1,1,5,5,5-hexamethyl-3,3-bis[(trimethylsilyl)oxy]trisiloxane

Toxicity to soil microorganisms

Link to relevant study record(s)

Reference

Endpoint:

toxicity to soil microorganisms

Type of information:

experimental study

Adequacy of study:

key study

Study period:

22nd March - 19th April 2019

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

according to guideline

Guideline:

OECD Guideline 216 (Soil Microorganisms: Nitrogen Transformation Test)

Deviations:

no

GLP compliance:

yes

Analytical monitoring:

no

Details on sampling:

Test substance was not analysed

Vehicle:

no

Details on preparation and application of test substrate:

AMENDMENT OF SOIL
- Type of organic substrate: Speyer 2.3 soil
- Other:

APPLICATION OF TEST SUBSTANCE TO SOIL
- Method: Since the test substance was received as a liquid, a carrier was not used. Approximately half of the soil batch was transferred to a 4-L jar. An appropriate amount of the test substance was dosed dropwise across the soil surface using a pipette. The remaining half of the soil batch was then added to the 4-L jar. The soil was mixed by rolling the capped jar for approximately 3 hours at approximately 15 rpm on a rolling mill. The fortified soil was transferred to the bowl of a food mixer and mechanically mixed for 10 minutes at low speed. During this mechanical mixing, the soil was amended with 5.0 g of lucerne meal (i.e. alfalfa). Also during this mechanical mixing a volume of purified reagent water was added to the soil in order to bring the moisture content to 45 ± 5% of the soil’s maximum water holding capacity. After dosing and mixing, each batch of soil was divided equally into three replicate test samples in 1-L glass bottle test vessels.

VEHICLE:
- Chemical name of vehicle (organic solvent, emulsifier or dispersant): N/a
- Concentration of vehicle in test medium (stock solution and final test solution): N/a
- Evaporation of vehicle before use:N/a

Test organisms (inoculum):

soil

Total exposure duration:

28 d

Test temperature:

20 ± 2 °C

Moisture:

The soil moisture content was maintained at approximately 45% of the maximum water holding capacity throughout the test.

Organic carbon content (% dry weight):

0.6

Details on test conditions:

TEST SYSTEM
- Testing facility: Smithers Viscient, 790 Main Street, Wareham, Massachusetts 02571-1037
- Test container (type, material, size): The 1-L glass bottle test vessels were covered with perforated parafilm to allow for air exchange while reducing water loss.
- Amount of soil: 333g
- No. of replicates per concentration: 3
- No. of replicates per control: 3
- No. of replicates per vehicle control: N/a

SOIL INCUBATION
- Method: bulk / series of individual subsamples

SOURCE AND PROPERTIES OF SUBSTRATE (if soil)
- Geographical reference of sampling site (latitude, longitude): Offenbach, Rheinland-Pfalz, Germany.
- History of site:Over the past 4 years, no pesticides or organic fertilizers have been applied to the collection site.
- Vegetation cover: Not reported
- Treatments with pesticides or fertilizers: Over the past 4 years, no pesticides or organic fertilizers have been applied to the collection site.
- Accidental contamination: Not reported
- Other:
- Depth of sampling: 0-20cm
- Soil texture: USDA
- % sand: 68
- % silt: 26
- % clay: 6
- Soil taxonomic classification: Not reported
- Soil classification system: Not reported
- pH (in water): 6.5
- Initial nitrate concentration for nitrogen transformation test (mg nitrate/kg dry weight): 84.0-87.0 (unknown whether dry weight or not)
- Maximum water holding capacity (in % dry weight): 26.4
- Cation exchange capacity (mmol/kg): 5.7meq/100 g
- Pretreatment of soil: N/a
- Storage (condition, duration): refrigerated (approximately 4 °C) and in the dark.
- Initial microbial biomass as % of total organic C: 2.2

DETAILS OF PREINCUBATION OF SOIL (if any): Before use, the soil was transferred to an environmental chamber set to maintain a temperature of 20 °C and allowed to equilibrate to incubation conditions for 2 to 28 days

EFFECT PARAMETERS MEASURED (with observation intervals if applicable) : Nitrogen formation rate

