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Toxicity to microorganisms

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Description of key information

The PNEC is derived from a nitrification study with a NOEC of 50mg/L  KSCN, equivalent to 30 mg/L SCN-

Key value for chemical safety assessment

EC10, LC10 or NOEC for microorganisms:
30 mg/L

Additional information

Three studies were considered for this endpoint. The first study is from a Robra test with Pseudomonas putida bacteria. This was was considered to be of low quality because there were no controls, no positive reference substance tested, the exposure time and the purity of the test substance were unclear. The second study from a OECD 301D ready biodegradability test, with a test concentration of 2 mg/L. At this test concentration, ammonium thiocyanate had no negative effect on the activated sludge tested.

The key study chosen for this endpoint was from a nitrification study with industrial activated sludge showing that up to 50mg/L KSCN (equivalent to 30 mg/L SCN-) thiocyanate has no inhibitory effects on nitrification. Although this study did not follow international guidelines, the methodology is well described, and the study was well conducted.

A variety of bacteria have been reported to degrade SCN-. Thiocyanate degrading Pseudomonas spp. have been isolated from sewage waters in coke factories, and activated sludge tanks.Thiobacillusspp. that degrade SCN- have been isolated from such varied sources as marine mud, cattle manure, and pond water, contaminated well water. A SCN- degrading Arthrobacter spp. has been isolated from soil adjacent to a railroad embankment.

Due to the wide nature/variety of microorganisms that can use thiocyanates as sulfur, nitrogen and carbon source, and to the wide occurrence of thiocyanates in nature, thiocyanates are not expected to pose problems for microorganisms, especially those in a STP.