Physical and Chemical Forms of Cyanide

The specific form of cyanide determines the environmental fate and transport of cyanide, as well as its toxicity. Cyanide occurs as various physical metal-cyanide complexes and metal-cyanide solids in water and soil. Figure 18.1 has described distribution of various forms of cyanide in aqueous, solid and gaseous phases.

Chemically, cyanide can be classified into inorganic and organic forms, as indicated in Fig. 18.1. 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 and metal-cyanate complexes and cyanogen halides. The cyanide anion, CN-, and HCN, are very volatile under environmental conditions, and occur as both aqueous and gaseous species (Dzombak et al. 2006). The cyanide anion is a versatile ligand that reacts with metal cations to form metal-cyanide complexes. Dissociation of these complexes can yield free cyanide. These are again subdivided into weak and strong metal cyanide complexes. Free cyanide can also be oxidized to form cyanate, CNO-, which is less toxic than free cyanide. Free cyanide can react with sulphur to form thiocyanate, SCN-, which is almost nontoxic. Metal-thiocyanate and organocyanate are other aqueous forms of cyanide present in the atmosphere.

There are three major solid forms of cyanide that exist in the nature. These are metal::cyanide, metal::metal-cyanide and alkali earth metal::metal solids.

Water

Cyanogei

Water

Simple Metal cyanide solids

NaCN, KCN, CuCN

Simple Metal cyanide solids

NaCN, KCN, CuCN

Fig. 18.1 Various forms of cyanide distributed in solid, liquid and gaseous phases

Metal::cyanides are simple structures consisting of metal and cyanide (NaCN and KCN are common examples). Bonding of another metal with the previous form of complex gives birth to metal::metal-cyanide complexes. In the third category, one or more alkali earth metal cations are ionicaly bonded to an anionic metal-cyanide metal complexes. A common example is potassium ferrocyanide (K4Fe(CN)6(s)). This form can readily dissociate into aqueous solution, releasing alkali metal cyanide and anion metal complexes (Ghosh et al. 2006).

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