Chapter I INTRODUCTION AND REVIEW OF LITERATURE Chapter I INTRODUCTION AND REVIEW OF LITERATURE 1.1 Cyanides, thiocyanates and nitrites Cyanide is a compound, which played a principle role in the evolution of life on Earth (Oro and Araujo, 1981). It is not only a notorious poison but also an indispensable industrial chemical. Cyanide is a singly-charged anion containing unimolar amounts of carbon and nitrogen atoms triply-bonded (-C=N) together. It is a strong ligand, capable of complexing at low concentrations with virtually any heavy metal. The same complexing capability makes it useful in different industries such as metal extraction (particularly gold), metal finishing, manufacturing of herbicides and pesticides, paint industries, production of organic chemicals such as nitrile, nylon, synthetic rubber and acrylic plastics. (Akcil et al, 2003). Considering the toxicity there are four major categories of cyanide compounds viz. free cyanides, strong acid dissociable cyanides (SAD), weak acid dissociable cyanides (WAD)'and cyanide related compounds. Table 1.1 enlists cyanides, thiocyanates and organic cyanides based on these categories. Free cyanides are highly volatile and most toxic forms of cyanides and include hydrogen cyanide (HCN) and CN' ions. WAD cyanides refer to cyanide complexes with metals such as cadmium, copper, nickel and zinc. SAD cyanides are cyanide- complexes with metals such as cobalt, gold, iron and silver. Cyanide related compounds include inorganic as well as organic forms of cyanides. The inorganic forms of cyanide include different thiocyanates (SCN"). Thiocyanate although a WAD cyanide, is often considered as a separate category. The organic cyanides include different aliphatic and aromatic nitriles. Nitriles are cyanide-substituted, carboxylic acids of the general structure R-CN. These different forms of cyanides are present in the environment as natural or industrial products. These cyanides are converted from one form to other depending on the environmental conditions. Table 1.1 Major categories of cyanides based on toxicity Toxicity to Types of Example Groups humans cyanides Highly toxic Free cyanide HCN, CN- Cadmium cyanide, Weak acid copper cyanide, nickel Toxic dissociable cyanide and zinc cyanides (WAD) cyanide Cyanides Strong acid Cobalt cyanide, gold Toxic dissociable cyanide, iron cyanide cyanides (SAD) and silver cyanide Potassium thiocyanate, sodium thiocyanate, Thiocyanates Less toxic ammonium (SCN-) Cyanide related thiocyanate, mercuric compounds thiocyanate Aliphatic Nitriles Organic Less toxic Aromatic nitriles cyanides 1.2 Uses of cyanides, thiocyanates and nitriles Although a deadly poison, cyanide is one of the most indispensable industrial chemicals. Due to the highly complexing nature of cyanide, it has been used to extract precious metals from crushed rock for more than 100 years. Modern recovery methods that utilize cyanide in water-based solution can recover nearly 100% of the contained precious metals, making it profitable for mining companies to process low-grade ores. Because of properties similar to cyanide, sodium and potassium thiocyanate as well as nitrile are also widely used in metal leaching. In the electroplating industry, alkali cyanides and thiocyanates are used to a great extent. Noble metals such as silver and gold can be satisfactorily deposited from solutions containing cyanides. Thiocyanate is used as the electrolyte in the production of black nickel. In cases where the highest plating quality is required, cyanide pretreatment and degreasing baths are in use even today. Both cyanide and thiocyanates play an important role in hardening of steel. Cyanides and nitriles are used in various organic syntheses (acetophenone, alpha-naphthalene acetic acid, thiamine, acetamidine). The introduction of a CN- group into an organic substance is the first step in developing a wide variety of well-known materials that find application in pharmaceuticals as well as polymer chemistry. For example acetonitrile and malononitrile are used as the starting material for the synthesis of organic compounds such as acetophenone, alpha- naphthalene acetic acid, thiamine and acetamide. Nitriles are also used in the synthesis of polyacrylonitrile plastics. In laboratories, acetonitrile is widely used in high-performance liquid chromatographic (HPLC) analysis and as a solvent for DNA synthesis and peptide sequencing. Other important uses of thiocyanates and nitriles are in the synthesis of different pesticides and herbicides. Ammonium thiocyanate and copper thiocyanate are used as pesticides whereas sodium thiocyanate is a raw material in the synthesis of number of pesticides and fungicides, for example methylene bis thiocyanate. Copper thiocyanate is used in anti-foulant paints for application to ships and boats for the control of aquatic organisms. Bromoxil is a contact herbicide containing benzonitrile. Nitriles and thiocyanates are also used in the synthesis of acrylic fibers and different pharmaceutical products such as vitamin Bi2 (Beekhuis, 1975). 