A study on radionuclide association with soil components using a sequential extraction procedure

Measurements performed in 1986–1988 demonstrate that most of the radiocesium isotopes (¹³⁷Cs and¹³⁴Cs) deposited after the Chernobyl accident are still located in the upper soil layers (0–2 cm). The vertical migration appears to be slow, and only a small fraction of the radiocesium has been transferred into the biological cycle. Sequential extraction techniques have been utilized in order to investigate the degree of binding or association between deposited radionuclides (¹³⁷Cs,¹³⁴Cs and⁹⁰Sr) and components in soil. The results indicate that a major fraction of the radiocesium is associated strongly with organic and mineral materials in the litter or upper soil layers: less than 10% is easily leachable. The distribution of¹³⁷Cs throughout the fractions was similar to that determined for naturally occurring stable cesium (¹³³Cs), implying that isotopic exchange had been extensive. For⁹⁰Sr, the results show a relatively high leachable fraction. Therefore, present results indicate that radiocesium should be less mobile, and less available for root uptake, than⁹⁰Sr in soil.     

Prediction capacity of a sequential extraction scheme

The predictions of a sequential extraction scheme with respect to the mobility of some radionuclides (⁸⁵Sr,¹³⁴Cs and^110mAg) in two Mediterranean sandy and sandy-loam soils, are compared to short-term soil-to-plant transfer factors and soil migration. Total soil-to-plant transfer is higher in sandy soil than in sandy-loam soil, as expected and predicted by the scheme. The relative transfer to plants of¹³⁴Cs and⁸⁵Sr follows the scheme predictions about exchangeable radionuclide fraction, radiosilver being less mobile than expected. Migration in soil of radiocesium and radiostrontium is also higher in sandy soil, especially for the latter radionuclide, the relative behavior of these two radionuclides being nearer to the bioavailable radionuclide fraction defined by the scheme. However, the scheme fails in predicting radiosilver migration, which is lower than deduced by the scheme.     

Local distribution of radioactivity in tree leaves contaminated by fallout of the radionuclides emitted from the Fukushima Daiichi Nuclear Power Plant

We analyzed fresh and dead leaves collected in forests in Fukushima after the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, using autoradiography. Both fresh and dead leaves of Cryptomeria japonica were contaminated by radionuclides (¹³⁴Cs and ¹³⁷Cs). Contamination of the fresh leaves was possibly attributed to interception of radionuclides by tree canopies, whereas the dead leaves indicated the direct deposition of radionuclides by fallout and/or washout of radionuclides intercepted by tree canopies. Translocation of radiocesium from a contaminated branch to new leaves growing after the FDNPP accident was not clearly observed, although transfer of radiocesium from leaf parts to male flowers occurred. Fallen leaves of Quercus serrata, which started growing after the FDNPP accident, did not show radioactivity, indicating that significant amounts of translocation from other parts to new leaves did not occur. Fallen leaves of Q. serrata collected from a litter showed hot spots originating from direct fallout. Needles of Pinus densiflora were also contaminated by fallout. Leaching with pure water removed soluble fractions of radiocesium and hot particles from the surface of the contaminated leaves, but significant amounts of radioactivity remained. This means that foliar absorption occurred in both fresh and dead leaves. Further leaching experiments using surfactant and acetone could not remove the remaining radiocesium from the leaves. The leaching experiments indicate that radiocesium in the contaminated leaves is strongly fixed in leaf tissues and is not readily released unless leaf tissues are decomposed.     

Factors affecting interaction of radiocesium with freshwater solids

The paper aims at the analysis of principal factors affecting the interaction of radiocesium with freshwater solids, important for the migration of radiocesium in rivers. Uptake of radiocesium by bottom sediments and suspended solids from small streams was studied as a function of pH and composition of aqueous phase, of the concentration of cesium in water and of the composition of freshwater solids, using laboratory model experiments. pH had negligible effect on the uptake in the pH range 5–9, the uptake decreased at pH values less than 3–5 depending on the nature and concentration of the solids. Addition of cations suppressed the uptake in the order K⁺>Na⁺>Ca²⁺, the suppression began at 0.001, 0.01 and 0.1 mol.dm^?3 concentration, respectively. Increase in cesium concentration in water caused a decrease of radiocesium uptake, but at very low concentrations of cesium combined with higher concentration of sediment (2g·dm^?3) the uptake was independent of cesium concentration. Removal of carbonates, oxidic coatings and organic matter from a sediment did not affect the sorption properties of the sediment. The nature of the effects found confirms that cesium is sorbed mainly by clay components of freshwater solids. Results obtained are compared with literature data and conclusions are drawn on the importance of the factors studied for modelling of radiocesium migration in rivers.     

