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Biodegradation in soil

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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:
comparable to guideline study with acceptable restrictions
Principles of method if other than guideline:
L3 degradation was studied in two sets of experiments: one at constant temperature and another at constant humidity, in both open and closed systems. 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.
Two different soils were used: Londo soil from Bay City, Michigan, USA and Loamy silt soil from Buxton, Derbyshire, UK.
GLP compliance:
no
Remarks:
This study was conducted using best available scientific methodology. However, this was a non-regulated 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 classification:
not specified
Soil no.:
#1
Soil type:
other: Londo soil
% Clay:
22
% Silt:
28
% Sand:
50
% Org. C:
2.4
pH:
7.6
Soil no.:
#2
Soil type:
silt loam
% Clay:
22
% Silt:
56
% Sand:
22
% Org. C:
3.4
pH:
6.6
Details on soil characteristics:
Londo soil from Bay City, Michigan, USA
Loamy silt soil from Buxton, Derbyshire, UK.
Air-dried and stored in cold room at 6°C before use.
Duration:
> 6 - < 60 d
Initial conc.:
0.01 g/kg soil d.w.
Based on:
test mat.
Parameter followed for biodegradation estimation:
test mat. analysis
Details on experimental conditions:
Air-dried soil (5g each) was weighed into pre-weighed 25-ml Teflon tubes. The soil in the tubes was pre-conditioned for at least one week in four containers at different humidities controlled by saturated salt solutions.
Following pre-conditioning, each soil sample was spiked with 10 µg/g (dry weight basis). The spiking solution was dropped to multiple positions in the surface layer of the soil. Immediately following addition of test material, the tubes were capped and vortexed for 5 minutes. The tubes were then purged with the moisture-controlled air for 1 minute and capped for incubation for the closed-system experiments and opened in a constant moisture chamber for the open-system experiments.
At appropriate sampling times, whole test tubes (in duplicates) were sacrificed for analysis.
Soil No.:
#1
DT50:
119.5 d
Type:
(pseudo-)first order (= half-life)
Remarks on result:
other: 100% RH 22.5°C Closed. NOTE: 24 days when corrected for head-space effect
Soil No.:
#1
DT50:
6.19 d
Type:
(pseudo-)first order (= half-life)
Remarks on result:
other: 92% RH 22.5°C Closed
Soil No.:
#1
DT50:
3.62 d
Type:
(pseudo-)first order (= half-life)
Remarks on result:
other: 42% RH 22.5°C Closed
Soil No.:
#1
DT50:
1.48 d
Type:
(pseudo-)first order (= half-life)
Remarks on result:
other: 32% RH 22.5°C Closed
Soil No.:
#2
DT50:
0.26 d
Type:
(pseudo-)first order (= half-life)
Remarks on result:
other: 32% RH 22.5°C Closed
Soil No.:
#1
DT50:
19.9 d
Type:
(pseudo-)first order (= half-life)
Remarks on result:
other: 4°C 42% RH Closed
Soil No.:
#1
DT50:
0.96 d
Type:
(pseudo-)first order (= half-life)
Remarks on result:
other: 38.5°C 42% RH Closed
Soil No.:
#1
DT50:
96.3 d
Type:
(pseudo-)first order (= half-life)
Remarks on result:
other: 100% RH 25.0°C Closed (From activation energy calculated for 42% RH). NOTE: 19.3 days when corrected for head-space effect
Soil No.:
#1
DT50:
4.98 d
Type:
(pseudo-)first order (= half-life)
Remarks on result:
other: 92% RH 25.0°C Closed (From activation energy calculated for 42% RH).
Soil No.:
#1
DT50:
12.8 h
Type:
(pseudo-)first order (= half-life)
Remarks on result:
other: 42% RH 25.0°C Closed (From activation energy calculated for 42% RH).
Transformation products:
yes
No.:
#1
No.:
#2
No.:
#3
Details on transformation products:
For any 14C-L3-spiked soil sample, most radioactivity was recovered in THF and acidic water extracts. Reversed phased HPLC/RAM analysis indicated the three degradation products detailed above. The test compound was mainly labeled in the dimethylsiloxyl unit, with only a small fraction of 14C atoms from the trimethylsilyl end group. The reletive peak areas in the HPLC/RAM chromatograms were consistent with the expected relative radioactivities of the hydrolysis products.

