2 Physical and Chemical Forms of Cyanide Rajat S. Ghosh, David A. Dzombak, and George M. Wong-Chong CONTENTS 2.1 Gaseous Forms of Cyanide 17 2.2 Aqueous Forms of Cyanide 17 2.2.1 Free Cyanide 17 2.2.2 Metal–Cyanide Complexes 19 2.2.2.1 Weak Metal–Cyanide Complexes 19 2.2.2.2 Strong Metal–Cyanide Complexes 19 2.2.3 Cyanate and Thiocyanate 19 2.2.4 Organocyanide Complexes 20 2.3 Solid Forms of Cyanide 20 2.3.1 Simple Metal–Cyanide Solids 21 2.3.2 Metal–Metal Cyanide Solids 21 2.3.2.1 Alkali/Alkaline Earth Metal–Metal Solids 21 2.3.2.2 Other Metal–Metal Cyanide Complex Salts 22 2.4 Summary and Conclusions 22 References 23 Cyanide occurs in many different forms in water and soil systems. The specific form of cyanide determines the environmental fate and transport of cyanide, as well as its toxicity. Understanding the specific form(s) of cyanide present in a particular water, soil, or sediment is critical for assessment of how to manage or treat the cyanide present. This cannot be overemphasized! While “cyanide” is often discussed as a single entity in the popular press and even in professional publications, this is a misleading portrayal. The various forms of cyanide are quite different in their reactivity and their toxicity. Proper professional evaluation, assessment, and design activities pertaining to cyanide contamination management requires knowledge about and careful consideration of cyanide speciation. This chapter provides an introductory overview of the various forms of cyanide that can exist in water and soil systems. All of the remaining chapters of this book assume a basic knowledge of the speciation of cyanide as presented here. A detailed examination of the properties and reactivity of the In water and soil systems, cyanide occurs in various physical forms, including many different kinds of species dissolved in water, many different solid species, and several gaseous species. The Chemically, cyanide can be classifiedinto inorganic and organic forms, as indicated in Figure 2.1. Inorganic forms, which occur in all three physical states, include free cyanide, weak metal–cyanide complexes, strong metal–cyanide complexes, thiocyanate andmetal–thiocyanate complexes, cyanate 15 © 2006 by Taylor & Francis Group, LLC most commonly occurring aqueous, gaseous, and solid forms of cyanide is provided in Chapter 5. cyanide species that occur in the aqueous, solid, and gas phases are indicated in Figure 2.1. 16 Cyanide in Water and Soil WATER GAS SOLID Free cyanide Metal–cyanide complexes Cyanate, thiocyanate Organocyanides Free cyanide HCN(g) Cyanogen halides CNCl(g), CNBr(g) Simple metal cyanide solids NaCN(s), KCN(s), CuCN(s) … Alkali or alkaline earth metal-metal cyanide solids K 3 Fe(CN) 6 (s), K 4 Fe(CN) 6 (s), KAg(CN) 2 (s), … Other metal-metal cyanide solids Fe 4 [Fe(CN) 6 ] 3 (s), Fe 3 [Fe(CN) 6 ] 2 (s), … HCN, CN – Weak complexes: Ag(CN) 2 – , CdCN – , … Strong complexes: Fe(CN) 4– Fe(CN) 3– … CNO – , SCN – Nitriles, cyanohydrins, … 6 , 6 , FIGURE 2.1 Forms and species of cyanide in water and soil. and metal–cyanate complexes, and cyanogen halides. Aqueous free cyanide is the sum of hydrogen cyanide, HCN, and its deprotonated form, the cyanide anion, CN − . HCN is volatile under environ- mental conditions and occurs as both aqueous and gaseous species. Many metals can bond with the cyanide anion to form dissolved metal–cyanide complexes, as well as metal–cyanide solids. Cyanate, CNO − , requires the presence of strong oxidizing agents for its formation and thus is rarely found in the environment. Thiocyanate, SCN − , can be formed in the environment and is also present in a variety of industrial wastewater discharges. The cyanogen halides of interest, CNCl and