IV Selenites

The compound is precipitated from 2 to 3N aqueous solutions of H2Se03 or its alkali salts and 00(864)2 at 25°0 [2]. An excess of H2Se03 does not seem necessary. The bright orange amorphous product was dried 10 to 24 h at 70 to 90°O after washing with hot and cold H2O [10]. 

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Selenium occurs in natural waters principally in two oxidation states, Se(IV) (selenite) and Se(VI) (selenate). Elemental selenium, Se(0) (red and black forms), and selenide, Se(—II), are essentially insoluble in water Se(VI)04 occurs mainly in oxidizing waters while HSe(IV)03 and Se(IV)Oi dominate under reducing conditions (Figure 9). The concentration ratio of Se(IV) to Se(VI) species in natural waters does not necessarily follow that of other redox couples (e.g., Fe /Fe ). This reflects the slow kinetics involved (White and Dubrovsky, 1994). 

Figure 5 (A) Selenium(IV) (selenite), (B) selenium(VI) (selenate), and (C) organic selenide (organic Se(ll)) in the eastern North Pacific Ocean, 18°N, 108°W. (Data from Cutter GAand Bruland KW (1984) The marine biogeochemistry of selenium a re-evaluation. Limnology and Oceanography 29-. 1179-1192.) (D) Tellurium(IV) and (E) tellurium(VI) in the eastern North Pacific Ocean, 7°N, 78°40 W. (Data from Lee DS and Edmond JM (1985) Tellurium species in seawater. Nature 313 782-785.)...

To develop an effective, optimized ELM process for selenium removal, it is necessary to understand the forms of the selenium ions present in contaminated aqueous streams. Selenium can exist in several oxidation states -II, 0, IV, and VI (13). In naturally occurring ground or surface waters, inorganic selenium is found in the IV (selenite) and VI (selenate) states (14). In petroleum refinery wastewater effluents, the form of selenium depends on the process stream. In stripped sour water the selenium exists primarily in the Se(-II) oxidation state as either organoselenium compounds or selenocyanate. In the final biotreated effluent, selenium exists primarily as selenite [Se(IV)] and selenate [Se(VI)] oxyanions (1). 

Another possible application has emergfed recently. Cerium(IV) selenite, prepared previously by Markovskii and Safina (1968), shows promising ion exchange properties, especidly toward Hg(II) (Rawat et al., 1984 Husain et al., 1984). 

The properties of anodic layers of HgS formed on mercury in sulfide solutions have been investigated in comparison with anodic sulfide layers of cadmium and bismuth. Also, the electrochemistry of mercury electrodes in aqueous selenite solutions has been studied (see Sect. 3.2.1). The problem with the presence of several cathodic stripping peaks for HgSe in acidic Se(IV) solutions has been addressed using various voltammetric techniques at a hanging-mercury-drop electrode [119]. 

Epitaxial effects are not limited to single-crystalline substrates. The possibility for substrate-induced epitaxial development in the difficult case of ZnSe (cf. conventional electrodeposition) has been established also by using strongly textured, albeit polycrystalline, zinc blende (111) CdSe electrolytic films to sustain monolithic growth of ZnSe in typical acidic selenite baths [16]. Investigation of the structural relations in this all-electrodeposited ZnSe/CdSe bilayer revealed that more than 30-fold intensification of the (111) ZnSe XRD orientation can be obtained on the textured (111) CdSe films, compared to polycrystalline metal substrates (Fig. 4.2). The inherent problems of deposition from the Se(IV) bath, i.e., formation of... 

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. 

In coastal environment, detrital and authigenic Fe and Mn oxides, which accumulate in oxic surface sediments, play a pivotal role in determining the geochemical behaviour of arsenic (Mucci et al., 2000) and selenium (Belzile et al., 2000). Arsenic and selenium differ in their affinities for metal oxide surfaces. Although both adsorb onto iron oxides, arsenate (As(V)) adsorbs more strongly than arsenite (As(lll)), and selenite (Se(IV)) adsorbs more strongly than selenate (Se(VI)) (Belzile et al., 2000). 

Nanosized selenium particles were deposited onto Ti02 by the photocatalytic reduction of selenate (Se(VI) and selenite (Se(IV) ions) [317], The deposition of Se particles on TiOz was only observed in... 

Selenium, in the form of selenate or selenite, is toxic to D. desulfuricans (Tomei et al. 1995) and Wolinella succinogenes (Tomei et al. 1992) at elevated levels. At sublethal levels of 0.1-1. OmM selenite or 10 mM selenate, minimal levels of growth is observed with both D. desulfuricans and Wolinella succinogenes. With both selenate and selenite, colloidal elemental selenium (Se°) is produced inside the cell and released into the culture fluid after cell death. This reduction of Se(VI) and Se(IV) by these anaerobes is not coupled to growth and proceeds by mechanisms that have not yet been identified. Selenite and selenate reduction with formation of elemental selenium by these nonrespiratory processes serve to detoxify the environment for future bacteria and may be important for the geochemical cycle of selenium. 

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