Reaction mass of 1,1,1,5,5,5-hexamethyl-3-phenyl-3-((trimethylsilyl)oxy)trisiloxane and 1,1,1,7,7,7-hexamethyl-3,5-diphenyl-3,5-bis[(trimethylsilyl)oxy]tetrasiloxane and 1,1,1,9,9,9-hexamethyl-3,5,7-triphenyl-3,5,7-tris((trimethylsilyl)oxy)pentasiloxane

Biodegradation in soil

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

biodegradation in soil: simulation testing

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Remarks:

The study was well documented and meets generally accepted scientific principles, but was not conducted in compliance with GLP.

Principles of method if other than guideline:

L4 degradation was studied in two sets of experiments: one at constant temperature (22°C) and four different humidities (32, 42, 92, 100% RH), and another at constant humidity (42% RH) and two different temperatures (4°C and 37°C). In addition, an open system at 22°C and 100% RH was studied).
Soil samples were spiked with 10 μg/g (dry weight basis). Substance-specific analysis was performed
at various time points and mass balance was calculated.

GLP compliance:

no

Remarks:

This study was conducted using best available scientific methodology. However, this was a nonregulated study and as such was not conducted to meet all of the requirements described in Good Laboratory Practices Regulations.

Test type:

laboratory

Radiolabelling:

yes

Oxygen conditions:

aerobic

Soil type:

other: Londo soil, Bay City Country, Michigan, USA.

% Clay:

22

% Silt:

28

% Sand:

50

% Org. C:

2.4

pH:

7.6

Details on soil characteristics:

N.B. A single soil (Londo soil) was used in this study, and the effect of relative humidity (RH) and temperature on degradation rates of L4 was determined in closed and open systems. Where 'Soil No.' is referenced in this RSS, it refers to the sample conditions (i.e. Soil No. #1, Londo Soil, 32%RH, 22°C, closed system).

SOIL COLLECTION AND STORAGE
- Geographic location: Bay City, Michigan, USA

- Pesticide use history at the collection site: not recorded

- Collection procedures: not recorded

- Sampling depth (cm): not recorded

- Storage conditions: Air-dried and stored in cold room at ~6°C before use.

- Storage length: not recorded

- Soil preparation (e.g., 2 mm sieved; air dried etc.): air dried.

PROPERTIES OF THE SOILS (in addition to defined fields)

- Moisture at 1/3 atm (%): not relevant (soils were pre-conditioned at different relative humidities)

- Bulk density (g/cm3): not recorded

Soil No.:

#1

Duration:

28 d

Soil No.:

#2

Duration:

27 d

Soil No.:

#3

Duration:

43 d

Soil No.:

#4

Duration:

60 d

Soil No.:

#5

Duration:

57 d

Soil No.:

#6

Duration:

18 d

Soil No.:

#7

Duration:

21 d

Initial conc.:

0.01 g/kg soil d.w.

Based on:

test mat.

Parameter followed for biodegradation estimation:

radiochem. meas.

test mat. analysis

Soil No.:

#1

Temp.:

22°C

Humidity:

32% relative humidity (RH); closed system

Microbial biomass:

not measured

Soil No.:

#2

Temp.:

22°C

Humidity:

42% relative humidity (RH); closed system

Microbial biomass:

not measured

Soil No.:

#3

Temp.:

22°C

Humidity:

92% relative humidity (RH); closed system

Microbial biomass:

not measured

Soil No.:

#4

Temp.:

22°C

Humidity:

100% relative humidity (RH); closed system

Microbial biomass:

not measured

Soil No.:

#5

Temp.:

4°C

Humidity:

42% relative humidity (RH); closed system

Microbial biomass:

not measured

Soil No.:

#5

Temp.:

37°C

Humidity:

42% relative humidity (RH); closed system

Microbial biomass:

not measured

Soil No.:

#7

Temp.:

22°C

Humidity:

100% relative humidity (RH); open system

Microbial biomass:

not recorded

Details on experimental conditions:

1. PRELIMINARY EXPERIMENTS: not applicable.

2. EXPERIMENTAL DESIGN
- Soil preincubation conditions (duration, temperature if applicable): The soil was pre-conditioned for at least one week in four containers at four different constant humidities controlled by saturated salt solutions. For example, at room temperature (22°C), saturated CaCl2.6H2O solution, saturated Zn(NO3)2.6H2O solution and saturated K2HPO3 solution were used to control humidities of 32% relative humidity (RH), 42% RH, and 92%RH, while purified (Milli-Q) water was used to maintain 100% RH. In the treatments at 4°C and 37°C, saturated CaCl2.6H2O and Mg(NO3)2.6H2O were used to maintain a constant humidity at ~42% RH in desiccators placed in two environmental chambers. A Vaisala humidity and temperature meter was used to confirm the humidity and temperature inside the desiccators at the beginning and end of the incubations.