CNBr, form upon chlorination or bromination of water containing free cyanide. These species are volatile under environmental conditions, and thus occur as both aqueous and gaseous species. Organic cyanides contain carbon–carbon covalent bonding between hydrocarbon and cyanide moieties, and are usually present as dissolved species. Natural as well as anthropogenic sources discharge a wide range of cyanide species to the envir- onment. Over 2650 species of plants (130 families) produce cyanogenic glycosides as part of natural coexisting plant enzyme and release HCN. In addition, almost all fruit-bearing plants release HCN during ethylene synthesis, which aids in the fruit ripening process (Chapter 3). Cyanide (as free, organic and metal-complexed cyanide compounds) is used as a raw mater- ial during the production of chemicals (nylon and plastics), pesticides, rodenticides, gold, wine, anticaking agents for road salt, fire retardants, cosmetics, pharmaceuticals, painting inks, and other and hydrometallurgical gold extraction (Chapter 4). One of the earliest uses of cyanide dates back to 1704, when the solid phase iron–cyanide compound ferric ferrocyanide (FFC), Fe 4 [Fe(CN) 6 ] 3 (s), also referred to as Prussian Blue, was first used as a pigment for artist colors [1,2]. In addition, free cyanide, weak and strong metal–cyanide complexes, and thiocyanates also occur as by-products of many current and former industrial processes (Chapter 4). Current industries that produce cyanide as a by-product include chemical manufacturing, iron and steel making, petroleum refining, and aluminum smelting. An example of a past industry that generated cyanide-bearing wastewaters and solid wastes in substantial quantities is gas manufacture by coal gasification. There are thousands of former manufactured gas plant (MGP) sites throughout the eastern and midwestern United States and © 2006 by Taylor & Francis Group, LLC defense mechanisms (Chapter 3). Upon stress or injury, cyanogenic glycosides are hydrolyzed by a materials (Chapter 4). Cyanide is also used directly in a variety of processes, including electroplating Physical and Chemical Forms of Cyanide 17 Europe with soil containing FFC, which was generated as a process by-product and often managed onsite as fill [3]. Cyanide contamination exists at many other former industrial sites. It is one of the most common contaminants identified at Superfund sites in the United States [4]. The aim of this chapter is to provide an overview of the common physical and chemical forms of cyanide that occur in water and soil systems. In the following sections, the cyanide species of primary interest in gaseous form, dissolved in water, and in solid form are listed and briefly described. 2.1 GASEOUS FORMS OF CYANIDE Three gaseous forms of cyanide are of interest in water and soil systems: hydrogen cyanide (HCN), cyanogen chloride (CNCl), and cyanogen bromide (CNBr). Cyanogen chloride and cyanogen brom- ide are disinfection by-products formed in water and wastewater treatment [5,6]. HCN is present in wastewater discharges and leachates from certain industrial waste sites, and can be formed in nature as well. Hydrogen cyanide gas is colorless with an odor of bitter almonds. It is highly toxic to humans HCN has a high vapor pressure (630 mm Hg at 20 ◦ C; Ref [7]) and is readily volatilized from water at pH values less than 9, where HCN remains fully protonated. The cyanogen halides CNCl and CNBr are also colorless gases with high vapor pressures (1230 mm Hg and 121 mm Hg at 25 ◦ C for CNCl and CNBr, respectively [8,9]). Like hydrogen cyanide gas, CNCl and CNBr are highly toxic to humans if inhaled or absorbed. These are soluble in water, but degrade by