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1009513 
Technical Report 
The effects of cyanides on aquatic organisms with emphasis upon freshwater fishes 
Leduc, G; Mccracken, IR; Pierce, RC 
1982 
RISKLINE/1988070023 
NRCC No 19246 
1982 
English 
Free cyanide: Free cyanide, composed of the equilibrium summation of molecular HCN and the cyanide ion, is considered the primary toxic agent in the aquatic environment. In natural waters with a pH of 8.5 or lower, most of the free cyanide is in the form of molecular HCN. Simple cyanide: Simple cyanides, represented by the formula M(CN)X, exhibit a diverse range of aqueous solubilities and degrees of ionization. Soluble simple cyanides, notably some alkali cyanides, readily release cyanide ion in solution; the cyanide ion may then enter into equilibrium with molecular HCN. Complex cyanide: Transition metals of the "d" block and their near neighbours combine with cyanide ions to produce various complex cyanides, depending upon such factors as solution strength and stoichiometric proportions. Several metallocyanide compounds dissociate in aqueous solution, thereby generating complex cyanide ions. These ions may, depending upon conditions, further dissociate yielding cyanide ions. In general, complex ions are more stable than the parent metallocyanide and their subsequent dissociation may be relatively small. However, the extent to which a complex cyanide dissociates is of little importance in environmental systems; instead, the important factor is the rate at which cyanide bound in the complex exchanges with the cyanide ion (i.e. free cyanide) in solution. Toxicity of complex cyanides is usually related to the capacity of complex cyanide ions to release cyanide ions in solution, which then enter into an equilibrium with molecular HCN. Relatively small fluctuations in pH may have a significant effect upon toxicity, as has been demonstrated for the tetracyanonickelate II complex. Organocyanides (nitriles): Organic compounds containing the -CN group can be termed cyanides or nitriles, depending upon the nomenclature used. From the point of view of free cyanide toxicity, many organocyanide compounds are considered relatively innocuous in aqueous systems because of their low chemical reactivity and biodegradability. Most degradative mechanisms involve a cleavage of the CN bond and hence the cyanide ion is not generated. It should be kept in mind, however, that some nitriles are toxic per se, although such toxicity is not attributable to release of the cyanide ion. Cyanohydrins (R2C(OH)CN) and cyanogenic glycosides (R1R2C(OR3)CN), are special classes of nitriles in that under appropriate conditions, they will release cyanide ions (i.e. free cyanide) in solution. Other related compounds: Cyanides (mainly free and simple) may be converted to cyanates (compounds containing the group -OCN) during waste treatment. Aqueous solutions of cyanate hydrolyze to ammonia and bicarbonate. In the presence of reduced sulfur compounds, particularly polysulfide sulfur, cyanides are converted to thiocyanates (compounds containing the group -SCN). Thiocyanates are relatively more stable in aqueous solution than cyanates and may form complexes with a number of elements. Analytical methodology: Free cyanide can be selectively determined by variations of gas stripping and aqueous diffusion techniques. Analysis of total cyanides can be accomplished by acid distillation with detection by selective ion electrode, titration or colorimetry. Pretreatment and modifications to the acid distillation step are commonly employed to ascertain some fractions of total cyanide. Such modifications include cyanide amenable to chlorination, weak acid dissociable cyanide and simple versus complex cyanide. Qualitative differences existing between the measured fractions may have some bearing upon their toxicity. A comparison of different analytical methods used to measure various cyanide species is presented in Table 2-4. Environmental dynamics: Very little reliable information is available on sources, levels, fates and environmental dynamics of cyanide compounds in dilute aqueous systems. Sources: The presence of cyanides in natural waters is attributed to both natural processes and man-made activities. Although molecular HCN may be produced naturally by microorganisms as well as from the cyanogenic degradation of glycosides, the environmental significance related to such sources has not been assessed. Compounds containing the cyanide group also enter the aquatic environment via point sources associated with certain industrial activities as well as nonpoint sources associated with the application of materials containing cyanide compounds. Detectable levels of cyanide in the environment (usually measured as some form of the total cyanide) are considered to result from improper management practices such as improper storage, handling and disposal of wastes. The pyrolysis of some cyanide-containing materials leads to the production of hydrogen cyanide; the atmospheric transport, transformation and possible entry into the aquatic environment have not been examined. Levels: The limited data that are available on the distribution of cyanide compounds in the aquatic environment have usually been obtained using techniques to measure only total cyanide, irrespective of the chemical species that may actually be present in such samples. Fragmentary information indicates that total cyanide concentrations in rural areas are related to the size of the watershed and seasonal run-off. Levels of total cyanide are found to be significantly lower in rural and wilderness waters than in waters of industrial origin. Fate in the aquatic environment: Very little information has appeared in the literature pertaining to the fate of cyanide compounds in natural waters. Depending upon the cyanide species in question and physicochemical conditions, such interactions as oxidation-reduction, volatilization, hydrolysis, photolysis, dissociation, photoaquation, biodegradation, biogenesis and precipitation-dissolution may be important. With few exceptions, studies have been conducted only under conditions which simulate highly concentrated process water or effluents. Dynamic interactions in an environmental context are virtually unexplored. Hydrogen cyanide is volatile, having a vapor pressure of 53.1 kPa at 10.2?C and will be in equilibrium with atmospheric HCN approximately according to Henry's Law. Volatilization is expected to be a significant means of