Selenium solubility

Selenium and its compounds are strong poisons. Selenium halides, hydrogen selenide, and the oxygen compounds of selenium soluble in water are the most toxic. 

Selenium dioxide is a volatile solid obtained when selenium is burnt in air or oxygen. It is very soluble in water, forming a solution of... 

On the other hand, wet chlorination of refinery slimes has proven to be a rapid and simple method of obtaining high extractions of selenium from slimes. A simple wet chlorination flow sheet is shown in Figure 3. Slimes chlorination per se is not a simple deselenization operation, but rather a process wherein virtually all the constituents of slimes which form soluble chlorides report as a complex solution of mixed chlorides. Thus the use of wet chlorination requires a complete change in the process to recover the metal values in slimes. The first plant to use wet chlorination of slimes was started by Kennecott (Salt Lake City, Utah) in 1995. 

Losses of selenium and tellurium from the solution are negligible, provided the reactor is equipped with a reflux condenser. The wet chlorination is easily controlled. The reaction is rapid, allowing fast turnover of the precious metals in the slimes and yielding all the selenium and tellurium in soluble form. 

Bina Selenides. Most biaary selenides are formed by beating selenium ia the presence of the element, reduction of selenites or selenates with carbon or hydrogen, and double decomposition of heavy-metal salts ia aqueous solution or suspension with a soluble selenide salt, eg, Na2Se or (NH 2S [66455-76-3]. Atmospheric oxygen oxidizes the selenides more rapidly than the corresponding sulfides and more slowly than the teUurides. Selenides of the alkah, alkaline-earth metals, and lanthanum elements are water soluble and readily hydrolyzed. Heavy-metal selenides are iasoluble ia water. Polyselenides form when selenium reacts with alkah metals dissolved ia hquid ammonia. Metal (M) hydrogen selenides of the M HSe type are known. Some heavy-metal selenides show important and useful electric, photoelectric, photo-optical, and semiconductor properties. Ferroselenium and nickel selenide are made by sintering a mixture of selenium and metal powder. 

Selenium trioxide, SeO, is white, crystalline, and hygroscopic. It can be prepared by the action of sulfur trioxide on potassium selenate or of phosphorous pentoxide on selenic acid. It forms selenic acid when dissolved in water. The pure trioxide is soluble in a number of organic solvents. A solution in Hquid sulfur dioxide is a selenonating agent. It is stable in very dry atmospheres at room temperature and on heating it decomposes first to selenium pentoxide [12293-89-9] and then to selenium dioxide. 

Dilution with water reverses the reaction, and heating the solution Hberates sulfur dioxide. Upon being added to a solution of teUurides, teUurium forms colored polyteUurides. Unlike selenium, teUurium is not soluble in aqueous sodium sulfite. This difference offers a method of separating the two elements. Like selenium, teUurium is soluble in hot alkaline solutions except for ammonium hydroxide solutions. Cooling reverses the reaction. Because teUurium forms solutions of anions, Te , and cations, Te" ", teUurium films can be deposited on inert electrodes of either sign. 

Production and Economic Aspects. Thallium is obtained commercially as a by-product in the roasting of zinc, copper, and lead ores. The thallium is collected in the flue dust in the form of oxide or sulfate with other by-product metals, eg, cadmium, indium, germanium, selenium, and tellurium. The thallium content of the flue dust is low and further enrichment steps are required. If the thallium compounds present are soluble, ie, as oxides or sulfates, direct leaching with water or dilute acid separates them from the other insoluble metals. Otherwise, the thallium compound is solubilized with oxidizing roasts, by sulfatization, or by treatment with alkaU. The thallium precipitates from these solutions as thaUium(I) chloride [7791 -12-0]. Electrolysis of the thaUium(I) sulfate [7446-18-6] solution affords thallium metal in high purity (5,6). The sulfate solution must be acidified with sulfuric acid to avoid cathodic separation of zinc and anodic deposition of thaUium(III) oxide [1314-32-5]. The metal deposited on the cathode is removed, kneaded into lumps, and dried. It is then compressed into blocks, melted under hydrogen, and cast into sticks. 