hydrolysis, very rapidly at high pH [5]. Degradation is rapid at any pH if there is free chlorine or sulfite present [5]. At pH 10, degradation of CNCl and CNBr by hydrolysis occurs with half-lives in the range of 20 to 40 min [5]. The hydrolysis degradation product is cyanate ion (CNO − ), which can subsequently hydrolyze to CO 2 and NH 3 at alkaline pH conditions (see 2.2 AQUEOUS FORMS OF CYANIDE free cyanide, metal–cyanide complexes, cyanate and thiocyanate species, and organocyanide com- pounds. Free cyanide comprises molecular HCN and cyanide anion. Metal–cyanide complexes range from weakmetal–cyanide complexes (e.g., complexes ofcopper, zinc, and nickelwith CN − ) tostrong metal–cyanide complexes (e.g., complexes of cobalt and iron with CN − ). Cyanate and thiocyanate form by oxidation of free cyanide, in the presence of sulfide compounds in the case of thiocyanate. Both of these species are anionic for the environmental pH range, and form complexes with metals. Finally, there are organocyanide complexes, where the cyanide anion is covalently bonded to a hydrocarbon group. 2.2.1 FREE CYANIDE soluble hydrogen cyanide, HCN(aq), or soluble cyanide anion (CN − ). HCN(aq) is a weak acid with a pK a of 9.24 at 25 ◦ (Chapter 5). It can dissociate into cyanide ion according to the following dissociation reaction: HCN(aq) = H + +CN − ,pK a = 9.24 at 25 ◦ C (2.1) where the “=” sign denotes a two-way, equilibrium reaction. Thus, at pH values less than 9.24, HCN is the dominant free cyanide species, while at greater pH values cyanide ion dominates free cyanide. © 2006 by Taylor & Francis Group, LLC (see Chapter 13). HCN(g) is very soluble in water, forming a weak acid, HCN(aq), upon dissolution. Chapter 5). Free cyanide represents the most toxic cyanide forms (see Chapters 13 and 14). It refers to either Common aqueous forms of cyanide, listed in Table2.1, can be broadly divided intofour majorclasses: 18 Cyanide in Water and Soil TABLE 2.1 Common Aqueous Cyanide Species Classification Cyanide species Free cyanide HCN, CN − Weak metal–cyanide AgCN(OH) − , Ag(CN) − 2 , Ag(CN) 2− 3 , Ag(OCN) − 2 complexes CdCN − , Cd(CN) 0 2 , Cd(CN) − 3 , Cd(CN) 2− 4 Cu(CN) − 2 , Cu(CN) 2− 3 , Cu(CN) 3− 4 Ni(CN) 0 2 , Ni(CN) − 3 , Ni(CN) 2− 4 , NiH(CN) − 4 , NiH 2 (CN) 0 4 , NiH 3 (CN) + 4 Zn(CN) 0 2 , Zn(CN) − 3 , Zn(CN) 2− 4 HgCN + , Hg(CN) 0 2 , Hg(CN) − 3 , Hg(CN) 2− 4 , Hg(CN) 2 Cl − , Hg(CN) 3 Cl 2− , Hg(CN) 3 Br 2− Strong metal–cyanide BaFe(CN) 2− 6 , BaFe(CN) − 6 complexes CaFe(CN) 2− 6 , CaFe(CN) − 6 ,Ca 2 Fe(CN) 0 6 , CaHFe(CN) 2− 6 Fe(CN) 4− 6 , HFe(CN) 3− 6 ,H 2 Fe(CN) 2− 6 ,Fe 2 (CN) 0 6 K 2 H 2 Fe(CN) 0 6 ,K 3 HFe(CN) 0 6 , KHFe(CN) 2− 6 K 2 Fe(CN) 2− 6 , KFe(CN) 3− 6 LiFe(CN) 3− 6 ,Li 2 Fe(CN) 2− 6 , LiHFe(CN) 2− 6 Fe(CN) 3− 6 MgFe(CN) − 6 , MgFe(CN) 2− 6 NH 4 Fe(CN) 3− 6 , (NH 4 ) 2 Fe(CN) 2− 6 ,NH 5 Fe(CN) 2− 6 NaFe(CN) 3− 6 ,Na 2 Fe(CN) 2− 6 , NaHFe(CN) 2− 6 SrFe(CN) − 6 TlFe(CN) 3− 6 Au(CN) − 2 Co(CN) 3− 6 Pt(CN) 2− 4 Cyanate HOCN, OCN − Metal–cyanate complexes Ag(OCN) − 2 , and others Thiocyanate HSCN, SCN − Metal–thiocyanate MgSCN + complexes MnSCN + FeSCN + FeSCN 2+ , Fe(SCN) + 2 , Fe(SCN) 0 3 , Fe(SCN) − 4 , FeOHSCN + CoSCN + , Co(SCN) 0 2 CuSCN + , Cu(SCN) 0 2 NiSCN + , Ni(SCN) 0 2 CrSCN 2+ , Cr(SCN) + 2 CdSCN + , Cd(SCN) 0 2 , Cd(SCN) − 3 , Cd(SCN) 2− 4 ZnSCN + , Zn(SCN) 0 2 , Zn(SCN) − 3 , Zn(SCN) 2− 4 , and others Organocyanides Nitriles (e.g., acetonitrile) Cyanohydrins Cyanocobalamin and others © 2006 by Taylor & Francis Group, LLC Physical and Chemical Forms of Cyanide 19 2.2.2 METAL–CYANIDE COMPLEXES The cyanide anion is a versatile ligand that reacts with many metal cations to form metal–cyanide complexes. These species, which aretypically anionic, have ageneral formulaof