1,1,3,3-tetramethyldisiloxane

Toxicity to soil microorganisms

Description of key information

The endpoint is waived for the parent substance due to its rapid hydrolysis and for the intermediate hydrolysis product dimethylsilanol, based on the Chemical Safety Assessment.

A study is available with dimethylsilanediol: In a toxicity to soil microorganisms study, conducted according to BBA, Part VI, 1-1 and in compliance with GLP, it was concluded that the substance dimethylsilanediol did not cause any significant long-term effects on the soil microflora respiration or the nitrogen transformation in soil. Thereby, a NOEC was concluded to be equal to or greater than 300 ppm, equivalent to 300 mg/kg dw (SEHSC, 1997).

Key value for chemical safety assessment

Long-term EC10 or NOEC for soil microorganisms:

300 mg/kg soil dw

Additional information

The endpoint is not applicable to the parent substance 1,1,3,3-tetramethyldisiloxane (CAS 3277-26-7) due to its rapid hydrolysis. However, its silanol hydrolysis products, dimethylsilanol and dimethylsilanediol are assessed. For further information on the terrestrial chemical safety assessment strategy please refer to Section 6.3 in IUCLID.

Dimethylsilanol:

In accordance with Column 2 of REACH Annex IX, the short-term toxicity test with terrestrial microorganisms (required in Section 9.4.2 of REACH Annex IX) does not need to be conducted as the chemical safety assessment according to Annex I indicates that this is not necessary.

Dimethylsilanediol:

In a toxicity to soil microorganisms study, conducted according to BBA, Part VI, 1-1 (equivalent to OECD guidelines 216 and 217) and in compliance with GLP, it was concluded that the substance dimethylsilanediol did not cause any significant long-term effects on the soil microflora respiration or the nitrogen transformations in sandy loam or loam soils. Thereby, the NOECs were concluded to be equal to or greater than 300 ppm, equivalent to 300 mg/kg dw, in studies up to 90-days.

Soil microflora respiration

A statistically significant stimulatory effect on the soil microbial respiration was observed for 28 days when 100 ppm of dimethylsilanediol was applied to a loam soil. However, after 28 days, no statistically significant effects in excess of ±15% were observed fornitrate and N-min[HD1] . Following application of 300 ppm of dimethylsilanediol, no treatment-related effects were observed throughout the study.

In the sandy loam soil, no statistically significant effects were observed at either 100 or 300 ppm levels. However, some percentage deviations were found to be in excess of ±15% with respect to control values although, after 90 days, no effects in excess of ±15% were observed. The relatively high variability within these samples is believed to depend on the low rates of respiration encountered in moist soil. The soil microflora respiration study did not produce any evidence of a dose response resulting from the application of the test substance.

Soil Nitrogen transformations

When the test substance was applied to a loam soil at day 0, some transient effects on the ammonium and nitrite levels occurred, regardless of the application levels. Additionally, a statistically significant effect after 28 days was observed on the nitrate concentrations at the 100 ppm treatment level. However, after 55 days, no statistically significant effects were observed at any of the treatment levels.

When the test substance was applied to a sandy loam soil at day 0, statistically significant effects were observed at both 100 and 300 ppm on nitrogen levels. Additionally, a slight temporary stimulation of nitrification was observed during the study. However, this was not considered as concentration related and returned to normal after 90 days incubation. Generally, greater variability within the data occurred for this type of soil. It is considered to be due to the required high moisture content to prevent the test substance polymerising to polydimethylsiloxane (PDMS). This variability caused a number of percentage deviations in excess of ±15, however, these were not significant at the 5% level.

Consequently, it was concluded that the substance did not cause any significant long-term effects on the soil microflora respiration or the nitrogen transformation in soil.