VEHICLE CONTROL PERFORMED: N/a

RANGE-FINDING STUDY
- Test concentrations: N/a
- Results used to determine the conditions for the definitive study:N/a

Nominal and measured concentrations:

Nominal: Untreated control, 1.2, 3.7, 11, 33, and 100 mg/kg dry weight soil

Reference substance (positive control):

no

Duration:

28 d

Dose descriptor:

EC10

Effect conc.:

> 100 mg/kg soil dw

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

nitrate formation rate

Duration:

28 d

Dose descriptor:

EC25

Effect conc.:

> 100 mg/kg soil dw

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

nitrate formation rate

Duration:

28 d

Dose descriptor:

EC50

Effect conc.:

> 100 mg/kg soil dw

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

nitrate formation rate

Details on results:

- Effect concentrations exceeding solubility of substance in test medium: Not reported
- Other:

Results with reference substance (positive control):

No reference test reported

Reported statistics and error estimates:

The nitrate formation rates were input into CETIS (Ives, 2019). Inhibition between the untreated control and the treated soils were tested for significance using the ANOVA and Dunnett’s Multiple Comparison Test statistical programs. All statistical conclusions were made at the 95% level of certainty. The nitrate formation rates were analyzed using the linear interpolation (ICPIN) statistical program in CETIS to determine EC10, EC25, and EC50 values (the concentrations at which nitrate formation is affected by 10, 25, and 50% relative to the untreated control, respectively). The results were investigated for outliers using the Grubbs Extreme Value Test statistical program in CETIS.

Table 1 - Nitrate concentration and formation rate

Nominal concentration (mg/kg)	Value type	Day 0 Nitrate concentration (mg/kg)	Day 0 Nitrate concentration (mg/kg)	Nitrate formation rate (mg/kg/day
Untreated control	Mean	87.0	254	5.98
	SD	0.26	5.97	0.21
	RSD	0.30	2.35	3.49
1.2	Mean	85.8	285	7.11
	SD	0.05	2.26	0.08
	RSD	0.05	0.79	1.14
3.7	Mean	86.3	255	6.03
	SD	0.09	7.48	0.27
	RSD	0.11	2.93	4.45
11	Mean	84.7	244	5.68
	SD	0.62	3.89	0.14
	RSD	0.74	1.60	2.42
33	Mean	84.0	256	6.13
	SD	0.92	2.87	0.09
	RSD	1.10	1.12	1.45
100	Mean	84.6	246	5.77
	SD	0.93	1.02	0.07
	RSD	1.10	0.41	1.17

Validity criteria fulfilled:

yes

Remarks:

Relative standard deviation in nitrate concentration and formation rate for the untreated control samples ranged from 0.3 to 3.5%, meeting the 15% validity criteria.

Conclusions:

No effects were observed in a 28-day exposure of the test substance to soil microorganisms, producing EC10, EC25 and EC50 values of >100 mg/kg.

Description of key information

28-day EC₁₀: >100 mg/kg dw (highest concentration tested) in accordance with OECD 216, based on read-across within the Reconsile Siloxane Category.

Additional information

No data are available with the registration substance. Data from the structural analogue, dodecamethylpentasiloxane (L5, CAS 141-63-9), are read across.

 

A 28-day toxicity to soil microorganisms study for effects of L5 on nitrate formation rate of soil microflora has been conducted in accordance with OECD TG 216 (Soil Microorganisms: Nitrogen Transformation Test) at concentrations up to 100 mg/kg soil dry weight. No effects on nitrate formation rate were observed.

 

A 28-day EC₅₀ value of >100 mg/kg soil dw and an EC₁₀ value of >100 mg/kg soil dw (highest concentration tested) have been determined for the effects of dodecamethylpentasiloxane (L5, CAS 141-63-9) on nitrate formation rate of soil microflora based on nominal concentrations.

 

Analytical verification of test substance concentrations during the earthworm toxicity test indicate that the test substance is relatively stable in the soil. The soil was prepared using a similar method in both the earthworm and microorganisms tests, therefore it is likely that the soil microorganisms were exposed to the test substance throughout the test.