M(CN) n− x , where M is a metal cation, x is the number of cyanide groups, and n is the ionic charge of the metal–cyanide complex. The stability of metal–cyanide complexes is variable and requires moderate to highly acidic pH conditions in order to dissociate. Metal–cyanide complex dissociation yields free cyanide: M(CN) n− x = M + +xCN − (2.2) Metal–cyanide complexes are classified into two broad categories, namely, weak metal–cyanide complexes and strong metal–cyanide complexes, based on the strength of the bonding between the metal and the cyanide ion. Complexes with greater strength of the metal–cyanide bond are more stable in aqueous solution, that is, they dissociate only to a limited extent, and the dissolution process may be very slow. 2.2.2.1 Weak Metal–Cyanide Complexes Weak metal–cyanide complexes are those in which the cyanide ions are weakly bonded to the metal cation, such that they can dissociate under mildly acidic conditions (pH = 4 to 6) to produce free cyanide. Because of their dissociative nature, they are often regulated along with free cyanide in water. Common examples of weak metal–cyanide complexes include copper cyanide (Cu(CN) 2− 3 ), zinc cyanide (Zn(CN) 2− 4 ), nickel cyanide (Ni(CN) 2− 4 ), cadmium cyanide (Cd(CN) 2− 4 ), mercury cyanide (Hg(CN) 2 ), and silver cyanide (Ag(CN) − 2 ). 2.2.2.2 Strong Metal–Cyanide Complexes Strong metal–cyanide complexes include cyanide complexes with transition heavy metals such as, iron, cobalt, platinum, and gold that require strong acidic conditions (pH < 2) in order to dissociate and form free cyanide. Strong metal–cyanide complexes are much more stable in aqueous solution than the weak ones and are relatively less toxic. Common examples of strong metal–cyanide com- plexes include ferrocyanide (Fe(CN) 4− 6 ), ferricyanide (Fe(CN) 3− 6 ), gold cyanide (Au(CN) − 2 ), cobalt cyanide (Co(CN) 3− 6 ), and platinum cyanide (Pt(CN) 2− 4 ). 2.2.3 CYANATE AND THIOCYANATE Free cyanide can be oxidized to form cyanate, CNO − , or, depending on the pH, its protonated form HOCN (pK a = 3.45 at 25 ◦ C). Cyanate is substantially less toxic than free cyanide. It is rarely encountered in aqueoussystems, as a strongoxidizing agent and a catalystare required for conversion of free cyanide to CNO − or HOCN [10]. When cyanate does form it can react with metals to form Free cyanide can reactwith various formsof sulfur toform thiocyanate, SCN − , which is relatively nontoxic. The two forms of sulfur in the environment most reactive with free CN − are polysulfides, S x S 2− , and thiosulfate, S 2 O 2− 3 (Chapter 5). Thiocyanate can protonate to form HCNS 0 , but this rarely occurs in natural systems as the pK a for this reaction is 1.1. Thiocyanate can form complexes with many metals (Chapter 5). © 2006 by Taylor & Francis Group, LLC metal–cyanate complexes, though these reactions have not been studied extensively (Chapter 5). 20 Cyanide in Water and Soil (sugar O) n C C ϵ N H,R R FIGURE 2.2 General structure of cyanogenic glycosides (R represents CH 3 group). O OCH CN O CH 2 OH CH 2 OH O HO OH OH HO O HO HO OCH 2 HO O HO OH OCH CN Amygdalin (Cherry, Apricot) Dhurrin (Cassava) O C CH 3 CN CH 3 FIGURE 2.3 Common plant cyanogenic glycosides. 