Iron selenite

and Moore [95RA1/FEL3] made a serious attempt to determine the solubility product of iron(lll) selenite as described in Appendix A. Their work shows that Fe2(Se03)3-6H20 is the stable phase in contact with an aqueous phase with a pH below 4. Above this value the selenite starts to transform into other, basic phases of unknown composition. 

The interpretation of careful experimental work resulted in an equilibrium model, which included the two equilibria  

The uncertainties are those of the paper. The stability constant of FeSeOj appears exceptionally large although there is known to be a great affinity between Fe(lll) and Se(IV). The value of the solubility product is not greatly affected by the value of log o P° (V.136), however. These results are selected by the review and assumed valid at standard conditions. 

Chukhlantsev and Tomashevsky [57CHU/TOM] prepared iron(III) selenite by mixing a 0.2 M solution of iron(lll) sulphate and a 0.1 M sodium selenite solution. The pH of the reaction mixture was 5 to 6. The precipitate was aged for 24 hours in the mother liquor. No information on the quality of the specimen is available but it is most likely amorphous. Chemical analysis showed a 2 3 ratio between Fe(lll) and Se(IV). The solubility of the specimen in dilute solution of nitric or sulphuric acid was measured at 293 K. The experiments were performed and the data recalculated as described in Appendix A, [56CHU]. Two experimental points were deleted, as they would be much affected by the hydrolysis of Fe. The result for 

No calorimetric determination of the standard enthalpy of formation of iron(lll) selenites from aqueous solution has been found. 

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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 —... 

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). 

The kinetics of reaction of Fe " aq, of FeOH +aq, and of Fe2(OH)2 " "aq with variously proto-nated forms of phosphate, phosphite, hypophosphite, sulfate, and selenite have been investigated, mainly at 283 K. The formation mechanism from the dimer is somewhat complicated, e.g., by formation of mononuclear complexes, probably via /i-hydroxo-/r-oxoanion di-iron intermediates, after the initial 4 complexation step. ... 

EXAFS data showed that cations and oxyanions (e.g. selenite and arsenite) can form two kinds of bidentate, inner sphere complexes on iron oxides depending upon the surface site at which the adsorbate adsorbs (Manceau, 1995 Randall et al.. 

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... 

Iron metaniobates(iv) with structures of the natural minerals, ilmenite and pseudo-brookite have been prepared by direct reaction of the oxides under vacuum at 1000—1100°C. The compounds are stable in air up to 500°C, but are oxidized at higher temperatures.326 a-Sr3Fe07 x has been found to be isostructural with Sr3Ti07.327 Synthesis and thermal decomposition of iron(m) normal selenite mono-or tri-hydrate, Fe203,3Se02,xH20 (x = 1 or 3) have been reported.328... 

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. 

The following interfere with the test strong reducing agents (hydrogen sulphide, dithionites, sulphites and selenites) V, U, Te, Hg, Bi, Au, Pd, Se, Te, Sb, Mo, W, Co and Ni. The reaction is not selective, but is fairly sensitive it can be used in the analysis of the Group IIB precipitate. Since iron(II) ions have no influence on the test, it may be applied to the tin solution which has been reduced with iron wire. 

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 ). 

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

Little is known about the mobility of selenium in plants. The lesser uptake of selenium may be due to a lower mobility of selenium, to the formation of an insoluble complex between selenite and iron at the acidic pH of the -nutrient solution or to selenium toxicity. The large amount of Se accumulated by the roots of soybeans may reflect a seleniw toxicity. 

Many samples have redox potentials such fiiat fiiey can be oxidized by iodine. Therefore, file iodine in file titrant may be consumed by readily oxidizable samples fiiat will give a false high value for file water content. Some common substances fiiat can be oxidized by iodine are ascorbic acid, arsenite (As02 ), arsenate (As04 ), boric acid, tetraborate (3407 ), carbonate (COs ), disulfite (8205 ), iron(ll) salts, hydrazine derivatives, hydroxides (OH ), bicarbonates (HCOs"), copper(l) salts, mercaptans (RSH), nitrite (N02 ), some metal oxides, peroxides, selenite (SeOs "), silanols (RsSiOH), sulfite (SOs ), tellurite (TeOs ), fiiiosulfate (8203 ), and tin(ll) salts. For situations such as fiiese where file material under analysis reacts wifii iodine, an oven can be used to liberate fiie moisture from file sample, which is fiieii carried into file reaction vessel and titrated wifiiout interference. 

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 iron oxide and clay content of soils and sediments can affect the bioavailability of selenium markedly. The strong pH dependence of adsorption is an important control. Maximum adsorption occurs between pH 3 and pH 5 and decreases as the pH rises. Organic matter also removes selenium from soil solution, possibly as a result of the formation of organometallic complexes. Addition of PO4 to soils increases selenium uptake by plants, because the PO ion displaces selenite from soil particles making it more bioavailable. Conversely, increasing the concentrations of PO4 in soils can... 

Chromium as a metallic element was first discovered two hundred years ago, in 1797. But the history of chromium really began several decades before this. In 1761, in the Beresof Mines of the Ural Mountains Johann Gottlob Lehmann obtained samples of an orange-red mineral, which he called Siberian red lead . He analyzed this mineral in 1766 and discovered that it contained lead mineralised with a selenitic spar and iron particles. The mineral he found was crocoite, a lead chromate (PbCr04). 

Subject to limitations discussed in section 4.1., the effects of pH on anions are similar to those observed using iron oxides. Thus, the sorption of phosphate and of selenite generally decrease with increasing pH (Fig. 12.) for the reasons discussed earlier. Similar principles apply for borate. However sorption increases with increasing pH because the pK for borate dissociation is near 9. The concentration of borate ions therefore increases 10-fold for each... 

Parida, K.M. et al.. Studies on ferric oxide hydroxides. III. Adsorption of selenite (SeOj") on different forms of iron oxyhydroxides, J. Colloid Interf. Sci., 185, 355, 1997. 

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