Iron selenates

No other thermodynamic data for iron selenates have been identified. 

In mineral technology, sulfur dioxide and sulfites are used as flotation depressants for sulfide ores. In electrowinning of copper from leach solutions from ores containing iron, sulfur dioxide prereduces ferric to ferrous ions to improve current efficiency and copper cathode quaHty. Sulfur dioxide also initiates precipitation of metallic selenium from selenous acid, a by-product of copper metallurgy (326). 

The selenate reductase from Enterobacter cloacae SLDla-1 functions only under aerobic conditions, and is not able to serve as an electron acceptor for anaerobic growth, in contrast to the periplasmic enzyme from Thauera selenatis (Schroder et al. 1997). In E. cloacae there are separate nitrate and selenate reductases, both of which are membrane-bound. The selenate reductase is able to reduce chlorate and bromate though not nitrate, contains Mo, heme and nonheme iron, and consists of three subunits in an a3p3y3 configuration. 

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). 

Peak, D. Sparks, D.L. (2002) Mechanisms of selenate adsorption on iron oxides and hydroxides. Envir. Sci. Techn. 36 1460-1466 Peak, D., Ford, R.G. Sparks, D.L. (1999)... 

Su, C. Suarez, D.L. (2000) Selenate and selenite sorption on iron oxides An infrared and electrophoretic study. Soil Sci. Soc. Am. J.101-111... 

Ferrous selenate can be prepared by the action of selenie acid on ferrous sulphide selenie acid does not dissolve iron (see p. 335) ... 

Fig. 12.49. Action of corrosion inhibitors, (a) Anodic inhibitors. Examples Chromate, nitrite, molybdate, tungstate, orthophosphate, silicate, benzoate, (b) Cathodic inhibitors. Examples Ca(HC03)2, ZnS04, Cr2(S04)3, NiS04 phosphate, aminoethylene phosphate (AMP), Ag3+, Sb3+ (on iron) Hg (on zinc), (c) Mixed inhibitors. Examples organic inhibitors containing nitrogen and/or sulfur (e.g., amines, triazoles, thiazoles, alkylthioureas) inorganic inhibitors (e.g., arsenite, arsenate, selenate). (Reprinted from G. Ranglen, Corrosion of Metals, p. 165,1985 with permission from Chapman Hall.)...

Since selenite has two pKa values, 2.3 and 7.9, as opposed to selenate which exhibits a pKg of approximately 1.7, it follows that selenite adsorption is more likely to be pH dependent. The data in Figure 12.17 show adsorption of selenite by various minerals. As expected, iron-oxide is more effective in adsorbing selenite than vermiculite or montmorillonite. 

Selenious acid oxidizes pyrrole to a blue dyestuff of unknown composition ( pyrrole blue ). Iron salts accelerate the reaction when it is carried out in phosphoric acid solution. Selenic, tellurous and telluric acids do not react under the conditions given below the test therefore provides a method of distinguishing selenites and selenates. 

As Se is one of the few elements absorbed by plants in sufficient amounts that can be toxic to livestock, soils containing more than 0.5 mg Se kg are considered as seleniferous because the forages produced on such soils absorb Se more than the maximum permissible level suitable for animal consumption. Se binding onto soils and sediments depends upon the pH, Eh, Se species, competing anions, hydrous oxides of iron, and the type of clay minerals. Se in contaminated soils and water exists mainly as water-soluble selenate (SeO ) and selenite (SeOf ). 

The purer forms of iron (wrought iron and steel) appear to be much more susceptible to this kind of reaction than cast iron.3 If the attacking acid is readily reducible by hydrogen, many secondary reactions may take place. Thus with nitric acid, oxides of nitrogen and ammonia may be evolved, whilst with selenic acid a deposit of elementary selenium is obtained (see below). When iron is exposed to the action of acids that are also powerful oxidisers—such as, for example, fairly concentrated solutions of nitric and chromic acids,—it is frequently rendered inert or passive.4 Its surface may remain perfectly bright, but the metal does not show any appreciable solution in the acid, and if removed and immersed in dilute solutions of such salts as copper and silver sulphates, no reaction takes place, although ordinary active iron would cause an immediate precipitation of the more electronegative metal. 