- Soil condition: air dried

- Soil (g/replicate): 5g

- Control conditions, if used (present differences from other treatments, i.e., sterile/non-sterile, experimental conditions): not relevant

- No. of replication controls, if used: not relevant

- No. of replication treatments:
soil #1: 8 replicates;
soil #2: 7 replicates;
soil #3: 8 replicates;
soil #4: 6 replicates;
soil #5: 7 replicates;
soil #6: 7 replicates;
soil #7: 7 replicates;

- Test apparatus (Type/material/volume): 25mL Teflon tubes

- Details of traps for CO2 and organic volatile, if any: not relevant for the test system design

- If no traps were used, is the system closed/open: soils #1-6: closed; soil #7 open

- Identity and concentration of co-solvent: pentane

Test material application
- Volume of test solution used/treatment: 250µL of 14C-L4 in pentane applied at an initial soil concentration of approximately 10µg/g L4 (dwt).

- Application method (e.g. applied on surface, homogeneous mixing etc.): The spiking solution was deposited to multiple positions in the surface layer of the soil. Immediately following the addition of test material, tubes were capped and vortexed for 5 minutes. The tubes were then purged with moisture controlled air for 1 minute and capped for incubation in closed-system experiments and opened in a constant moisture chamber for the open system experiments.

- Is the co-solvent evaporated: Not specified, but presumed to occur during purging.

Any indication of the test material adsorbing to the walls of the test apparatus: none recorded.

Experimental conditions (in addition to defined fields)
- Moisture maintenance method: see description of test system above.

- Continuous darkness: Not specified.

Other details, if any: no other details available

3. OXYGEN CONDITIONS (delete elements as appropriate)
- Methods used to create the an/aerobic conditions: aerobic

- Evidence that an/aerobic conditions were maintained during the experiment (e.g. redox potential): not measured

4. SUPPLEMENTARY EXPERIMENTS: none

5. SAMPLING DETAILS
- Sampling intervals: incubation times ranged from 6 days to 60 days (see duration of test time/contact time), depending on the observed times required for degradation trends to be established. At appropriate sampling times, whole test tubes (in duplicates) were sacrificed for analysis.

- Sampling method for soil samples: whole test tubes (in duplicates) were sacrificed for analysis using the following protocols:
1. THF extraction:
At the time of test system sacrifice, 5 mL of THF was transferred into each soil tube, followed by addition of 1 mL of the saturated MgSO4 solution. The soil sample with THF was vortexed for 2 minutes and then shaken for 2 hours. After the sample was centrifuged for 5 minutes at 2000 rpm (671 x g), the upper THF phase was collected in a polypropylene tube. This soil sample was extracted twice more with 3 mL of THF each time and all of the THF extracts from each sample were pooled together for HPLC-RAM analysis.
2. Acidic-water extraction:
After the THF extraction, 6 mL of 0.1M HCl / 0.01 M CaCl2 solution was transferred into each tube. The tube was shaken overnight. After the sample was centrifuged for 5 minutes at 2000 rpm, the water was collected in a polypropylene tube. The water extract was analysed by HPLC-RAM and LSC.
3. Combustion of the extracted soil samples:
After acidic water extraction, each soil sample was weighed and three weighed aliquots of soil residue (~1 g each) were combusted in a biological oxidiser with CO2-absorbing Harvey cocktail to capture the evolved 14C-CO2. The sample cocktail was then analysed by LSC.
4. HPLC-RAM analysis:
Quantitative analysis by HPLC-RAM was performed on three equal-volume aliquots of the soil extracts (0.2 or 0.4 mL each for THF extracts, and 1.0 mL each for acidic water extracts).