Amount of non-extractable organosilicon species increased linearly with incubation times with little difference observed for the two soils tested, but was very sensitive to humidity and incubation temperature. As the temperature increased or humidity decreased, the slopes of linear relation between percentage of non-extractable and the incubation time increased.. The nature of the non-extractable fraction is not completely understood. However, the coupling of the increase in the amount of non-extractable organosilicon compounds with the decreased amount of monomer diol as the incubation proceeded suggested that the non-extractable fraction may be the strongly sorbed monomer diol.
Details on results:
Average total spiked radioactivity recovery ranged from 99.1 ± 2.2% to 100.8 ± 1.5% in closed systems.
In open systems, loss of spiked radioactivity due to volatilisation could be up to 1/3 at 92% RH, and greater than 95% at 100% RH. Volatilisation loss in open systems was highly sensitive to moisture levels. Volatilisation was not significant at 32% RH (the recovery in both open and closed systems were close to 100%), and the degradation rate was rapid at this RH in both open and closed systems. In contrast, more than 60% of spiked radioactivity was lost from the open systems within one day at 100% RH (i.e. volatilisation t1/2 <1 day), and more than 90% was lost with less that 4 days of incubation. In the closed systems, an average recovery of 99.1% was found over a 2-month period for the same Londo soil at 100% RH.

For closed systems, the first-order kinetics described the decrease of L3 concentration in both soils very well.

The UK Loamy soil degradation at 32% RH was more rapid than that in the Michigan Londo soil under that same conditions.

Rates at high humidity may be underestimated due to complication of soil/air partitioning in the test tubes during incubation (L3 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 L3 actually distributed in soil at 100% RH will be around 18%. The rest will be in the headspace. Assuming that attainment of soil/air partition equilibrium for L3 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 L3 in soil without headspace should be ~5 times larger than observed. In addition, the expected large fraction of L3 in headspace above wet soil also explains the rapid volatilisation loss of L3 from soil at 100% RH observed in open system experiments. The fact that the degradation rate of L3 is much slower than volatilisation rate under this condition suggested that volatilisation may be more important for removal of L3 under wet condition in the field.

First-order kinetics also describes the decrease of L3 concentration at the two other temperatures (4°C and 38.5°C at 42% RH) very well. As expected, the rate constants derived increased substantially with the increase of the incubation temperatures. The logarithm of the rate constants was linearly related to the reciprocal of the incubation temperature, with a slope corresponding to an activation energy of 63.5 kJ/mol. If the activation energy is assumed the same for other soil humidities as that at 42% RH, the degradation rate constant for L3 at 25°C at various soil humidities can be calculated. The calculated half-lives for Michigan soil at 25°C are 12.8 hours at 42% RH, 4.98 days at 92% RH and 96.3 days at 100% RH.

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 L3 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 presence of L3 in agricultural soil should be shorter than that at 100% RH determined in closed systems in the current study.

Conclusions:
Soil degradation rates were determined in a reliable study conducted according to generally accepted scientific principles.
The soil degradation/volatilisation study for L3 was conducted with a Londo soil from Bay City, Michigan, USA and a UK Loamy silt soil. 14C-labeled L3 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. In Michigan Londo soil, degradation half-lives (closed tubes) ranged from 1.48 d at 32% RH and at 22.5°C to 119.5 d at 100% RH and at 22.5°C (estimated to be 24 days when corrected for amount of L3 predicted to be in the headspace at this RH).
The degradation products were Dimethylsilanediol, Trimethylsilanol and 3, 3, 3, 1, 1-pentamethyldisiloxanol.
In open systems, volatilisation was the predominant process for removal of L3 from soil at 100% RH with a volatilisation half-life of <1 day, much faster than the degradation of L3 at the same moisture level in the closed system.
In loamy silt soil, the degradation half-life (closed tubes) was 0.26 d at 32% RH and at 22.5°C.
Endpoint:
biodegradation in soil, other
Type of information:
read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
key study
Justification for type of information:
Please refer to the endpoint summary for justification of read-across.
Reason / purpose:
read-across source
Soil No.:
#1
DT50:
1.48 d
Type:
(pseudo-)first order (= half-life)
Temp.:
22.5 °C
Remarks on result:
other:
Remarks:
at 32% RH and at 22.5°C to 119.5 d at 100% RH and at 22.5°C (estimated to be 24 days when corrected for amount of L3 predicted to be in headspace at this RH)
Transformation products:
yes
No.:
#1
No.:
#2
No.:
#3