Selenic acid dissolves iron with the production of ferrous selenate, FeSe04, and a deposit of selenium which collects on the surface of the undissolved metal, thereby greatly retarding the reaction. No hydrogen is evolved, and the selenium is presumably obtained by the action of nascent hydrogen upon the excess of selenic acid. Thus —... 

Iron Selenites.—Although metallic iron does not appear to be soluble in selenous acid, yet selenites of iron are readily obtained in a variety of ways. When sodium selenite is added to ferrous sulphate solution, a white precipitate of ferrous selenite, FeSe03, is obtained.4 This becomes darker on exposure to air in consequence of oxidation. If the white precipitate is dissolved in hydrochloric acid, a portion of the selenium separates out, whilst ferric chloride and selenous acid remain in solution. Thus —... 

Ferrous sdenate, FeSe04.—Iron dissolves in aqueous selenic acid, yielding ferrous selenate and a deposit of selenium,7 which collects upon the surface of the undissolved metal and shields it from further attack. The rate of solution is thereby so greatly retarded that as a method of preparing ferrous selenate it is not to be recommended.8 The net result of the action may be represented by the equation —... 

Iron and Selenium—Selenides—Selenites—Selenates—Double Selenates. 

The behavior of selenium in soils mirrors that of the pure oxides (Goldberg, 1985). In acid soils, selenium is likely to occur mainly as Se(lV) strongly adsorbed to iron oxides. Less commonly, Se(IV) may form highly insoluble iron compounds such as ferric selenite (Fe2(0H)4Se03) or iron selenide (FeSe). In alkaline, oxidized and selenium-rich soils, most of the selenium is likely to be present as Se(Vl) which is very weakly adsorbed. Furthermore, there are no common insoluble selenate minerals. Hence, selenate accumulates in soluble form particularly in arid and semi-arid areas where evaporation tends to concentrate selenium along with other soluble salts (Deverel et al., 1994). 

The strong affinity of iron oxides for Se(IV) has been well documented (Dzombak and Morel, 1990) and calculations based on the Dzombak and Morel (1990) diffuse double-layer model and default HFO database show the principal response to pH and redox speciation changes (Figure 11). The selenate species is less strongly adsorbed by iron oxides at near-neutral pH than the selenite species (Figure 11). Clay minerals (Bar-Yosef and Meek, 1987) also adsorb Se(IV). 

The bio-availabilities of these elements through the gastrointestinal tract vary markedly. Molybdenum, iodine, fluorine, and arsenic are apparently highly bio-available, whereas medium uptake occurs with haem iron, cobalt, zinc, chromium in the presence of a glucose tolerance factor, selenium, either as selenate or as organo selenium compounds, whereas only low bio-availability is experienced with non-haem iron,... 

Iron(II) fluoride Iron(II) hydroxide Iron(III) hydroxide Iron(III) phosphate dihydrate Lanthanum iodate Lead(II) bromide Lead(II) carbonate Lead(II) chloride Lead(II) fluoride Lead(II) hydroxide Lead(II) iodate Lead(II) iodide Lead(II) selenate Lead(II) sulfate Lithium carbonate Lithium fluoride Lithium phosphate Magnesium carbonate Magnesium carbonate trihydrate... 

It has been shown that the predominate state of the selenium in these SSW streams is as the selenocyanate (SeChT) [1]. The selenocyanate in SSW is oxidized by refinery biotreaters to selenite (SeOs ), which can then be removed via an iron based treatment process. One of the most effective of these iron-based processes is iron co-precipitation process. However, the selenium can also occur as selenate (Se04 )> which is not removed during the iron coprecipitation process [2]. In order to design effective treatment protocols to remove the selenium, more than just the concentration of the metal must be determined. Without knowledge of the types of selenium species present in the water, design of an effective removal treatment protocol... 

Building upon a study that was performed for the Western States Petroleum Association (WSPA) [1] to optimize iron-based selenium removal processes, a scheme was developed to measure total selenium, and the individual selenium species present in the SSW and aqueous streams. Total selenium was measured by ICP-MS after a peroxide/acid digestion pretreatment of the samples. Speciation of the selenium was accomplished using an anionic ion chromatographic separation followed by quantification of the selenite, selenate, and selenocyanate using ICP-MS. 

---