The radiation dose and distribution coefficient of 210Po and 210Pb concentrations in aquatic environs of major rivers of coastal Karnataka

The activity of ²¹⁰Po and ²¹⁰Pb was measured in different matrices of aquatic ecosystem of the major rivers of Coastal Karnataka viz, Kali, Sharavathi and Netravathi. The environmental samples such as surface water, suspended particulate matter and sediment have been subjected to analyses. The activity of these two radionuclides were determined by radiochemical separation of ²¹⁰Po and counting the activity using a ZnS(Ag) alpha counter. The activity ratio of ²¹⁰Po and ²¹⁰Pb and correlation between the activity of these radionuclides were studied. From the measured concentration of ²¹⁰Po and ²¹⁰Pb, the internal Committed Effective Dose to the population for the study area was calculated. The distribution coefficient K_d between water, suspended particulate matter and sediments have been calculated to understand the distribution and accumulation of these radionuclides in different matrices of the aquatic environment.

     

Relationship between particle size and radiocesium in fluvial suspended sediment related to the Fukushima Daiichi Nuclear Power Plant accident

We collected fluvial suspended sediments in Fukushima after the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident and analyzed the ¹³⁷Cs concentration in bulk and size-fractioned samples to investigate the particle-size-dependent distribution of radiocesium. The ¹³⁷Cs concentration in bulk suspended sediments decreased from August to December 2011, possibly reflecting a decrease of radiocesium concentration in its source materials. Smaller particles had higher radiocesium concentrations, reflecting larger specific surface areas. Silt- and sand-size fractions occupied more than 95 % of the total ¹³⁷Cs in the suspended sediments. The contribution of clay-size fractions, which had the highest ¹³⁷Cs concentration, was quite small because of their low frequency. A line of the data showed that the particle size distribution of radiocesium was essential to evaluate the migration and distribution of radiocesium in river systems where radiocesium is mainly present as particulate form after the FDNPP accident.     

Distribution and mean residence time of natural radionuclides in the forest ecosystem

Activity concentrations of radionuclides,⁷Be,²¹⁰Pb and²¹⁰Po, in precipitation (rain, throughfall and stemflow), wood and soil were determined by using gamma-ray and alpha-ray spectrometry to estimate the migration behavior of these radionuclides in the forest canopy. The activity ratios between output and input precipitations for the forest canopy were 0.53 of⁷Be, 0.79 of²¹⁰Pb and 1.5 of²¹⁰Po for Pasania edulis forest. A dynamic model of the transport and fate of radionuclides in the forest ecosystem was constructed. Mean residence times of radionuclides were 56 days for⁷Be, 765 days for²¹⁰Pb and 653 days for²¹⁰Po for Pasania edulis forest.     

Heterogeneous distribution of radiocesium in aerosols, soil and particulate matters emitted by the Fukushima Daiichi Nuclear Power Plant accident: retention of micro-scale heterogeneity during the migration of radiocesium from the air into ground and river systems

We analyzed ¹³⁷Cs in aerosols, rock, soil and river suspended sediment collected after the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. Based on the results, we discuss the post-event behavior and transportation of radiocesium in the environment from the air into ground and river systems. First, radionuclides were emitted from the FDNPP as airborne ‘hot’ particles, which contained water-soluble fractions of radiocesium. Radiocesium was still present in a water-soluble fraction after deposition on the ground. Subsequent interaction of the ‘hot’ particles with water (e.g. rainfall) dissolved and strongly fixed the radiocesium on rock and soil particles, thus changing the radiocesium into insoluble forms. The distribution of ‘hot spots’ was possibly controlled by the initial position of deposition on the ground. Consequently, ‘hot spots’ were studded on the rock surface rather than being uniformly distributed. The distribution of radiocesium in river suspended particles was not homogeneous during water transportation, reflecting the heterogeneity of radiocesium in rock and soil. Leaching experiments demonstrated that radiocesium in rock, soil and river suspended sediment was fairly insoluble, showing that the adsorption reaction is irreversible. The micro-scale heterogeneous distribution of radiocesium in aerosols, soil and suspended particles was due to the presence of ‘hot’ particles in aerosols. Dissolution of radiocesium in the ‘hot’ particles in the aerosols and subsequent irreversible adsorption onto the soil particle complex are responsible for the preservation of the heterogeneity both in soil and in river suspended particles.     

