Toxic hexavalent chromium

Redox reactions may cause mobile toxic ions to become either immobile or less toxic. Hexavalent chromium is mobile and highly toxic. It can be reduced to be rendered less toxic in the form of trivalent chromium sulfide by the addition of ferrous sulfate. Similarly, pentavalent (V) or trivalent (III) arsenic, arsenate or arsenite are more toxic and soluble forms. Arsenite (III) can be oxidized to As(IV). Arsenate (V) can be transformed to highly insoluble FeAs04 by the addition of ferrous sulfate. 

While chromium metal or trivalent chromium is not very toxic, hexavalent chromium (Cr +) is carcinogenic and moderately toxic. Cr + is corrosive to skin and causes denaturation and precipitation of tissue proteins. Inhalation of Cr6+ dust or mist can cause perforation of the nasal septum, lung irritation, and congestion of the respiratory passsages. Chronic exposure may produce cancer of the respiratory tract. 

Chemical reduction alone will only convert metals from a higher valence start to a lower one to either decrease toxicity or to encourage a given chemical reaction. Explain how a combination of chemical reduction and chemical precipitation together (i.e., chemical reduction precipitation process) can remove highly toxic hexavalent chromium ions from an industrial efQuent. 

After the toxic hexavalent chromium(VI) is reduced to less toxic trivalent chromium(III) ion, lime or another base chemical is added to convert soluble chromium(III) to insoluble chromium hydroxide in accordance with Eq. (7). A water-solid separation process unit is needed to remove the insoluble chromium hydroxide from the pretreatment wastewater. What are the feasible water-solid separation processes available ... 

List the possible sources of highly toxic hexavalent chromium (Cr " ") and methods to remove it from a wastewater stream. 

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 ... 

When taken orally or rectally it is irritant to the mucosa and can cause acute tubular necrosis, hepatitis and colitis. The toxic hexavalent chromium becomes rapidly bound to tissue (in the bivalent form). Therefore clinical measures to reduce absorption must be administered immediately in order to have an effect. When inhaled, it causes chronic bronchitis, interstitial... 

It is also becoming increasingly apparent that the specific chemical form of a metal is a factor in its toxicity. Water-lipid partitioning, cell-wall permeability, and metabolic transport are just a few of the factors influenced by chemical form which, in turn, influence toxicity. Hexavalent chromium, pentavalent vanadium, divalent manganese, arsine, and methylmercury are more toxic than other forms of the corresponding metal. ... 

A buildup of Al + in the zinc phosphating baths will restrict the amount of coating formed unless these ions are ranoved by addition of fluoride or borate. An advantage of zinc phosphating, however, is that it avoids the use of toxic hexavalent chromium compounds. Some simple bath compositions for zinc and manganese phosphating of aluminium are... 

This is similar for mercury [15], caesium, technetium and uranium [16], as well as for molybdenum, which can be bound to iron oxides even in the form of the molybdate anion [17]. In some soils, hydrous oxides represent dominant substrates of copper [18] and cadmium [19]. The highly toxic, hexavalent chromium is by far more mobile than the trivalent, which makes its reduction in the environment especially important [20], although it also sorbs onto hydrous oxides [21]. 

Gurung A, Oh SE, Kim KD, Shin BS (2012) Semi-continuous detection of toxic hexavalent chromium using a sulfur-oxidizing bacteria biosensor. J Environ Manage 106 110-112. doi 10.1016/j.jenvman.2012.04.010... 

In removing excess free chlorine from municipal or industrial water and from wastewater, sodium sulfite competes with bisulfite or sulfur dioxide. Other commercial appHcations of sodium sulfite in wastewater treatment include the reduction of hexavalent chromium to the less toxic Cr " salts as well as the precipitation of silver and mercury. 

Chromium complexes of long-chain fatty acids are exceUent water repeUents which are also used for their food-release properties in certain packaging appHcations. The presence of chromium has raised environmental concerns, despite the fact that the metal is in the trivalent rather than in the highly toxic hexavalent state. This material is available as Qudon (DuPont). 

The primary routes of entry for animal exposure to chromium compounds are inhalation, ingestion, and, for hexavalent compounds, skin penetration. This last route is more important in industrial exposures. Most hexavalent chromium compounds are readily absorbed, are more soluble than trivalent chromium in the pH range 5 to 7, and react with cell membranes. Although hexavalent compounds are more toxic than those of Cr(III), an overexposure to compounds of either oxidation state may lead to inflammation and irritation of the eyes, skin, and the mucous membranes associated with the respiratory and gastrointestinal tracts. Skin ulcers and perforations of nasal septa have been observed in some industrial workers after prolonged exposure to certain hexavalent chromium compounds (108—110), ie, to chromic acid mist or sodium and potassium dichromate. 

Metal Finishing and Corrosion Control. The exceptional corrosion protection provided by electroplated chromium and the protective film created by applying chromium surface conversion techniques to many active metals, has made chromium compounds valuable to the metal finishing industry. Cr(VI) compounds have dominated the formulas employed for electroplating (qv) and surface conversion, but the use of Cr(III) compounds is growing in both areas because of the health and safety problems associated with hexavalent chromium and the low toxicity of trivalent chromium (see... 

Health Effects Assessment for Hexavalent Chromium, EPA/540/1-86-019, United States Environmental Protection Agency (EPA), Sept. 1984 Toxicological Profile for Chromium, Agency for Toxic Substances and Disease Registry (ASTDR), ASTDR/TP-88/10,1989. 

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. 

Among warm-blooded organisms, hexavalent chromium was fatal to dogs in 3 months at 100 mg/kg in their food and killed most mammalian experimental animals at injected doses of 1 to 5 mg Cr/kg body weight, but it had no measurable effect on chickens at dietary levels of 100 mg/kg over a 32-day period. Trivalent chromium compounds were generally less toxic than hexavalent chromium compounds, but significant differences may occur in uptake of anionic and cationic CL3 species, and this difference may affect survival. 

Gendusa, T.C., T.L. Beitinger, and J.H. Rodgers. 1993. Toxicity of hexavalent chromium from aqueous and sediment sources to Pimephales promelas and Ictalurus punctatus. Bull. Environ. Contamin. Toxicol. 50 144-151. 

Laj, S., V.K. Jain, and S.K. Tandon. 1984. Comparative toxicity of trivalent and hexavalent chromium IV biochemical changes in blood and liver of rat. Jour. Environ. Biol. 5 29-35. 

Pickering, Q.H. 1980. Chronic toxicity of hexavalent chromium to the fathead minnow. Arch. Environ. Contam. Toxicol. 9 405-413. 

Tandon, S.K., D.K. Saxena, J.S. Gaur, and S.V. Chandra. 1978. Comparative toxicity of trivalent and hexavalent chromium. Alterations in blood and liver. Environ. Res. 15 90-99. 

Van der Putte, I., M.A. Brinkhorst, and J.H. Koeman. 1981b. Effect of pH on the acute toxicity of hexavalent chromium to rainbow trout (Salmo gairdneri). Aquat. Toxicol. 1 129-142. 

Hexavalent chromium is also a toxic compound (like lead, cadmium, mercury) and can be easily detected with UV spectrophotometry [20]. This system works for the quality control of electroplating treated wastewater with a detection limit of 5 pg It1. 

Dorn, P.B., Rodgers. J.H.. Jr.. Jop,K.M.,Raia,J.C., and Dickson, K.L. Hexavalent chromium as a reference toxicant in effluent toxicity tests. Environ. Toxicol. Chem., 6(6) 435-444, 1987a. 

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