Cesium transfer to millet and mustard as a function of Cs availability in soils

¹³⁷Cs is one of the most persistent radioactive contaminants in soil after a nuclear accident. It can be taken up by plants and enter the human food chain generating a potential human health hazard. Although a large amount of literature has highlighted the role of the different processes involved in Cs uptake by plants, there is still no simple way to predict its transfer for a specific plant from a particular soil. Based on the assumption that the concentration ratio (CR) of Cs can be predicted from one plant taxon if the CR of another taxon is known and taken as reference, whatever the supporting soils, a series of plant/soil Cs transfer experiments were performed on Rhizotest during 21 days using three soils with different textures, clay and organic matter contents and two plants (millet and mustard) with potentially contrasting Cs uptake capacity based on their phylogeny. CRs of each plant varied by 2-3 orders of magnitude depending on the soil and contrary to expectations, the CRs of mustard were either higher (for clay soil), equal (for clay-loam soil) or lower (for sandy soil) than the one of millet. Considering Cs availability in soils and defining a new CR based on the amount of Cs available in the soil (CR_avail) decreased the range of variation in CR between the different soil types for a given plant by one order of magnitude. Differences in Cs (and K) translocation to shoots, possibly specific to millet within Poales, could partly explain the relative CRs of millet and mustard as a function of soils.

Meta-analysis of radiocesium contamination data in Japanese cedar and cypress forests over the period 2011-2017

Since Fukushima accident, dozens of field studies have been conducted in order to quantify and understand the behaviour of atmospheric radiocesium (¹³⁷Cs) fallouts in contaminated forests of Fukushima and neighbouring prefectures. In this paper, we carry out a detailed review of data acquired over 2011-2017 in Japanese cedar and cypress plantations, focusing on aerial tree organs, soil layers and tree-to-soil depuration fluxes. To enable comparison and reinforce the consistency between sites, radiological measurements were normalized by the deposit and interpolated onto the same spatio-temporal frame. Despite some (poorly explained) residual variability, we derived a "mean" pattern by log-averaging data among sites. These "mean" results were analysed with the help of a simple mass-balance approach and discussed in the light of post-Fukushima literature. We demonstrated that the activity levels and dynamics in all compartments were consistent and generally well reproduced by the mass balance approach, for values of the interception fraction between 0.7 and 0.85. The analysis indicated that about 5% of the initial deposit remained in the aerial vegetation after 6 years, more than two thirds of intercepted ¹³⁷Cs being transferred to the soil due to throughfall. The simulations indicated that foliar uptake might have contributed between 40% and 100% to the activity transferred to stem wood. The activity concentration in canopy organs rapidly decreased in the first few months then more slowly, according to an effective half-life of about 1.6 years. The activity level in the organic layer peaked in summer 2011 then decreased according to an effective half-life of 2.2 years. After a rapid increase in 2011, the contamination of mineral horizons continued to increase more slowly, 85% of ¹³⁷Cs incoming through the organic layer being retained in the 0-5 cm layer according to a mean residence time longer than in the upper layer (7 against 1.5 years).

Kinetics of selenate sorption in soil as influenced by biotic and abiotic conditions: a stirred flow-through reactor study

This study (i) quantified the kinetics of selenate sorption and (ii) measured the influence of biotic processes in soil selenate stabilisation. Stirred flow-through reactor experiments were conducted on samples of a silty clay soil (pH = 8, Eh = 240-300 mV) from Bure (France) in both non-sterile and sterile conditions. Parameters of the proposed two-site sorption model (EK), adapted from van Genuchten and Wagenet (1989), were estimated by nonlinear regression. Fast selenate sorption on type-1 sites was moderate, with an equilibrium constant of 25.5 and 39.1 L/kg for non-sterile and sterile conditions. Rate-limited sorption on type-2 sites increased with time, and was predominant for longer periods of time in non-sterile conditions. At equilibrium, it would represent over 96% of the sorbed inventory, with mean sorption times of 17 h and 191 h for non-sterile and sterile conditions. Our results showed for Bure soil that (i) selenate sorption in flowing and mildly-oxidising conditions was strongly kinetically controlled, especially in non-sterile conditions, (ii) selenate desorption was much slower than sorption, which suggests its pseudo-irreversible stabilisation, and (iii) microbial activity increased the contribution of rate-limited sorption on type-2 sites, for which it increased sorption rate by a factor 7 but also facilitated its reversibility. This work stresses the limits of the Kd approach to represent selenate sorption in flowing conditions and supports an alternative formulation like the EK model, but also points out that biotic conditions are significant sources of variability for sorption parameters.

