Binding of ²³⁹Pu and ⁹⁰Sr to organic colloids in soil solutions: evidence from a field experiment

Impact of a Biological Chelator, Lanmodulin, on Minor Actinide Aqueous Speciation and Transport in the Environment

Minor actinides are major contributors to the long-term radiotoxicity of nuclear fuels and other radioactive wastes. In this context, understanding their interactions with natural chelators and minerals is key to evaluating their transport behavior in the environment. The lanmodulin family of metalloproteins is produced by ubiquitous bacteria and Methylorubrum extorquens lanmodulin (LanM) was recently identified as one of nature's most selective chelators for trivalent f-elements. Herein, we investigated the behavior of neptunium, americium, and curium in the presence of LanM, carbonate ions, and common minerals (calcite, montmorillonite, quartz, and kaolinite). We show that LanM's aqueous complexes with Am(III) and Cm(III) remain stable in carbonate-bicarbonate solutions. Furthermore, the sorption of Am(III) to these minerals is strongly impacted by LanM, while Np(V) sorption is not. With calcite, even a submicromolar concentration of LanM leads to a significant reduction in the Am(III) distribution coefficient (Kd, from >104 to ∼102 mL/g at pH 8.5), rendering it even more mobile than Np(V). Thus, LanM-type chelators can potentially increase the mobility of trivalent actinides and lanthanide fission products under environmentally relevant conditions. Monitoring biological chelators, including metalloproteins, and their biogenerators should therefore be considered during the evaluation of radioactive waste repository sites and the risk assessment of contaminated sites.

90Sr in seawater of the East China Sea: Inventory, new potential source, and environmental implications

⁹⁰Sr is useful for tracing water mass movement in oceans. We collected a suit of seawater samples from the East China Sea (ECS) in the May 2011 to investigate the spatial and vertical distribution of ⁹⁰Sr and to understand its transportation and fate. To understand the sources and transportation of ⁹⁰Sr more clearly, published ¹³⁷Cs data from the same cruise were used to obtain the ⁹⁰Sr/¹³⁷Cs activity ratios. The results showed that ⁹⁰Sr activities were controlled by the circulation system of the ECS, with high values in coastal regions and low values in oceanic waters. The plume with a high ⁹⁰Sr/¹³⁷Cs ratio showed that in late spring, the Changjiang Diluted Water could flow southeastward and extend to 126-127° E, which is farther than the previously known value of 124° E. The high ⁹⁰Sr/¹³⁷Cs ratios (1.35 ± 0.62) and a long effective half-life of ⁹⁰Sr (20.0 ± 0.3 y) in the ECS surface water revealed that ⁹⁰Sr is surplus in comparison with ¹³⁷Cs. However, historical variations in the ⁹⁰Sr/¹³⁷Cs ratio seem to preclude the simple explanation that riverine input causes a ⁹⁰Sr surplus in the ECS. Groundwater discharge with a high ⁹⁰Sr but very low ¹³⁷Cs may be a new potential source. However, it is difficult to quantify the contribution of groundwater discharge at present, and more detailed studies are required in this regard. Additionally, we compiled ⁹⁰Sr and ¹³⁷Cs water column inventory data in the western North Pacific and found that the cumulative fallout onto the ocean was different from that on land in the 20-40° N band.

Origin and stability of uranium accumulation-layers in an Alpine histosol

Uranium (U) accumulation in organic soils is a common phenomenon that can lead to high U concentration in montane wetlands. The stability of the immobilized U in natural wetlands following redox fluctuations and re-oxidation events, however, is not currently known. In this study, we investigated a saturated histosol that had accumulated up to 6000 ppm of U at 30 cm below ground level (bgl). Uranium in the waters feeding the wetland originates from the weathering of surrounding gneiss rocks, a process releasing trace amounts (<3 ppb) of soluble U into nearby streams. Redox oscillations in the first 20 cm bgl led to the accumulation of U, Ca, S in low permeability layers at 30 and 45 cm bgl. XRF measurements along the core showed that U strongly correlates with sulfur (S) and calcium (Ca), but not iron (Fe). We tested the stability of uranium in the histosol over a nine-month laboratory amendment of a large core of the histosol (∅ 30 cm; length 55 cm) with up to 500 ppm nitrate. Nitrate addition was followed by complete nitrate reduction and re-generation of oxidizing E_h conditions in the top 25 cm of the soil without U release to the soil pore waters above background levels (1-2 ppb). Our results demonstrate that, fast reduction of nitrate, sulfate, and Fe(III) occur in the soil without U release. The remarkable stability of sorbed U in the histosol may result from buffering by sulfide and S_n° and/or strong U(IV)-OM or U(VI)-OM enhanced by organic S moieties or bridging complexation by Ca. That U in the soil was immobile under nitrate addition for up to 9 months can inform remediation strategies based on the use of artificial wetlands to limit U mobility in contaminated sites.

Fate of 137Cs, 90Sr and 239+240Pu in soil profiles at a water recharge site in Basel, Switzerland

An important process in the production of drinking water is the recharge of the withdrawn ground water with river water at protected recharge fields. While it is well known that undisturbed soils are efficiently filtering and adsorbing radionuclides, the goal of this study was to investigate their behaviour in an artificial recharge site that may receive rapid and additional input of radionuclides by river water (particularly when draining a catchment including nuclear power plants (NPP)). Soil profiles of recharge sites were drilled and analysed for radionuclides, specifically radiocesium (¹³⁷Cs), radiostrontium (⁹⁰Sr) and plutonium (^239+240Pu). The distribution of the analysed radionuclides were compared with an uncultivated reference soil outside the recharge site. The main activity of ¹³⁷Cs was located in the top soil (4.5-7.5 cm) and reached down to a depth of 84 cm and 48 cm for the recharge and the reference site, respectively. The found activities of ^239+240Pu originate from the global fallout after 1950. ^239+240Pu appeared to be strongly adsorbed onto soil particles. The shape of the depth profile was similar to ¹³⁷Cs, but also similar between the recharge and the reference site. In contrast, ⁹⁰Sr showed a uniform distribution over the entire depth of the recharge and reference profiles indicating that ⁹⁰Sr already entered the gravel zone and the ground water. Elevated inventories of the radionuclides were observed for the recharge site. The soil of the recharge field exhibited a threefold higher activity of ¹³⁷Cs compared to the reference soil. Also for ^239+240Pu higher inventories where observed for the recharge sites (40%). ⁹⁰Sr behaved differently, showing similar inventories between reference and recharge site. We estimate that 75-89% of the total inventory of ¹³⁷Cs in the soil at the recharge site (7.000 Bq/m²) originated from the fallout of the Chernobyl accident and from emissions of Swiss NPPs. This estimate is based on the actual activity ratio of ¹³⁷Cs/^239+240Pu of 22 for global fallout. The investigations identified radiostrontium as potential threat to the ground water.