Chromium reductant systems

It should be noted that dissolved air flotation (DAF) is a more effective process for clarification.8-10 As shown in Appendix D, with an additional step of chromium reduction, the secondary treatment system effectively removed chromium (over 99%), copper (89%), cadmium (64%), lead (67%), and zinc (77%). 

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... 

System component Reaction tanks, a reagent storage and feed system, mixers, sensors, and controls for continuous chromium reduction. A single above-ground concrete tank with retention time of 45 min is provided. For batch operation, dual above-ground concrete tanks with 4h retention time are provided. 

Salts of [Cr(bpy)3] + are best prepared by chemical or electrochemical oxidation of chromium(II) systems (857). Care must be taken to minimize redox reactions that give rise to [Cr(bpy)2L2] + (52, 75, 219, 245, 589, 621, 721, 856, 857, 872, 917, 921). Salts of [Cr(bpy)3]2+ have been evaluated as photographic developers (741) and used for the reduction of the binuclear Fe, Fe center in hemerythrin (26, 27). A mass spectroscopic study of [Crfbpylsl salts has been reported (133). 

A common batch system for chromium reduction with sodium bisulfite consists of a collection tank and a reaction tank with a 4-h retention time. Sodium bisulfite solution is metered into the reaction tank and the pH is controlled by sulfuric acid addition. 

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. 

Continuous chromium reduction treatment requires a tank for acidification and reduction with separate tanks for precipitation and sedimentation. The retention time in the reduction tank is dependent on the pH employed but should be at least four times the theoretical time for complete reduction. In cases where the chromium content of the wastewater varies markedly, equalization should be provided prior to the reduction tank to minimize fluctuations in the chemical feed system. Successful operation of a continuous chrome reduction process requires instrumentation and automatic control. Redox and pH control should also be provided. 

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). 

Gould and co-workers have reported acid dependence of the rates of reduction of acetato- and propionato-penta-amminecobalt(m) by chromium(n), vanadium(n), and europium(ii). In all cases they note a decrease in rate with increasing acidity, which in the case of the acetatopenta-ammine + chromium(n) system confirms earlier work. Assuming the protonation equilibria (22 R=Me or Et) they have fitted... 

Modem chrome-tanning methods are weU controUed and employ an extensive knowledge of the chemistry of the system. The most common chromium-tanning material used is basic chromium sulfate [12336-95-7] Cr(0H)S04, made by the reduction of sodium bichromate with sulfur dioxide or by sulfuric acid and a sugar. 

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