Soil nematode assemblages as bioindicators of radiation impact in the Chernobyl Exclusion Zone

In radioecology, the need to understand the long-term ecological effects of radioactive contamination has been emphasised. This requires that the health of field populations is evaluated and linked to an accurate estimate of received radiological dose. The aim of the present study was to assess the effects of current radioactive contamination on nematode assemblages at sites affected by the fallout from the Chernobyl accident. First, we estimated the total dose rates (TDRs) absorbed by nematodes, from measured current soil activity concentrations, Dose Conversion Coefficients (DCCs, calculated using EDEN software) and soil-to-biota concentration ratios (from the ERICA tool database). The impact of current TDRs on nematode assemblages was then evaluated. Nematodes were collected in spring 2011 from 18 forest sites in the Chernobyl Exclusion Zone (CEZ) with external gamma dose rates, measured using radiophotoluminescent dosimeters, varying from 0.2 to 22 μGy h(-1). These values were one order of magnitude below the TDRs. A majority of bacterial-, plant-, and fungal-feeding nematodes and very few of the disturbance sensitive families were identified. No statistically significant association was observed between TDR values and nematode total abundance or the Shannon diversity index (H'). The Nematode Channel Ratio (which defines the relative abundance of bacterial- versus fungal-feeding nematodes) decreased significantly with increasing TDR, suggesting that radioactive contamination may influence nematode assemblages either directly or indirectly by modifying their food resources. A greater Maturity Index (MI), usually characterising better soil quality, was associated with higher pH and TDR values. These results suggest that in the CEZ, nematode assemblages from the forest sites were slightly impacted by chronic exposure at a predicted TDR of 200 μGy h(-1). This may be imputable to a dominant proportion of pollutant resistant nematodes in all sites. This might result from a selection at the expense of sensitive species after the accident.

Morphological variability of the shape of striped red mullet Mullus surmuletus in relation to stock discrimination between the Bay of Biscay and the eastern English Channel

Truss analysis and length measurements were made on 168 striped red mullet Mullus surmuletus. Multivariate statistical analyses with principal component analysis and partial redundancy analysis (pRDA) were used on these measurements to evaluate the influence of maturity, sex and geographical area distribution on body shape. Truss measurements were important to quantify and discriminate changing body shape, presumably due to changing environmental conditions. Sexual dimorphism was not observed and juveniles could be distinguished from adults based on their body shape. More importantly, M. surmuletus occurring in different geographical areas could be differentiated using this method. Based on pRDA, a significant difference of head morphological dimensions was observed between populations occurring in the eastern English Channel and those occurring in the Bay of Biscay, suggesting that fish from these areas could represent two subpopulations.

Investigating the embryo/larval toxic and genotoxic effects of γ irradiation on zebrafish eggs

Eggs/larval of freshwater fish (Danio rerio) were exposed to low dose rates of external gamma radiation (from 1 to 1000 mGy d(-1)) over a 20-day period, with the objective of testing the appropriateness of the 10 mGy d(-1) guideline suggested by the IAEA. The present study examines different endpoints, mortality and hatching time and success of embryos as well as the genotoxicity of γ-irradiations (after 48 h). The 20-day embryo-larval bioassay showed an enhanced larval resistance to starvation after chronic exposure to γ irradiation (from low 1 mGy d(-1) to high dose rate 1000 mGy d(-1)) and an acceleration in hatching time. Gamma irradiation led to increased genotoxic damage Ito zebrafish egg (40-50% DNA in tail in Comet assay) from the lowest dose rate (1 mGy d(-1)). Possible mechanisms of γ radiotoxicity and implications for radioprotection are discussed.

