Se Loss Research Articles

Distribution and bioaccumulation of selenium in aquatic microcosms

Closed-system microcosms were used to study factors affecting the fate of selenium (Se) in aquatic systems. Distribution and bioaccumulation of Se varied among sediment types and Se species. A mixture of dissolved 75Se species (selenate, selenite and selenomethionine) was sorbed more rapidly to fine-textured, highly organic pond sediments than to sandy riverine sediments. Sulfate did not affect the distribution and bioaccumulation of 75Se over the range 80–180 mg SO 4 liter −1. When each Se species was labeled separately, selenomethionine was lost from the water column more rapidly than selenate or selenite. Selenium lost from the water column accumulated primarily in sediments, but volatilization was also an important pathway for loss of Se added as selenomethionine. Loss rates of dissolved Se residues were more rapid than rates reported from mesocosm and field studies, suggesting that sediment: water interactions are more important in microcosms than in larger test systems. Daphnids accumulated highest concentrations of Se, followed by periphyton and macrophytes. Selenium added as selenomethionine was bioaccumulated preferentially compared to that added as selenite or selenate. Organoselenium compounds such as selenomethione may thus contribute disproportionately to Se bioaccumulation and toxicity in aquatic organisms.

Crystallization Studies of Glassy Te-Se-Br Thin Films

AbstractWe have studied the effect of laser irradiation on Te-Se-Br thin films. The major effects were found to be dominated by changes in composition because of the complete loss of Br and variable loss of Se. These losses are measured by EIDS and are reasonable in view of the temperatures obtained from a heat flow calculation. The remaining Te-Se material can be made crystalline under specific conditions of laser pulse length and laser power (which determine the cooling rate). We have been able to establish that the crystallization starts with surface filamentary growth exhibiting fractal network formation. With higher laser energy it tends to coalesce to form three dimensional crystals.

Profiles of dissolved and acid-leachable selenium in a sediment core from the lower St. Lawrence estuary

Concentrations of Se in the pore-water and in the solid phase, and the concentrations of other diagenetic constituents (Fe, Mn, phosphate, ammonium and I) in pore-water, were determined in a sediment core from a 350-m deep station in the lower St. Lawrence Estuary. The concentration of dissolved Se in pore-water was 2.1 nmol kg −1 at the surface of the core, increasing to a maximum of 7.6 nmol kg −1 at a depth of 12 cm, and thereafter decreasing gradually with depth. This profile is similar to the profiles of Fe and phosphate, whose concentration maxima occur around 10 cm. The concentration of total sedimentary Se remained almost constant with depth (≈ 0.75 mg kg −1); however, a significant enrichment of oxalate-leachable Se was observed in the top 2 cm. The sedimentary cycling of Se appears to be closely related to that of Fe: adsorption of Se onto Fe oxyhydroxide at or near the sediment-water interface, release of the adsorbed Se by the reduction of Fe oxyhydroxide, and removal by formation of ferroselite (FeSe 2) at depth. The pore-water flux of Se was estimated by two different methods, firstly from the pore-water gradient, and secondly by applying a box model to the oxalate-leachable solid-phase Se data. The methods agree well both giving values for the flux of 0.11 nmol cm −2 year −1. This agreement suggests that the loss of labile Se in the sediment is balanced by the upward flux of dissolved Se.

Selenium in Japanese baby foods

The selenium content of various baby food was determined by spectrofluorimetry. Mean levels of Se (ng ml −1) decreased in the sequence: pasteurized cow's milk (28.4) > raw cow's milk (23.1) > mature human milk (22.5) > milk-based infant formulae (6.6). The sequence of mean values of Se (ng mg −1 protein) was: human milk (1.57) > raw cow's milk (0.96) > infant formulae (0.37). Dietary Se intake (μg day −1) of 3 month old infants fed on infant formula and various milks decreased in the order: human milk (21.0) > pasteurized cow's milk (18.9) r raw cow's milk (15.0) > infant formulae (5.4). There was a significant positive correlation ( r = 1.00 p < 0.001) between the Se content of the infant formula product and that of the pasteurized milk product of three manufacturers. Hot air treatment reduced the Se content of cow's milk with increasing temperature and time of heating: loss of Se amounted to 11.1% at 210°C for 25 min. The Se content of human milk was positively correlated ( r = 0.64, p < 0.001) with its protein content and negatively correlated ( r = −0.77, p < 0.01) with is fat content. These correlations were not found for infant formulae.