- Method of collection of CO2 and volatile organic compounds: not relevant

- Sampling intervals/times for:
> Sterility check, if sterile controls are used - not relevant
> Moisture content: refer to experimental design section, above.
> Redox potential/other: not measured
> Sample storage before analysis: not recorded.
- Other observations, if any: not relevant

Soil No.:

#1

% Recovery:

114.2

St. dev.:

8.6

Remarks on result:

other:

Remarks:

range 104.0-126.2%

Soil No.:

#2

% Recovery:

99

St. dev.:

6.5

Remarks on result:

other:

Remarks:

range 89.5-107.6%

Soil No.:

#3

% Recovery:

101.4

St. dev.:

3.9

Remarks on result:

other:

Remarks:

range 95.1-106.6%

Soil No.:

#4

% Recovery:

89.7

St. dev.:

1.1

Remarks on result:

other:

Remarks:

range 88.2-91.3%

Soil No.:

#5

% Recovery:

94.8

St. dev.:

2.5

Remarks on result:

other:

Remarks:

range 90.9-98.5%

Soil No.:

#6

% Recovery:

104.2

St. dev.:

8.5

Remarks on result:

other:

Remarks:

range 92.6-116.7%

Soil No.:

#7

% Recovery:

45.5

St. dev.:

34.6

Remarks on result:

other:

Remarks:

range 5.0-92.1%

Soil No.:

#1

DT50:

3.7 d

Type:

(pseudo-)first order (= half-life)

Temp.:

22 °C

Remarks on result:

other:

Remarks:

at 32% RH

Soil No.:

#2

DT50:

4.5 d

Type:

(pseudo-)first order (= half-life)

Temp.:

22 °C

Remarks on result:

other:

Remarks:

at 42% RH

Soil No.:

#3

DT50:

10 d

Type:

(pseudo-)first order (= half-life)

Temp.:

22 °C

Remarks on result:

other:

Remarks:

at 92% RH

Soil No.:

#4

DT50:

106.6 d

Type:

(pseudo-)first order (= half-life)

Temp.:

22 °C

Remarks on result:

other:

Remarks:

at 100% RH

Soil No.:

#5

DT50:

29 d

Type:

(pseudo-)first order (= half-life)

Temp.:

4 °C

Remarks on result:

other:

Remarks:

at 42% RH

Soil No.:

#6

DT50:

1.2 d

Type:

(pseudo-)first order (= half-life)

Temp.:

37 °C

Remarks on result:

other:

Remarks:

at 42% RH

Transformation products:

yes

No.:

#1

No.:

#2

Details on transformation products:

- Formation and decline of each transformation product during test:
For any 14-C L4-spiked soil sample, most radioactivity was recovered in THF and acidic water extracts. Reversed phased HPLC-RAM chromatograms showed three major distinguishable peaks in addition to that of L4, with reduced retention times relative to L4, suggesting that the major degradation products in those extracts were polar hydrolytic products. The first peak was always the predominant peak. Based on retention times under the same conditions, this peak was attributed to DMSD. The next peak matched that of the TMS standard. The test compound was mainly labelled in the dimethylsiloxyl unit, with only a small fraction of 14C atoms from the trimethylsilyl end group. The relative peak areas in the HPLC-RAM chromatograms were consistent with the expected relative radioactivities of the hydrolysis products.

- Pathways for transformation: abiotic degradation.

- Other: none

Evaporation of parent compound:

yes

Remarks:

in open system (#7)

Volatile metabolites:

not specified

Residues:

yes

Remarks:

See 'Any other information' field for further details.

Details on results:

TEST CONDITIONS
- Aerobicity (or anaerobicity), moisture, temperature and other experimental conditions maintained throughout the study: Yes
- Anomalies or problems encountered (if yes): none

MAJOR TRANSFORMATION PRODUCTS
- Range of maximum concentrations in % of the applied amount and day(s) of incubation when observed: raw data not provided. See attached information

- Range of maximum concentrations in % of the applied amount at end of study period: raw data not provided. See attached information

MINOR TRANSFORMATION PRODUCTS
- Range of maximum concentrations in % of the applied amount and day(s) of incubation when observed: not relevant
- Range of maximum concentrations in % of the applied amount at end of study period: not relevant

TOTAL UNIDENTIFIED RADIOACTIVITY (RANGE) OF APPLIED AMOUNT: not specified

EXTRACTABLE RESIDUES - see below.
- % of applied amount at day 0: not specified
- % of applied amount at end of study period: not specified.