2.2.4 ORGANOCYANIDE COMPLEXES Organic cyanide compounds contain a cyanide functional group that is attached to a carbon atom of the organic molecule via covalent bonding. Common examples include nitriles, such as acetonitrile (CH 3 CN) or cyanobenzene (C 6 H 5 CN), which are used as industrial solvents and as raw materials for making nylon products and pesticides. Nitriles can also exist in the natural environment in shale oils [11], in plants [12], or as a plant-growth hormone [13]. Several classes of nitriles can be produced naturally or synthesized chemically, the most common of which are the cyanogenic glycosides and cyanohydrins. Cyanohydrins, also known as α-hydroxynitriles, are organic cyanides with the general structure R 1 R 2 C(OH)(CN), where the hydroxide group and the cyanide group are attached to the same carbon atom. Cyanogenic glycosides are produced by the plants under natural environmental conditions to aid bonded to a carbon atom, which in turn is bound by a glycosidic linkage to one or more sugars depicted in Figure 2.2. Some common cyanogenic glycosides produced by plants are shown in Figure 2.3. Certain groups of nitriles such as, cyanogenic glycosides, exhibit high stability in water as far as dissociation to free cyanide is concerned. Other organocyanide compoundsof interest includecyanocobalamin, also known as Vitamin B 12 . It consists of single cyanide group bonded to a central trivalent cobalt cation. Vitamin B 12 is syn- thesized by microorganisms, not by plants, and is found in animal tissues as a result of intestinal synthesis [14]. It is essential for human life, serving numerous functions and being an especially important vitamin for maintaining healthy nerve cells and aiding the production of genetic building blocks DNA and RNA [15]. There are cyanide and noncyanide forms of Vitamin B 12 . The noncyan- ide forms include methylcobalamin, adenosylcobalamin, chlorocobalamin, and hydroxycobalamin. These compounds, also produced by microorganisms, are less stable than cyanocobalamin but also essential to human life. 2.3 SOLID FORMS OF CYANIDE In systems with metals and cyanide present in sufficient quantities, metals can react with cyan- ide to form a wide range of solids. The solid forms of cyanide may be divided into two general © 2006 by Taylor & Francis Group, LLC in their defense mechanism (Chapter 3). These species comprise a cyanide anion that is covalently Physical and Chemical Forms of Cyanide 21 TABLE 2.2 Common Solid Phase Cyanide Species Classification Cyanide species Simple metal–cyanide solids KCN(s) NaCN(s) AgCN(s) CuCN(s) Hg(CN) 2 (s) Alkali or alkaline earth metal–metal K 4 Fe(CN) 6 (s) cyanide solids K 3 Fe(CN) 6 (s) K 4 Ni 4 (Fe(CN) 6 ) 3 (s) K 2 CdFe(CN) 6 (s) K 2 Cu 2 Fe(CN) 6 (s) KZn 1.5 Fe(CN) 6 (s) Other metal–metal cyanide solids Fe 4 [Fe(CN) 6 ] 3 (s) Fe 3 [Fe(CN) 6 ] 2 (s) Fe[Fe(CN) 6 ](s) Fe 2 [Fe(CN) 6 ](s) Ag 4 Fe(CN) 6 (s) Cd 2 Fe(CN) 6 (s) Cu 2 Fe(CN) 6 (s) Zn 2 Fe(CN) 6 (s) categories: simple metal–cyanide solids, which are relatively soluble, and metal–metal cyanide com- plex solids with varying degree of solubility. Some common metal–cyanide and metal–metal cyanide solids are listed in Table 2.2. 2.3.1 SIMPLE METAL–CYANIDE SOLIDS This class of cyanide solids consist of structurally simple, metal cyanides of the form M(CN) x , where M is an alkali, alkaline earth metal or a heavy metal. Common examples include sodium cyanide (NaCN(s)), potassium cyanide (KCN(s)), calcium cyanide, (Ca(CN) 2 (s)), zinc cyanide (Zn(CN) 2 (s)), and others (see Table 2.2). Most of these solids are highly soluble in water and readily dissociate, releasing the cyanide ion, and therefore are potentially toxic. 2.3.2 METAL–METAL CYANIDE SOLIDS This class of cyanide solids consists of one or more alkali, alkaline earth, or transition metal cations combined with an anionic metal–cyanide complex. Based on whether the metal cation is alkali/alkaline earth or transition metal, this class of compounds is again subdivided into two cat- egories: alkali/alkaline earth metal–metal cyanide solids and other metal–metal cyanide solids. In the latter, the metals involved are B-type or transition metals [16]. 