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Environmental fate & pathways

Biodegradation in soil

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Endpoint:
biodegradation in soil: simulation testing
Type of information:
experimental study
Adequacy of study:
weight of evidence
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. This study is of limited use due to uncertainty in interpretation of the results (discussed further in Overall remarks).
Principles of method if other than guideline:
Nine soils (some sludge-amended) were tested for their ability to biodegrade 14C-Dimethylsilanediol (DMSD) and 14C-Methylsilanetriol (MST). 5-6g soil (moist) in plastic vials were spiked with DMSD or MST. Each vial placed in centrifuge bottle with another vial containing KOH, which served as CO2 trap. Soils monitored for the production of 14-CO2 over 63d.
GLP compliance:
not specified
Test type:
laboratory
Radiolabelling:
yes
Oxygen conditions:
aerobic
Details on soil characteristics:
SOIL COLLECTION AND STORAGE

- Geographic location: Two soils (Cohotah and Sloan) from Chippewa Nature Centre, Midland, Michigan. Seven soils, both sludge-ammended and non-sludge amended, from farms in Ohio (Groveport, Braun and Kenton).

Details on experimental conditions:
2. EXPERIMENTAL DESIGN

- Soil (g/replicate): 5-6g moist soil

- Control conditions, if used: Sterilized controls - autoclaved and sodium azide (50ul of a 10% solution).

- No. of replication treatments: All soils, live and sterile, were prepared in duplicate.

- Test apparatus (Type/material/volume): 5-6g moist soil in plastic vial and spiked with radiolabelled TS.

- Details of traps for CO2 and organic volatile, if any: Plastic vials containing soil samples placed in a 300 ml centrifuge bottle with another vial containing 5ml of 0.2 M KOH to serve as a CO2 trap.

Test material application
- Volume of test solution used/treatment: Soil samples spiked with either 250 ul of a 2000ppm solution [14C]-DMSD (soil 100ppm wrt DMSD) or 65.8ul of a 760 ppm solution [14C]-MST (soil 10ppm wrt MST).

Experimental conditions (in addition to defined fields)
- Moisture maintenance method: Moisture level adjusted either by adding sterile, deionized water or drying the soil at room temperature, so that after all spiking materials were added the soil moisture would be equivalent to the level at 1 bar.

4. SUPPLEMENTARY EXPERIMENTS: (see Any other information on materials and methods, below)

5. SAMPLING DETAILS
- Sampling intervals: Soils were monitored for the production of 14-CO2 on days 4, 11, 17, 24, 42 and 63.
- Sampling method for soil samples: Radioactivity balance was determined at the completion of experiments. Soil samples extracted with CaCl2 and HCl. Extracts analyzed by LSC; soil dried for a week and then combusted to determine 14-C counts.
- Method of collection of CO2 and volatile organic compounds: In soil bottles, the entire KOH trap removed, capped and a new trap added.
Soil No.:
#1
% Degr.:
ca. 0.16 - ca. 1.9
Parameter:
CO2 evolution
Remarks on result:
other: See 'Remarks'
Remarks:
An initial soil biodegradation of 0.16-1.9% during first four days of incubation per month was obtained. It was found that previous addition of sludge to soil had no statistically significant effect on the rate of biodegradation.
Transformation products:
no
Evaporation of parent compound:
not measured
Volatile metabolites:
not measured
Residues:
not measured

DMSD (100ppm): Rates varied depending on soil location. Initial rate during first four days 0.16 - 1.9% per month. Total conversion after 63d 0.33 - 1.35% (initial rate of biodeg slowed over time). Previous addition of sludge to the soil had no statistically significant effect on the rate of biodegradation.

MST (10ppm): Initial rate during first four days for all soils around 0.38% per month. Total conversion after 63d 0.08 - 0.18% (initial rate of biodeg slowed over time). Since none of these rates is >0.4% (the level of possible impurity in [14C]-MST) it is difficult to make any conclusions about the biodeg of [14C]-MST in soil.

MST (100ppm): Initial rate during first six days 0.34% per month. Total conversion after 45d 0.13% (initial rate of biodeg slowed over time). In liquid culture, however, the biodegradation rates of DMSD and MST were similar. These results suggest that rate of MST biodeg in soil is slow due to poor bioavailability of MST due to adsorption to soil.

Adsorption experiment: 50ppm DMSD ~5% adsorbed to soil in first hr, after which no additional loss (up to 70hrs). 10 ppm MST 10% adsorbed to soil in first hr, ~25% adsorbed to soil over 45hrs, after which no additional loss (up to 70hrs). 50ppm MST 10% adsorbed to soil in first hr, 90% adsorbed to soil in 620 hrs. (Control = No loss DMSD to glass vial; 1% or 5% loss MST at 50 and 10ppm.)

