Wastes Containing Hexavalent Chromium

Liquid wastes containing hexavalent chromium require reduction of chromium to the trivalent state prior to metal removal. Commonly used reducing agents are sodium metabisulfite, sulfur dioxide, ferrous sulfide, and other ferrous ions (ferrous sulfate, ferrous chloride, or electrochemically generated ferrous ion). All of these reagents create some form of chromium sludge, which must be separated and dewatered before disposal. 

Solubility of Metal Hydroxides and Sulfides as a Function of pH. (Source EPA publication, EPA-600/2-82-011C, 1981) 

---

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... 

Corrective Action Application At a RCRA site in the southwest, a waste stream containing hexavalent chromium was reduced to the trivalent form. The trivalent chromium was then removed using ion exchange. The influent hexavalent chromium... 

In addition to the incomplete oxidation of some chemicals with the reflux methods, laboratory wastes generated from the standard COD tests contain hexavalent chromium, mercury, and silver metals, all of which are classified as hazardous wastes by the US Environmental Protection Agency and their disposal is regulated under Resource Conservation and Recovery Act. Consequently, various modifications of the standard procedures or alternative methods have been reported for the COD test. These include the replacement of hexavalent chromium, mercury, and silver metals microwave digestion automation and online COD measurement and electrochemical oxidation to measure the sample COD. 

Solid Waste - The steam-reforming process contains up to 8-9 catalytic steps catalysts are replaced after 2-6 years of service. Partial oxidation and coal gasification use 3-4 catalysts. The catalysts contain " hexavalent chromium, nickel, zinc, iron, and mineral supports therefore these materials could not be disposed into landfills, Companies that specialize in producing catalysts or metal-processing companies customarily recycle catalysts. 

Spent pickle liquor is considered a hazardous waste (K062) because it contains considerable residual acidity and high concentrations of dissolved iron salts. For example, spent pickle liquor and waste acid from the production of stainless steel is considered hazardous. The hazardous constituents in K062 are lead, nickel, and hexavalent chromium. Waste pickle liquor sludge generated by lime stabilization of spent pickle liquor is not considered hazardous unless it exhibits one or more of the characteristics of hazardous waste. An estimated 6 million tons of spent pickle liquor are generated annually in the U.S.1... 

Stoichiometric hexavalent chromium oxidants have been used for a variety of oxidation reactions but, due to environmental problems of chromium-containing waste, catalytic versions with soluble chromium catalysts have been developed, for example by Muzart ° using mainly TBHP as oxygen source. For instance, Muzart and coworkers... 

The decomposition of the catalyst beads can cause a secondary air pollution emission consisting of the particulate dust generated by abrasion of the surface of the catalyst. Operating cost for catalyst replacement varies directly with catalyst attrition rate. The system can process waste streams with VOC concentrations of up to 25% of the lower explosive limit (LEL). The proprietary catalyst contains up to 10% chromium, including 4% hexavalent chromium. This could lead to the emission of hexavalent chromium in some applications of the technology. 

The EPA regulates both lead and hexavalent chromium in its hazardous waste regulation. Solid waste containing lead chromate or lead molybdate should be tested for toxicity prior to disposal via the TCLP test. 

Still, some harmful compounds can appear in waste plastics, e.g. additives containing antimony, cadmium, lead, zinc, or hexavalent chromium, and plasticizers. Brominated flame retardants spreading into the environment is another reason for concern and a rational argument for dealing with WEEE and ASR-plastics [37]. 

Cleaning Up The used alumina, since it contains adsorbed hexavalent chromium ion, should be placed in the hazardous solid waste container for Cr " contaminated alumina. Place dichloromethane in the halogenated waste container. 

Hexavalent chromium (6 ") is a recognized carcinogen, and industrial exposure to fiimes and dusts containing this metal is associated with increased incidence of lung cancer, dermatitis, and skin ulcers. Environmental health risks arise from soil contamination by Cr " waste disposal sites left by the leather tanning and dyestuff industries. Cr is more efficiently absorbed than Cr " and its toxicity and carcinogenic effects involve reduction to Cr and Cr " by cysteine, with the formation of intracellular DNA adducts. Cr species are relatively nontoxic partly because of their poor intestinal absorption and rapid excretion in urine. 

EnvirOTimental trends are having an impact on electrical applications. Waste legislation includes WEEE (Waste of Electrical and Electronic Equipment) directive 2002/%/EC which holds producers responsible for collection and recovery of materials at end of Ufe. Additionally, materials that contain bromine-based flame retardants must be removed from the waste and handled separately. In restrictions on use of hazardous substances (ROHS) directive 2002/95/EC, the use of various hazardous materials is restricted. These include lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyls, and polybrominated diphenyl ether. Since the introduction of Blue Angel in Germany in 1978, several other eco-labels have been implemented. These include TCO (Sweden), Nordic Swan, Milieukeur (Netherlands), and the EU Ecolabel. The general purpose of these labels is to provide cmisumers with information relating to the environmental impact of the products they purchase. 

The pretreatment miit operations for various types of treatment facilities are shown in Table 24. The pretreatment processes generally involve separate treatment of cyanide wastes and other acid wastes containing metal ions. The cyanide wastes can be treated with ferrous sulfate and lime to convert highly toxic cyanides to less toxic cyanates or cyanide complexes, or can be oxidized to COj and Nj with chlorine under alkaline conditions. The acid waste streams are treated first to reduce hexavalent chromium to trivalent chromium, using ferrous sulfate, scrap iron, or sulfur dioxide, and then precipitating the metal ions (Cr +) as metal hydroxides. 

Jacobs, J. H. 1992. Treatment and stabilisation of hexavalent chromium containing waste material. Environmental Progress, 11(2), 123-126. 

---