Chemical hexavalent chromium

Strong reducing agents such as sulfur dioxide, sodium bisulfite, sodium metabisulfite, and ferrous sulfate are used in the iron and steel finishing sites to reduce hexavalent chromium to the triva-lent form, which allows the metal to be removed from solution by chemical precipitation.21 23 Metal-containing wastewaters may also be treated by chemical precipitation or ion-exchange. 

Chemical treatment of chromium wastewater is usually conducted in two steps. In the first step hexavalent chromium is reduced to trivalent chromium by the use of a chemical reducing agent. The trivalent chromium is precipitated during the second stage of treatment.15... 

Another innovative flotation-filtration wastewater treatment system adopts the innovative use of the chemical ferrous sulfide (FeS), which reduces the hexavalent chromium and allows separation of chromium hydroxide, nickel hydroxide, and ferric hydroxide in one single step at pH 8.5. Figure 6.7 illustrates the entire system. Again, a DAF-filtration clarifier plays the most important role in this wastewater treatment system. 

Chromate conversion coatings for aluminum are carried out in acidic solutions. These solutions usually contain one chromium salt, such as sodium chromate or chromic acid and a strong oxidizing agent such as hydrofluoric acid or nitric acid. The final film usually contains both products and reactants and water of hydration. Chromate films are formed by the chemical reaction of hexavalent chromium with a metal surface in the presence of accelerators such as cyanides, acetates, formates, sulfates, chlorides, fluorides, nitrates, phosphates, and sulfamates. 

The selected treatment option involves the reduction of hexavalent chromium to trivalent chromium either chemically or electrochemically. The reduced chromium can then be removed using a conventional precipitation-solids removal system. Alternative hexavalent chromium treatment techniques include chromium regeneration, electrodialysis, evaporation, and ion exchange.16... 

There are three treatment methods applicable to wastes containing hexavalent chromium. Wastes containing trivalent chromium can be treated using chemical precipitation and sedimentation, which is discussed below. The three methods applicable to treatment of hexavalent chromium are... 

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. 

Basha, S., Murthy, Z.V.P., and Jha, B., Biosorption of hexavalent chromium by chemically modified seaweed, Cystoseira indica, Chemical Engineering Journal, 137 (3), 480-488, 2008. 

Hexavalent chromium (Cr+6) is the most biologically active chromium chemical species, although little is known about the properties of organochromium compounds, water-soluble species, or their interactions in complex mixtures... 

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

In 1996, the ISEE system was used to remove 200 g of hexavalent chromium from 16 yd of soil at the U.S. Department of Energy s (DOE s) Unlined Chromic Acid Pit located at the Sandia National Laboratory s (SNL s) Chemical Waste Landfill in Albuquerque, New Mexico. The treatment costs for this 4-week demonstration were 1368/yd. According to the U.S. EPA, these costs were calculated for the ISEE prototype used during the demonstration. The costs for a full-scale system would be lower due to design improvements and efficiency of scale (D22781Q, pp. 65, 66 D22758R, pp. 44-51). 

Sevenson Environmental Services, Inc. (Sevenson), is the owner of the MAECTITE chemical treatment process for the precipitation and stabilization of toxic heavy metals. Chemical treatment by the MAECTITE process converts teachable lead, hexavalent chromium, or other heavy metals into insoluble minerals and mixed mineral forms within the material or waste matrix. The technology can be used as an in situ or an ex situ method and does not use pozzolanic or siliceous binders to stabilize the treated material. 

Versar, Inc., has developed a method for the ex situ remediation of soils contaminated with hexavalent chromium. In this process, soil is mixed with a sodium bisulfite solution to chemically reduce the chromium to the less toxic trivalent form. 

The popular movie Erin Brockovich, although focusing on the power industry, is an example of how the popular media can simplify the public s perception of chemical exposure. In Erin Brockovich the chemical of concern was hexavalent chromium, Cr +. Hexavalent chromium is listed as a carcinogen by EPA, but complicated questions dealing with exposure (drinking, inhalation, absorption through skin), toxicity levels, and specific health effects were lost in Hollywood s version. 

The application of isosterism for the design of safer commercial chemicals is much less common than it is for the design of safer drugs or pesticides. There are some examples, however, of how isosterism has been used to design safer commercial chemical substances. One very successful application is in the case of metallized azo dyes [82, 83]. Historically, chromium was a metal of choice in many metallized azo dyes because it imparts the desired color and fastness. Hexavalent chromium (Cr6+) was often used in making such dyes. 

There are two treatability groups of dissolved metals for chemical precipitation, complexed and non-complexed metals. Non-complexed metals can be removed by a direct precipitation with such a chemical as lime (Ca(OH) ), caustic (NaOH), sodium sulfide (Na2S), ferrous sulfide (FeS), or sodium carbonate (NajCOj). Complexed metals require coprecipitation with ferrous sulfate (FeS04), ferrous chloride (FeClj), or sodium dimethyl dithiocarbamate (DTC) in addition to a regular precipitant such as caustic or lime. Electrochemically generated ferrous ion is also effective in removing a wide variety of heavy metals, including hexavalent chromium. 

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. 

The European Union has been quite visible in recent years with such directives. RoHS (Restriction in the use of Hazardous Substances), WEEE (Waste of Electrical and Electronic Equipment), and REACH (Registration, Evaluation, Authorization, and Restriction of Chemical substances). RoHS places restrictions on use of lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyls, and some polybrominated diphenylethers. WEEE targets responsible recycling of electronic equipment. REACH is a new European Community Regulation on chemicals and their safe use... 

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