Sources of Cyanide

INTRODUCTION

Cyanide is a chemical compound possessing highly toxic potential. It is present in three spheres that are atmosphere (air), hydrosphere (water) and in lithosphere (soil). Cyanide is a unique chemical compound used to manufacture hundreds of everyday goods, and chances are that today you will use many products that included cyanide in its manufacture. These products may include vitamins (B12), jewellery, adhesives, computer electronics, fire retardants, airplane brakes, cosmetics, dyes, nylon, nail polish remover, paints, pharmaceuticals, Plexiglas, rocket propellant, and table salt. The manufacturing of these products takes place every day around the world with little knowledge from the general public that cyanide is a critical ingredient in their manufacture. Indeed, without cyanide it would not be possible to manufacture such widely used items as nylon and many vitamins and other medications.

Products made by using Cyanide 

Cyanide is a relatively toxic compound but has been safely used for well over a hundred years around the world. Many people have a natural fear of cyanide that arises from a general understanding of its toxicity but a less thorough understanding of its actual properties and usefulness in our everyday lives. As with any chemical, cyanide must be properly and safely handled to avoid harm to people or the environment. The benefit people derive from cyanide and its many products far outweigh the risk posed to people and the environment.

The human body has a natural ability to detoxify small quantities of cyanide, and there is normally a small amount of cyanide and its breakdown products in the body as a result of everyday activities. These activities may include the metabolism of vitamin B12, eating of foods naturally containing cyanide (for example, almonds, Lima beans, coffee and table salt), exposure to automobile exhaust and smoking cigarettes. In some form, we are exposed to low levels of natural and man-made cyanide every day without risk to our health or the environment.

WHAT IS CYANIDE?

Cyanide is a general term for a group of chemicals containing carbon (C) and nitrogen (N). The term cyanide in toxicological profile means a compound that contains the cyanogen (CN) radical (A triple-bonded molecule with a negative one charge consisting of one atom of carbon in the +2 oxidation state and one atom of nitrogen in the -3 oxidation state). Since the CN portion of the compound is of concern in poisons. Cyanide compounds include both naturally occurring and human-made chemicals. Naturally, cyanide can be produced by certain bacteria, fungi, algae, and it is found in a number of foods and plants. The principal human-made cyanide forms are hydrogen cyanide (HCN), sodium cyanide (NaCN) and potassium cyanide (KCN). Hydrogen cyanide is a colourless gas with a faint, bitter, almond-like odour. Cyanide is acute toxic and is lethal if ingested or inhaled.

Cyanide molecule 

Cyanide chemical formula

TYPES; HOW IT IS USED?

Cyanide combines with many organic and inorganic compounds. Because of its unique properties, cyanide is used in the manufacture of metal parts and numerous common organic products. About 1.4 million tonnes of hydrogen cyanide (HCN) are produced annually worldwide, of which only about 20% is converted into sodium cyanide (NaCN) and mainly used in the extraction of precious metals such as gold and silver, and others (e.g. copper). The remaining 80 % of the hydrogen cyanide (HCN) is used in electroplating, metallurgy, and in the production of a wide range of chemicals, such as plastics, fire retardant, cosmetics, dyes, nylon, paints, pharmaceuticals, Plexiglas, rocket propellant, and road and table salts. 

Industrial use of cyanide production

SOURCES OF CYANIDE

Cyanide is a chemical compound possessing highly toxic potential. There are many sources of cyanide; that exists naturally and anthropogenically (human made). The two main categories of the cyanide sources are natural source and the anthropogenic source. Humans are the main contributors of the cyanide production as compared to the nature.

Natural sources

Cyanide and chemically related compounds are formed, excreted and degraded in nature by hundreds of species of bacteria, algae, fungi, plants and insects. Plants that produce cyanide are cassava, sorghum, alfalfa, bamboo, peach pits, wild cherry, almond, apricot kernels apple seeds, Lima beans etc. Insects that produce cyanide are Spiny Dragon, Pleuroloma flavipes, Narcissus annularis etc.

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. At least 1,000 species of plants and micro-organisms from 90 families have been shown to contain one or more of nearly twenty compounds capable of producing cyanide.

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 (containing a cyanide group in the molecule).

Fungi and bacteria are prevalent producers of cyanide. In addition to plants and microorganisms, insects have been shown to produce cyanide. Species of centipedes, millipedes, beetles, moths and butterflies synthesize and excrete cyanide for defensive purposes. Coffee and table salt also contain cyanide. Laetrile (an anti-cancer preparation made from apricot kernels) and sodium nitroprusside (a drug used to reduce high blood pressure), release cyanide upon metabolism. Non-anthropogenic sources also include damaged or decaying tissues of plants from the family.

The cyanide content in certain varieties of Lima beans can be as high as 3 mg/g, although values between 0.10 and 0.17 mg/g are common in Lima beans. Much lower cyanide concentrations in various cereal grains and cereal products have been reported, ranging from 0.001 to 0.45 μg/g. Mean cyanide concentrations in soybean products have been found to range from 0.07 to 0.3 μg/g, whereas the mean cyanide concentration in soybean hulls was 1.24 μg/g. Due to the lack of data on cyanide content in total diet samples, the average daily intake could not be estimated. 