Liquid culture: F. oxysporum was able to biodegrade MST at a rate comparable to or faster than the biodeg of DMSD. Arthrobacter was able to biodegrade MST at a rate slightly slower than DMSD.

Rate-determining soil simulation biodeg experiment on DMSD. 1ppm DMSD average initial rate in first 7d 1.4% per month. 10ppm DMSD average initial rate 0.9% per month. 100ppm DMSD average initial rate 0.6% per month. Total biodeg at 59d 1.8%, 1.8% and 1.1% respectively.

Conclusions:
An initial rate of soil biodegradation (during first four days of incubation) of 0.16 - 1.9% per month was determined in a reliable study conducted according to generally accepted scientific principles. It was found that previous addition of sludge to soil had no statistically significant effect on the rate of biodegradation.
Endpoint:
biodegradation in soil: simulation testing
Type of information:
experimental study
Adequacy of study:
weight of evidence
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. This study is of limited use due to uncertainty in interpretation of the results (discussed further in Overall remarks).
Principles of method if other than guideline:
Four soils were tested for their ability to biodegrade 14C-dimethylsilanediol (DMSD). 50g moist soil spiked with radio-labelled TS and ammonium chloride (added to eliminate possibility of nitrogen limitation in the soils) were mixed together in a 500-ml Gledhill flask. Soils monitored for the production of 14-CO2 over 290d.
GLP compliance:
not specified
Test type:
laboratory
Radiolabelling:
yes
Oxygen conditions:
aerobic
Details on soil characteristics:
SOIL COLLECTION AND STORAGE

- Geographic location: Four soils taken from either a residential garden or a sludge disposal area. Two soils, a sandy soil from Santa Barbara, California, and a mixture of forest and residential garden soils and composted cow manure from Cobleskill, N.Y., were not believed to have been previously exposed to PDMS. The remaining two soils were taken from a residential garden (Guilderland, N.Y.) and a sludge disposal area (Glendale, Ohio).
Details on experimental conditions:
2. EXPERIMENTAL DESIGN
- Soil (g/replicate): 50g moist soil

- Control conditions, if used: Sterilized controls of both the Cobleskill and the Santa Barbara soils - autoclaved and sodium azide (2ml of a 2.5% solution).

- No. of replication treatments: Duplicates were prepared for each of the four soils.

- Test apparatus (Type/material/volume): 50g moist soil spiked with radiolabelled test substance and 100ul of ammonium chloride (50g/l; added to eliminate possibility of nitrogen limitation in the soils) were mixed together in 500-ml Gledhill flasks.

- Details of traps for CO2 and organic volatile, if any: The Gledhill CO2 traps were filled with 10 ml of 0.2 M KOH, although in some cases this was increased to 2 M KOH.

Test material application
- Volume of test solution used/treatment: Soil samples spiked with 2.5 ml of a 2000ppm solution [14C]-DMSD.

3. OXYGEN CONDITIONS
- Methods used to create the an/aerobic conditions: Oxygen was supplied by diffusion. 12-in 20-gauge needle, connected to an oxygen manifold, was inserted through the septum at the top of the trap.

4. SUPPLEMENTARY EXPERIMENTS: Liquid culture experiments.

5. SAMPLING DETAILS
- Sampling method for soil samples: Radioactivity balance was determined at the completion of experiments. Soil samples extracted with THF, HCl and KOH. Extracts analyzed by LSC; soil combusted to determine 14-C counts.

- Method of collection of CO2 and volatile organic compounds: The entire contents or an aliquot of the KOH trap were withdrawn periodically with a sterile needle. Fresh KOH was added to the traps immediately.
Soil No.:
#1
% Degr.:
0.17
Parameter:
CO2 evolution
Sampling time:
30 d
Remarks on result:
other: Glendale soil. After 255 d, 1.1% had been converted
Soil No.:
#2
% Degr.:
0.41
Parameter:
CO2 evolution
Sampling time:
30 d
Remarks on result:
other: Santa Barbara soil. After 290d, 2.64% had been converted
Soil No.:
#3
% Degr.:
0.52
Parameter:
CO2 evolution
Sampling time:
30 d
Remarks on result:
other: Cobleskill soil. After 290d, 1.5% had been converted
Soil No.:
#4
% Degr.:
2.1
Parameter:
CO2 evolution
Sampling time:
30 d
Remarks on result:
other: Guilderland soil. After 244d 9% had been converted
Transformation products:
no
Evaporation of parent compound:
not measured
Volatile metabolites:
not measured
Residues:
not measured

Guilderland soil: 2.1% of the total 14-C counts added was converted to 14CO2 in the first month. After 244d 9% had been converted.