NON-EXTRACTABLE RESIDUES
- % of applied amount at day 0: not specified.
- % of applied amount at end of study period: Amount of non-extractable organosilicon species increased linearly with incubation times, temperature and soil dryness. As the temperature increased or humidity decreased, the slopes of linear relation between percentage of non-extractable and the incubation time increased. The fraction of radioactivity not extracted by the three repeated THF extractions was the sorbed radioactivity which increased with incubation times (see attached information

MINERALISATION
- % of applied radioactivity present as CO2 at end of study: not relevant

VOLATILIZATION
- % of the applied radioactivity present as volatile organics at end of study: Soil#7 (open system): At 100% RH, more than half of spiked radioactivity for L4 was lost within 3 days, and more than 90% was lost within two weeks of incubation. The rate constant for volatile loss of L4 was determined by first order kinetics to be -0.175 day-1, corresponding to a volatilisation half-life of 4.0 days. It has been demonstrated for other volatile methyl siloxanes that the volatilisation loss in open systems is highly sensitive to moisture levels. Although this was not determined for L4, it has been shown that for the analogue substance, L3, recovery in both closed and open systems was close to 100% and volatilisation loss is minimal when the soil is dryer than 92% RH.

STERILE TREATMENTS (if used)
- Transformation of the parent compound: not relevant
- Formation of transformation products: not relevant
- Formation of extractable and non-extractable residues: not relevant
- Volatilization: not relevant

RESULTS OF SUPPLEMENTARY EXPERIMENT (if any): none

Degradation of L4 in soil was obvious at all tested moisture levels as indicated by the decline of the amount of extracted L4 and increase of degradation products dimethylsilanediol and trimethylsilanol with incubation time. Kinetic data is presented in the table below:

Exp no.	%RH	Temp (°C)	No. Samples^a	Regression^b: lnC/C0=-kappt+ m			Half-life (days)
				k­app(day^-1)	m	r^2	At 22°C	At 25°C
1	32	22	8	0.188	-0.096	0.9663	3.7	2.8
2	42		7	0.153	-0.031	0.9877	4.5	3.4
3	92		8	0.069	-0.093	0.9321	10.0	7.6
4	100		6	0.0065	-0.045	0.7165	106.6	80.6
5	42	4	7	0.0239	-0.001	0.962	29.0	N/A
6	42	37	7	0.5923	-0.038	0.9931	1.2	N/A

a: number of pairs of duplicates used for regression;

b: where t=time of incubation, in days

For the closed systems, first-order kinetics described the decrease of L4 concentration well (refer to attached information <Figure 6_ First-order kinetic plots for L4 degradation in closed systems at 22°C (soils #1- #4)>). The kinetics of L4 degradation at 32% RH can be better described as two phases. In the first phase, the degradation was rapid and accounts for two thirds of the L4 degradation. Further degradation was slower but still followed first-order kinetics. The half-life estimation for L4 at 32% RH is based on the rapid degradation phase.

Rates at high humidity may be underestimated due to complication of soil/air partitioning in the test tubes during incubation (L4 present in headspace not available for degradation). For each sample in a closed tube at 100% RH, the estimated fraction, via soil-air partitioning calculation, of L4 actually distributed in soil is around 52.4%. The rest will be in the headspace. Assuming that attainment of soil/air partition equilibrium for L4 was rapid compared to its degradation, and that the system was always in equilibrium during the course of the degradation, the actual degradation rate for L4 in soil without headspace should be ~1.9 times larger than observed. In addition, the expected large fraction of L4 in headspace above wet soil also explains the rapid volatilisation loss of L4 from soil at 100% RH observed in open system experiments. In the open system, the volatilisation of L4 was the predominant process for removal of L4 from soil at 100% RG, with a volatilisation half-life of 4 days, almost 10x faster than the degradation of L4 at the same moisture level in the closed system. The fact that the degradation rate of L4 is much slower than volatilisation rate under this condition suggests that volatilisation may be more important for removal of L4 under wet condition in the field.

First-order kinetics also describes the decrease of L4 concentration at the two other temperatures (4°C and 38.5°C at 42% RH) very well (refer to attached information <Figure 7_ Temperature dependence of first-order kinetic degradation of L4 at 42%RH (4°C, 22°C, 37°C)> ). As expected, the rate constants derived increased substantially with the increase of the incubation temperatures. The logarithm of the apparent rate constants was linearly related to the reciprocal of the incubation temperature, with a slope corresponding to an activation energy of 69.1 kJ/mol. If the activation energy is assumed the same for other soil humidities as that at 42% RH and 25°C, the degradation rate constant for L4 at 25°C at various soil humidities can be calculated.

