Radioactive Se Research Articles

Kinetic studies on radio-selenium uptake by ion exchange resin

ABSTRACTKinetic studies on the sorption of radioactive Se (IV) ion on Amberlite MB9L have been carried out under a wide range of temperature, pH, initial concentration, and resin weight. Analyses of the experimental data by two theoretical models commonly used to explain the ion exchange kinetics and shrinking core model (SCM), and homogeneous diffusion model (HDM) against exponential diffusion decay model (EDM) show a greater preference of EDM over HDM and SCM at all reaction conditions. The obtained regression coefficient values in case of HDM and SCM are in general not high enough to give a satisfactory fit for the experimental data. The estimated values for EDM parameters provides a good satisfactory fit for the yielded adsorption results as the corresponding linear regression values are high. The sorption process is found to be accompanied with fast film diffusion and slow diffusion between the reacted layer and the shrinking un-reacted core within the ion exchange resin. It also shows that ion exchange as the controlling step offers the lowest probability due to the highest obtained diffusion parameters over the normal tested reaction conditions.

Use of transfer factors to characterize uptake of selenium by plants

The radioactive selenium isotope 79Se can be a component of radioactive waste produced in nuclear power plants. The accidental release of 79Se in the environment from power plants or nuclear waste repositories, and subsequent transfer of 79Se into soils, plants and food chain, are relevant concerns in Europe. In environmental risk assessment models, the ability of plants to take up Se is often characterized by means of soil-to-plant transfer factors (TF). However, these recommendations take little or no account of the variability in plant Se accumulation among plant species and differences in Se distribution between plant parts. Also, such factors as soil type, form of Se, climatic conditions, as well as the chemical form in which Se is present in the soil, are not taken into consideration. This paper reviews the current knowledge on Se uptake by plants and compiles published data on the transfer of Se from soils and nutrient solutions into plants. The data were categorized according to the form of Se supply, i.e. when added as either selenate or selenite to nutrient solutions or soils, or whether plant Se accumulation was studied on soil containing only natural-occurring Se (native Se). Plant Se accumulation can vary more than two orders of magnitudes at a given soil Se concentration for a specific form of soil Se (i.e. “native” soil Se, Se added as selenate, or as selenite) among different plant taxa. Differences observed in the transfer of Se from soil into plants appear to result primarily from differences in the solubility of Se species in soil and only to a minor degree from differences in plant uptake efficiency among these species. Values of the TF for Se were found to lie between 0.01 and 100 with few exceptions. Transfer factor values derived from studies in which Se was added to soil as selenate or selenite were generally found to be one or more orders of magnitude higher for a given plant species and plant part than TF values derived for “native Se”. Situations where radioactive Se has been accidentally released into the environment may be better represented by TF values for experimentally added Se than by TF values computed for native soil Se. The uncertainty in selecting a TF value for modeling Se transfer from soil into plants can be substantially reduced by identifying the predominant Se form in the soil, the plant species, and the target plant parts. The large variability in TF values suggests that it is still important to understand the complexity of the soil-plant system when selecting TF values to assess risks arising from the transfer of accidentally released 79Se from soil into plants.

One codon – two amino acids.

In ciliated protozoa Euplotes crassus, cysteine (Cys) is encoded by three codons, UGA, UGU and UGC. UGA is a stop signal in the universal genetic code, and this codon can also code for the 21st amino acid, selenocysteine (Sec), in all three domains of life. Metabolic labeling of E. crassus with 75Se revealed specific incorporation of radioactive Se into several proteins. We found that in this organism the UGA codon specifies insertion of both Sec and Cys, and that the dual use of this codon can occur within a single gene. We studied a Euplotes protein, thioredoxin reductase 1 (eTR1), which has seven in‐frame UGA codons. Our experiments demonstrated that Sec was only inserted into the classical Sec site in eTR1, whereas other UGA positions were not served by SECIS for Sec insertion and instead supported termination of translation in mammalian cells. LC‐MS/MS sequencing revealed eTR1 peptides containing Cys in positions 63, 68, 208, and 270, which are encoded by UGA codons, whereas peptides containing Sec at these positions were not detected. Thus, our data show that E. crassus utilizes UGA for insertion of both Cys and Sec, establishing it as the first known organism that utilizes one codon to code unambiguously for two different amino acids.

