Selenium, anionic species

Due, M., Lefevre, G. et al. 2003. Sorption of selenium anionic species on apatites and iron oxides from aqueous solutions. Journal of Environmental Radioactivity, 70, 61-72. 

Extraction procedures have also been developed for the determination of the anionic species in soils of elements such as sulfur which are important as binding sites for metals as well as for its own mobility and availability (Cordos et al., 1995). The important biosignificant element selenium has similarly received attention (Blaylock and James 1993 Seby et al., 1997) and procedures for the speciation of phosphorus have been developed (Vaz et al., 1992 Chapman et al., 1997). 

A novel class of chalcogen(II) halogen compounds was established recently by the preparation of the first simple binary mononuclear and oligomeric haloanions of divalent selenium and tellurium. The chemistry of these compounds is especially variable for selenium, and a number of highly interesting mono-, di-, tri, tetra- and pentanuclear anionic species with composition X Ym have been synthesized and characterized by structure analyses and spectroscopic studies up... 

Examples for anionic species are rare and only some (RS Br) have been observed. 156 Radical cations have, however, been detected much more frequently. They include, for example, intramolecular sulfur-bromine, -iodine, -selenium, -nitrogen, and -phosphorus species of the general types 15, 16, and 17.140,151,157-161... 

Recently, IC has also foimd increased application in the field of metal speciation. Short anion exchange columns have been used for the rapid separation of anionic species of arsenic, selenium, and chromium. 

Metal ions such as arsenic(Ill), selenium and tellurium (Me"" ) may be determined by stripping voltammetry after adding a second metal, such as copper (Me h ), to the test solution and co-electrolysing the two metals onto the surface of the HMDE. The copper acts as a co-deposition agent and facilitates the deposition of the analyte, Me . on the electrode surface as an inter-metallic compound. The analyte may then be stripped from the electrode either by oxidation (ASV) or by further reduction (CSV) to an anionic species according to the following reaction scheme. 

An emulsion liquid membrane process is reported in Chapter 24 by Gleason et al which rapidly reduces aqueous phase selenium concentrations to low levels within 15 minutes of contact time, while enhancing the concentration of selenium in the internal phase by more than a factor of 40. The emulsion formulation is very stable, and swelling does not significantly dilute the selenium in the internal phase. The presence of other anionic species, such as sulfate, is not a significant interference for removal of the selenium anions. 

The results also indicate that the ELM process has a reciprocal effect on the external phase pH. As the ELM process is carried out, besides exchanging chloride ions in the internal phase and the selenium anions in the external phase, it also extracts hydroxide ions from the external phase due to the OH concentration difference between the external and internal phases. The result is a slight reduction in the external phase pH (from pH 10.5 to 9.3 and from pH 8 to 6) during the course of the ELM process. At the higher pH (10.5), this should not affect the extraction process since it is not changing the speciation of the selenium compounds. However, at a lower pH of 8, the reduction in pH will change the relative amounts of Se(IV) species from an equal amoimt of selenite and biselenite to almost exclusively biselenite. As we saw above, the extraction rate of selenite [Se(IV)] is very sensitive to pH. Therefore, as the extraction process reduces the pH, this will in turn reduce the extraction rate even further. 

Scheme 4 illustrates the application of another selenium-stabilized species, a formyl anion equivalent, in the synthesis of aldehydes. a-Metallated enamines, formed by reaction of a-chloroenamines with Li metal, are also acyl-lithium equivalents. ... 

A current area of interest is the use of AB cements as devices for the controlled release of biologically active species (Allen et al, 1984). AB cements can be formulated to be degradable and to release bioactive elements when placed in appropriate environments. These elements can be incorporated into the cement matrix as either the cation or the anion cement former. Special copper/cobalt phosphates/selenates have been prepared which, when placed as boluses in the rumens of cattle and sheep, have the ability to decompose and release the essential trace elements copper, cobalt and selenium in a sustained fashion over many months (Chapter 6). Although practical examples are confined to phosphate cements, others are known which are based on a variety of anions polyacrylate (Chapter 5), oxychlorides and oxysulphates (Chapter 7) and a variety of organic chelating anions (Chapter 9). The number of cements available for this purpose is very great. 

