Arsenic selenium oxides

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. 

For removing low levels of priority metal pollutants from wastewater, using ferric chloride has been shown to be an effective and economical method [41]. The ferric salt forms iron oxyhydroxide, an amorphous precipitate in the wastewater. Pollutants are adsorbed onto and trapped within this precipitate, which is then settled out, leaving a clear effluent. The equipment is identical to that for metal hydroxide precipitation. Trace elements such as arsenic, selenium, chromium, cadmium, and lead can be removed by this method at varying pH values. Alternative methods of metals removal include ion exchange, oxidation or reduction, reverse osmosis, and activated carbon. 

Zinc(ll). gallium(lll), and germanium(IV) are the most stable oxidation states for these elements, but the later nonmetals (arsenic, selenium, and bromine) show a reluctance to assume their highest possible oxidation state. 

The usefulness of the As(m)-As(V) couple as a redox indicator in hydrothermal experiments is limited by the availability of thermochemical data and by the range of Eh dictated by analytical and reaction rate constraints. For low temperature (<150°C) short-term experiments, selenium oxidation state analysis may be more appropriate than arsenic, since the Se(VI)-Se(IV) reduction occurs at relatively high Eh (36). 

Whatever instrument is used, provision must be made for using both air and nitrous oxide-supported flames. A fume exhaust must be provided. If arsenic, selenium, or mercury are to be determined, an apparatus for vapor generation should be used. Such apparatus is usually available from the instrument manufacturer. Mercury is usually determined by a flameless or cold vapor technique. 

In contrast to arsenic, the reduced form of selenium, Se(IV), is very strongly adsorbed by HFO. This may account, in part, for the very low selenium concentration in many strongly reducing environments. Furthermore, also in contrast with arsenic, the oxidized form of selenium, Se(VI), is less strongly adsorbed to HFO than the reduced species. These differences, also reflected by other oxide-based sorbents including clays, account for the markedly different behavior of arsenic and selenium in natural waters. 

A considerable number of organometallic species have been detected in the natural environment in recent years. A number of these are nonmethyl compounds which have entered the environment after manufacture and use (e.g. butyltin and phenyltin compounds in antifouling paints for boats). Only a few methyl compounds are now manufactured and used (e.g. methyltin compounds for oxide film precursors on glass and methylarsenic for compounds used as desiccants or defoliants). It is now well established that certain organometallic compounds are formed in the environment, unequivocally so for those of mercury, arsenic, selenium, tellurium and tin, and deduced on the basis of analytical evidence for lead, germanium, antimony and thallium . 

The concentrations of several trace elements increase sharply with increased time, and they continue to increase even after most of the other elemental concentrations have reached steady-state values. These elements, arsenic, selenium, and cadmium, are associated with sulfide mineral phases, possibly as sulfides (or selenides). The mobilization of these elements may depend not only on the pH of the leachate, but also on the rate at which the respective mineral phases are oxidized. 

Surface strip mining of coal destroys the native vegetation at the mine site. The topsoil is sufficiently disturbed that it is difficult to rehabilitate the site and re-start the native growth pattern. Oxidation of waste coal and mine residues produces sulfuric acid and releases toxic materials such as arsenic, selenium and beryllium. These leach into the ground water and drain into streams and lakes. At a minimum, these materials strongly alter the types of plant and animal life able to survive in their presence. At the worst, they kill all life near the mine. 

Within the context of toxicological and clinical importance, speciation studies have been focused on relatively few elements, mainly aluminum, antimony, arsenic, chromium, iodine, lead, mercury, platinum, selenium and tin. However, coupled HPLC-ICP-MS has most often been used for speciation of arsenic, selenium, iodine and, to a lesser extent, mercury. The primary species of these elements include different oxidation states, alkylated metal and/or metalloid compounds, selenoamino acids and selenopeptides.In addition, applications in smdies on the pharmacokinetics of metal-based drugs (mainly platinum complexes) and metalloproteins should be included. " In the following sections, the advances in speciation smdies of individual elements are reviewed. 

Arsenic, selenium, and antimony can be measured at low concentrations with HG-AAS. Digestion with aqua regia in a microwave oven is necessary prior to the determination. Next, hydroxylammonium chloride is added as a neutral solution iodide may be added to accelerate the reaction. For arsenic or antimony a reducing agent such as ascorbic acid has to be added to convert all arsenic or antimony to a single oxidation state. A background correction is required for this purpose apparatus with a deuterium lamp corrector can be used but Zeeman background correction is preferred. 

Iron and manganese occur in a number of soil minerals. Sodium and chlorine (as chloride) occur naturally in soil and are transported as atmospheric particulate matter from marine sprays (see Chapter 10). Some of the other micronutrients and trace elements are found in primary (unweathered) minerals that occur in soil. Boron is substituted isomorphically for Si in some micas and is present in tourmaline, a mineral with the formula NaMg3AlgB3Sig027(0H,F)4. Copper is isomorphically substituted for other elements in feldspars, amphiboles, olivines, p5Toxenes, and micas it also occurs as trace levels of copper sulfides in silicate minerals. Molybdenum occurs as molybdenite (M0S2). Vanadium is isomorphically substituted for Fe or A1 in oxides, pyroxenes, amphiboles, and micas. Zinc is present as the result of isomorphic substitution for Mg, Fe, and Mn in oxides, amphiboles, olivines, and pyroxenes and as trace zinc sulfide in silicates. Other trace elements that occur as specific minerals, sulfide inclusions, or by isomorphic substitution for other elements in minerals are chromium, cobalt, arsenic, selenium, nickel, lead, and cadmium. 

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