Description of key information

Degradation in soil: The following results are read-across from the structurally-related substance L3. Michigan Londo soil, half-lives (closed tubes) 1.48 d at 32% RH and at 22.5°C to 119.5 d at 100% RH and at 22.5°C (estimated to be 24 days when corrected for amount of L3 predicted to be in headspace at this RH). UK loamy silt soil, half-life (closed tubes) 0.26 d at 32% RH and at 22.5°C. The degradation products were dimethylsilanediol, trimethylsilanol and 3, 3, 3, 1, 1-pentamethyldisiloxanol.  In open systems at higher RH, volatilisation became the predominant removal process, with a volatilisation half-life <1 day at 100% RH and at 22.5°C. 
In exposure modelling (EUSES 2.1.2) a half-life value of 10 days at 20°C will be used, based on the value of 6.19 d (#1) (92% RH 22.5°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 soil stability data available for Vi2-L2, therefore good quality data for the structurally-related substance, octamethyltrisiloxane (L3, CAS 107-51-7), have been read across in a Category approach.

Vi2-L2 and L3 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 Kow>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 sediment 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.

Vi2 -L2 and the source substance L3 are linear siloxanes with two silicon atoms and three silicon atoms respectively, alternated by oxygen atoms. In L3, the Si atoms are fully methyl substituted, whereas in Vi2-L2 each Si atom is bonded to one vinyl group and two methyl groups. L3 and Vi2-L2 possess similar physicochemical properties. A comparison of the key physico-chemical properties is presented in the table below. Both substances have negligible biodegradability and similar moderate hydrolysis rates.

Table3.1.1Key physico-chemical properties of Vi2 -L2 and surrogate substance L3

Property

Vi2 -L2 (2627 -95 -4)

L3 (107 -51-7)

Molecular weight

186.4

236.54

Log Kow

5.4

6.60

Log Koc

3.2

4.34

Water solubility (mg/l)

2.1E-01(at 20°C)

3.4E-02 (at 23°C)

Vapour pressure at 25°C (Pa)

1.7E+03

5.3E+02

Hydrolysis half- life at pH 7 (h)

140

329

Given the similar properties and structural similarities, it is considered valid to read across sediment toxicity data from L3 to Vi2 -L2.

 

Table 4.1.6presents all of the available data for removal of substances from soil within the Siloxane CategoryIn 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 thatin generalthe 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. The read-across substance, L3, is more adsorbing than Vi2-L2 and, based on a slower hydrolysis rate, degradation in soil is likely to be slower for L3 than for Vi2-L2. It is therefore considered valid to read across the results for L3 to fill the data gap for the registered substance, as a worst-case scenario.

Table 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 & 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 & 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 & 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)

30.1 d (#1) (4°C 42% RH Closed)

4.5 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(2014)

 

The soil degradation/volatilisation study for L3 was conducted with a Londo soil from Bay City, Michigan, USA and a UK Loamy silt soil.14C-labelled L3 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. In Michigan Londo soil, degradation half-lives (closed tubes) ranged from 1.48 d at 32% RH and at 22.5°C to 119.5 d at 100% RH and at 22.5°C (estimated to be 24 days when corrected for amount of L3 predicted to be in the headspace at this RH).

The degradation products were dimethylsilanediol, trimethylsilanol and 3, 3, 3, 1, 1-pentamethyldisiloxanol (the intermediate hydrolysis product of L3).

In open systems, volatilisation was the predominant process for removal of L3 from soil at 100% RH with a volatilisation half-life of <1 day, much faster than the degradation of L3 at the same moisture level in the closed system.

In loamy silt soil, the degradation half-life (closed tubes) was 0.26 d at 32% RH and at 22.5°C.