Sorption of selenate on soils and pure phases: kinetic parameters and stabilisation

This study was conducted to identify the principle selenate carrier phases for two selected soils, by comparing their reactivity with selenate to that of pure phases of the solids. Silica, calcium carbonate, aluminium hydroxide, goethite, bentonite and humic acid were selected as the main soil carrier phases. Comparisons were made first on the parameter values obtained with the best fit of a kinetic sorption model which can discriminate instantaneous sorption from kinetically limited sorption. Then comparisons were made of the ability for each solid to stabilise selenate by measuring the ratio of the partition coefficient for sorption (Kd(sorption)) over that of the desorption (Kd(desorption)). Kinetics and stabilisation were used to help elucidate the nature of interactions with the test solid phases for a large range of selenate concentrations. The experiments were conducted over 165 h in batch reactors, the solid being isolated from the solution by dialysis tubing, at two pH (5.4 and 8) and three selenate concentrations (1 × 10(-3), 1 × 10(-6) and 1 × 10(-8) mol L(-1)). The results obtained showed that only aluminium hydroxide can sorb selenate throughout the studied pH range (pH 5.4 to 8.0). The sorption capacity on this mineral was high (Kd(sorption) > 100 to 1 × 10(4) L kg(-1)) and the selenate was mainly stabilized by the formation of inner sphere complexes. The sorption on goethite occurred at pH 5.4 (Kd(sorption) 52 L kg(-1)), mainly as outer sphere complexes, and was null at pH 8. On silica, a weak sorption was observed only at pH 5.4 and at 165 h (Kd(sorption) 4 L kg(-1)). On bentonite, calcium carbonate and humic acid no significant sorption was observed. Concerning the two soils studied, different behaviours were observed for selenate. For soil Ro (pH 5.4), Kd(sorption) was low (8 L kg(-1)) compared to soil Bu (pH 8) (70 L kg(-1)). The sorption behaviour of selenate on soil Ro was mainly due to outer sphere complexes, as for goethite, whereas for soil Bu the sorption was mainly attributed to inner sphere complexes followed by reduction mechanisms, probably initiated by microorganisms, in which no steady state was reached at the end of the 165 h experiments. The sorption of selenate decreased when concentrations reached 1 × 10(-3) mol L(-1), due to solid saturation, except for aluminium hydroxide. Reduction of selenate seemed also to occur on goethite and soil Ro, for the same concentration, but without preventing a decrease in sorption. Thus, this work shows that the comparison of selenate behaviour between soil and pure phases helps to elucidate the main carrier phases and sorption mechanisms in soil.

An alternative method of particulate fluorescent tracer analysis in sediments using a microplate fluorimeter

Conservative particulate fluorescent tracers (e.g. luminophores and microspheres) are commonly used in a wide range of sediment transport studies. Traditionally, their spatial redistribution is estimated by counting them in sediments under ultraviolet light (e.g. by epifluorescence microscopy), a time-consuming but effective method. While alternative methods have recently been developed (e.g. photodetection, digital image analyses), this 'classical' counting method remains the most commonly used. This article describes an alternative procedure for measuring the concentration of fluorescent tracers (here, microspheres) using a microplate fluorimeter. This technique enables simultaneous analysis of numerous samples while reducing the sediment preparation and quantification time. After a calibration step from sediment samples with known microsphere content, the method was validated by comparing results from the epifluorescence microscopy method. Different adjustments were also reported, as well as application examples. The different calibration tests showed high linear relationships between the microsphere concentration of sediment samples and the measured fluorimetric intensities (R2-0.99) with a detection limit of 6%. In comparison with the previously used method, very similar results were obtained, as illustrated in recent studies using both luminophores and microspheres. The rapid and reliable method proposed here will enable increasingly complex experiments to be performed with less time-consuming qualitative analyses.

