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Toxicity to microorganisms

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Description of key information

Available reliable data on the toxicity of molybdenum to towards the aquatic microbial communities of sewage treatment plants (activated sludge) were collected. There were insufficient useful data for aquatic micro-organisms to apply statistical extrapolation. From the extracted results, two test results were considered relevant for deriving the PNEC for micro-organisms in STPs: the activated sludge test with pure MoO3 (HRC, 1994) and the activated sludge test with sodium molybdate (Stearns, 1985). The lowest reliable observed EC10-value for respiration (inhibition of respiration after a 3h incubation period) using activated sludge was, 216.5 mg Mo/L (HRC, 1994d). 

As the second study resulted in the highest, unbounded NOEC (i. e., > 950 mg/L), the derivation of a PNECmicro-organism is based on the HRC (1994) study. The test was conducted with molybdenum trioxide (MoO3) as test substance.

Key value for chemical safety assessment

EC10 or NOEC for microorganisms:
216.5 mg/L

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