Factors affecting interaction of radiocesium with freshwater solids

The paper aims at the analysis of principal factors affecting the interaction of radiocesium with freshwater solids, important for the migration of radiocesium in rivers. Uptake and release of radiocesium by bottom sediment and suspended solids from a small stream were studied as a function of contact time during the uptake and releases, of concentration of the solid phases and of temperature, using laboratory model experiments. Kinetics of the uptake were found to be singificantly affected by temperature and concentration of the solid phase. The kinetics and the concentration effect can be quantitatively described using kinetic model of two parallel or consecutive reactions. Kinetic parameters for the model were determined. Distribution coefficient K_d is independent of sediment concentration in the range of 20–70 mg·dm^?3 but passes through a maximum at higher concentration values. Release of radiocesium adsorbed on the freshwater solids was found to be quite rapid and a simple kinetics of the release from freshwater solids was observed. The amount released decreased with increasing contact time of radiocesium with solid phase. Quantitative evaluation of the release revealed partial irreversibility of radiocesium uptake on the solids studied. Results obtained are compared with literature data and conclusions are drawn on the importance of the factors studied for modeling of radiocesium migration in rivers.     

The validation of a method for determining the migration of trace elements from food packaging materials into food

A new radiotracer method has been developed to measure the migration of trace elements from food contact packaging into four standard food simulants; acetic acid, ethanol, olive oil, deionised water. A sample of material is irradiated in a thermal neutron flux of 10¹⁶n·m^–2·s^–1 to activate the trace elements and produce a range of radionuclides. The sample is then placed in the food simulant and the migration of the radionuclides is monitored by performing -ray spectrometry on a sample of the simulant. Any radionuclides measured must be due entirely to the migration of the elements present in the plastic, since the simulant itself is not radioactive. Preliminary studies have shown that detection limits of around 0.2g·dm^–2 (0.002 mg/kg) can be achieved for antimony in a sample of polyethylene terephthalate. This method can now been extended to measure migration into real foods. This will highlight any differences between the standard simulants currently used and real foods. Since the method only involves irradiation of the packaging material, any food matrix can be studied.     

Mobility of 137Cs Related to Speciation Studies in Contaminated Soils of the Chernobyl Area

Chernobyl derived ¹³⁷Cs present in upper soil layers can still remain available to cation exchange and hence to downward migration. More than its physico-chemical deposit forms (fuel particles and/or condensed aerosols), radiocesium mobility is influenced by soil properties. Although all the soils studied contain illitic clay minerals, different ¹³⁷Cs fixation levels were observed due to: (1) the reactivity state of sorption sites with a low radiocesium content in soil, and (2) the presence, or absence, of organic matter complexes inhibiting cation sorption on clays.     

Influence of freezing on physicochemical forms of natural and technogenic radionuclides in Chernozem soil

Sharp variations of different climatic parameters influence the transport, transfer, and deposition of contaminants in nature. Investigations of the impact of environmental temperature on the fractionation of radionuclides in soil are necessary for adequate assessment of their distribution and bioavailability in case of a nuclear accident. The impact of a sharp decrease of environmental temperature shortly after radioactive contamination on the physicochemical fractionation of natural and technogenic radionuclides in Chernozem soil and its influence on their potential migration ability and bioavailability in case of subsequent warming were evaluated. The soil was contaminated in a laboratory with ²⁴¹Am, ⁶⁰Co, ¹³⁷Cs, ²²⁸Ra, ²³⁴Th, and U and two temperature regimes were used for storage. Changes of the radionuclides association with various soil phases in the first weeks after contamination were studied. Physicochemical forms of ²⁴¹Am, ⁶⁰Co, ²²⁸Ra, ²³⁴Th, and U were determined using two sequential extraction procedures. The ion-exchangeable forms of ¹³⁷Cs were evaluated by single extraction with 1 M NH₄NO₃. The data showed that the freezing, following the radioisotope contamination of the soil, causes an increase of the amount of potentially mobile forms of radiocobalt, radiocesium, radium, and thorium and has an insignificant impact on the fractionation of americium and uranium.     