The molecular and crystal stmctures of 2-phenylbenzotellurazole were determined by X-ray (89KGS1690). The dihedral angle between the planes of the five-membered heterocycle and 2-phenyl ring is 31.2°. In contrast to benzoisotel-lurazole, no shortened intermolecular Te- N contacts were found in the crystal stmcture of 2-phenylbenzotellurazole. Consequently, no anomalies in solubility and melting point were revealed for this compound as compared with its sulfur and selenium congeners. 

Determination of copper as copper(I) thiocyanate Discussion. This is an excellent method, since most thiocyanates of other metals are soluble. Separation may thus be effected from bismuth, cadmium, arsenic, antimony, tin, iron, nickel, cobalt, manganese, and zinc. The addition of 2-3 g of tartaric acid is desirable for the prevention of hydrolysis when bismuth, antimony, or tin is present. Excessive amounts of ammonium salts or of the thiocyanate precipitant should be absent, as should also oxidising agents the solution should only be slightly acidic, since the solubility of the precipitate increases with decreasing pH. Lead, mercury, the precious metals, selenium, and tellurium interfere and contaminate the precipitate. 

Selenic acid, H2Se04, is a strong acid (/fai 2) and the solubility of its salts parallels that of the corresponding sulfates. It is formed by the oxidation of selenous acid or elemental selenium with strong oxidizing agents in the presence of water. Telluric acid, H2Te04, or tellurates are obtained by oxidation of tellurides,... 

According to these previous studies, the most dominant dissolved states of Au and Ag in ore fluids are considered to be bisulfide and chloride complexes, depending on the chemistry of ore fluid (salinity, pH, redox state, etc.). However, very few experimental studies of Au solubility due to chloride complex and Ag solubility due to bisulfide complexes under hydrothermal conditions of interest here have been conducted. Thus, it is difficult to evaluate the effects of these important species on the Ag/Au of native gold and electrum. Other Au and Ag complexes with tellurium, selenium, bismuth, antimony, and arsenic may be stable in ore fluids but are not taken into account here due to the lack of thermochemical data. 

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. 

Coprecipitation is a partitioning process whereby toxic heavy metals precipitate from the aqueous phase even if the equilibrium solubility has not been exceeded. This process occurs when heavy metals are incorporated into the structure of silicon, aluminum, and iron oxides when these latter compounds precipitate out of solution. Iron hydroxide collects more toxic heavy metals (chromium, nickel, arsenic, selenium, cadmium, and thorium) during precipitation than aluminum hydroxide.38 Coprecipitation is considered to effectively remove trace amounts of lead and chromium from solution in injected wastes at New Johnsonville, Tennessee.39 Coprecipitation with carbonate minerals may be an important mechanism for dealing with cobalt, lead, zinc, and cadmium. 

Water-soluble selenium compounds are poured down the sink in the hood and flushed with much water. Rubber stoppers and gloves can be freed from toxic compounds by soaking them for a few minutes in bromine or chlorine water and then in dilute sodium hydroxide solution. 

Water soluble Se in the North West region varied from 0.0002-0.0429 mg/kg. Water soluble Se accounted for 2.13-6.34% of the total Se in the soils of North China. Selenium is an essential element to animals and humans. When water soluble Se in soils is less than 0.003 mg/kg, Se deficiency in animal and human beings may occur. EDTA-extractable Se in the alkali desert soils of North China was in the range of 0.011-0.090 mg/kg this was about 5-11% of the total Se in the soils. Selenium deficiency was mostly found in the Loess Plateau and Tibet region. NH4OAc-cxtractable Ni in soils from Beijing was 0.29 mg/kg. 

Elrashidi, M.A., Adriano D.C., Lindsay W.L. Solubility, Speciation, and Transformations of Selenium in Soils. In Selenium in Agriculture and the Environment, L.M. Jacobs, ed. Madison, IL Soil Science of America, Inc. 1989. 

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