Hexavalent chromium wastes

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... 

Hexavalent chromium wastes resulting from rinsewater and the concentrated acid bleed accumulate in the chromium waste sump [T-20], The chromium wastes are then pumped into the chromium treatment module [T-21] for reduction to the trivalent form. This pump is activated only if the oxidation-reduction potential (ORP) and pH are at the proper levels and if the level in the chromium wastewater sump [T-20] is sufficiently high. 

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... 

Perlmutter, Lieber, and Frauenthal, Movement of Waterborne Cadmium and Hexavalent Chromium Wastes H. W. Davids and M. Lieber, Underground Water Contamination by Chromium Wastes, Water and Sewage Works, vol. 98, pp. 528-534(1951). 

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... 

Alternatively, hexavalent chromium can be reduced, precipitated, and floated by ferrous sulfide. By applying ferrous sulfide as a flotation aid to a plating waste with an initial hexavalent chromium concentration of 130 mg/L and total chromium concentration of 155 mg/L, an effluent quality of less than 0.05 mg/L of either chromium species can be achieved if a flotation-filtration wastewater treatment system is used.15... 

After these subcategorization bases were evaluated, raw waste characterization was selected as the basis for subcategorization. The raw waste characterization is divided into two components, inorganic and organic wastes. These components are further subdivided into the specific types of wastes that occur within the components. Inorganics include common metals, precious metals, complexed metals, hexavalent chromium, and cyanide. Organics include oils and solvents. 

Table 9.2 lists the unit operations associated with each of the seven industry subcategories (raw waste characteristics). Common metals are found in the raw waste of all 44 unit operations. Precious metals are found in only seven unit operations complexed metals are found in three unit operations hexavalent chromium is found in seven unit operations and cyanide is found in eight unit operations. Within the organics, oils are found in 22 unit operations and solvents are found in nine unit operations. A unit operation will often be found in more than one subcategory. 

Concentrations of hexavalent chromium from metal finishing raw wastes are shown in Table 9.8. Hexavalent chromium enters wastewater as a result of many unit operations and can be very concentrated. Because of its high toxicity, it requires separate treatment so that it can be efficiently removed from wastewater. 

FIGURE 9.7 Hexavalent chromium reduction. (Adapted from U.S. EPA, Meeting Hazardous Waste Requirements for Metal Finishers, Report EPA/625/4-87/018, U.S. Environmental Protection Agency, Cincinnati, OH, 1987.)... 

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... 

Andco Environmental Processes, Inc., has developed an electrochemical iron generation process to remove hexavalent chromium and other metals from gronndwater and aqneons wastes. As contaminated water flows through a treatment cell, electrical cnrrent passes between electrodes, releasing ferrons and hydroxyl ions. The small gap between electrodes allows almost instantaneons rednction of chromium ions. Depending on the pH, varions solids may form. 

The process can be used to treat dissolved metals and is commonly used in groundwater treatment for the reduction and precipitation of hexavalent chromium, as well as in the oxidation of cyanide wastes (at concentrations up to 10%). Other potential applications of electrochemical treatment include remediation of arsenic, cadmium, molybdenum, aluminum, zinc,... 

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. 

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. 

Replacement of hexavalent chromium with trivalent chromium offers important environmental advantages. Trivalent chromium is considerably less toxic than hexavalent. Trivalent systems use chromium concentrations that are typically two orders of magnitude less than in hexavalent systems. Thus, far less chromium enters the waste stream. Trivalent systems also generate few toxic air emissions, while hexavalent systems involve a reaction that produces hydrogen bubbles which entrain chromium compounds and carry them out of the baths. Trivalent chromium is readily precipitated from wastewater, while hexavalent chromium solutions must go through an additional step in a treatment system in which the chromium is reduced to its trivalent form before precipitation. It has been shown that trivalent chromium systems can successfully replace hexavalent ones for decorative chrome applications. Trivalent chromium systems are not suitable for hard chrome applications. More information regarding trivalent chromium plating can be obtained from Roy (1984), Robison (1978), Chementator (1982), and Smart (1983). 

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