Dipotassium tetraoxomolybdate

Toxicity to microorganisms

Administrative data

Endpoint:

activated sludge respiration inhibition testing

Type of information:

experimental study

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: see 'Remark'

Remarks:

Reported nominal toxic concentration levels were substance-based instead, i.e., not expressed as the Mo-ion, bus as the salt. Test was conducted according to OECD Guideline 209. Relevant endpoints were reported, or could be derived with the reported raw data.

Data source

Reference

Reference Type:

study report

Title:

Unnamed

Year:

1994

Report date:

1994

Materials and methods

Test guideline

Qualifier:

according to guideline

Guideline:

OECD Guideline 209 (Activated Sludge, Respiration Inhibition Test

GLP compliance:

yes

Test material

Test material information

Constituent 1

Reference substance name:

Molybdenum trioxide

EC Number:

215-204-7

EC Name:

Molybdenum trioxide

Details on test material:

- Name of test material (as cited in study report): molybdenum oxide (pure)
- Physical state: light grey powder
- Analytical purity: 66.63% molybdenum
- Expiration date of the lot/batch: 13 June 1993
- Storage condition of test material: room temperature in the dark
- Method of preparation: direct dispersion in water

Sampling and analysis

Analytical monitoring:

no

Test solutions

Vehicle:

no

Test organisms

Test organisms (species):

activated sludge of a predominantly domestic sewage

Details on inoculum:

Type and source of the inoculum was a mixed population of activated sewage sludge micro-organisms, taken from the aeration stage of the HRC Ltd. Sewage treatment plant treating predominantly domestic sewage. The collected sample was maintained at 20 ± 2 degrees Celsius with continuous aeration and used on the day of collection. pH was 7.4 and the suspended solid level of the sample was 4.1 g/L.

Study design

Test type:

static

Water media type:

freshwater

Limit test:

no

Total exposure duration:

3 h

Remarks on exposure duration:

also results after 30 min

Test conditions

Hardness:

dilution water: 150-200 mg/L as CaCO3

Test temperature:

20 degrees Celsius

pH:

dilution water: 6.0-8.5

Nominal and measured concentrations:

20, 60, 180, 540, 1620 mg/L of the substance

Details on test conditions:

- Contact time: 30 minutes and 3 hours
- Vessels: 500 mL conical flasks
- Aeration: compressed air via narrow bore glass tubes
- Measuring apparatus: 280 mL darkened glass BOD bottle fitted with a Yellow Springs BOD probe (Model 50B) and a magnetic stirrer. Separate measuring bottles were used for control, test and reference cultures to eliminate the possibility of cross-contamination.
- Photoperiod: normal laboratory lighting
- Control: inoculated medium without test or reference toxicant

- Test procedure: At time "0", 16 mL of synthetic sewage was made up to 300 mL with water in a 500 mL conical flask. 200 mL inoculum were added and the mixture aerated at approximately 0.5-1 L per minute. This preparation was designated Control R1. Thereafter, at 15 min intervals, further cultures were prepared following the same procedure but with the addition of appropriate amounts of test or reference substance to the vessel prior to adjusting the volume and adding the inoculum. The series was completed by a second Control R2 culture.
As each culture reached 30 minutes contact time its rate of respiration was measured. The measuring vessel was completely filled with the culture and sealed by the dissolved oxygen probe before being placed on a magnetic stirrer. THe stirrer was started and the dissolved oxygen level was recorded for approximately 10 minutes. At the end of the recording period the sample was returned to the culture vessel and aeration continued until the 3-hour contact period was reached. At this time, the determination of respiration rate was repeated.

Synthetic sewage composition: 16g peptone, 11g meat extract, 3g urea, 0.7g NaCl, 0.4g CaCl2.2H2O, 0.2g MgSO4.7H2O, 2.8g K2HPO4 dissolved in 1L tap water

Dilution water: Laboratory tap water, dechlorinated by passage through activated carbon and softened by reverse osmosis.

Reference substance (positive control):

yes

Remarks:

3,5-dichlorophenol, 97% purity

Results and discussion

Effect concentrations

Effect concentrations 1

Duration:

30 min

Dose descriptor:

EC50

Effect conc.:

1 100 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

inhibition of total respiration

Remarks:

respiration rate

Effect concentrations 2

Duration:

3 h

Dose descriptor:

EC50

Effect conc.:

820 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

inhibition of total respiration

Remarks:

respiration rate

Effect concentrations 3

Duration:

3 h

Dose descriptor:

EC10

Effect conc.:

325 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

inhibition of total respiration

Remarks:

respiration rate

Remarks on result:

other: 253.7-416.2

Effect concentrations 4

Duration:

3 h

Dose descriptor:

EC10

Effect conc.:

216.5 mg/L

Nominal / measured:

nominal

Conc. based on:

element

Basis for effect:

inhibition of total respiration

Remarks:

respiration rate

Remarks on result:

other: 169.1-277.3

Details on results:

The raw data for the test that was conducted with pure Mo-oxide allowed the derivation of an EC10-value. The use of this value is preferred, and reduces the uncertainty of a PNEC that may be derived using this specific study (i.e. an assessment factor of 10 on an EC10-value versus an assessment factor of 100 on an EC50-value). Fitting a log-logistic distribution through the reported raw data of the 3h-test with pure MoO3 resulted in a 3h-EC10 of 325.0 mg MoO3/L (95%CL: 253.7-416.2) or 216.5 mg Mo/L (95%CL: 169.1-277.3). It should be noted that this concentration level of molybdenum, added as MoO3, may strongly reduce the pH-level. As the pH during the exposure period was not reported, it is always possible that the observed effects are more pH-related than Mo-related.

Rate of respiration for each culture was measured for a 10 minute period
Variation in respiration rates of Controls R1 and R2: +/-1.7% after 30 min, and +/- 4.0% after 3h.

Results with reference substance (positive control):

30 min-EC50: 20 mg/L
3h-EC50: 17 mg/L

Reported statistics and error estimates:

Percentage inhibition plotted against concentration

Applicant's summary and conclusion

Validity criteria fulfilled:

yes

Conclusions:

Reported nominal toxic concentration levels were substance-based instead, i.e., not expressed as the Mo-ion, bus as the salt. Method is well described and in line with OECD 209. EC10 level could be determined with the reported data . EC50-values are categorised as Klimisch 2 data points (reliable with restrictions), as pH may have had an influence on the reported effect levels.