free cyanide (and in some cases simple cyanide) removal from concentrated effluents: however, it may be a rate-determining step in dilute aqueous systems. Data on chemical oxidation and adsorption to solids of molecular HCN in dilute aqueous systems are unavailable. Numerous microorganisms have been identified that can degrade molecular HCN. Microbial degradation has been employed as a biological treatment method; however, acclimation to low levels of cyanides are required in order for the mixed cultures to be effective. The relationship between biological synthesis and degradation of molecular HCN, and its environmental significance, has not been investigated. If conditions in dilute aqueous systems are such that complex cyanide ions release cyanide ions, which will then enter into equilibrium with molecular HCN, the removal mechanisms indicated above will apply. Numerous metallocyanide complex ions are relatively stable in aqueous solution in the absence of ultraviolet and visible light. However, under certain conditions, photodecomposition, with subsequent release of cyanide ions, will occur. The significance of biological degradation, adsorption to solids and other relevant processes in dilute aqueous solution has not been assessed. It is postulated that some of the various mechanisms involved in the removal of cyanide compounds from the water column may result in localized pools or sinks of such material that, upon changing conditions, may release free cyanide or other cyanide-containing material back into the water column. Toxicology: Free cyanide: The acute mode of action of HCN is limited to binding those porphyrins that contain iron III, such as cytochrome oxidase, hydroperoxidases, and methemoglobin. At lethal levels, cyanide is therefore primarily a respiratory poison and one of the most rapidly effective toxicants known. Concentrations in the range of 30-150 ug.L-1 HCN may be acutely toxic and include the lethal thresholds and median tolerance concentrations for most cold- and warm-water fishes. The acute toxicity of HCN can be influenced by several modifying factors; however, temperature appears to be most significant. The influence of temperature is related to the HCN concentrations present. At high, rapidly lethal concentrations, cyanide is more toxic at elevated temperatures whereas at relatively low, slowly lethal levels the opposite is true. At the chronic level of exposure, metabolic processes other than biological oxidation may be responsible for detrimental effects observed at HCN concentrations as low as 5 ug.L-1. Various responses of ecological significance have been measured. Reproductive impairment has been demonstrated under continuous chronic exposure to HCN at concentrations as low as 4 ug.L-1. All such tests have revealed significant detrimental effects upon reproductive processes at approximately 10 ug.L-1. Fertilized eggs are relatively resistant to free cyanide prior to blastulation, however, teratogenesis and mortality following blastulation, in addition to mortality of newly hatched larvae, have occurred at concentrations of approximately 60-100 ug.L-1. The growth of juvenile fish may be severely affected during chronic exposures to 10 ug.L-1, although transient growth enhancement has been demonstrated. Tolerance (e.g. levels affecting growth) may change with body size. Chronic exposure to 10 ug.L-1 HCN has been shown to elevate resting metabolic rate above basal levels. However, this oxidative stimulation in relatively inactive fish remains unexplained. The active swimming ability required for migration, feeding, and escape from predators is severely affected by free cyanide. An experimental situation simulating anadromous migration revealed osmoregulatory disturbances at 10 ug.L-1 HCN in fresh water, following a period of several days in salt water. Other related compounds: Few detailed toxicity tests have been conducted into the effects of simple cyanide-containing compounds upon aquatic organisms. Cyanates are believed to be relatively nontoxic although the 96-h LC50 for mercuric thiocyanate was determined to be 120 ug.L-1 for a marine invertebrate and 180 ug.L-1 for a freshwater fish. Cyanogen chloride is toxic to fish at 40 ug.L-1. Metallo- and organocyanides: The toxicity of metallocyanides may in part be related to molecular structures, as discussed in Section 2.5 of the text. An inherent toxicity is associated with tetracyanonickelate II, dicyanoargentate I, and dicyanocuprate I. The nickel complex, which may have an accumulative mode of action, has been estimated as lethally toxic to fish at 1.0 mg.L-1 as total cyanide. The toxic silver and copper complexes may not be significant in the environment because of their removal by wastewater treatment and their rapid dissociation upon dilution of effluents. Iron cyanide complexes are extremely stable in the absence of ultraviolet or visible light and are essentially nontoxic. However, they can undergo photolysis under certain circumstances and release toxic levels of free cyanide. The inherent toxicity associated with cobalt and mercury complexes (other than mercuric thiocyanate) has not been investigated. Toxicity of cyano complexes of chromium, cadmium, zinc, and lead has not been demonstrated; however, solutions of these complexes may produce toxic levels of free cyanide. Within the organocyanides is a group of cyanohydrins (e.g. lactonitrile) which may produce sufficient HCN via hydrolysis to elicit toxic responses in short-term exposure situations. Other nitriles are less toxic in acute conditions. For example, the 100-d LC50 for rainbow trout was estimated as 1.6 mg.L-1 as acrylonitrile. Chronic toxicity has not been adequately investigated. Multiple toxicity: Binary mixtures of free cyanide plus hexavalent chromium, zinc, or ammonia resulted in more fish lethalities than predicted by either response addition or concentration addition models. However, at sublethal levels, only the NH3-HCN binary mixture had an interactive effect whereas Cr-HCN and Zn-HCN combinations demonstrated independent modes of action. Binary mixtures of arsenic plus free cyanide may elicit effects at sublethal levels which are predictable according to a response addition model. 
ANIMAL; reproductive effect; Acute toxicity; Chronic toxicity; reproductive and developmental tests; ENVIRONMENT; AQUATIC TOXICITY; fish; invertebrate; ENVIRONMENTAL CONCENTRATIONS; water; DEGRADATION; MOBILITY; soil/sediment 
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