Trivalent chromium wastes

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... 

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... 

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). 

Normally, hexavalent chromium in waste rinsewater or plating solutions is treated by being reduced to trivalent chromium with a reducing agent, followed by precipitation with lime. 

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. 

EPA. 1984d. Health effects assessment for trivalent chromium. Report to Office of Solid Waste and Emergency Response, Office of Emergency and Remedial Response, U.S. Environmental Protection Agency, Washington, DC by Environmental Criteria and Assessment Office, Office of Health and Environmental Assessment, Office of Research and Development, U.S. Environmental Protection Agency, Cincinnati, OH. EPA-540/1 -86-035. 

Chromium wastes are commonly treated in a two-stage batch process. The primary stage is used to reduce the highly toxic hexavalent chromium to the less toxic trivalent chromium. There are several ways to reduce the hexavalent chrome to trivalent chrome including the use of sulfur dioxide, bisulfite, or ferrous sulfate. The trivalent chrome is then removed by hydroxide precipitation. Most processes use caustic soda (NaOH) to precipitate chromium hydroxide. Hydrated lime [Ca(OH)2] may also be used. The chemistry of the reactions is described as follows ... 

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). 

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. 

Guan et al. (2009) removed trivalent chromium ions from waste water using synthetic zeolites. It was observed that zeolite can selectively adsorb chromium even in the presence of other alkali and alkaline earth metal cations including sodium and potassium. Xie et al. (2012) stated that if zeolites are modified with chitosan by forming a monolayer of chitosan on zeolite surface their adsorption capacities increases and they can preferentially adsorb phosphorous along with many other heavy metals from waste water. It was attributed to the increased porous structure of zeolites and non-zeolite fraction of different oxides. Monolayer of chitosan acted as binding material for adsorption of different heavy metal ions. 

In addition to the heavy metals stated in Table 22.10, ferro- and ferricyanide are also part of the pollutants in the wastewater generated in a chrome pigment plant. These wastes are generally combined and treated through reduction, precipitation, equalization, and neutralization to be followed by clarification and filtration processes. Most of the heavy metals are precipitated using lime or caustic soda at specific pH. Chromium is reduced by S02 to a trivalent form, wherein it is precipitated as chromium hydroxide at specific pH. Sodium bisulfide is also employed to precipitate some of the metals at a low pH. The treated water is recycled for plant use while the sludge is sent to landfills (Figure 22.7). 

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. 

Chromic acid is a very viscous solution in the concentrations used in decorative chrome plating. The concentration is generally 26 to 44 ounces per gallon. It is one of the main metals that must be treated in a waste treatment system for pollution control. Hexavalent chromium must be converted to trivalent chrome before it can be precipitated out of the waste waters. 

Falerios M, Schild K, Sheehan P,et al. 1992. Airborne concentrations of trivalent and hexavalent chromium from contaminated soils at unpaved and partially paved commercial/industrial sites. J Air Waste Manage Assoc 42 40-48. 

Chrome bearing waste water requires pretreatment, since hexavalent chromium will not react with hydroxide. Hexavalent chrome must be reduced to the trivalent form by reaction with ferrous sulfate, sodium meta-bisulfite or sulfur dioxide. The reduction must be carried out at a pH below 3.0. Achieving complete reduction is important, since any remaining hexavalent chromium will remain in solution in the effluent. The pH is then raised to pH... 

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