plutonium Research Articles

Plutonium mobility and reactivity in a heterogeneous clay rock barrier accented by synchrotron-based microscopic chemical imaging

The long-term safe disposal of radioactive waste corresponds to a challenging responsibility of present societies. Within deep geological waste disposal concepts, host rocks correspond to the ultimate safety barrier towards the environment. To assess the performance of such barriers over extended time scales, mechanistic information on the interaction between the radiotoxic, long-lived radionuclides like plutonium and the host rock is essential. Chemical imaging based on synchrotron microspectroscopic techniques was used to visualize undisturbed reactive transport patterns of Pu within pristine Opalinus Clay rock material. Pu+V is shown to be progressively reduced along its diffusion path to Pu+IV and Pu+III due to interaction with redox-active clay rock constituents. Experimental results and modeling emphasize the dominant role of electron-transfer reactions determining the mobility of Pu in reactive barrier systems. The effective migration velocity of Pu is controlled by the kinetic rates of the reduction to Pu+IV and Pu+III and the redox capacity of the involved electron donor pools. To advance our predictive capabilities further, an improved understanding of the nature and capacity of redox-active components of the reactive barrier material is fundamental. The findings represent an essential contribution to the evaluation of the long-term safety of potential nuclear waste repositories and have implications regarding the development of effective geological disposal strategies.

Plutonium Accumulation by Vegetation on Different Soils

The high variability of the plutonium transfer factors presented in the literature makes it relevant to study the mechanisms that affect its migration ability and its availability for vegetation. The variability of transfer factors is explained by different properties of soils, since the redox potential and soil acidity can significantly affect the mobility of plutonium. In the vegetation experiment, the migration of plutonium in the “soil-agricultural plant” system was studied for different types of soils. The content of 239 + 240Pu was analyzed by alpha spectrometry with preliminary radiochemical isolation. Plutonium migration parameters were determined using barley (Hordeum) and beans (Fabaceae) as test cultures. The plutonium accumulation coefficients obtained in the course of vegetation experiments are in the range of 3.1 × 10–4–6.8 × 10–3, with an average value of 3.8 × 10–3 for the aboveground part of barley and 9.2 × 10–3–7.6 × 10–2, with an average value of 3.8 × 10–2 – for the root system of barley. The transfer factor range was 1.5 × 10–3–5.7 × 10–3 with an average value of 3.7 × 10–3 and 5.8 × 10–2–6.5 × 10–2 with an average value of 6.2 × 10–2 for the aboveground part and the root system of the beans, respectively. It has been determined that the nature of the plutonium distribution over the vegetative organs of the crops under consideration is non-uniform. On average, the plutonium transfer factor for the aboveground part of plants is lower than for the root part by more than 40 times. It has been determined that the accumulation of plutonium in the aboveground parts of plants growing on different types of soil is not the same for individual species/organs of agricultural plants. For the aboveground part of the agricultural crops under consideration, the transfer factors differ up to several orders of magnitude. In general, the accumulation of plutonium by vegetation growing on different soils types is arranged in the following row: soddy-podzolic (Retisol) and gray forest soil (Phaeozem) lowland peat-bog (Histosol) \( \gg \) typical chernozem (Chernozem). The smallest accumulation of plutonium by vegetation is observed in soils with a high content of organic matter. For the root system of plants, the dependence of the plutonium accumulation coefficients on the soil type is uncertain.