Effects of soil type, moisture content, redox potential and methyl bromide fumigation on Kd values of radio-selenium in soil

Understanding the processes that determine the solid–liquid partitioning ( K d value) of Se is of fundamental importance in assessing the risk associated with the disposal of radio-selenium-containing waste. Using a mini-column (rather than batch) approach, K d values for 75Se were determined over time in relation to soil moisture content (field capacity or saturated), redox potential and methyl bromide fumigation (used to disrupt the soil microbial population) in three contrasting soil types: clay loam, organic and sandy loam. The K d values were generally in the range 50–500 L kg −1, with mean soil K d increasing with increasing organic matter content. Saturation with water lowered the measured redox potentials in the soils. However, only in the sandy loam soil did redox potential become negative, and this led to an increase in 75Se K d value in this soil. Comparison of the data with the Eh–pH stability diagram for Se suggested that such strong reduction may have been consistent with the formation of the insoluble Se species, selenide. These findings, coupled with the fact that methyl bromide fumigation had no discernible effect on 75Se K d value in the sandy loam soil, suggest that geochemical, rather than microbial, processes controlled 75Se partitioning. The inter-relations between soil moisture content, redox potential and Se speciation should be considered in the modelling and assessment of radioactive Se fate and transport in the environment.

Sorption behavior of selenium on humic acid under increasing selenium concentration or increasing solid/liquid ratio

The sorption of selenium (Se) on humic acid (HA) was investigated in order to better understand the fate of stable and radioactive Se in soils and sediments. An ultrafiltration technique was used to determine size distributions of HA-sorbed-Se when increasing Se concentration and solid/liquid ratio. The results showed that the Se sorption onto HA followed the Freundlich isotherm. No solid/liquid ratio-dependence was observed especially when <3 kDa molecular size fraction was used from solid/liquid separation. The Freundlich isotherm parameters K F and n obtained using the <3 kDa molecular size fraction for solid/liquid separation were 3.7 × 10 2 and 0.82, respectively. In addition, since dissolved HA increased with decreased ionic strength in the HA suspension, ionic strength could promote aggregation of HA. The conformational change of HA could affect the sorption behavior of Se on HA.

Use of stable isotopic selenium as a tracer to follow incorporation of selenium into selenoproteins.

Stable isotopes of selenium (Se) have been used in human studies to measure Se absorption, retention and excretion. The purpose of this study was to examine whether stable Se could also be used to follow the incorporation of Se into selenoproteins and whether selenoproteins are labeled with stable isotopes the same way they are with radioactive Se. Rats fed either a Se-deficient or a high-Se diet were injected with either a radioactive (75Se) or a stable isotope of Se (77Se), and the liver cytosol was chromatographed on Sephadex G-200. Compared with 75Se, a greater percentage of 77Se was incorporated into cytosol, but the distribution and the effect of dietary Se was similar for both isotopes. New Zealand long-eared rabbits were also injected with either 77Se or 75Se, and the plasma was chromatographed. More of the 75Se was incorporated into the plasma, but again the patterns of incorporation were similar for both isotopes. Plasma from a male subject who ingested 60 micrograms of 77Se was chromatographed, and the stable Se was detected in column fractions and showed a distribution similar to that observed for rabbit plasma. Finally, a polyacrylamide gel electrophoresis (PAGE) method was developed that allowed loading of sufficient protein to analyze for 77Se in individual protein fractions. The distribution of 77Se and 75Se in rabbit plasma was similar. Human plasma was electrophoresed by a similar method and peaks of 56 and 23 kDa were detected. These data show that stable isotopes of Se can be used for selenoprotein production in the same way as radioactive isotopes. They also show that, when physiological amounts of stable Se are ingested by humans, the isotope can be detected in blood-borne proteins separated by column chromatography and PAGE.

Toxicity of selenium to developingXenopus laevisembryos

Se in the form of sodium selenite is toxic to Xenopus laevis embryos and tadpoles continuously exposed to concentrations above 1 ppm. Concentrations of 2 ppm and above result in severe developmental abnormalities and increased mortality. Uptake and loss of radioactive Se from water are rapid, but depuration is not complete indicating that some Se can remain bound by the organism. The facts that Se is toxic at low levels to Xenopus embryos and tadpoles, can cause developmental abnormalities, and accumulates in tissues suggest that increased release of Se compounds into the environment poses a potential threat to aquatic organisms.

Anisotropic Segregation in InSb

The anisotropic segregation of Se in has been studied using radioactive Se as a tracer element. A core consisting of a high concentration of Se has been found in the center of crystals pulled in the [111] direction. This phenomenon is thought to result from an extremely rapid lateral growth occurring on the (111) facet. Well‐defined striations were also observed. Crystals grown from seeds oriented in directions other than the [111] are found to have high concentrations of Se in the (111) facets near the edge of the crystal, thus removal of the edges leaves the major portion of the crystal relatively homogeneous and with lower impurity content.