Chromium chemical reduction

Method Chemical reduction of hexavalent chromium by sulfur dioxide under acid conditions for the continuous operating system and by sodium bisulfite under acid conditions for the batch operating system. The reduced trivalent form of chromium is subsequently removed by precipitation as the hydroxide. 

Chemical reduction is used to transform a toxic substance with a higher valence to a nontoxic or less-toxic substance with lower valence. The most promising application is the reduction of hexava-lent chromium to trivalent chromium. This method is also applicable to other multivalent metals such as lead and mercury. Commonly used chemical agents for this purpose are sulfite salts, sulfur dioxide, and base metals (e.g., iron and aluminum).22 24... 

Chemical precipitation for removal of dissolved metals Settling or filtration to remove SS Ion exchange for silver recovery Chemical reduction of chromium Settling... 

T0858 Versar, Inc., Chemical Reduction of Hexavalent Chromium Contaminated Soils... 

The reduction of aqueous chromium(III) solutions can be carried out electrolytically o chemically with zinc amalgam, zinc and acid or a Jones reductor.2,24 Electrolytic procedures ca be cumbersome, and with chemical reductants contamination with other products can occur Chromium metal and acid can be used to reduce chromium(III) salts, and this requires less c the metal than in the method described in Section 35.3.1.1.i. 

It has been established that most cathode metals are to some extent soluble in chromic acid solutions, and ions will enter the solution in the highest available oxidation state [e.g. copper(II), gold(III)]. Polarization of the cathode will then cause reduction to lower oxidation states [kinetic factors will prevent the prior reduction of chromate(VI)], then new low-valent species may then initiate a chemical reduction of the chromium(Vl). Chromium deposition occurs within the potential range for the evolution of dihydrogen and, indeed, the latter is the dominant cathode process with the result that typically cathode current efficiencies of only 10-20% are achieved (see equation 9). 

The chromium(II) cation [Cr(terpy)2] is conveniently prepared by the interaction of aqueous chromium(II) solutions with excess terpy, and may be isolated as its perchlorate or iodide salt 239,255,299). The magnetic properties of [Cr(terpy)2][C104]2 have been recorded over the temperature range 20-300 K the complex is low spin its exhibits a nearly linear temperature dependence over this range 299). The complex may be reduced electrochemically (255) or chemically 239). Herzog and Aul have isolated the various [Cr(terpy)2]" complexes that may be obtained by chemical reduction [Cr(terpy)2]l2 (red-brown, = 2.80 BM), [Cr(terpy)2]I (wine-red, = 1.85 BM), and [Cr(terpy)2] (green, /r ff = 0.63 BM) 239). 

Chromium hexacarbonyl is extremely photolabile (equation 6) therefore photochemical substitution is an efficient means of preparing derivatives. Oxidation of the Cr center requires nitric or sulfuric acid, or chlorine. Alternatively, some hgands induce complete carbonyl dissociation with concomitant oxidation, for example, acetylacetonate. Chemical reduction with alkali or alkaline-earth metals or electrochemical reduction proceeds in two-electron steps with loss of two CO molecules to first give [Cr2(CO)io]" and then [Cr(CO)s]. Nucleophilic attack at CO generates a number of stable (Nu = R) and unstable (Nu = N3, OH, H, NEt2) products. The stable [(OC)5CrCOR] ion is a carbene precursor. 

In chemical reduction processes chromium(III) oxide is always the starting material, for which it has to be as pure as possible particularly with regard to sulfur content (nickel alloys otherwise form nickel sulfide at grain boundaries). The reduction can be carried out with silicon and, in particular, aluminum and carbon. The reaction ... 

Reduction of soluble chromium(VI) to soluble chromium(III) as insoluble chromium hydroxide is a typical example of the chemical reduction precipitation process. 

Technology for large-scale application of chemical reduction is well developed. The reduction of residual chlorine in a chlorination or superchlorination process system is termed dechlorination, which is the most common process in municipal water and wastewater treatment. The reduction of chromium waste by sulfur dioxide is another classic process and is in use by numerous plants employing chromium compounds in operations such as electroplating. 

The major advantage of chemical reduction when used to reduce hexavalent chromium, or residual chlorine, is that it is a fully proven technology based on many years of experience. Operation at ambient conditions results in minimal energy consumption. 

One limitation of chemical reduction of hexavalent chromium, residual chlorine, and others, is that for high concentrations of chromium the cost of treatment chemicals may he prohibitive. When this situation occurs, other treatment processes are hkely to be more economical. Chemical interference by oxidizing agents is possible in the treatment of mixed wastes, and the treatment itself may introduce pollutants if not properly controlled. Storage and handling of sulfur dioxide is somewhat hazardous. 

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 chemical reduction process normally generates only small amounts of sludge due to minor shifts in the solubility of the contaminants. In a chromium reduction process, the reduced chromium and other metal ions are precipitated and removed in the subsequent precipitation-sedimentation process. An exception would be hexavalent chromium reduction with ferrous sulfate, where sludge generation may be significant. In a dechlorination process, there will be no chlorine residue, nor reducing agent residue, if the dechlorination process is operated properly. 

Chemical reduction alone will only convert metals from a higher valence start to a lower one to either decrease toxicity or to encourage a given chemical reaction. Explain how a combination of chemical reduction and chemical precipitation together (i.e., chemical reduction precipitation process) can remove highly toxic hexavalent chromium ions from an industrial efQuent. 

Sedimentation and/or filtration (26,28) will be feasible for separating the insoluble chromium hydroxide precipitates (or chemical floes) from a wastewater. Other feasible solid-water separation processes for removing the insoluble chromium hydroxide include membrane filtration (such as ultrafiltration and microfiltration), continuous DAF, PC-SBR-sedimentation, PC-SBR-DAF. The following is a summary of the solid-water separation processes feasible for the combined application of chemical reduction and precipitation. 

A study of an operational waste treatment facility that chemically reduces hexavalent chromium has shown mat 99.7% reduction efficiency is easily achieved (Appendix A2). Final concentrations of 0.05 mg/L are readily attained, and concentrations of 0.01 mg/L are considered to be attainable by properly maintained and operated equipment. Condnct a literature search, showing how chemical reduction, sedimentation, and/or filtration togemer can effectively treat the industrial effluents from me mdnstrial operations of metal finishing, aluminum forming, and inorganic chemicals mannfactnring. 

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). 

Chemical Reduction and Filtration Used in Metal Finishing Industry (Common Metals and Hexavalent Chromium)... 

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