137Cs and 239,240Pu activity concentrations distribution in waterlogged and non-boggy soils of Lithuania

Vertical distributions of 239,240Pu and 137Cs activity concentrations in the waterlogged and non-boggy soil cores sampled in the vicinity of Vilnius City and in the Varėna District were analyzed. The radionuclides appeared in the soils mainly as a result of the fallout after the nuclear weapon testing in atmosphere in 1960s and due to the Chernobyl NPP accident in 1986. The deposited radionuclides on the surface of waterlogged soils on the shores of lakes experienced processes of translocation and accumulation, therefore their total activity in the column differs from the activity in non-boggy soils. The mobility of plutonium is the highest in waterlogged organic-rich soils. Clay in the waterlogged soil significantly limits the migration of radiocesium and does not affect noticeably the mobility of plutonium. Compared with radiocesium, the mobility of plutonium in non-boggy soils is somewhat limited. So, in waterlogged and non-boggy soils, the radionuclide mobilizing factors act in the opposite directions for radiocesium and plutonium. A clear correlation between the radionuclide activity and soil organic matter content was not observed. The investigation showed that radionuclide mobility can be determined by studying radionuclide physicochemical forms in the soil, vertical profiles of radiocesium to plutonium ratios, soil composition as well as its oxidation regime.

Plutonium Co-precipitation with Calcite

The mobility of plutonium (Pu) in the environment is affected by Pu–mineral interactions, such as adsorption–desorption and structural incorporation. Calcite (CaCO3) is a common secondary phase in ...

Formation of plutonium(IV) silicate species in very alkaline reactive media.

Studying the speciation of Pu(IV) in very alkaline and silicate ion rich reactive media allowed identification of the formation of plutonium(IV)-silicate colloidal suspensions which were stable for months. These colloids were stabilized in aqueous solution for pH > 13 and for concentrations around 10-2 mol L-1. Successive filtration processes allowed evaluation of their size, which was found to be smaller than 6 nm. Their structural characterization by XAS evidenced that their structure was similar to those identified for the other tetravalent actinide-silicate colloidal systems like thorium, uranium and neptunium. Their formation could explain the increase of plutonium solubility usually observed in alkaline silicate-rich solutions and could affect the plutonium mobility as a result in contaminated sites or in other environmental permeable media.

Chelation Treatment by Early Inhalation of Liquid Aerosol DTPA for Removing Plutonium after Rat Lung Contamination.

Occupational contamination is a potential health risk associated with plutonium inhalation. DTPA remains the chelating drug of choice to decorporate plutonium. In this study, plutonium was found to be more effectively removed from lungs by a single inhalation of nebulized DTPA solution at only 1.1 µmol.kg-1 than by a single intravenous (i.v.) dose of DTPA at 15 µmol.kg-1. When DTPA was inhaled promptly after contamination, it removed the transportable fraction of plutonium prior blood absorption, thereby preventing both liver and bone depositions. Conversely, DTPA injection was better than inhalation at reducing the extrapulmonary burden, probably due to the much greater circulating dose, favoring the mobilization of plutonium already translocated. Thus, prompt inhalation, concomitantly supplemented with i.v. injection, of DTPA induced an important decrease in extrapulmonary deposits. Repeated DTPA inhalations over several weeks were more efficient than a single inhalation in limiting both pulmonary and extrapulmonary plutonium retention, due at least in part to the chelation of the transportable fraction of lung plutonium. Furthermore, repeated DTPA injections remained better at reducing liver and bone plutonium retentions. Taken together, our results suggest that multiple DTPA inhalations may be considered an effective treatment after inhalation of plutonium, particularly given the ease of this needle-free delivery, for the two following conditions: 1. A treatment combining i.v. injection and inhalation should be given in an emergency scenario to efficiently chelate the activity already absorbed; 2. Inhalations should be administered daily to effectively trap the early transferable fraction.

The speciation, transformation kinetics and fate of spiked Pu (IV) in highly saline groundwater