Inner-sphere, outer-sphere and ternary surface complexes: a TRLFS study of the sorption process of Eu(III) onto smectite and kaolinite

The surface sorption process of Eu(III) onto smectite and kaolinite was investigated by time-resolved laser fluorescence spectroscopy (TRLFS) in the trace concentration range. The experiments were performed in 0.025 M and 0.45 M NaClO4. The sorption process of Eu(III) onto smectite was obtained by TRLFS under atmospheric conditions and in absence of CO2. The pH was varied between 3.5 and 9 at a fixed metal ion concentration of 3.3 × 10−6mol/L Eu(III). At low pH (< 4) the metal ion keeps its complete hydration sphere indicating outer-sphere complexation. With increasing pH the formation of an inner-sphere Eu(III) surface complex was observed. The differences in the spectra and the fluorescence emission lifetimes of the surface sorbed Eu(III) in presence and absence of carbonate indicate the formation of ternary clay/Eu(III)/carbonate complexes. The different europium/clay surface complexes were characterized by their fluorescence emission spectra (5D0→7F1/5D0→7F2intensity ratio) and their fluorescence emission lifetime.

Time-resolved laser fluorescence spectroscopy study of the sorption of Cm(III) onto smectite and kaolinite

For long-term performance assessment of nuclear waste repositories knowledge concerning interactions of actinides with mineral surfaces is imperative. The mobility and bioavailability of released radionuclides is strongly dependent on sorption/desorption processes onto mineral surfaces. Therefore it is necessary to characterize the surface species formed and to elucidate the reaction mechanisms involved. The high fluorescence spectroscopic sensitivity of Cm(III) has attracted our interest regarding the complexation process of Cm(III) onto smectite and kaolinite as a model system for the sorption of trivalent actinides in the trace concentration range. We conclude that at low pH Cm(III) is sorbed onto kaolinite and smectite as an outer-sphere complex and retains its complete primary hydration sphere. With increasing pH inner-sphere adsorption onto kaolinite and smectite occurs via the aluminol edge sites. The same evolution of the Cm(III)-clay surface species as a function of pH was observed for both minerals. Starting at a pH > or = 5 we observe the formation of a [triple bond]Al-O-Cm2+(H2O)5 surface complex, which is replaced by a second species at higher pH. The second surface complex may be a monodentate [triple bond]Al-O-Cm+(OH)(H2O)4 species or bidentate [triple bond](Al-O)2-Cm+(H2O)5 species. The Cm(III)/clay surface complexes are characterized bytheir emission spectra (peak maxima at 598.8 and 603.3 nm) and their fluorescence lifetime (both 110 +/- 7 micros). An important result in view of the mobility and bioavailability of radionuclides is that no incorporation of Cm(III) into the bulk clay structure was observed.

Determination of cefotaxime and desacetylcefotaxime in plasma and urine by high-performance liquid chromatography

A high-performance liquid chromatographic method is described for the analysis of the anti-bacterial agent cefotaxime and desacetylcefotaxime in physiological fluids. Plasma or serum samples were mixed with chloroform--acetone to remove proteins and most lipid material. The aqueous phase was then freeze-dried, reconstituted in mobile phase and chromatographed on a reversed-phase column using UV detection at 262 nm. Urine was analysed directly after centrifugation to remove particulate matter. The detection limit was 0.5--1.0 micrograms/ml for serum and 5 micrograms/ml for urine. The method has been applied to the analyses of cefotaxime and desacetylcefotaxime in plasma, serum, urine, cerebrospinal fluid, saliva, and pus from infected wound secretions. Two additional metabolites, which are lactones in which the beta-lactam ring has been opened, could be separated by this method.