2.3.2.1 Alkali/Alkaline Earth Metal–Metal Solids This class of structurally complex solids comprises one or more alkali or alkaline earth metal cations ionically bonded to an anionic metal–cyanide complex with the general formula of A x [M(CN) y ]·nH 2 O, where A is an alkali or alkaline earth metal cation (or ammonium ion), M is a transition metal atom, x is the number of alkali metal atoms, y is the number of cyanide groups, © 2006 by Taylor & Francis Group, LLC 22 Cyanide in Water and Soil and n is the number of water molecules incorporated in the solid structure. A common example of this class of compound is potassium ferrocyanide (K 4 Fe(CN) 6 (s)). Alkali/alkaline earth metal– metal cyanide complex salts can readily dissociate in aqueous solutions, releasing the alkali metal cation and the anionic metal cyanide complex according to the following equation: A x [M(CN) y ]·nH 2 O = xA + +[M(CN) y ] m− (2.3) where m is the ionic charge of the metal–cyanide complex released to solution. 2.3.2.2 Other Metal–Metal Cyanide Complex Salts This class of structurally complex compound comprises one or more transition metal cations ionically bonded to an anionic transition metal cyanide complex with the general formula of M x [M(CN) y ] z ·nH 2 O where M is a B-type or transition metal cation, x number of transition metal cations, y is the number of cyanide groups, z is the number of metal–cyanide complexes, and n is the number of water molecules in the structure. Due to the versatility of the cyanide anion as a ligand, there are many different kinds of metal–metal cyanide compounds that exhibit a wide range of structural properties [17]. Metal–metal cyanide solids involving all B-type and transition metals are very stable and relat- these compoundsare relatively soluble, releasingmetal cations andanionic metal–cyanide complexes to solution according to the following general reaction: M x [M(CN) y ] z ·nH 2 O = xM + +z[M(CN) y ] m− (2.4) where m is the ionic charge of the metal–cyanide complex released to aqueous solution. A well-known example of a transition metal–metal cyanide is ferric ferrocyanide 4 6 3 2.4 SUMMARY AND CONCLUSIONS • Cyanide is present in gas, liquid, and solid forms in water and soil systems. • Many different species of cyanide occur in water and soil systems. The specific form of cyanide determines the environmental fate and transport of cyanide, as well as its toxicity. Understanding the specific form(s) of cyanide present in a particular water, soil, or sediment is critical for assessment of how to manage or treat the cyanide present. • Cyanide mostly occurs in inorganic forms. The dissolved forms of primary interest are free cyanide (HCN and CN − ) and metal–cyanide complexes. Solid forms of cyanide include simple metal–cyanide solids (e.g., NaCN(s), KCN(s)), which are relatively sol- uble, and more complex, less soluble metal–metal cyanide solids (e.g., Fe 4 (Fe(CN) 6 ) 3 (s), or Prussian Blue). The gaseous form of cyanide of primary interest is HCN(g). • Free cyanide, either in dissolved (HCN and CN − ) or gaseous form (HCN(g)), are the species of primary interest with respect to human health and aquatic toxicity. • Dissolved inorganic metal–cyanide complexes can be categorized as weak metal–cyanide complexes and strong metal–cyanide complexes, based on the strength of the bonding between the metal and the cyanide ion. • Cyanate (CNO − ) is formed from oxidation of free cyanide. It can react with metals and form metal–cyanate complexes. • Thiocyanate (SCN − ) is formed from reaction of free cyanide with various forms of sulfur. It can react with metals to form metal-thiocyanate complexes. © 2006 by Taylor & Francis Group, LLC ively insoluble under acidic and neutral conditions (Chapter 5). However, under alkaline conditions, Fe (Fe(CN) ) (s), or Prussian Blue, which has various commercial and medicinal uses (Chapter 4). Physical and Chemical Forms of Cyanide 23 • Organic compounds containing cyanide are produced by both natural and anthropogenic activities. They consist of molecules with carbon–carbon covalent bonding with the –CN group. Common organocyanide compounds include the nitriles, such as acetonitrile (CH 3 CN). REFERENCES 1. ACC, The Chemistry of the Ferrocyanides, American Cyanamid Co., New York, NY, 1953. 