The mobility of plutonium (Pu) in groundwater is dependent of its speciation distribution and transformation. The speciation and transformation kinetics of Pu(IV) and its colloids in highly saline groundwater have, however, been rarely studied. In the present study, groundwater (Ionic strength 1 M) from Dunhuang region, NW China, was collected for investigating the speciation, transformation kinetics and fate of spiked Pu (IV) with aging time. The results showed that ~99% of the spiked Pu (IV) (over initial concentration c0 range 2.5 × 10−10–7.8 × 10−7 mol·L−1) was easily associated with the natural colloids and transformed into relatively unstable Pu pseudo-colloids in 1 day, which then gradually deposited and/or adsorbed on the container walls with aging. The suspended Pu pseudo-colloids decreased in similar exponential models, with rate equations r(t) = −3.1 × 10−10e– t/4 and −1.3 × 10−8e–/3 for c0 = 1.25 × 10−9 mol·L−1and 4.17 × 10−8 mol·L−1, respectively. The chemical speciation of the suspended colloidal Pu was dominated by “Fe/Mn Oxides” at the early time, while “Carbonates” with slower depositing rate (r(t) = −6.9 × 10−12e– 0.149t) dominated it (~82%) at equilibrium state. Whatever the c0 was, the concentration of dissolved Pu (i.e., the apparent solubility of Pu) kept at 0.7 × 10−11 mol·L−1 over aging. The valence of dissolved Pu was dominated by Pu(IV) at early time, while Pu(V + VI) would become dominant (~95%) at equilibrium state with transformation rate of r(t) = −92.9e– t/16.6 + 96.9. The equilibrium times of Pu deposition (and/or adsorption), speciation transformation of the suspended colloidal Pu, and valence change of the dissolved Pu were 30 d, 80 d and 120 d, respectively. The kinetic process for each Pu species could be well fitted with exponential model. These results suggest that the majority of released Pu(IV) into highly saline groundwater will be easily associated with natural aquatic colloids and then become immobile in short time due to deposition (and/or adsorption) onto the environmental medium, but potential migration risk caused by stable suspended Pu colloids cannot be ignored.

Reduction of Plutonium(VI) to (V) by Hydroxamate Compounds at Environmentally Relevant pH.

Natural organic matter is known to influence the mobility of plutonium (Pu) in the environment via complexation and reduction mechanisms. Hydroxamate siderophores have been specifically implicated due to their strong association with Pu. Hydroxamate siderophores can also break down into di and monohydroxamates and may influence the Pu oxidation state, and thereby its mobility. In this study we explored the reactions of Pu(VI) and Pu(V) with a monohydroxamate compound (acetohydroxamic acid, AHA) and a trihydroxamate siderophore desferrioxamine B (DFOB) at an environmentally relevant pH (5.5-8.2). Pu(VI) was instantaneously reduced to Pu(V) upon reaction with AHA. The presence of hydroxylamine was not observed at these pHs; however, AHA was consumed during the reaction. This suggests that the reduction of Pu(VI) to Pu(V) by AHA is facilitated by a direct one electron transfer. Importantly, further reduction to Pu(IV) or Pu(III) was not observed, even with excess AHA. We believe that further reduction of Pu(V) did not occur because Pu(V) does not form a strong complex with hydroxamate compounds at a circum-neutral pH. Experiments performed using desferrioxamine B (DFOB) yielded similar results. Broadly, this suggests that Pu(V) reduction to Pu(IV) in the presence of natural organic matter is not facilitated by hydroxamate functional groups and that other natural organic matter moieties likely play a more prominent role.

Microbial mobilization of plutonium and other actinides from contaminated soil

We examined the dissolution of Pu, U, and Am in contaminated soil from the Nevada Test Site (NTS) due to indigenous microbial activity. Scanning transmission x-ray microscopy (STXM) analysis of the soil showed that Pu was present in its polymeric form and associated with Fe- and Mn- oxides and aluminosilicates. Uranium analysis by x-ray diffraction (μ-XRD) revealed discrete U-containing mineral phases, viz., schoepite, sharpite, and liebigite; synchrotron x-ray fluorescence (μ-XRF) mapping showed its association with Fe- and Ca-phases; and μ-x-ray absorption near edge structure (μ-XANES) confirmed U(IV) and U(VI) oxidation states. Addition of citric acid or glucose to the soil and incubated under aerobic or anaerobic conditions enhanced indigenous microbial activity and the dissolution of Pu. Detectable amount of Am and no U was observed in solution. In the citric acid-amended sample, Pu concentration increased with time and decreased to below detection levels when the citric acid was completely consumed. In contrast, with glucose amendment, Pu remained in solution. Pu speciation studies suggest that it exists in mixed oxidation states (III/IV) in a polymeric form as colloids. Although Pu(IV) is the most prevalent and generally considered to be more stable chemical form in the environment, our findings suggest that under the appropriate conditions, microbial activity could affect its solubility and long-term stability in contaminated environments.