Under the current study conditions, the water potential of the soil should be around 0 J/kg at 100% RH and -11400 J/kg at 92% RH. In the growing season, the water potential in the rooting zone in agriculture soils should not be less than -4000 J/kg (corresponding to about 97% RH, but at the least upper 5cm soil is frequently much drier than 92% RH, more closely related to air moisture levels (50 - 70% RH), especially in the dry season. Under this condition, a net transfer of L4 is expected form wet soil in rooting zone by volatilisation to air dry soil in the surface layer. When both degradation and volatilisation mechanisms are taken into consideration, the dissipation of L4 in agricultural soil should be much faster than the degradation of L4 at 100% RH determined in the closed systems in the current study.

Conclusions:

Soil degradation rates of decamethyltetrasiloxane (L4) were determined in a reliable study conducted according to generally accepted scientific principles.
The soil degradation/volatilisation study for L4 was conducted with a Londo soil from Bay City, Michigan, USA. 14C-labeled L4 was added to soil that was pre-conditioned at the desired relative humidity (RH), and incubated at different moisture levels and temperatures. Closed and open systems were used. The rate of degradation was greater as the soil became drier. Degradation half-lives (closed tubes) ranged from 3.7 d at 32% RH and at 22°C to 106.6 d at 100% RH and at 22°C (estimated to be 42 days when corrected for amount of L4 predicted to be in the headspace at this RH). The main degradation products were dimethylsilanediol and trimethylsilanol.
In open systems, volatilisation was the predominant process for removal of L4 from soil at 100% RH, with a volatilisation half-life of 4 days, almost 10 times faster than the degradation of L4 at the same moisture level in the closed system.

Reference 2

Endpoint:

biodegradation in soil: simulation testing

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

key study

Reason / purpose for cross-reference:

read-across source

Soil No.:

#1

DT50:

10 d

Type:

(pseudo-)first order (= half-life)

Temp.:

20 °C

Remarks on result:

other: 92% RH

Transformation products:

yes

Description of key information

Degradation in soil: The following results are read-across from the structurally-related substance L4. Michigan Londo soil, half-lives (closed tubes) 3.7 d at 32% RH and at 22°C to 106.6 d at 100% RH and at 22°C (estimated to be 56 days when corrected for amount of L4 predicted to be in headspace at this RH). The main degradation products were dimethylsilanediol and trimethylsilanol. In open systems, the volatilisation of L4 was the predominant process for removal of L4 from soil at 100% RH, with a volatilisation half-life of 4 days, almost 10 times faster than the degradation of L4 at the same moisture level in the closed system. In exposure modelling (EUSES 2.1.2) a half-life value of 10 days at 20°C (equivalent to 19 days at 12°C) will be used, based on the value of 10 d (#1) (92% RH 22°C Closed). This is an estimate. The exact value is not significant in respect of the overall risk characterisation for soil.

Key value for chemical safety assessment

Half-life in soil:

10 d

at the temperature of:

20 °C

Additional information

There are no reliable biodegradation in soil data available for phenyl silsesquioxanes, therefore good quality data for the structurally-related substance, decamethyltetrasiloxane, L4, (CAS 141-62-8), have been read across in a Category approach.

Phenyl silsesquioxanes and L4 are members of the Reconsile Siloxanes Category. This Category consists of linear/branched and cyclic siloxanes which have a low functionality and a hydrolysis half-life at pH 7 and 25°C >1 hour and log K_ow>4. There is a limited amount of soil stability data available with siloxanes. Substances that are highly absorbing are expected to have slow degradation rates in soil. The category hypothesis is that stability in soil is linked to the organic carbon-water coefficient and hydrolysis rates, which are dependent in turn on the structural features and constituent functional groups within the molecule. In the context of the Read-Across Assessment Framework (RAAF), Scenario 4 is applicable to this endpoint.

Additional information on the structure of the category and the supporting evidence for the application of the Scenario is given in a supporting report (PFA, 2017) attached in Section 13 of the IUCLID dossier.

