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Environmental fate & pathways

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Thiocyanate, a naturally occurring compound

The biogenic production of thiocyanate is quite widespread. However, thiocyanate does not occur in the intact plant but is a component of destroyed tissues of plants. When plants are crushed, glucosinolates hydrolyze to form thiocyanate. Natural sources also include the urine of mammals. In animals thiocyanate may be derived from the food source or from a detoxification reaction. Cyanides and nitriles are, for instance, detoxified to thiocyanate.

Biodegradation in OECD ready biodegradability tests

Ready biodegradability tests are primarily used for regulatory purposes. The Closed Bottle test has been specified to determine the ready biodegradability of organic compounds. There is a general consensus that natural compounds are readily biodegradable. Indeed, ammonium thiocyanate degraded 80% within 28 days in the Closed Bottle test (Garttener and van Ginkel, 1999). The biodegradation percentage obtained in the Closed Bottle test allows classification of thiocyanate as readily biodegradable. A chemical that passes a ready biodegradability test is expected to biodegrade rapidly under aerobic conditions in biological treatment systems and ecosystems.

Degradation by pure cultures of microorganisms

Microbial degradation of thiocyanate by pure cultures of microorganisms has been well documented. Several studies have reported on the heterotrophic (microorganisms utilizing thiocyanate as nitrogen and/or sulfur source and another compound as carbon and energy source) and autotrophic biodegradation (microorganisms utilizing thiocyanate as energy source and carbon dioxide as carbon source). Pure strains were isolated by enrichment with thiocyanate as energy source, or as nitrogen or sulfur source. Thiobacillus thiocyanoxidans, an autotrophic microorganism, was first isolated by Happold et al (1954). Thiobacillus species are wide-spread in nature and are especially known for their utilization of reduced inorganic sulfur compounds as energy source. Isolation of twenty strains of bacteria including three strains of Thiobacillus thiocyanoxidans capable of transforming thiocyanate demonstrates the wide-spread occurrence of these organisms (Hutchinson et al 1965). Thiocyanate not only serves as source for energy but also as nitrogen source for the Thiobacillus sp. Thiobacillus sp were isolated from many sources including effluent from an activated sludge plant, sludge treating saline wastewater, and soils. In addition to being oxidized for energy purposes, several heterotrophic bacteria are able to utilize thiocyanate as nitrogen source. These bacteria were isolated from different sources such as activated sludge and soils. Stafford and Callely (1969) isolated a Pseudomonas stutzeri, which utilized thiocyanate as nitrogen source and succinate as carbon and energy source. An Arthrobacter sp which utilized thiocyanate as a nitrogen source and glucose as carbon and energy source was isolated by Betts et al (1979). Finally, utilization of thiocyanate-sulfur has been described. The acquisition of sulfur was detected with Neisseria meningitidis. This bacterium was grown in a medium containing glutamic acid, glucose, uracil and arginine (Port et al, 1984).

Biochemical pathways

The overall reaction catalyzed by microorganisms is as follows; HSCN + 2H2O + 2O2 ⇒ H2SO4 + NH3 + CO2 Thiocyanate degradation by microorganisms is catalyzed by series of enzymes. Currently two distinct pathways for microbial degradation of thiocyanate are recognized and either H2S or NH3 is the first product. Degradation of thiocyanate therefore requires the primary action of specific enzymes to release the sulfur or nitrogen from the carbon atom. For Thiobacillus thioparus it has been postulated that thiocyanate is degraded via cyanate. The liberated sulfide is utilized as an energy source. The hydrolysis of cyanate to ammonia and carbon dioxide is catalyzed by a specific enzyme cyanase (Youatt, 1954; Happold et al, 1958). Another strain of Thiobacillus thioparus initially produces ammonium. The first reaction is the hydrolysis of thiocyanate to carbonyl sulfide and ammonium, which is catalyzed by the enzyme thiocyanate hydrolase (Katawaya et al 1993). The carbonyl sulfide produced is hydrolyzed to hydrogen sulfide and carbon dioxide. The hydrogen sulfide is oxidized to sulfate to provide energy. It has been suggested that microorganisms utilizing thiocyanate as the nitrogen and sulfur source employ the same degradation pathways.

Biological wastewater treatment

Biological treatment systems constitute common practice in industrialized countries. These treatment systems use naturally occurring microorganisms to convert organic compounds. Thiocyanate may be a major constituent of wastewater. It is generally accepted that a substance judged as readily biodegradable will be removed from wastewater in biological treatment systems. Indeed, thiocyanate-containing wastewater can be treated efficiently in biological treatment systems. Acclimatization of biological treatment systems to very high thiocyanate concentrations has been reported (Karavaiko et al, 2000). Especially Thiobacillus sp are able to maintain themselves in activated sludge plants operated under different conditions (Catchpole and Cooper, 1972; Stott et al, 1999). Recently, Du Plessis et al (2001) operated an activated sludge system on a continuous basis. In this system thiocyanate was degraded to concentrations of < 1 mg/L at a hydraulic retention time of 8 hours and a solids retention time of only 18 hours. The thiocyanate feed concentration was 55 mg/L. The removal achieved in this reactor demonstrates that microorganisms are capable of biodegrading thiocyanates at very high rates. Other work successfully made use of fed-batch reactors and batch reactors. (Neufeld et al, 1981; Hung and Pavlostathis, 1999). Acclimatization of aerobic thiocyanate degrading microorganisms in the biological treatment systems led to complete thiocyanate degradation to ammonia, carbon dioxide and sulfate.