Mobility Of Radionuclides In Soils Research Articles

A comparative study of the effect of desferrioxamine B, oxalic acid, and Na-alginate on the desorption of U(VI) from goethite at pH 6 and 25 °C

Organic ligands affect the sorption and mobility of radionuclides in soils. Batch desorption experiments on goethite particles reveal the extent of uranyl desorption and hence bioavailability with different organic acids. The desorptive strength increases in the following order: background electrolyte < Na-alginate < desferrioxamine B (DFO-B) < oxalate. The sequence is consistent with decreasing molecular size and mass from alginate via DFO-B to oxalate. The concomitant Fe release in the desorption experiments indicates that desorption from goethite and not dissolution of goethite governs the mobility of adsorbed U(VI). A compilation of DFO-B surface excesses on goethite from our experiments together with literature values indicate that DFO-B adsorbs at a constant ∼3% to the goethite surface. It is surprising that such a small fraction suffices to account for the considerable uranyl desorption and thus remobilization of a radionuclide into solution. Oxalate displays higher surface concentrations but still lower than the determined uranyl surface excess. It follows that based on the high U(VI) stability constants, both organic ligands induce the desorption of uranyl species by increasing the chemical affinity of the aqueous phase. In the case of alginate, desorption of uranyl is weak and adsorbed alginate hampers any considerable detachment of U(VI) in the presence of the more potent ligands, DFO-B and oxalate. This inhibition is based on biosorption and in this respect polysaccharides in soils may retard and even halt the advance of actinides through the soil column. This hypothesis calls for further studies into the interaction of siderophores and polysaccharides with soil adsorbents and their role in the mobilization of contaminant metals.

Utilization of the Mapping System for the Evaluation of the Mobility of Radionuclides in Soil (Program for 2005 Annual Meeting of Japanese Society of Soil Science and Plant Nutrition)

Utilization of the Mapping System for the Evaluation of the Mobility of Radionuclides in Soil (Program for 2005 Annual Meeting of Japanese Society of Soil Science and Plant Nutrition)

Complexing capacity of low-level radioactive waste leachates for 60Co and 109Cd using an ion-exchange technique

Low-level radioactive wastes (LLRW) intended for disposal at Chalk River laboratories (CRL) are composed primarily (>90% by volume) of organic material (paper, used clothing, packaging material, mop heads, etc.). Upon contact with water, microbial degradation of this material will produce dissolved organic matter (DOM) which has the potential to change the speciation and mobility of radionuclides in soils. The determination of the aqueous speciation of nuclides will provide a tool that will enable a better prediction of the behaviour of nuclides and their potential to migrate in soils.This work is one aspect of the characterisation of the DOM produced by the microbial degradation of LLRW. An ion-exchange technique was set up to demonstrate the applicability of the method to determine the complexing capacity of the DOM in a difficult matrix. Leachates from a LLRW degradation experiment were used with two radioisotopes (109Cd and 60Co). The method allows the determination of the free-ion content of a metal in a solution, which also leads to the determination of the complexed fraction.The leachate matrix consisted of ∼0.1M ionic strength of inorganic salts, with 4177mg C/l of DOM. The complexing capacity of the DOM in the leachates was 50μg/l for Co, and 0.54μg/l for Cd. This represents 0.0028% and 0.000015% of the available DOM sites to Co and Cd, respectively. The conditional stability constant of the DOM with Cd was slightly higher (logβ=1.98) than that of acetic acid, whereas it was ambiguous for Co (logβ=2.02 or 2.54). DOM fouling did not constitute a problem, and the method could be used for other radionuclides and metals.

