CYANIDE in WATER and SOIL: Chemistry, Risk, and Management - Chapter 2 pdf

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

2 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 classified into 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 and metal–thiocyanate complexes, cyanate

15

most commonly occurring aqueous, gaseous, and solid forms of cyanide is provided inChapter 5

cyanide species that occur in the aqueous, solid, and gas phases are indicated inFigure 2.1

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

K3Fe(CN)6(s), K4Fe(CN)6(s), KAg(CN)2(s), …

Other metal-metal cyanide solids

Fe4[Fe(CN)6 ]3(s),

Fe3[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), Fe4[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 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

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 CO2and NH3 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 weak metal–cyanide complexes (e.g., complexes of copper, zinc, and nickel with CN−) to strong 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 FREECYANIDE

soluble hydrogen cyanide, HCN(aq), or soluble cyanide anion(CN−) HCN(aq) is a weak acid

with a pKaof 9.24 at 25◦(Chapter 5) It can dissociate into cyanide ion according to the following dissociation reaction:

HCN(aq)= H++ CN−, pKa= 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

(seeChapter 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 and14) It refers to either Common aqueous forms of cyanide, listed inTable 2.1, can be broadly divided into four major classes:

TABLE 2.1

Common AqueousCyanide 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)02, Ni(CN) −

3 , Ni(CN)2−

4 , NiH(CN) −

4 , NiH 2 (CN)04, NiH 3 (CN) +

4

Zn(CN)02, Zn(CN) −

3 , Zn(CN)2−

4

HgCN +, Hg(CN)0 , Hg(CN) −

3 , Hg(CN)2−

4 , Hg(CN)2Cl −, Hg(CN)

3 Cl2−, Hg(CN)

3 Br2−

Strong metal–cyanide BaFe(CN)2−

6 , BaFe(CN) −

6

complexes CaFe(CN)2−

6 , CaFe(CN) −

6 , Ca2Fe(CN)0, CaHFe(CN)2−

6

Fe(CN)4−

6 , HFe(CN)3−

6 , H2Fe(CN)2−

6 , Fe2(CN)0

K2H2Fe(CN)0, K3HFe(CN)0, KHFe(CN)2−

6

K2Fe(CN)2−

6 , KFe(CN)3−

6

LiFe(CN)3−

6 , Li2Fe(CN)2−

6 , LiHFe(CN)2−

6

Fe(CN)3−

6

MgFe(CN) −

6 , MgFe(CN)2−

6

NH4Fe(CN)3−

6 , (NH4)2 Fe(CN)2−

6 , NH5Fe(CN)2−

6

NaFe(CN)3−

6 , Na2Fe(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 +

FeSCN2+, Fe(SCN)+

2 , Fe(SCN)0, Fe(SCN) −

4 , FeOHSCN +

CoSCN +, Co(SCN)0

2

CuSCN +, Cu(SCN)0

NiSCN +, Ni(SCN)0

CrSCN2+, Cr(SCN)+

2

CdSCN +, Cd(SCN)0 , Cd(SCN) −

3 , Cd(SCN)2−

4

ZnSCN +, Zn(SCN)0 , Zn(SCN) −

3 , Zn(SCN)2−

4 , and others Organocyanides Nitriles (e.g., acetonitrile)

Cyanohydrins Cyanocobalamin and others

2.2.2 METAL–CYANIDECOMPLEXES

The cyanide anion is a versatile ligand that reacts with many metal cations to form metal–cyanide complexes These species, which are typically anionic, have a general formula of 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−

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 ANDTHIOCYANATE

Free cyanide can be oxidized to form cyanate, CNO−, or, depending on the pH, its protonated form HOCN(pKa = 3.45 at 25◦C) Cyanate is substantially less toxic than free cyanide It is rarely encountered in aqueous systems, as a strong oxidizing agent and a catalyst are 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 react with various forms of sulfur to form thiocyanate, SCN−, which is relatively nontoxic The two forms of sulfur in the environment most reactive with free CN−are polysulfides,

SxS2−, and thiosulfate, S

2O2−

3 (Chapter 5) Thiocyanate can protonate to form HCNS0, but this rarely occurs in natural systems as the pKafor this reaction is 1.1 Thiocyanate can form complexes with many metals (Chapter 5)

metal–cyanate complexes, though these reactions have not been studied extensively (Chapter 5)

(sugar O)n C C ⬅ N

H,R

R

FIGURE 2.2 General structure of cyanogenic glycosides (R represents CH3group)

O

OCH

CN

O

CH2OH

CH2OH

O

OH

HO

O

2

OCH

CN

3

CN

CH3

FIGURE 2.3 Common plant cyanogenic glycosides.

2.2.4 ORGANOCYANIDECOMPLEXES

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

(CH3CN) or cyanobenzene (C6H5CN), 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 asα-hydroxynitriles, are organic cyanides with the general

structure R1R2C(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 compounds of interest include cyanocobalamin, also known as Vitamin B12

It consists of single cyanide group bonded to a central trivalent cobalt cation Vitamin B12is 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 B12 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 wcyan-ide range of solids The solid forms of cyancyan-ide may be divcyan-ided into two general

in their defense mechanism (Chapter 3) These species comprise a cyanide anion that is covalently

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 K3Fe(CN)6(s)

K 4 Ni 4 (Fe(CN) 6)3 (s)

K2CdFe(CN)6(s)

K2Cu2Fe(CN)6(s) KZn1.5Fe(CN)6(s) Other metal–metal cyanide solids Fe4[Fe(CN)6]3(s)

Fe 3 [Fe(CN)6] 2 (s) Fe[Fe(CN)6](s)

Fe 2 [Fe(CN) 6 ](s)

Ag4Fe(CN)6(s)

Cd2Fe(CN)6(s)

Cu2Fe(CN)6(s)

Zn2Fe(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 SIMPLEMETAL–CYANIDESOLIDS

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–METALCYANIDESOLIDS

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

Ax [M(CN)y] · nH2O, 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,

and n is the number of water molecules incorporated in the solid structure A common example

of this class of compound is potassium ferrocyanide(K4Fe(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:

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

Mx[M(CN)y]z · nH2O 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 compounds are relatively soluble, releasing metal cations and anionic metal–cyanide complexes

to solution according to the following general reaction:

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

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., Fe4(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

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)

• 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

(CH3CN).

REFERENCES

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p 2333

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Ltd., London, 1991

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acetylenes and insect juvenile hormones, Plant Physiol., 44, 1051, 1969.

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