Waste treatment chromium removal

Medina, B.Y., Torem, M.L., and De Mesquita, L.M.S., Removal of chromium III from liquid effluent streams by precipitate flotation, in Waste Treatment, and Clean Technology, The Global Symposium on Recycling, REWAS 04, Vol. II, Gaballah, I., Mishra, B., Solosabal, R., and Tanaka, M., Eds., Madrid, Spain, September 26-29, 2004. 

Sylvester, P. Rutherford, L. A. Jr. Gonzalez-Martin, A. Kim, J. Rapko, B. M. Lumetta, G. J. Ferrate Treatment for Removing Chromium from High-Level Radioactive Tank Waste, Environ. Sci. Technol. 2001,35, 216-221. 

Use Decolorizing of sugar, water and air purification, solvent recovery, waste treatment, removal of sulfur dioxide from stack gases and clean rooms, deodorant, removal of jet fumes from airports, catalyst for natural-gas purification, brewing, chromium electroplating, air- conditioning. 

While heavy metals (i.e., chromium, copper, nickel) are not typical pollutants in a pharmaceutical waste water stream, removal becomes an issue in some segments of the industry, namely chemical intermediates. These streams are generally treated at the process source in order to minimize the waste water volume. Also, heavy metal streams must be treated prior to any biological treatment that the waste water also requires. Since heavy metals are toxic to microorganisms (even at very low concentrations), their presence reduces biological treatment efficiency. 

Ostertag s committee painted the picture of water pollution in shades quite different from those Dickey had used. Industrial wastes received equal billing with sewage. While the committee repeated the usual platitudes about industry s desire to reduce pollution, the facts it compiled made clear that real progress was slow. Numerous sites of industrial pollution were mapped, with chemical plants in Buffalo seen as a particular problem. Aside from the Long Island chromium removal systems, only three major industrial treatment facilities were built anywhere in the state in 1948. The total construction cost of needed industrial waste treatment... 

Sylvester P et al. (2001) Ferrate treatment for removing chromium from high-level radioactive tank waste. Environ Sci Technol 35 216-221... 

Chemical precipitation is used in hazardous waste treatment primarily for the removal of heavy metal ions from water. The most widely used means of precipitating metal ions is by the addition of base (Ca(OH)2), NaOH, or Na2C03), leading to the formation of hydroxides such as chromium(III) hydroxide... 

The ideal disposal method is a chemical treatment that can convert hazardous waste into environmentally benign materials. For example, trichloroethylene (CI2 C I CHCl) is highly toxic to aquatic life, but this compound can be made nontoxic by chemical treatment that converts its chlorine atoms into chloride anions. Similarly, the chromium-containing waste from electroplating operations contains highly toxic CrOq anions, but a chemical treatment that converts CrOq into Cr causes the chromium to precipitate from the solution as insoluble Cr (OH). This removal of chromium detoxifies the water. 

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. 

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. 

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). 

As discussed, the aqueous waste with hexavalent chromium requires reduction of chromium to the trivalent state prior to metal removal because hexavalent chromium does not form a precipitate. Demonstrated reducing agents are sodium metabisulfite (Na2S205), sulfur dioxide (S02), ferrous sulfide (FeS), and other ferrous ion (ferrous sulfate, ferrous chloride, or electrochemically generated ferrous ion). The treatment processes using these are described below. 

Electrolytic methods have been applied to the treatment of other metal waste streams generated in the electroplating or metal finishing industries. Pollution engineering processes have been designed and implemented for the removal of hexavalent chromium, trivalent chromium, nickle, copper, zinc and cadmium.Besides the Edwards patent, there seems to be no documentation of electrolytic methods for removal and recovery of mercury metal from waste streams. 

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). 

Trace toxic metals may escape from the municipal incineration process. Various agents, such as 0.25 M Na-citrate, have been used to aid the removal of heavy metals during electrodialytic treatment of municipal solid waste incineration (MSWI) fly-ash (Pedersen 2002). One study found that the bottom ash in a municipal incineration system had 1000-fold higher levels of chromium(VI) in test leachates than the hopper cyclone and filter ashes (Abbas et al. 2001), but another study found the chromium in fly-ash to be mostly trivalent chromium (Coodarzi and Huggins 2001). 

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