Sequential extraction as a tool for mobility studies of radionuclides and metals in soils and sediments

Abstract To assess the long term environmental impact of radioactive contamination of ecosystems, information on source terms including radionuclide speciation, mobility and biological uptake is of high importance. The speciation of radionuclides deposited or occurring naturally depends on source term and release scenario characteristics, transport and dispersion mechanisms and ecosystem properties. If mobile species are present, ecosystem transfer is relatively fast, whereas the ecosystem transfer is delayed if radionuclides are present as particles or incorporated in mineral lattices. This paper discusses cases showing important factors influencing the mobility of different radionuclides and metals. As examples can be given: the difference between 137Cs and 90Sr from Chernobyl fallout and stable Cs and Sr in the soil showing that the fallout has not reached a steady state and therefore shows different mobility properties, thus the time since contamination is important. the presence of organic matter affect the mobility of plutonium in soils and sediments even though the source term is the same. physical properties (i.e. pH, OM, clay content, grain sice etc.) is important to interpret mobility data between similar sites. the influence of industrial activity on radionuclides and metals shows that any man-made activity might change the mobility.

Goethite colloid enhanced Pu transport through a single saturated fracture in granite

α-FeOOH, a stable iron oxide in nature, can strongly absorb the low-solubility plutonium (Pu) in aquifers. However, whether Pu transports though a single saturated fracture can be enhanced in the presence of α-FeOOH colloids remains unknown. Experimental studies were carried out to evaluate Pu mobilization at different water flow velocity, as affected by goethite colloids with various concentrations. Goethite nanorods were used to prepare (α-FeOOH)-associated Pu suspensions with α-FeOOH concentration of (0–150) mgL−1. The work experimentally evidenced that α-FeOOH colloid does enhance transport of Pu through fractured granites. The fraction of mobile 239Pu (RPu, m=41.5%) associated with the α-FeOOH of an extremely low colloid concentration (0.2mgL−1) is much larger than that in absence of α-FeOOH (RPu, m=6.98%). However, plutonium mobility began to decrease when α-FeOOH concentration was increased to 1.0mgL−1. On the other hand, the fraction of mobile Pu increased gradually with the water flow velocity. Based on the experimental data, the mechanisms underlying the (α-FeOOH)-associated plutonium transport are comprehensively discussed in view of its dynamic deposition onto the granite surfaces, which is decided mainly by the relative interaction between the colloid particle and the immobile surface. This interaction is a balance of electrostatic force (may be repulsive or attractive), the van der Walls force, and the shear stress of flow.

Plutonium in wildlife and soils at the Maralinga legacy site: persistence over decadal time scales

