Chromium trivalent, chemistry

The chemistry of aluminium combines features in common with two other groups of elements, namely (i) divalent magnesium and calcium, and (ii) trivalent chromium and iron (Williams, 1999). It is likely that the toxic effects of aluminium are related to its interference with calcium directed processes, whereas its access to tissues is probably a function of its similarity to ferric iron (Ward and Crichton, 2001). The effective ionic radius of Al3+ in sixfold coordination (54 pm) is most like that of Fe3+ (65 pm), as is its hydrolysis behaviour in aqueous solution ... 

Careful control of temperature, pH, process chemical concentrations, and other process parameters is important in obtaining the maximum lifetime from baths. In some applications, such as in trivalent chromium plating systems, it is essential to keep anolyte solutions contained in anode boxes strictly segregated from the electrolytes in the rest of the bath. Mixing of the two chemistries can ruin the effectiveness of the baths. 

Aside from its color, trivalent chromium differs most markedly from Al(III) in its complexing ability. There are no analogs in aluminum chemistry for many of the octahedral complexes formed by Cr(III) with CN, SCN, NOj, NH3, and with a number of organic amines. The octahedral complexes of aluminum are confined to those having Al—0 or Al—F bonds. In contrasting the complexes derived from the two... 

Chromium wastes are commonly treated in a two-stage batch process. The primary stage is used to reduce the highly toxic hexavalent chromium to the less toxic trivalent chromium. There are several ways to reduce the hexavalent chrome to trivalent chrome including the use of sulfur dioxide, bisulfite, or ferrous sulfate. The trivalent chrome is then removed by hydroxide precipitation. Most processes use caustic soda (NaOH) to precipitate chromium hydroxide. Hydrated lime [Ca(OH)2] may also be used. The chemistry of the reactions is described as follows ... 

Chromium can exist in several oxidation states from Cr(0), the metallic form, to Cr(Vl). The most stable oxidation states of chromium in the environment are Cr(lll) and Cr(Vl). Besides the elemental metallic form, which is extensively used in alloys, chromium has three important valence forms. The trivalent chromic (Cr(lll)) and the tetravalent dichromate (Cr(Vl)) are the most important forms in the environmental chemistry of soils and waters. The presence of chromium (Cr(Vl)) is of particular importance because in this oxidation state Cr is water soluble and extremely toxic. The solubility and potential toxicity of chromium that enters wetlands and aquatic systems are governed to a large extent by the oxidation-reduction reactions. In addition to the oxidation status of the chromium ions, a variety of soil/sediment biogeochemical processes such as redox reactions, precipitation, sorption, and complexation to organic ligands can determine the fate of chromium entering a wetland environment. 

Chemistry is used for many aspects of wastewater treatment. Lime or caustic soda can be used to raise the pH, while sulfuric acid lowers it. Cyanide can be destroyed by introducing chlorine, as a gas or in liquid from (with sodium hypochlorite at a pH of 11) [9]. When enough chlorine is added the cyanide is converted to carbon dioxide and nitrogen. Hexavalent chromium can be reduced to trivalent chromium by using SO2 gas or SO2 releasing chemicals [9]. 

The simultaneous extraction of trivalent chromium, hexavalent chromium, and zinc from cooling lower blowdown water was studied in detail by Fuller and Li. They investigated Alamine 336, an oil-solaUe tertiary amine, and Aliquat 336-S, an oil-soluble quaternary amine salt, both manufactured by General Mills, as carriers in various liquid-membrane systems. Alamine was found to be an effective extractant for Cr , Ct, and Zn. However, for Cr and Cr, a pH of 3.0 or less in the external (feed) phase was required while Zn could be extracted only at a pH > 7. Thus, a combined process could not be developed. Aliquat 336-S was more promising. The chemistry of extraction with this carrier involves the following steps ... 

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