1.3 Sources of environmental exposure to cyanides, thiocyanates and nitriles Cyanides, because of the industrial uses, are widely present in the environment. Apart from its anthropogenic origin, cyanides are also produced naturally. 1.3.1 Natural origin Cyanide and chemically related compounds are formed, excreted and degraded in nature by hundreds of species of bacteria, algae, fungi, plants and insects (Knowles, 1976). As a result, low levels of cyanide can appear in naturally occurring surface or groundwater samples, which normally would not be expected to contain it. 3 At least 1,000 species of plants from 90 families have been shown to contain one or more cyanide compounds (Seigler, 1976). About 800 species of higher plants from 70 to 80 families, including agriculturally important species such as the cassava, flax, sorghum, alfalfa, bamboo, peach, pear, cherry, plum, corn, potato, cotton, almond and beans are cyanogenic (Eyjolfsson, 1970). Cyanogenic glycosides such as linamarin and amygdalin, present in several plants, can be converted to cyanide under certain conditions (Legras ef a/, 1990). Cyanide poisoning of livestock by forage sorghums and other cyanogenic plants is well documented (Mudder, 1997). Vegetables in the family Brassicaceae contain high levels of thiocyanate with concentrations ranging upto 660 pg/g, whereas other commonly consumed vegetables (e.g., spinach, radish, celery) generally contain thiocyanates at concentrations <2 pg/g. The reported thiocyanate concentrations in milk and dairy products as well as in meat are <1.0-9.0 and 0.5-0.7 pg/g, respectively (Thurkow e^ a/, 1982). In crushed plant tissues, cellular glucosinolates (thioglucosides) are hydrolyzed by glucosidase to produce thiocyanate (Wood, 1975). Glucosinolates are widely distributed in plant families such as Cruciferae (Kelly ef a/, 1993). Nitrile compounds are numerous and fairly widespread in the natural environment mainly in the form of cyanogenic glycosides (Legras et al, 1990). Toxicity of different plants, such as cassava, white clover bamboo, peach etc. is due to the presence of different cyanogenic glycosides. Cyanide is one of the virulence factors in fungi and bacteria. For example, Pseudomonas aeruginosa PA01 kills Caenoiiiabditis elegans by cyanide poisoning (Blumer and Haas, 2000, Larry and Colin, 2001). In the pathogenic fungus, Basidiomycete W2, cyanide is the main factor involved in etiology (Ward etal, 1971, Lebeau etal, 1959). In addition to plants and microorganisms, insects have been shown to produce cyanide and cyanogenic glycosides. Species of centipedes, millipedes, beetles. moths and butterflies synthesize and excrete cyanide for defensive purposes (Duffey, 1981). 1.3.2 Anthropogenic origin Cyanides, thiocyanates and nitriles are produced industrially in large amounts for use in metal, pesticide, herbicide, organic chemical and polymer industries. The major sources of cyanide in water are discharges from some metal mining processes, organic chemical industries, iron and steel works, and electroplating industries. The release of different cyanides in the environment by these industries is estimated to be >14million kg/year (Agency for toxic substance and disease registry (ATSDR): Toxicological Profile for Cyanide). The concentration of total cyanide released varies from industry to industry. Apart from water streams, soil surrounding gas work sites are also contaminated with cyanide and thiocyanate ions (Meehan, 1999). The iron-complexed cyanides are often found as a Prussian blue (ferri ferrocyanide Fe(lll)4(Fe(ll)(CN)6)3) coloration in the soil (Kjeldsen, 1999). Other cyanide sources include vehicle exhaust, exhaust releases from certain chemical industries, municipal waste burning and use of cyanide-containing pesticides. Hydrogen cyanide is also contained in vehicle exhaust and in tobacco smoke. The smoke of burning plastics contains hydrogen cyanide so also house fires often result in cyanide poisonings. Another important source of cyanide in the water bodies is cyanide fishing, which was first practiced in Philippines and has spread to other places including Indonesia, Cambodia, the Maldives, Thailand and Vietnam (Barber and Pratt, 1997). Cyanides are used to capture live fish near coral reefs for the aquarium and seafood market. In this method, a diver uses a large, needle-less syringe to squirt a cyanide solution into areas where the fish are hiding, stunning them so that they can be easily gathered (Rubec,