Santa Barbara: 0.41% of the total 14-C counts added was converted to 14CO2 in the first month. After 290d, 2.64% had been converted.

Cobleskill: 0.52% of the total 14-C counts added was converted to 14CO2 in the first month. After 290d, 1.5% had been converted.

Glendale: 0.17% of the total 14-C counts added was converted to 14CO2 in the first month. After 255 d, 1.1% had been converted.

The rate of conversion in all of the soils slowed over the course of the experiments

Mixed liquid culture experiments indicated that a fungus (Fusarium oxysporum Schlechtendahl) was likely responsible for the conversion of DMSD to CO2 in liquid cultures inoculated with Guilderland soil, and a bacteria (Arthrobacter sp.) for the conversion in liquid cultures inoculated with Santa Barbara soil.

Conclusions:
An initial rate of soil biodegradation of 0.17 - 2.1% per month was determined in a reliable study conducted according to generally accepted scientific principles.
Endpoint:
biodegradation in soil: simulation testing
Type of information:
experimental study
Adequacy of study:
weight of evidence
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:
200-cs 14C-PDMS (1, 10, 100 mg/kg) was placed in 50g Londo sandy clay loam, and the soil was dried to generate the desired silanols in situ. The soil was remoistened and incubated at 25°C, 12% moisture, and 21% O2. Soils monitored for the production of 14-CO2 (KOH trap in side flask), other volatiles (carbon plug) over 20 weeks. In addition, extracts from extractions with THF, HCL and KOH were sampled for total 14-C (THF extracts also analysed via HPLC-GPC).
GLP compliance:
no
Test type:
laboratory
Radiolabelling:
yes
Oxygen conditions:
aerobic
Details on soil characteristics:
SOIL COLLECTION AND STORAGE
- Geographic location: London sandy clay loam (0 to 5 cm) was collected in late April of 1992 from a farm in Bay County, Michingan.

- Soil preparation (e.g., 2 mm sieved; air dried etc.): Sieved (2mm) and stored moist at 4degC until use in July/December.
Details on experimental conditions:
2. EXPERIMENTAL DESIGN
- Soil (g/replicate): 50g moist soil weighed into 11 incubation flasks

- Control conditions, if used: Silanol degradation in sterilised soils was not examined because the process of generating silanols required that flasks be open to the air for 2 weeks as the soil dried, and sterility would have been impossible to maintain.

- Details of traps for CO2 and organic volatile, if any: 100 ml 0.2M KOH solution in side flask. A 1.5 x 1.5-cm plug (activated cabon on a polyester support) was inserted in the neck to trap other volatiles.

Test material application
- Volume of test solution used/treatment: 0.5 ml of spiking solution added dropwise to the surface of the soil in incubation flasks to yield 100 mg/kg of PDMS in soil. Soil stirred and flasks left open for 2 weeks at 25 deg C for the soil to dry and generate silanols.

Experimental conditions (in addition to defined fields)
- Continuous darkness: Yes

- Other information: At 7 weeks, 1g of ground, dried alfalfa was mixed into the soil in three of the flasks, 1 ml water added, and the KOH traps replaced. A second incubation was begun after obtaining results from the first incubation. Soil added (as above) to 24 flasks, and 1 g alfalfa added to 12 of the flasks. The flasks were maintained moist for 2 weeks to allow microbial activity on the alfalfa to subside, after which PDMS was added as above to yield either 1, 10 or 100 mg PDMS/kg soil. Flasks were then allowed to dry for 2 weeks, remoistened, and incubated as above for 0, 3, 9 and 20 weeks (carbon plugs not used).

3. OXYGEN CONDITIONS (delete elements as appropriate)
- Methods used to create the an/aerobic conditions: Flasks attached to O2 manifold.

4. SUPPLEMENTARY EXPERIMENTS:

5. SAMPLING DETAILS
- Sampling intervals for soil only: 0, 1, 2, 4, 7, 10, 13 and 19 weeks.
- Sampling intervals for soil with alfalfa: 10, 13 and 19 weeks

- Sampling method for soil samples: Aliquotes (5g) of soil were extracted 4 times with THF, and the pooled extracts (25 ml) were sampled for total 14-C, while additional extracts (25 ml) were removed for HPLC-GPC. Soils were then extracted overnight with 20 ml of 0.1 M HCL. Certain soils showed low recoveries and so were extracted overnight with 20 ml of 0.5 M KOH. All extracts were sampled in duplicate for total 14C. Soils were then air-dried and ashed. All samples were counted on a Packard 2500 scintillation counter using external standards for quench correction.

