Selenium is usually known as the ‘double-edged sword element' for its dual toxic and beneficial character to health. Since the pioneer works by Schwarz and Foltz on the relationships between selenium deficiency and liver, muscle and heart diseases, many efforts have been undertaken to better understand the role of selenium in health. At the same time, an increasing number of publications have appeared during these last years on the selenium physico–chemical interactions within the environment. Both types of research represent ongoing efforts to correctly estimate the bioavailability of selenium species for health and the environment. Redox reactions, diffusion, adsorption and precipitation processes or interactions with organic matter and biota govern the speciation and mobility of selenium in the environment. This review intends to emphasize and collect the important advances made during these last years in the mechanistic understanding of processes which govern selenium cycling and bioavailability, like adsorption at the mineral/water interface, precipitation of elemental selenium, or bioavailability of nanoscaled precipitates. The advent of powerful spectroscopic techniques, like X-ray absorption spectroscopy, has allowed the structural description of adsorption and substitution processes that selenium undergoes in a variety of minerals. These and other structural details about selenium precipitates are reviewed here, together with their relationships to the bioavailability of the element in the environment.

Selenium environmental cycling and bioavailability: a structural chemist point of view

Suboxic soils and sediments often contain the Fe(II)-bearing minerals mackinawite (FeS), siderite (FeCO3) or magnetite (Fe3O4), which should be able to reduce aqueous selenite, thereby forming solids of low solubility. While the reduction of selenate or selenite to Se(0) by green rust, pyrite and by Fe2+ sorbed to montmorillonite is a slow (weeks), kinetically limited redox reaction as demonstrated earlier, we show here that selenite is rapidly reduced within one day by nanoparticulate mackinawite and magnetite, while only one third of selenite is reduced by micrometer-sized siderite. Depending on Fe(II)-bearing phase and pH, we observed four different reaction products, red and gray elemental Se, and two iron selenides with structures similar to Fe7Se8 and FeSe. The thermodynamically most stable iron selenide, ferroselite (FeSe2), was not observed. The local structures of the reaction products suggest formation of nanoscale clusters, which may be prone to colloid-facilitated transport, and may have a hig...

Selenite reduction by mackinawite, magnetite and siderite: XAS characterization of nanosized redox products.

X-ray absorption and photoelectron spectroscopy investigation of selenite reduction by FeII-bearing minerals

The state of the art in accelerator mass spectrometry (AMS) is reviewed. The review is divided in two parts. The first covers the general methodology, followed by its specific elaborations for the commonly measured long-lived isotopes such as 10Be, 14C, 26Al, 36Cl and 129I, as well as other isotopes with emerging applications. The second part considers the very wide spectrum of applications that now employ AMS, and groups these according to research area rather than isotope. The period up until late 1998 is covered, with an emphasis on post-1990 developments and literature.

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Accelerator mass spectrometry and its applications

79Se : geochemical and crystallo-chemical retardation mechanisms

In the recent charts of the nuclides an accepted half-life of 79 Se is only an upper limit, 4 a. In this paper, we report a measurement of the half-life of 79 Se. The number of 79 Se atoms was determined from measured 79 Se Se ratios with the accelerator mass spectrometry technique and the decay rate of 79 Se was determined by counting the emitted β-rays with a liquid scintillation spectrometer. The half-life of 79 Se was measured to be (1.1 ± 0.2) × 10 6 a. We have also re-examined the 79 Se half-life value in the recent charts, which has been reported in Ref. [1], and have found that from the data related to the calculation of the half-life the upper limit of the half-life was re-calculated to be 6.5 × 10 5 a, other than 6.5 × 10 4 a as reported in the original paper in which a mistake could be made in the calculation.

Determination of the half-life of 79se with the accelerator mass spectrometry technique

This paper presents a radiochemical separation procedure of79Se from high level liquid radioactive waste, with nitrobenzene extraction and with sulfur dioxide precipitation. The procedure gives a chemical yield of selenium greater than 60%, and satisfactory decontamination is observed for all the relevant radioactive nuclides. Liquid scintillation counting were used for the measurement of79Se radioactivity. The79Se samples obtained from the procedure were used successfully in79Se half-life detemination.

Li Chunsheng

Papers

Determination of the half-life of 79se with the accelerator mass spectrometry technique

A procedure for the separation of79Se from fission products and application to the determination of the79Se half-life