2. Feller, R.L., Ed., Artist’s Pigments: A Handbook of Their History and Characteristics, National Gallery of Art, Washington, DC, 1986. 3. Hayes, T.D., Linz, D.G., Nakles, D.V., and Leuschner, A.P., Eds., Management of Manufactured Gas Plant Sites, Vol.1&2, Amherst Scientific Publishers, Amherst, MA, 1996. 4. USEPA, Common chemicals found at Superfund sites, U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response, accessed: March 22, 2005. 5. Xie, Y. and Hwang, C.J., Cyanogen chloride and cyangen bromide analysis in drinking water, in Encyclopedia of Analytical Chemistry, Meyers, R.A., Ed., John Wiley & Sons, Chichester, UK, 2000, p. 2333. 6. Zheng, A., Dzombak, D.A., and Luthy, R.G., Formation of free cyanide and cyanogen chloride from chlorination of POTW secondary effluent: laboratory study with model compounds, Water Environ. Res., 76, 113, 2004. 7. ATSDR, Toxicological profile for cyanide (update), U.S. Department of Health and Human Services, Public Health Service, Agency for Toxic Substances and Disease Registry, Atlanta, GA, 1997. 8. CDC, NIOSHemergencyresponse card:Cyanogen chloride, Centers for DiseaseControland Prevention, 9. IPCS/INCHEM, Cyanogen bromide, International Programme on Chemical Safety and the Commission of the European Communities, accessed: April 3, 2005. 10. Smith, A. and Mudder, T., The Chemistry and Treatment of Cyanidation Wastes, Mining Journal Books Ltd., London, 1991. 11. Evans, E.J., Batts, B.D., Cant, N.W., and Smith, J.W., The origin and significance of nitriles in oil shale, Org. Geochem., 8, 367, 1985. 12. Knowles, C.J., Microorganisms and cyanide, Bacteriol. Rev., 40, 652, 1976. 13. Stowe, B.B. and Hudson, V.W., Growth promotion in pea stem sections. III. By alkyl nitriles, alkyl acetylenes and insect juvenile hormones, Plant Physiol., 44, 1051, 1969. April 3, 2005. 16. Stumm, W. and Morgan, J.M., Aquatic Chemistry, Wiley-Interscience, New York, 1996. 17. Dunbar, K.R. andHeintz, R.A., Chemistry of transition metal cyanide compounds: modern perspectives, Prog. Inorg. Chem., 45, 283, 1997. © 2006 by Taylor & Francis Group, LLC 14. Gershoff, S.N., Vitamin B12, AccessScience@McGraw-Hill, http://www.accessscience.com, accessed: 15. UMD, Vitamin B12 (Cobalamin), University ofMaryland Medical Center, http://www.umn.edu/altmed/ ConsSupplements/VitaminB12Cobalamincs.html, accessed: April 3, 2005. http://www.epa.gov/superfund/resources/chemicals.htm, http://www.bt.cdc.gov/agent/cyanide/erc506-77-4.asp, accessed: April 3, 2005. http://www.inchem.org/documents/icsc/icsc/eics0136.htm, . Cyanide 17 2. 2 .2 Metal Cyanide Complexes 19 2. 2 .2. 1 Weak Metal Cyanide Complexes 19 2. 2 .2. 2 Strong Metal Cyanide Complexes 19 2. 2.3 Cyanate and Thiocyanate 19 2. 2.4 Organocyanide Complexes 20 2. 3 Solid. of Cyanide 20 2. 3.1 Simple Metal Cyanide Solids 21 2. 3 .2 Metal–Metal Cyanide Solids 21 2. 3 .2. 1 Alkali/Alkaline Earth Metal–Metal Solids 21 2. 3 .2. 2 Other Metal–Metal Cyanide Complex Salts 22 2. 4. (Cu(CN) 2 3 ), zinc cyanide (Zn(CN) 2 4 ), nickel cyanide (Ni(CN) 2 4 ), cadmium cyanide (Cd(CN) 2 4 ), mercury cyanide (Hg(CN) 2 ), and silver cyanide (Ag(CN) − 2 ). 2. 2 .2. 2 Strong Metal–Cyanide