Chromium reduction using

The sources of theories and principles for chromium reduction using an acid, chemical precipitation using a base, and clarification are detailed in Refs. 8 to 10. 

The first system, shown in Figure 6.6, is identical to the conventional reduction-precipitation in chemistry (i.e., neutralization, chromium reduction, pH adjustment, metal hydroxide precipitation, and so on). However, a flotation-filtration clarifier (Tank T101SF, as shown in Figure 6.6) is used. The unit consists of rapid mixing, flocculation, high-rate DAF, and sand filtration.1557... 

The acid used for chromium reduction is sulfuric acid. 

Hexavalent chromium reduction through the use of sulfur dioxide and sodium metabisulfite has found the widest application in the metal finishing industry. It is not truly a treatment step, but a conversion process in which the hexavalent chromium is converted to trivalent chromium. The hexavalent chromium is reduced through the addition of the reductant at a pH in the range of 2.5-3 with a retention time of approximately 30-40 min (Figure 9.7). 

Sodium metabisulfite or bisulfite is a commonly used chemical for chromium reduction. The metabisulfite hydrolyzes to sodium bisulfite, and the bisulfite in turn dissociates to sulfurous acid, which reduces the hexavalent chromium at a pH of 2-3. 

Sulfur dioxide is another commonly used chemical for chromium reduction. The reduction occurs when sulfurous acid, produced by the reaction of sulfur dioxide and water, reacts with chromic acid as follows ... 

Difficulties have been observed in the preservation of samples for speciation of chromium. Chromium speciation in seawater was determined on board ship shortly after samples had been collected (Abollino et at., 1991). Some samples were frozen, and analysed later in a laboratory. However, significantly lower concentrations of Crvl were observed in these latter samples. Thus, sea-going analytical methods for the determination of Crm and total chromium are of particular importance (Mugo and Orians, 1993). The volatile trifluoroacetyl-acetone derivative of Crm was formed and then concentrated by extraction into toluene. Chromium was determined by means of a gas chromatograph equipped with an electron capture detector. Total chromium was determined as Cr111 after reduction. The detection limits were 0.062 and 0.255 nmol dm 3 total chromium. A useful method was described for sampling natural water in the field, and for the preservation of Crm and Crvl species for subsequent analyses in a laboratory (Cox and McLeod, 1992). Water samples were drawn through small columns packed with activated alumina, which had been prepared previously. Chromium species were retained on the columns. 

Reduction of triphenylcyclopropenyl salts with other reagents including Mg and Zn have been reported. Reduction of triphenylcyclopropenyl cations by these metals yields 22 (equation 14), whereas the diphenyl species yield tetraphenylbenzene (equation 12) . Isolation of bis-cyclopropenes has also been reported from reductions using chromium(II)... 

Chromium (0.02% lithosphere) is produced from chromite, FeCr204. Direct reduction of this compound with carbon yields ferrochrome, an iron-chromium alloy used in making stainless steel. Chromium is obtained free from iron by converting the chromite first to a soluble chromate, then to the sesquioxide CrgOg, and finally to metal ... 

The most common chemicals used for chromium reduction and other chemical reduction applications are sulfur dioxide (SO2), sodium metabisulfite (Na2S205), sodium bisulfite (NaHSOg), and sulfuric acid (H2SO4). 

The chromium reduction process can be employed as batch treatment or continuous treatment. For small daily volumes of water or wastewater that are less than 150,000 L (40,000 gal), the most economical system is batch treatment in which two tanks are provided, each with a capacity of one day s flow. Reduction, precipitation, and sedimentation are carried out in one tank, while the other is used to collect the waste. In a typical batch system, the required dosage of acid and sodium metabisulfite is added to the tank and the contents are mixed for 15 min to ensure complete reduction of the chromium. 

Chemical redox is a full-scale, well-established technology used for disinfection of drinking water and wastewater, and it is a common treatment for cyanide (oxidation) and chromium [reduction of Cr(VI) to Cr(III) prior to precipitation] wastes. Enhanced systems are now being used more frequently to treat hazardous wastes in soils. Figure 4 shows a typical site remediation project involving the use of chemical redox (chemical reduction/oxidation) for removal of chromium from the environment (22). 

The third approach to obtain diarylmethylpiperazine derivatives uses the highly stereospecific chiral oxazaborolidine-catalyzed reduction, using catecholborane as the reductant of the 4-bromobenzophenone chromium tricarbonyl complex, as described by Corey and Helal [59], followed by the stereospecific displacement of the hydroxyl benzyl group by the /V-substituted-piperazine [44]. As outlined in Scheme 2, Delorme et al. [44] used this approach for the enantioselective synthesis of compound 31, (+)-4-[ (aS)-a-(4-benzyl-l-piperazinyl)benzyl]-lV,lV-diethylben-zamide. Lithiation of the readily available benzene chromium tricarbonyl with n-BuLi in the presence of TMEDA in THF at —78 °C, followed by addition of... 

Canfield, D.E., Raiswell, R., Westrich, J.T., Reaves, C.M. Berner, R.A. (1986) The use of chromium reduction in the analysis of reduced inorganic sulfur in sediments and shales. Chemical Geology 16, 59-62. 

Chromium reduction to measure reduced inorganic sulfur compounds in sediments was proposed by Zhabina and Volkov (1978). Since then it has found wide use internationally (Sullivan etal. 1999), particularly when pyritic sediments and acid volatile mono-sulfides are expected. The method is not measurably affected by sulfur in organic matter or sulfates (Canfield etal. 1986 Morse Cornwell 1987). Accordingly, this chromium reducible sulfur method (SCr) is especially useful on samples with appreciable organic matter and also for sandy soils where the %S action criterion is very low (e.g. as low as 0.03%S). 

However, it is assumed that the active centers are coordinatively unsaturated Cr or Cr centers that are generated by reaction with ethylene (Eq. 8-11). It is also possible to convert the chromate deposited on the silica surface to an active form by high-temperature reduction with CO. In an alternative method of catalyst production, low-valent organochromium compoimds such as chromocene and tris( -allyl)-chromium are used as catalyst precursors. 

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