The mobility of plutonium (Pu) in soils, and its uptake into a range of wildlife, were examined using recent and ∼25 year old data from the Taranaki area of the former Maralinga weapons test site, Australia. Since its initial deposition in the early 1960s, the dispersed Pu has been incorporated into the soil profile and food chain through natural processes, allowing for the study of Pu sequestration and dynamics in relatively undisturbed semi-arid conditions.The data indicate downward mobility of Pu in soil at rates of ∼0.2–0.3 cm per year for the most mobile fraction. As a result, while all of the Pu was initially deposited on the ground surface, approximately 93% and 62% remained in the top 0–2 cm depth after 25- and 50-years respectively. No large-scale lateral spreading of the Taranaki plume was observed. Pu activity concentrations in 0–1 cm soils with biotic crusts were not elevated when compared with nearby bare soils, although a small number of individual data suggest retention of Pu-containing particles may be occurring in some biotic crusts.Soil-to-animal transfer, as measured by concentration ratios (CRwo-soil), was 4.1E−04 (Geometric Mean (GM)) in mammals, which aligns well with those from similar species and conditions (such as the Nevada Test Site, US), but are lower than the GM of the international mammal data reported in the Wildlife Transfer Database (WTD). These lower values are likely due to the presence of a low-soluble, particulate form of the Pu in Maralinga soils. Arthropod concentration ratios (3.1E−03 GM), were similar to those from Rocky Flats, US, while values for reptiles (2.0E−02 GM) were higher than the WTD GM value which was dominated by data from Chernobyl. Comparison of uptake data spanning approximately 30 years indicates no decrease over time for mammals, and a potential increase for reptiles. The results confirm the persistence of bioavailable Pu after more than 50 years since deposition, and also the presence of larger-sized particles which currently affect CRwo-soil calculations, and which may serve as an ongoing source of bioavailable Pu as they are subjected to weathering into the future.

The dynamic role of natural colloids in enhancing plutonium transport through porous media

Unexpectedly fast migration of Pu in aquifers at some nuclear sites was known as the result of enhanced transport by natural colloids. Nevertheless, transport experiments still need to provide direct evidence for the role of colloids in promoting Pu transport. On the other hand, systematic laboratory studies should be carried out to comprehensively evaluate Pu mobilization as affected by colloids that are varied in concentrations, since naturally occurring colloids have greatly different concentrations in aquatic environments. We isolated mineral colloids with the diameter of 1nm–1μm from sandy soils and prepared the colloidal suspensions with the concentrations ranged from c=0 (solution) to c=2017.80mg/L. The results of transport experiments show that the mobile fraction of 239Pu (RPu=8.62%) associated with the colloids of an extremely low concentration (c=0.5mg/L) is much larger than that not associated with the colloids (RPu=1.31%, c=0). The mobile fraction is defined by the percent recovery of Pu (RPu). This is strong evidence for the enhanced transport of Pu by the colloids. Plutonium mobility (reflected by RPu) continually increases with the colloid concentrations of c<375.37mg/L and then declines at c>375.37mg/L, which is demonstrated as a dynamic role of colloids in Pu transport. The highest mobility (RPu=52.48%), occurring at c=375.37mg/L, is referred to as the critical transport-enhanced concentration. Plutonium transport mechanism with a focus on its dynamic deposition onto the surfaces of porous media is explored.

Long-Term and Long-Range Migration of Radioactive Fallout in a Karst System

Mountainous areas are often covered by little evolved soils from which deposited radionuclides can potentially leak into the vadose zone. In the Swiss Jura mountains, we observed unusual isotopic ratios of nuclear weapon test (NWTs) fallout with an apparent loss of NWTs plutonium relative to ¹³⁷Cs of Chernobyl origin in thinner soils. Here, we studied the karstic watershed of a vauclusian spring to determine the residence times of plutonium, ²⁴¹Am, and ⁹⁰Sr deposited by global fallout and their respective mobility in carbonaceous soils. The results show that ⁹⁰Sr is washed most efficiently from the watershed with a residence time of several hundred years. The estimated plutonium residence time is more than 10 times higher (in the range of 5000-10,000 years), and the ²⁴¹Am residence time is double that of plutonium. The spring water ²⁴¹Am/²³⁹+²⁴⁰Pu isotopic ratio is lower (0.12 - 0.28) than found in watershed soils (0.382 ± 0.077). Similar differences are found in aquatic mosses (²⁴¹Am/²³⁹+²⁴⁰Pu isotopic ratio 0.05-0.12), which are permanently submerged in spring waters. In contrast to plutonium, ⁹⁰Sr is leached from these mosses with 0.5M HCl, demonstrating that strontium is probably associated with calcium carbonate precipitations on the mosses. The higher plutonium to americium isotopic ratio found in the samples of spring water and mosses at the outlet of the karst shows that plutonium mobility is enhanced.

