Plants and soils from a natural thorium and rare-earth element occurrence (Morro do Ferro, Brazil) were analyzed by alpha spectrometry (Th) and ICP-AES (REE), after pre-concentration of the elements by solvent extraction, co-precipitation and ion exchange procedures. Leaching experiments with humic acid solutions and different soils were performed to estimate the fraction of elements biologically available. High concentrations of the light rare-earth elements (LREE) and of Th, reaching some hundreds of g/g-ash, were measured in plant leaves from the areas of the highest concentration of these elements in soil and in near-surface waters. Chondrite normalized REE plots of plant leaves and corresponding soils are very similar, suggesting that there is no significant fractionation between the REE during uptake from the soil solution and incorporation into the leaves. However, Ce-depletion was observed for some plant species, increasing forSolanum ciliatum in the sequence: leaves<fruits<seeds. Soil to plant concentration ratios (CR's) for Th and the REE, based on the total concentration of these elements in soils, are in the range of 10–3 to 10–2. Leaching experiments confirmed the importance of humic acid complexation for the bio-uptake of Th and REE and further showed that only a very small fraction of these elements in soil is leachable. The implications of these results on the calculated CR's will be discussed.
Authors:Y. Fujikawa, M. Sugahara, E. Ikeda, and M. Fukui
Chemical separation yield of actinide elements from soil samples containing high amounts of organic matters was often low as compared to that for samples containing low organic matters. We compared chemical yield of uranium and plutonium from organic-rich Japanese soil and humic acid samples which were subjected to three different pretreatment procedures ((1) pulverization only, (2) dry-ashing at 510 °C after pulverization, and (3) 680 kGy gamma-ray irradiation after pulverization) prior to acid digestion. Separation of U and Pu was performed by U-TEVA resin and AG1-X8 anion exchange resin, respectively. For organic samples, pretreatment procedures governed the yield of actinide elements significantly. Chemical separation yield of U was low (6±1%) for Aldrich humic acid sample after dry-ashing at 510 °C. For such samples, gamma-ray irradiation improved the yield of U to 35±22%. Recovery of Pu was lower for some organic soil samples which were dry-ashed at 510 °C (27±2%) as compared to those which were not dry-ashed (62±10%).
Authors:K. Lieser, R. Hill, U. Mühlenweg, R. Singh, Tu Shu-De, and Th. Steinkoff
The influence of the following parameters is discussed: oxidation state (Eh), hydrolysis (pH), solubility, complex formation, colloid formation, sorption and filtration effects. The oxidation states of the actinides in neutral aqueous media are plotted as a function of the redox potential. Hydrolysis, stability of 11 carbonato complexes, of humic acid complexes and solubilities of the hydroxides in the various oxidation states are considered. Mobility is investigated as a function of pH by paper chromatography. Partition of the actinides in groundwaters on the molecular fraction, the fine particle (colloid) fraction and the coarse particle fraction is determined as a function of pH by filtration and ultrafiltration. Sorption ratios are measured as function of pH, salt concentration and of EDTA concentration. When the groundwaters are passed through the sediments, mainly the coarse particle fractions are retained by size filtration. The pronounced influence of the redox potential on sorption is demonstrated for Np.
Authors:S. Nagao, Y. Sakamoto, T. Tanaka, and R. Rao
The association properties of Pu with aquatic humic substances in a 0.01M NaClO4 solution at pH 6–8 were studied on the basis of molecular size distribution. Seven humic substances were isolated from river
water and groundwaters using XAD extraction technique. They were used for comparing their effects on the association of Pu.
In the presence of humic acid, the dominant molecular size of Pu was 100–30 kDa. In the presence of fulvic acid, Pu exhibited
three dominant molecular sizes: 30–10 kDa, 30–5 kDa, and less than 5 kDa. The association of Pu-humus complexes might be controlled
by the molecular size distribution of humic substances and characteristics of their respective size fractions.
Three independent speciation techniques, the free liquid/moving boundary electrophoresis, ultrafiltration, and equilibrium dialysis, combined with the radiotracer method (using 152Eu) have been compared at the study of Eu interaction with humic acid (HA). The degree of complexation of Eu in 10 mg/l Aldrich HA solutions was determined within a broad range of metal loading (Eu total concentration 10-8-10-4 mol . l-1), at pH 4 and 6, ionic strength of 0.01 and 0.1 (NaClO4). From the anodic electrophoretic mobilities determined, additional information on the charge of the Eu-HA complexes was obtained. Uncertainty of the determination given by sorption losses of Eu on the walls and membranes of the experimental devices, and the effect of kinetic lability of the Eu-HA complexes are estimated. Based on the ultrafiltration experiments, the existence of an unknown, labile, high-molecular non-humic Eu species is discussed.
Authors:D. Banerjee, R. Guin, S. Das, and S. Thakare
A new method for the possible incorporation of nuclear wastes has been attempted here by using ceramic matrix of TiO2 as a host precursor for confinement. Hafnium is used as a simulant for actinide high-level waste. After incorporating 181Hf tracer into TiO2 matrix, the leaching property of the resulting matrix was studied in water, sodium chloride and humic acid solutions. The
leaching was measured in each of the case by following the radioactivity of 181Hf. TiO2 matrix has also been exposed to γ-radiation in order to simulate the radiation field for nuclear waste. It has been investigated
with a nuclear technique called time differential perturbed Angular Correlation (TDPAC) that the lattice structure of titania
remains undisturbed even under a strong radiation field. The leaching of 181Hf has also been studied after irradiating the TiO2 matrix with γ-radiation and the leaching behavior was observed not to change from that before irradiation.
MX-80 bentonite is considered as one of the best backfill materials for high-level radioactive nuclear waste. Herein, the
bentonite is characterized by using XRD and FTIR techniques. Sorption of radionickel to MX-80 bentonite in the presence/absence
of humic acid (HA) or fulvic acid (FA) as a function of pH is investigated. The results indicate that the presence of HA or
FA decreases the sorption of Ni2+ obviously. The different experimental processes do not affect the sorption of nickel to FA/HA bound bentonite. The sorption
of Ni2+ on FA/HA-bound bentonite decreases with the increasing FA/HA content in the systems. The mechanism of nickel sorption is
also discussed in detail.
Within this work the analysis of the kinetic stability of a series of yttrium complexes, i.e., Y-citrate, Y-NTA, Y-CDTA as
well as Y-humic acid (Y-HA) has been successfully performed by the free-ion selective radiotracer extraction (FISRE) method.
FISRE uses 90Y with a high specific activity to perform CHELEX extractions in a buffered aqueous solution at pH 6 in two different modes
by monitoring the dissociation and association reactions of the corresponding complexes. Whereas in the case of Y-citrate
the dissociation profile could be successfully described in terms of (pseudo) first order kinetics, the other complexes tend
to form two species with different kinetic properties, although only one species is predicted by speciation calculations.
In the batch FISRE method, all (except Y-NTA) corresponding association rate constants were determined by monitoring the formation
rate of the yttrium complexes.