The cyanide is also present in many other naturally occurring things that include Cassava, Wild Cherries, Almonds, Sorghum, Peach pits, Apricot kernels, Apple seeds, Bamboo shoots etc. Sorghum forage either green or dry is the main source of livestock feed in dry areas of Pakistan. However, a lethal risk of hydrogen cyanide (HCN) is associated with this forage.

A nitriloside is a naturally occurring compound which upon hydrolysis by a beta-glucosidase yields a molecule of a non-sugar, or aglycone, a molecule of free hydrogen cyanide.

The metabolism of all the higher animals and most of the invertebrates as well, involves the hydrolysis of plant-derived nitrilosides ingested in the plant components of the diet. This hydrolysis is produced by betaglucosidase occurring in the gastro-intestinal tract and produced in various tissues of the animal. The enzyme occurring in the intestinal tract is produced by various bacteria or microflora. When the enzyme so produced or that enzyme existing in the organs acts to hydrolyse the nitrilosides to free HCN, sugar and a non-sugar moiety the CN [cyanide] ion released is detoxified or converted by an enzyme normally occurring in the organism and known as rhodanese or thiosulfate transulfurase. The product of such conversion is thiocyanate, a compound found in the tissues of all vertebrates, many invertebrates and a number of plants.

Anthropogenic sources

Anthropogenic (of human origin) sources are responsible for much of the cyanide in the environment. Cyanide has been used worldwide in the extraction of gold and silver mainly. The major cyanide releases to water are discharges from metal-finishing industries, iron and steel mills, electroplating and organic chemical industries plastics and pharmaceuticals. These industries discharge large quantities of cyanide as in the form of liquid waste. Effluents from the cyanidation process used in precious metal extraction contain high amounts of cyanide. Vehicle exhaust and biomass burning are major sources of cyanide released into the air. The major sources of simple and complex cyanide releases to soil appear to be from the disposal of cyanide wastes in landfills and the use of cyanide-containing road salts.

Cyanogen chloride is formed in drinking water from reaction of humic substances with chloramine produced during chlorination. Thiocyanate is released to water primarily from discharges of industrial waste waters from coal processing and extraction of gold and silver; the thiocyanate is formed from the reaction of sulphur donors that are present in coal and crushed rock with the cyanide that is used in the processing of these materials. Thiocyanate is also found in mining waste waters where it results from the interaction of the cyanide anion (CN^–) with sulphur. Releases of thiocyanate to soil result from anthropogenic and natural sources. Anthropogenic releases occur primarily from direct application in herbicidal formulations and from disposal as by-products from industrial processes. The mean cyanide concentration in most surface waters is not greater than 3.5 μg/L.

WHAT HAPPENS TO CYANIDE WHEN RELEASED TO THE ENVIRONMENT?

Cyanide produce a wide variety of new compounds. Such compounds are often categorized as simple cyanide compounds, cyanide complexes, and cyanide-related compounds. The following sections focus on cyanide breakdown compounds most commonly found at mine sites.

Simple Cyanide Compounds: Simple cyanide compounds consist of only a single metal ion in combination with CN. Simple cyanide compounds include sodium cyanide, potassium cyanide, and calcium cyanide—all of which are readily soluble. Some simple cyanide compounds are insoluble.

Metal-Cyanide Complexes: Cyanide complexes are compounds of cyanide bound together with numerous other organic and inorganic compounds. Only the metal-cyanide, complexes are compounds.

Cyanides are not persistent in water or soil. Cyanides may accumulate in bottom sediments, but residues are generally as low as ˂1 mg/kg even near polluting sources. Majority of an accidental release of cyanide is volatilized to the atmosphere where it is quickly diluted and degraded by ultra violet. Other factors, such as biological oxidation, precipitation and the effects of sunlight also contribute to cyanide degradation. Cyanide is released into air mainly as hydrogen cyanide gas and, to a lesser extent, as particulate cyanides. Hydrogen cyanide can potentially be transported over long distances before reacting with photo chemically generated hydroxyl radicals. The residence time of hydrogen cyanide in the atmosphere has been estimated to be approximately 2.5 years, with a range of 1.3–5.0 years, depending on the hydroxyl radical concentration. Neither photolysis nor deposition by rainwater is expected to be a significant removal mechanism. Only 2% of the tropospheric hydrogen cyanide is expected to be transported to the stratosphere.

In water, cyanide occurs most commonly as hydrogen cyanide. Hydrogen cyanide is expected to be removed from water primarily by volatilization. Cyanide may also be removed by aerobic or anaerobic bio-degradation.

At soil surfaces, volatilization of hydrogen cyanide is a significant loss mechanism for cyanides. In subsurface soil, cyanide at low concentrations would probably biodegrade under both aerobic and anaerobic conditions. In case where cyanide levels are toxic to microorganisms (i.e., landfills, spills), the concentrations of water-soluble cyanides may be sufficiently high to leach into groundwater.

When metal-cyanide complexes are formed and released into the near-surface environment, they begin to decompose at varying rates, some quickly, others quite slowly. This breakdown releases cyanide into the soil or water, generally at relatively low concentrations. Those complexes that most readily decompose are referred to as weak complexes and those most resistant to decomposition are called strong complexes. Examples of weak cyanide complexes include zinc and cadmium cyanides. Moderately strong complexes include copper, nickel, and silver cyanides. And strong complexes include iron, cobalt, and gold cyanides.