Binding of 239Pu and 90Sr to Organic Colloids in Soil Solutions: Evidence from a Field Experiment

Colloidal transport has been shown to enhance the migration of plutonium in groundwater downstream from contaminated sites, but little is known about the adsorption of ⁹⁰Sr and plutonium onto colloids in the soil solution of natural soils. We sampled soil solutions using suction cups, and separated colloids using ultrafiltration to determine the distribution of ²³⁹Pu and ⁹⁰Sr between the truly dissolved fraction and the colloidal fraction of the solutions of three Alpine soils contaminated only by global fallout from the nuclear weapon tests. Plutonium was essentially found in the colloidal fraction (>80%) and probably associated with organic matter. A significant amount of colloidal ⁹⁰Sr was detected in organic-rich soil solutions. Our results suggest that binding to organic colloids in the soil solutions plays a key role with respect to the mobility of plutonium in natural alpine soils and, to a lesser extent, to the mobility of ⁹⁰Sr.

Fractionation of plutonium in environmental and bio-shielding concrete samples using dynamic sequential extraction

Fractionation of plutonium isotopes ( 238Pu, 239,240Pu) in environmental samples (i.e. soil and sediment) and bio-shielding concrete from decommissioning of nuclear reactor were carried out by dynamic sequential extraction using an on-line sequential injection (SI) system combined with a specially designed extraction column. Plutonium in the fractions from the sequential extraction was separated by ion exchange chromatography and measured using alpha spectrometry. The analytical results show a higher mobility of plutonium in bio-shielding concrete, which means attention should be paid to the treatment and disposal of nuclear waste from decommissioning.

Association of plutonium with sediments from the Ob and Yenisey Rivers and Estuaries

The present study applied sequential extraction techniques to investigate the binding and mobility of plutonium (Pu) in sediments from the rivers and estuaries of the Ob and Yenisey. As a study site, the Ob and Yenisey are particularly interesting as both rivers have weapons-grade Pu sources in their catchment areas, including the Russian Pu production and reprocessing plants at Mayak, Tomsk-7 and Krashnoyarsk, and the Semipalantinsk nuclear weapons testing site in Kazakhstan. Plutonium activity and 240Pu/ 239Pu ratios were determined using accelerator mass spectrometry (AMS). Sequential extractions showed that between 47 and 80% of the Pu in Yenisey River sediments and 35–53% of the Pu in soils around the Techa River are mobilized with weak oxidising agents, which can indicate that Pu is bound to organic material. In contrast, Pu in Ob and Yenisey Estuarine sediments was more strongly bound, with 60–100% being found in the HNO 3-extractable fraction. This change in speciation could reflect either that Pu bound to organic material in the Techa and Yenisey River sediments becomes more fixed to the sediments with time, or that organic-bound Pu is mobilized and released to the water when the sediments encounter the more saline water of the Ob and Yenisey estuaries. In general, 240Pu/ 239Pu ratios were relatively consistent between different extraction fractions, although, in whole sediments, an increase in ratio was observed with distance from the source. This reflects the increased influence of weapon fallout from catchment runoff within the river systems, as compared to the weapons-grade sources close to the production and reprocessing plants. Knowledge of Pu speciation in the Ob and Yenisey Rivers, and the processes controlling its behaviour in estuarine systems, can improve predictions of its transfer and subsequent environmental impact to Arctic Seas.