Decamethyltetrasiloxane (CAS 141-62-8, L4) is a linear siloxane with four Si linked by oxygen, with methyl groups bound to silicon. Linear constituents 1-7 and cyclic constituents C3-C7 of the registration substance are all siloxanes with four to sixteen silicon atoms linked by oxygen and methyl and phenyl groups bound to silicon. Increase of the chain length results in decreased water solubility. Even though the surrogate substance and the constituents cannot be considered as close structural analogues, the property that will dominate the behaviour of the substance in the environment and its toxicity, in particular in the sediment compartment, is the high adsorption potential (log Kowand Koc). Both the surrogate substance and the registration substance have high MW (310.7 and from 372.8 to 1635.1 respectively) very high log Kow (8.2 and 9 respectively), and high log Koc (5.16 and 6 respectively).

 

Given the similar properties and structural similarities, it is considered valid to read-across soil degradation data from L4 to phenyl silsesquioxanes..

Table 4.1.10 presents all of the available data for removal of substances from soil within the Siloxane Category.In these studies,¹⁴C-labelled siloxane was added to soil that was pre-conditioned at the desired relative humidity (RH), and incubated at different moisture levels and temperatures. Closed and open systems were used. The results show that in general the rate of degradation is greater at lower RH in closed systems, and in open systems volatilisation is the predominant process for removal from soil at higher RH. Removal half-lives are generally <10 days in closed systems at RH< 100. Degradation products are identified in all studies; the ultimate hydrolysis products are identified as degradation products in all studies, and in most cases the intermediate hydrolysis products are also observed. It is considered valid to read-across the results for L4 to fill the data gap for the registered substance.

Degradation in soil data for substances within the Siloxane Category

CAS	Name	Soil type	Results	Reliability	Reference
556-67-2	Octamethylcyclotetrasiloxane (D4)	Wahiawa soil (#1)   Londo soil (#2)	Half-life (DT50): 0.04 d (#1) (32% relative humidity) 0.08 d (#1) (92% relative humidity) 0.89 d (#1) (100% relative humidity) 3.54 d (#2) (relative humidity 32%) 5.25 d (#2) (relative humidity 92%)   Transformation products: Siloxane diols Dimethylsilanediol	2	Xu and Chandra, 1999
541-02-6	Decamethylcyclopentasiloxane (D5)	Wahiawa soil	Half-life (DT50): 0.08 d (32% relative humidity)   Transformation products: Siloxane diols Dimethylsilanediol	2	Xu and Chandra, 1999
540-97-6	Dodecamethylcyclohexasiloxane (D6)	Wahiawa soil	Half-life (DT50): 1.38 d (32% relative humidity)   Transformation products: Siloxane diols Dimethylsilanediol	2	Xu and Chandra, 1999
107-46-0	Hexamethyldisiloxane (L2)	Londo	Half-life (DT50): 407.6 d (#1) (100% RH 22.0°C Closed NOTE: 9.8 days when corrected for head-space effect) 5.8 d (#1) (92% RH 22.0°C Closed) 6.4 d (#1) (42% RH 22.0°C Closed) 1.8 d (#1) (32% RH 22.0°C Closed) 19.9 d (#1) (4°C 42% RH Closed) 0.96 d (#1) (37°C 42% RH Closed) 323.9 d (#1) (100% RH 25.0°C Closed (From activation energy calculated for 42% RH) NOTE: 7.9 days when corrected for head-space effect) 4.7 d (#1) (92% RH 25.0°C Closed (From activation energy calculated for 42% RH)) 5.2 d (#1) (42% RH 25.0°C Closed (From activation energy calculated for 42% RH)) 1.4 d (#1) (32% RH 25.0°C Closed (From activation energy calculated for 42% RH))   Transformation products: Trimethylsilanol	2	Dow Corning Corporation (2014)
107-51-7	Octamethyltrisiloxane (L3)	Londo (#1)   Loamy silt (#2)	Half-life (DT50): 119.5 d (#1) (100% RH* 22.5°C Closed NOTE: 24 days when corrected for head-space effect) 6.19 d (#1) (92% RH 22.5°C Closed) 3.62 d (#1) (42% RH 22.5°C Closed) 1.48 d (#1) (32% RH 22.5°C Closed) 0.26 d (#2) (32% RH 22.5°C Closed) 19.9 d (#1) (4°C 42% RH Closed) 0.96 d (#1) (38.5°C 42% RH Closed) 96.3 d (#1) (100% RH 25.0°C Closed (From activation energy calculated for 42% RH) NOTE: 19.3 days when corrected for head-space effect) 4.98 d (#1) (92% RH 25.0°C Closed (From activation energy calculated for 42% RH)) 12.8 h (#1) (42% RH 25.0°C Closed (From activation energy calculated for 42% RH))   Transformation products: Dimethylsilanediol Trimethylsilanol 3, 3, 3, 1, 1-Pentamethyldisiloxanol	2	Dow Corning Corporation, 2010
141-62-8	Decamethyltetrasiloxane (L4)	Londo (#1)	106.6 d (#1) (100% RH 22°C Closed. NOTE: 56 days when corrected for head-space effect) 10 d (#1) (92% RH 22°C Closed) 4.5 d (#1) (42% RH 22°C Closed) 3.7 d (#1) (32% RH 22°C Closed) 29 d (#1) (4°C 42% RH Closed) 1.2 d (#1) (37°C 42% RH Closed) 80.6 d (#1) (100% RH 25.0°C Closed (From activation energy calculated for 42% RH) NOTE: 42 days when corrected for head-space effect) 7.6 d (#1) (92% RH 25.0°C Closed (From activation energy calculated for 42% RH).) 3.4 h (#1) (42% RH 25.0°C Closed (From activation energy calculated for 42% RH).) 2.8 d (#1) (32% RH 25.0°C Closed (From activation energy calculated for 42% RH).) % Degradation of test substance: Transformation products: Dimethylsilanediol Trimethylsilanol	2	Dow Corning Corporation (2014a)