An Efficient Approach to Model the Long-Term Radiocesium Contamination of Mushrooms and Berry Plants

Since 1987, a coniferous forest in Bavaria has continuously been monitored for radiocesium. About 350 soil samples and about 450 samples of different species of mushrooms and berry plants were analyzed for ¹³⁴Cs and ¹³⁷Cs. Based on this extensive data set a radioecological model for the long-term contamination of mushrooms and berry plants was developed. To keep the model as simple as possible without losing predictive power, it was especially designed to describe the vertical migration of radiocesium in forest soil, a key process which governs the temporal evolution of activity levels in mushrooms and berry plants.     

Extraction of radiostrontium from the mixture of radionuclides in milk using dicarbolide of cobalt

The possibilities of radiostrontium extraction from milk after the isolation of radiocesium were investigated. The polyhedral complex of the type H⁺[(π-/3/-1,2-C₂B₉H₁₁)₂Co⁻], further referred to as dicarbolide-H⁺, in nitrobenzene was used for the extraction. The increase of values and larger differentiation of distribution ratios in the extraction of ion associates of radionuclides were achieved by hydrophobizing with polyethyleneglycol (PEG). The changes of the distribution ratios of⁹⁰Sr,⁸⁹Sr and⁹⁰Y with PEG concentration were studied, and ranges of the highest synergic effect, changing with different extraction agent concentrations in extraction from water solutions and a fresh milk were determined. The influence of milk dilution and nitric acid concentration upon the value of distribution ratio in radiostrontium extraction using dicarbolide-H⁺ and the optimum PEG concentration were investigated. Under suitably selected conditions of radiostrontium isolation, the distribution ratios of some potential contaminants were determined. The influence of nonisotopic carrier Ca²⁺ upon radiostrontium extraction was examined. The possibilities of purification of isolated radiostrontium using reextraction were investigated. A selective and quick analytical procedure was suggested for radiostrontium isolation after the separation of radiocesium from a mixture of fission and activated radionuclides in milk using extraction by dicarbolide-H⁺. The chemical yield is about 98% and⁹⁰Sr is determined by liquid scintillation technique.     

Distribution and circulation of radionuclides originating from fallout in a forest

Water, plant and soil samples from forested area were analyzed in order to study the behavior of radionuclides in the forest ecosystem. The concentration of ¹³⁷Cs in the soil decreased with depth, while stable Cs and ⁴⁰K were almost constant. It is suggested that most of the ¹³⁷Cs once entered in the forest ecosystem is trapped in the upper part of the soil.     

A novel method for the determination of migration of contaminants from food contact materials

A neutron activation method has been developed for the analysis of high density polyethylene, low density polyethylene, polypropylene, polyethylene terephthalate and polystyrene food contact plastics. The method provides determination of over 50 elements at concentrations below 1 mg kg^–1. This technique has now been extended to study migration from food contact materials into standard food simulants (olive oil, acetic acid, ethanol and water). Samples of plastic are irradiated in a thermal neutron flux to produce radionuclides of the elements present in the plastic. Over a period of time the radionuclides of these elements may travel from the plastic into the food simulants, and hence the migration can be determined. Gamma ray spectrometry is performed on the simulants at the end of the test to quantify the migration. Any activity present must be due only to the migration of radionuclides of elements in the plastic and nothing else. This eliminates the need for a blank determination, which is required with existing migration methods. Preliminary studies have shown that detection limits of around 0.002 mg kg^–1 can be achieved for Sb in a retail polyethylene terephthalate (PET) bottle. This can be compared to levels of 0.005     

Vertical migration of 85Sr, 137Cs and 131I in various arable and undisturbed soils

The vertical migration of ⁸⁵Sr, ¹³⁷Cs and ¹³¹I in some arable and undisturbed single-contaminated soils was studied by gamma-spectrometry measurements under lysimetric laboratory conditions during irrigation of the soil profiles with wet atmospheric precipitation for about one year, except ¹³¹I. A new simple exponential compartment (box) model was derived, which makes it possible to calculate the migration rate constants and migration rates in the individual soil layers (vertical sections) as well as the total vertical migration rate constants and total vertical migration rates of radionuclides in the bulk soil horizon. The relaxation times of radionuclides in respective soil horizons can also be evaluated.     

