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EC number: 208-754-4
CAS number: 540-72-7
Thiocyanate, a naturally occurring
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
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
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).
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
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