There have been no reports of complexes of " JV-substituted thiosemicarbazones derived from 2-formylpyridine, but 2-acetylpyridine JV-methyl-thiosemicarbazone, 3a, formed [Fe(3a-H)2]C104 and [Fe(3a-H)2]FeCl4 [117]. The nature of these two species was established by partial elemental analyses, molar conductivities, magnetic moments, electronic, infrared, mass and electron spin resonance spectra. A crystal structure of a related selenosemicarbazone complex confirmed the presence of a distorted octahedral iron(III) cation coordinated by two deprotonated anions so that each ligand is essentially planar and the azomethine nitrogens are trans to each other the pyridyl nitrogen and selenium donors are both cis. 

Biological activity can be used in two ways for the bioremediation of metal-contaminated soils to immobilize the contaminants in situ or to remove them permanently from the soil matrix, depending on the properties of the reduced elements. Chromium and uranium are typical candidates for in situ immobilization processes. The bioreduction of Cr(VI) and Ur(VI) transforms highly soluble ions such as CrO and UO + to insoluble solid compounds, such as Cr(OH)3 and U02. The selenate anions SeO are also reduced to insoluble elemental selenium Se°. Bioprecipitation of heavy metals, such as Pb, Cd, and Zn, in the form of sulfides, is another in situ immobilization option that exploits the metabolic activity of sulfate-reducing bacteria without altering the valence state of metals. The removal of contaminants from the soil matrix is the most appropriate remediation strategy when bioreduction results in species that are more soluble compared to the initial oxidized element. This is the case for As(V) and Pu(IV), which are transformed to the more soluble As(III) and Pu(III) forms. This treatment option presupposes an installation for the efficient recovery and treatment of the aqueous phase containing the solubilized contaminants. 

Numerous Zind anions are formed by selenium and tellurium, with some of the more prominent species being Se 2 (where n 2, 4, 5, 6, 7, 9, or 11). The species with n = 11 contains two rings that have five and six members that are joined by a selenium atom. Those with smaller numbers of selenium atoms generally consist of zigzag chains. Tellurium forms an extensive series of polyanions that are present in such species as NaTe (n = 1 to 4). One tellurium anion contains the Hg4Te124 ion, but other species such as [( lg2Tes) 2 are also known, such as the Te122 anion that is present in some cases where the cation is a +1 metal. 

Generally, oxo-selenates can be classified according to the oxidation state of the selenium atom as oxu-selenates(IV), and o.Yo-selenates(VI). The same applies for the respective tellurates. Besides this systematically correct naming, chemists usually use the terms selenites and tellurites instead of o. o-selenates(IV) and oxo-tellurates(IV), and selenates and tellurates instead of OAO-selenates(VI) and oxo-tellurates(VI). Therefore, both nomenclatures will be used in parallel throughout this chapter. Compared to the respective sulfur species, the oxo-selenate(IV) and oxo-tellurates(IV) ions are very stable so that numerous compounds with these anions have been prepared. 

One of the first series of reports on ultrasonically-enhanced electrosynthesis was by Gautheron, Tainturier and Degrand [69] who used the technique to explore routes to organoselenium and tellurium derivatives. Instead of employing a sacrificial cathode of elemental selenium, their procedure allowed the direct use of selenium powder with carbon cloth as cathode to produce Se and Se. A further benefit was that this method also allowed production of the corresponding tellurium anions. These species could be employed in situ in aprotic solvents such as DMF, THF and MeCN for the synthesis of selenides and tellurides by nucleophilic displacement from haloalkanes. 

On-line anion exchange LC/ICP-MS methods for selenium and chromium speciation have been published [30], In studies of selenium speciation in environmental samples LC/ICP-MS and LC/ ESI-MS methods have been used for the determination of both inorganic and organic selenium species [43]. For the determination of methyl selenide, strong anion exchange and RP chromatography have been employed [43], while for the determination of seleno-aminoacids, IP RP chromatography with on-line detection based on ICP-MS has been successfully employed [44],... 

A. Woller, H. Garraud, J. Boisson, A. M. Dorthe, P. Fodor and O. F. X. Donard, Simultaneuous speciation of redox species of arsenic and selenium using an anion-exchange microbore column coupled with a micro-concentric nebuliser and an inductively coupled plasma mass spectrometer as detector, J. Anal. At. Spectrom., 13, 1998, 141-149. 

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