Molybdenum hexavalent

Some metals used as metallic coatings are considered nontoxic, such as aluminum, magnesium, iron, tin, indium, molybdenum, tungsten, titanium, tantalum, niobium, bismuth, and the precious metals such as gold, platinum, rhodium, and palladium. However, some of the most important poUutants are metallic contaminants of these metals. Metals that can be bioconcentrated to harmful levels, especially in predators at the top of the food chain, such as mercury, cadmium, and lead are especially problematic. Other metals such as silver, copper, nickel, zinc, and chromium in the hexavalent oxidation state are highly toxic to aquatic Hfe (37,57—60). 

Lener, J. and B. Bibr. 1979. Biliary excretion and tissue distribution of penta- and hexavalent molybdenum in rats. Toxicol. Appl. Pharmacol. 51 259-263. 

The process can be used to treat dissolved metals and is commonly used in groundwater treatment for the reduction and precipitation of hexavalent chromium, as well as in the oxidation of cyanide wastes (at concentrations up to 10%). Other potential applications of electrochemical treatment include remediation of arsenic, cadmium, molybdenum, aluminum, zinc,... 

Hexavalent molybdenum and tungsten, pentavalent vanadium and, to a more limited extent, niobium and tantalum form a very large number of polyoxoanions ( heteropolyanions such as [PW12O40]3- and isopolyanions such as [M07O24]6-). Unlike the polyoxoanions of the post transition elements the heteropolyanions for the most part are discrete, compact species of high... 

At low concentrations hexavalent molybdenum in dilute acidic solutions is likely mononuclear.8 At higher acidity the ion Mo02(H20)4+ is probably present.8 In special situations the dimolybdate ion, Mo202, is formed whose structure is analogous to that of the better known dichromate ion, Cr202-. (Dinuclear MoVI ions are discussed separately below.)... 

Polyoxometalate anions, especially those of hexavalent molybdenum and tungsten interact with rare-earth cations to generate a seemingly endless variety of complexes in which the rare earths function as encrypted cations, as core heteroatoms, or as linkers of polyoxometalate fragments to yield discrete anionic assemblies currently incorporating as few as 8 and as many as 164 metal centers,... 

Hexavalent uranium is far more easily reduced than either hexavalent molybdenum or tungsten the potential for the UO —> U+4 couple is close to that for the Cu2+ —> Cu couple. 

Different metals and metal compounds have been in use since the beginning of human civilization. Metals include aluminum, antimony, cobalt, copper, chromium, iron, nickel, manganese, molybdenum, selenium, tin, vanadium, and zinc. The list of toxic metals includes but is not limited to arsenic, beryllium, cadmium, hexavalent chromium, lead, and mercury. Contamination of food, water, and the air by metals, particularly lead and cadmium, has caused global concern. Several studies have shown elevated levels of lead, nickel, chromium, and manganese in children s hair. 

Molybdate based scheelites have been intensively studied in this respect, one reason being that they are found with molybdenum in both the penta- and hexavalent state. Bismuth molybdates in particular are useful catalysts for selective oxidation of propylene to acrolein, propylene ammoxidation to acrylonitrile and the oxidative dehydrogenation of butene to butadiene. 

Molybdenum most frequently occurs in the hexavalent state in derivatives of the trioxide MoOg, this being the fully oxidised condition and its most stable form. The lower oxides and their salts can readily be oxidised to molybdates, and this is usually done before the metal is estimated. 

The interest in adsorption of molybdenum stems primarily from its use as a carbon-supported catalyst [22]. The use of activated carbon for the removal of Mo-99 (used in nuclear medicine) has also been reported ] 158]. Only hexavalent Mo is stable under a wide pH range and in the absence of other complexing agents. At pH > 8, the dominant species is M0O4-, while at very low pH there is precipitation of the hydrated oxide between these two extremes polymeric anions are formed [123,159]. 

In contrast, hexavalent transition metals, such as molybdenum and tungsten, polymerize in acid by aggregation to specifically favored geometries containing oxide octahedra of MoOg and W06 (the iso poly acids) (5a). These oxide structures have cavities which accept foreign ions, to form heteropoly ions (5b). The central cavity is a tetrahedral site, often occupied by P0, Si0, etc, a fact utilized in phosphate precipitation ... 

Note that the seven Zr-0 bond strengths are fairly uniform but that there is quite a range in Mo-0 distances, with a resultant range in their bond strengths in spite of this, they sum quite well for hexavalent molybdenum. 

Stainless steel welders are at high risk for developing asthma. The fumes emitted into the welders breathing zones contain hexavalent chromium, iron, nickel, manganese, titanium, molybdenum, copper, and chromium, as well as silica, calcium carbonate, volatile organic compounds from fluxes and fumes from burnt paints. I64 68 ... 

Among the hexavalent elements, molybdenum and tungsten seem to be the most technologically important metals because of their use as deposits on steels in the form of protecting metal or metal boride layers. Since the main features of molybdenum and tungsten electrolytes are similar, the following text will deal with the properties of the former. 

Furthermore, [NiCit] can readily be reduced to metallic nickel (c.f. Eq. 39) at the potential where the Ni-Mo alloy is formed. Yet, no such reduction is shown in Eqs. (42) and (43), in which Mo is first reduced from the hexavalent molybdate to the tetrava-lent molybdenum dioxide, and then further to metallic molybdenum. A solution, in which the concentrations of Ni and Cit - were equal, was used and it was assumed that all citrate ions were bound to Ni in [NiCit] . This assumption ignores the possible existence of complexes of the type [(Mo04)(Cit)(H)ni] (written for short as [l,l,m] where m is the number of protons in the complex), in spite of the fact that such complexes are well known (cf.. Fig. 13a). In addition, detailed calculation of the distribution of species in a NP and Cit - mixture shows that, when the ratio of the concentrations of the two ions is unity, there are still a few percents of free Ni and Cif" ions not bound in the complex, as seen if Fig. lb. 

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