Effect of inorganic selenium supplementation on selenium retention in postweaning swine.

A total of 64 weanling pigs was used in a randomized complete-block experiment to evaluate the efficacy of various inorganic Se levels on weekly Se balance measurements over a 5-wk post-weaning period. Four-week-old weaned pigs were fed a 20% protein, corn-soybean meal diet supplemented with 0, .3, .5 or 1.0 ppm Se from sodium selenite. Eight pigs were allotted by weight, litter and sex to each metabolism crate. A 2-d preliminary period followed by a 5-d collection period was conducted for five weekly periods. Feed intake was provided ad libitum but was similar between dietary Se levels. Fecal Se excretion increased each week and with increasing dietary Se level. Apparent digestibility of Se was relatively constant for each period when inorganic Se was fed, averaging approximately 70%, whereas it ranged between 30 to 40% for pigs fed the basal diet during wk 2 through 5. Urinary Se decreased during the postweaning periods for pigs fed the basal diet, but increased linearly as dietary Se increased during the initial 2-wk postweaning period and then quadratically during wk 3 through 5. There was a net loss of Se from the body when the Se-unsupplemented basal diet was fed during the initial 2 wk postweaning, whereupon, it became positive. Selenium retention in pigs supplemented with inorganic Se increased each week of the trial. When Se retention was expressed in relation to Se intake, the resulting regression was linear (R2 = .99), suggesting that Se retention in the postweaning pig increased in direct proportion to the amount consumed when diets contained up to 1.0 ppm Se.

The influence of growth conditions on selenium incorporation in GaAs grown by molecular beam epitaxy

An electrochemical cell Pt/Ag/AgI/Ag2Se/Pt has been used as a highly controllable source of Se2 for n-type doping of GaAs grown by molecular beam epitaxy (MBE). Se is incorporated with 100% efficiency under all normal MBE conditions up to growth temperatures of 620 °C. Above this temperature doping is still possible but with reduced efficiency. The general behavior is similar to that already reported for sulfur doping, with the added advantage that doping can be extended to temperatures some 30 °C higher in the case of Se. Chemical analysis has confirmed that when doping efficiency is reduced, it is due to actual loss of Se. Thermodynamic analysis indicates that loss as Ga2Se is to be expected under all conditions studied, so successful doping is due to kinetic barriers to this loss reaction. Excess As4 is shown to hinder the loss of Ga2Se even further.

New biologic functions — Selenium-dependent nucleic acids and proteins

Selenium occurs normally in living things as a highly specific component of certain enzymes and amino acid transfer nucleic acids (tRNAs). In bacteria, biosynthesis of essential selenoenzymes has been shown to be unaffected by wide variations in sulfur levels. The naturally occurrring selenoenzymes so far identified from bacterial sources include glycine reductase, certain formate dehydrogenases, a hydrogenase, nicotinic acid hydroxylase, xanthine dehydrogenase and thiolase. The selenoenzyme, glutathione peroxidase, and three other selenoproteins of unknown function have been isolated from animals. In certain enzymes, e.g. glycine reductase, formate dehydrogenase, hydrogenase and glutathione peroxidase, the chemical form of selenium has been identified as selenocysteine. One enzyme, a bacterial thiolase, contains selenomethionine rather than selenocysteine. A labile, unidentified form of selenium is present in nicotinic acid hydroxylase, and by inference, xanthine dehydrogenase. The seleno-tRNAs serve as examples of a different type of biological macromolecule that is specifically modified with selenium. The major seleno-tRNAs in Clostridium sticklandii and Escherichia coli have been identified as glutamate and lysine isoaccepting species. The selenium-modified nucleoside is 5-methyl-aminomethyl-2-selenouridine (mnm 5Se 2U), which is the chemical analog of 5-methylaminomethyl-2-thiouridine, a previously identified minor base of E. coli tRNA 2 Glu. The seleno-tRNA Glu of C. sticklandii contains one gram atom of Se per mole of biologically active tRNA. Loss of Se from the modified nucleoside, mnm 5Se 2U, in this tRNA results in concomitant loss of glutamate charging activity suggesting that selenium is essential for interaction of the synthetase and its cognate tRNA.