 

The soil degradation/volatilisation study for L4 was conducted with a Londo soil from Bay City, Michigan, USA. 14C-labelled L4 was added to soil that was pre-conditioned at the desired relative humidity (RH) and incubated at different moisture levels and temperatures. Closed and open systems were used.

The rate of removal was greater as the soil became drier. Degradation half-lives (closed tubes) ranged from 3.7 d at 32% RH and at 22°C to 106.6 d at 100% RH and at 22°C (estimated to be 56 days when corrected for amount of L4 predicted to be in the headspace at this RH).

The correction for amount of L4 predicted to be in the headspace is made to degradation rates at 100% RH. It is thought by the authors of the study that the rates at this RH may be underestimated due to complication of soil/air partitioning in the test tubes during incubation (L4 present in headspace not available for biotic or abiotic degradation). For each sample in a closed tube at 100% RH, the estimated fraction, via soil-air partitioning calculation, of L4 actually distributed in soil is around 52.4%. The rest will be in the headspace. Assuming that attainment of soil/air partition equilibrium for L4 was rapid compared to its degradation, and that the system was always in equilibrium during the course of the degradation, the actual degradation rate for L4 in soil without headspace should be ~1.9 times larger than observed.

The identified degradation products were dimethylsilanediol and trimethylsilanol.

In open systems, volatilisation was the predominant process for removal of L4 from soil at 100% RH, with a volatilisation half-life of 4 days, almost 10 times faster than the degradation of L4 at the same moisture level in the closed system.

This study result is read-across to phenyl silsesquioxanes. The corresponding ultimate degradation products for phenyl silsesquioxanes are expected to be trimethylsilanol and phenylsilanetriol (final products of abiotic degradation).

References:

Xu S and Chandra G (1999). The fate of cyclic methylsiloxanes in soils: 2. Rates of degradation and volatilization. Environ. Sci. Technol. 33, 4034-4039

Dow Corning Corporation 2014a: Non-regulated study: Degradation of Decamethyltetrasiloxane (L4) (CAS No. 141-62-8) in Soils (study report), Testing laboratory: Dow Corning Corporation, Health and Environmental Sciences (HES), 2200 W. Salzburg Road, Auburn, MI 48611, Report no: HES Study number: 12204-101. Owner company; Dow Corning Corporation, 2200 W. Salzburg Road, Auburn, MI 48611, Report date: Jan 8, 2014

Dow Corning Corporation (2014b). Non-Regulated Study: Degradation of Hexamethyldisiloxane (HMDS) (CAS No. 107-46-0) in Soils. Testing laboratory: Dow Corning Corporation, Health and Environmental Sciences (HES), 2200 W. Salzburg Road, Auburn, MI 48611. Report no.: HES Study Number: 12231-101. Owner company: Dow Corning Corporation. Report date: 2014-01-08.

Dow Corning Corporation (2010a). Degradation of Octamethyltrisiloxane (L3) in Soils. Testing laboratory: Dow Corning Corporation, Health and Environmental Sciences (HES), 2200 W. Salzburg Road, Auburn, MI 48611. Report no.: 11297-102. Owner company: SEHSC. Report date: 2010-06-10.