Influence of Sources on Plutonium Mobility and Oxidation State Transformations in Vadose Zone Sediments

Well-defined solid sources of Pu(III) (PuCl3), Pu(IV) (Pu (NO3)4 and Pu (C2O4)2), and Pu(VI) (Pu02(NO3)2) were placed in lysimeters containing vadose zone sediments and exposed to natural weather conditions for 2 or 11 years. The objective of this study was to measure the release rate of Pu and the changes in the Pu oxidation states from these Pu sources with the intent to develop a reactive transport model source-term. Pu(III) and Pu(IV) sources had identical Pu concentration depth profiles and similar Pu release rates. Source release data indicate that PuIV(C2O4)2 was the least mobile, whereas Pu(VI)O2(NO3)2 was the most mobile. Synchrotron X-ray fluorescence (SXRF) revealed that Pu was very unevenly distributed on the sediment and Mn concentrations were too low (630 mg kg(-1)) and perhaps of the wrong mineralogy to influence Pu distribution. The high stability of sorbed Pu(IV) is proposed to be due to the formation of a stable hydrolyzed Pu(IV) surface species. Plutonium X-ray absorption near-edge spectroscopy (XANES) analysis conducted on sediment recovered at the end of the studyfrom the Pu(IV)(NO3)4- and Pu(III)(III)Cl3-amended lysimeters contained essentially identical Pu distributions: approximately 37% Pu(III), 67% Pu(IV), 0% Pu(V), and 0% Pu(VI). These results were similar to those using a wet chemistry Pu oxidation state assay, except the latter method did not detect any Pu(III) present on the sediment but instead indicated that 93-98% of the Pu existed as Pu(IV). This discrepancy was likely attributable to incomplete extraction of sediment Pu(III) by the wet chemistry method. Although Pu has been known to exist in the +3 oxidation state under microbially induced reducing conditions for decades, to our knowledge, this is the first observation of steady-state Pu(III) in association with natural sediments. On the basis of thermodynamic considerations, Pu(III) has a wide potential distribution, especially in acidic environments, and as such may warrant further investigation.

Influence of Oxidation States on Plutonium Mobility during Long-Term Transport through an Unsaturated Subsurface Environment

Lysimeter and laboratory studies were conducted to identify the controlling chemical processes influencing Pu(IV) mobility through the vadose zone. A 52-L lysimeter containing sediment from the Savannah River Site, South Carolina and solid PuIV(NO3)4 was left exposed to natural wetting and drying cycles for 11 years before the lysimeter sediment was sampled. Pu had traveled 10 cm, with >95% of the Pu remaining within 1.25 cm of the source. Laboratory studies showed that the sediment quickly reduced Pu(V) to Pu(IV) (the pseudo-first-order reduction rate constant, Kobs, was 0.11 h(-1)). Of particular interest was that this same sediment could be induced to release very low concentrations of sorbed Pu under oxidizing conditions, presumably by oxidation of sorbed Pu(IV) to the more mobile Pu(V) species. Transport modeling supported the postulation that Pu oxidation occurred in the lysimeter sediment; the inclusion of an oxidation term in the model produced simulations that capture the Pu depth profile data. By not including the oxidation process in the model, Pu mobility was grossly underestimated by a factor of 3.5. It is concluded that both oxidation and reduction mechanisms can play an important role in Pu transportthrough the vadose zone and should be considered when evaluating disposal of Pu-bearing wastes.

Soil organic matter and migration properties of239,240Pu and241Am

Humus influence on mobility of plutonium and americium in soil has been investigated. To do this the entrance of radionuclides into soil water solutions with different organic matter concentration and the distribution of 239,240 Pu and 241 Am between humus fractions of soil has been analyzed. The mineral and organic soils with distinct quantitative and qualitative composition of humus, contaminated by radionuclides of Chernobyl origin have been studied. The groups of fulvic (FA) and humic acids (HA) which differ in their composition and solubility in water were extracted from soils and the content of 239,240 Pu and 241 Am in these organic fractions was detected. It was established that humus substances (HS) mainly promote the fixation of 239,240 Pu and 241 Am in solid phase of soil. The humus of organic soils promotes the fixation of plutonium and americium to a greater extent than HS of mineral soils. That is one of the reasons of lower mobility of 239,240 Pu and 241 Am in organic soils. A higher ability of plutonium to form the difficultly soluble complexes with HS compare to americium may be a factor of higher mobility of americium in soil.