Long-pulse, broadband-Xe*2 -fluorescence excitation of OCSe: Initiation of the Se(1S→3P) laser and implications for large-system design

The formation and loss of Se(1S) excited states have been examined using long-pulse (0.7–1.0 μsec), broadband (13 nm FWHM)-Xe*2- fluorescence photodissociation of carbonyl selenide. Experimental data are compared to results of a computer code that models the Xe*2 radiation transport and Se(1S) kinetics. The effective quantum yield for Se(1S) production over the wavelength interval 160–180 nm has been found to be near 0.6. The observed Se(1S) decay rates have been used in conjunction with the code to predict the effects of excitation pulse length on the efficiency of the selenium laser system. Laser oscillation has been demonstrated for the Se(1S0→3P1) transition in a long-active-length (54 cm) optical cavity. Oscillator performance was characterized as a function of OCSe pressure. The optimized laser output was 1.3 mJ, a value consistent with the Se(1S) kinetics and laser cavity conditions.

Modification of CdSe resistivity by laser annealing

The resistivity of CdSe thin films, vapor deposited on insulating substrates, has been reduced by more than two orders of magnitude by irradiation with a cw argon-ion laser. A threshold power density of about 0.6 W for a beam diameter of 250 μm is necessary to cause a measurable decrease in the resistivity of a 0.4-μm-thick film; above this value, the resistivity decreases rapidly with increasing power to a minimum value. Electron and x-ray diffraction analyses show that there is an increase in film grain size by only a factor of 2 on annealing, but SEM and Auger studies indicate that there are significant changes in the surface topography and composition. Calculations show that the power absorbed by the CdSe film is sufficient to cause melting. The decrease in resistivity can be attributed to the effect of Se vacancies created by preferential loss of Se from the film surface at these elevated temperatures.

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.

Retention of the metabolized trace elements in biological tissues following different drying procedures: I. Antimony, cobalt, iodine, mercury, selenium and zinc in rat tissues

Loss of Sb, Co, I, Hg, Se and Zn during freeze-drying and oven-drying at 80, 105 and 120 °C were studied in rat tissues that contained metabolized radioactive isotopes. No loss was observed for any of the 6 elements on freeze-drying. However, tissue-specific differences were observed in many cases for Hg, Se, I and Sb on oven-drying. A significant loss of Hg was observed in liver even at 80 °C, and for brain 105 °C. Se was lost from whole blood, brain, lung and muscle at 120 °C. Sb was lost from whole blood at 105 °C, but from brain, kidney, lung and spleen at 120 °C. Iodine was also lost from whole blood, kidney, blood serum, erythrocytes, brain, lung and muscle at 120 °C. Although the losses were statistically significant, they remained in most cases between 2 and 10%, with the exception of Hg at 120 °C, where the loss in some of the tissues were unpredictable. For urine, freeze-drying and oven-drying at 80 °C was found to be relatively safe for Hg and I. At 105 °C and above, serious loss of Hg was observed. In this experiment, the elements Zn, Co, Sb and Se were not studied for urine.

Thermal metamorphism of primitive meteorites—I. Variation of six trace elements in Allende carbonaceous chondrite heated at 400–1000°C

Determination by neutron activation of 6 trace elements retained in Allende (C3) samples heated at 400–1000°C for 1 week in a low-pressure (initially ~10 −5 atm H 2) atmosphere reveals loss of small proportions of Ga and Se and large proportions of Bi, In and Tl-Co being unaffected. The retentivity patterns for the 5 volatile elements differ and in no way duplicate a step-function. In contrast to these trace elements, sulfur is initially present in discrete mineral(s) and visually it appears to be released over a narrow temperature range. Elements are lost more easily from powder than from chips but the difference is ≤35 per cent. Above 600°C, the process of loss appears due to process(es) with apparent activation energies of 2 kcal/mole (Bi, Tl), 4 kcal/mole (Se) and 22 kcal/mole (In). Loss of Bi, Se and Tl below 600°C involves higher apparent activation energies. Two-element correlation diagrams involving Bi, In and Tl are consistent with the idea that trends among highly-volatile elements in enstatite chondrites arise from metamorphism.