Influence of plant roots upon the mobility of radionuclides in soil, with respect to location of contamination below the surface

The movement of85Sr,137Cs,54Mn and60Co in the 50 cm soil profile was studied with and without the presence of plant roots (triticum aestivum) in order to investigate the influence of roots and depth contamination upon the migration of radionuclides. The water table was maintained manually at 3 cm from the bottom. The physiochemical characteristics (Eh Fe−2, NH4+, pH and moisture content) as well as the total and extractable radioactivity were investigated. In the discrete contamination, where the location of contamination varied within the soil profile (0–5, 25–30 or 45–50 cm from the top), the influence of location upon the movement of these radionuclides was also studied. It was found that the changes in the soil physicochemical characteristics influenced the mobility of the four radionuclides. The extractability of54Mn and60Co was significantly increased in the reducing region of the soil, whereas that of85Sr,137Cs was not. Plant roots excerted significant effects upon the soil characteristics, via, reducing the Eh pH and moisture content of the soil; increasing the extractability of both54Mn and60Co from the depth of 35 cm downwards. Radionuclide migration occurred via physicochemical and biological transport. The biological transport via plant roots was of particular importance for137Cs. Location of contamination had a significant influence upon the mobility of radionuclides. The migration of radionuclides was in the sequence of contamination in middle>bottom>top. The degree of the influence varied with radionuclides concemed. In the top layer contamination, the rank of the migration from the contamination layers, on the other hand54Mn,60Co and137Cs were more mobile and the movement was:85Sr∼54Mn∼60Co>137Cs. In the middle and bottom contamination layers, on the other hand,54Mn and60Co and137Cs were more mobile and the movement was85Sr∼54Mn∼60Co>137Cs. The results provide evidence conceming the soil-root interaction influencing the transfer efficiency of radionuclides from below the soil surface into the human food chain.

Earthworms and radionuclides, with experimental investigations on the uptake and exchangeability of radiocaesium

The potential influence of earthworm activity on the mobility of radionuclides in soils and their subsequent availability for uptake by plants and transfer to higher trophic levels is briefly reviewed. The accumulation of caesium by the earthworm Aporrectodea longa from soil and from plant litter was investigated in laboratory experiments, as was the effect of reworking (through burrowing and ingestion) soil and soil with added organic material, on the extractability of caesium (ammonium acetate extraction). Soil was spiked with 134Cs, organic matter with 137Cs. In soil-fed worms, most of the radioactivity measured was eliminated with the gut contents; 5–25% of the ingested radioactivity was retained or assimilated. Loss of caesium from soil-fed worms followed a two component curve, with an initial rapid loss due to gut clearance (half-life of loss (Tb1/2) of about 0·2–0·6 days) and a slower loss of assimilated caesium (Tb1/2 of 15–26 days). Loss rates of assimilated caesium from worms fed on fragmented apple leaves were found to have half-lives of 18–54 days. Assimilation of caesium from apple leaves was higher than from soil, ranging from 55–100% of the activity measured before gut clearance. Dry weight transfer factors (concentration in worm tissue/concentration in substrate) for worms cleared of their gut contents were similar for the two substrates 0·04 and 0·04 for two loss experiments with worms fed on radioactive soil, and 0·03 and 0·05 for worms fed on apple leaves. After three months of reworking soil and soil/organic mixtures, A. longa was found to have no measurable effect on the extractable fraction of caesium. If earthworms have any subtle effects they were masked by changes in availability that occurred when the spiked soil and organic substrates were mixed together. Only about half of the extractable fraction in soil was recovered when soil was mixed with organic material suggesting that some of the labile fraction in soil had become complexed with organic material. This exchange occurred in substrate mixtures with and without worms. The limitation of chemical extraction procedures is discussed and suggestions for further work are presented.

THE INTERACTIONS OF LOW-LEVEL, LIQUID RADIOACTIVE WASTES WITH SOILS

ABSTRACT The characteristics of radioactive wastes and soils vary over a wide range. Liquid radioactive waste entering the environment will eventually contact the soil or geological matrix; interactions will be determined by the chemical and physical nature of the liquid, as well as the soil matrix. We report here the results from an investigation of certain of those characteristics as they relate to retention of radionuclides by soils. Three fractions were demonstrated in the waste as filterable, soluble-sorbable, and soluble-nonsorbable; the physical nature of each fraction was demonstrated using autoradiographic techniques. Isotopes of plutonium and uranium and americium-241 in the soluble fraction of the waste were shown to have a negative charge as determined by ion exchange techniques. In the soil-waste systems, the net charge for those radionuclides was shown to change from predominantly negative to predominantly positive. Nevertheless, cesium-137 was shown to be predominantly positively charged. The change in charge and the redistribution of radionuclides among the various fractions is important in assessing the degree of mobility of radionuclides in soil.