Behavior of <Superscript>134</Superscript>Cs, <Superscript>90</Superscript>Sr,and <Superscript>238</Superscript>Pu in different Syrian soils

The experiment aimed to evaluate the vertical migration of ¹³⁴Cs, ⁹⁰Sr and ²³⁸Pu in the main types of Syrian soils; entisol, inceptisol, alluvial (rock outcrops) and gypsiferous soils, using soil columns through which the aqueous solution of the radionuclides percolated. The results show that the vertical migration of the studied radionuclides through the soil profile depend on the radionuclide and the soil type. More than 97% of ¹³⁴Cs and ²³⁸Pu concentrated in the upper 2 cm of the entisol, inceptisol, and alluvial soils, whereas only 46.2% to 68.6% of the ⁹⁰Sr was retained in the upper 2 cm of these soils. The vertical migration of the studied radionuclides in the gypsiferous soil was different from the other soils. The distribution of the radionuclides in the gypsiferous soil was irregular through the soil profile and reached the deeper layer of the soil. This may be due to its physical characteristics; poor structure stability, high permeability and low retention capacity.     

Radionuclide migration experiments in the laboratory: Some radioanalytical aspects

Radionuclide transport studies in consolidated and unconsolidated geological media are being performed in the laboratory as part of the Canadian Nuclear Fuel Waste Management Program. These studies present an interesting challenge to radiochemists because many of the radionuclides are either not eluted or only partially eluted from fractures or from columns. Therefore, non-invasive radiometric analyses have been applied to determine the location of radionuclides along a flow path during a migration experiment, and invasive analyses have been developed to determine the transport behaviour of these radionuclides at the termination of a migration experiment. Linear gamma scanning has been used in migration experiments with^95mTc and¹³¹I through columns of crushed fracture-infilling material to follow their movement through the columns. Two-dimensional gamma scanning has been used to determine the distribution of¹⁴⁴Ce on fracture surfaces at the termination of migration experiments through natural fractures in granite. In addition, information on the interaction of radionuclides with specific minerals can often be obtained by separating minerals on the basis of density, or by a combination of density and magnetic properties, followed by radiometric assay of the separated fractions.     

Migration of 134Cs in unsaturated soils at a site in Egypt

To demonstrate the safety performance assessment for the disposal of ¹³⁴Cs radionuclide in a geological formation, several investigations were required to calculate the possible release of radionuclides into groundwater. This research examined the sorption behavior of radioactive cesium (¹³⁴Cs) in natural groundwater. Cesium chloride (10^-6 to 10^-2 mol^.l^-1) was used as a carrier, traced with ¹³⁴Cs radionuclide. Distribution coefficients of radiocesium for sorption and desorption were measured on natural soil samples of different grain size fractions (400 to 36 mm). Cesium sorption and desorption were found reversible at cesium chloride concentrations between 10^-6 mol^.l^-1 to less than 10^-3 mol^.l^-1. Sequential extraction procedures showed that the cesium sorption on soils were of various types: those easily desorbed, ion-exchanged, bound to carbonate, iron oxides, and organic matter. To demonstrate the safety performance assessment for the disposal of ¹³⁴Cs radionuclide in a geological formation, several investigations were required to calculate the possible release of radionuclides into groundwater. This research examined the sorption behavior of radioactive cesium (¹³⁴Cs) in natural groundwater. Cesium chloride (10^-6 to 10^-2 mol^.l^-1) was used as a carrier, traced with ¹³⁴Cs radionuclide. Distribution coefficients of radiocesium for sorption and desorption were measured on natural soil samples of different grain size fractions (400 to 36 mm). Cesium sorption and desorption were found reversible at cesium chloride concentrations between 10^-6 mol^.l^-1 to less than 10^-3 mol^.l^-1. Sequential extraction procedures showed that the cesium sorption on soils were of various types: those easily desorbed, ion-exchanged, bound to carbonate, iron oxides, and organic matter.