- Method of collection of CO2 and volatile organic compounds: Carbon plugs were extracted 4 times with THF for volatile organic 14-C. The KOH trapping solutions were sampled and 14-CO2 was determined via BaCl2 precipitation method.
Soil No.:
#1
Remarks on result:
other: See 'Remarks'
Remarks:
DMSD could biodegrade and volatilise from soil. However, rates of these processes were not well defined because the 14C-DMSD was generated in-situ from 14C-PDMS and the soil also contained incompletely hydrolysed 14C-PDMS. Moreover, the volatilisation rate inside a flask is probably lower than would be expected in open air
Transformation products:
no

First incubation: HPLC-GPC analysis indicated that after two weeks the PDMS fragmented into a mixture of large oligomers and low-molecular-weight molecules, with DMSD being the main breakdown product. Analysis via KOH traps implied that the 14C-Si bond was broken in the soil. Other pathways of loss from the soil solution included volatilisation and binding to soil components.

Over half of the 14C was in stabilised forms (large oligomers, HCL-extractable) at the beginning of the microbial degradation phase, the 14C found as volatile silanols, as 14CO2, and as humus-bound (KOH-extractable) probably came from only 40 to 50% of the applied radioactivity. With this in mind, the amount of 14C in the above three fractions after 4 months may represent from 25 to 50% of the original dimethylsilanediol.

Second incubation: HPLC-GPC analysis indicated that PDMS degraded more slowly in soils with alfalfa - probably the polymer has sorbed to the alfalfa preventing it from degrading. This meant that when soils were remoistened to begin the microbial degradation phase, the soils with alfalfa contained much less DMSD than did soils without. Therefore, the data was normalised according to the amount of small silanols actually present in the soil before comparison with data from soils without alfalfa.

At 1 and 10 mg/kg the soils with alfalfa produced higher rates of 14C volatilisation than did soil-only treatments, although at 100 mg/kg the increase was not clear. The secretion of H2O2 and enzymes by some fungi during cellulose metabolism may be responsible for the degradation fo silanols.

Conclusions:
It was determined, in a reliable study conducted according to generally accepted scientific principles, that DMSD could biodegrade and volatilise from soil. However, rates of these processes were not well defined because the 14C-DMSD was generated in-situ from 14C-PDMS and the soil also contained incompletely hydrolysed 14C-PDMS. Moreover, the volatilisation rate inside a flask is probably lower than would be expected in open air.
Endpoint:
biodegradation in soil: simulation testing
Type of information:
experimental study
Adequacy of study:
weight of evidence
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:
50g moist soil (four soils from sites under different vegetation) incubated in biometer flasks with 0.8 ml 14C-DMSD (89 µg/ml), to give 1mgDMSD/kg soil, for 6months. Production of 14CO2 was followed via KOH trap in sidearms, and carbon plugs were used to trap volatile DMSD. The incorporation of 14C into the soil was examined via CaCl2, HCL and KOH extractions.
GLP compliance:
no
Test type:
laboratory
Radiolabelling:
yes
Details on soil characteristics:
SOIL COLLECTION AND STORAGE
- Geographic location: Collected from near Midland, MI, in the summer of 1994. Three (Cohoctah - permanent grass - loam, Pipestone - pine woods - sand, and Sloan - beech, maple woodland - sandy loam) from Chippewa Nature Centre, one (Londo) from Cornfield - sandy clay loam.

- Soil preparation (e.g., 2 mm sieved; air dried etc.): Sieved (2mm) and refridgerated (4 degC) until use.
Details on experimental conditions:
2. EXPERIMENTAL DESIGN
- Soil (g/replicate): 50g

- Control conditions, if used: Two additional flasks of Sloan soil were incubated as for test vessels but with addition of 0.5 ml THF as a microbial inhibitor. One 50g sample per soil was ammended with DMSD as above and refrigerated until concurrent analysis with other soils.

- No. of replication treatments: 2

- Test apparatus (Type/material/volume): 50 g moist soil and spiking solution in 16 biometer flasks.

- Details of traps for CO2 and organic volatile, if any: Carbon pugs inserted into necks of flasks to trap volatile DMSD. 100 ml of 0.2 M KOH placed in the sidearms to trap 14CO2.

Test material application
- Volume of test solution used/treatment: 0.8 ml of aqueous DMSD (89 µg/ml) added and soil stirred to yield about 1 mg DMSD/kg soil. 8 flasks also amended with 2g of the appropriate plant litter.

Experimental conditions (in addition to defined fields)
- Continuous darkness: Yes

3. OXYGEN CONDITIONS (delete elements as appropriate)
- Methods used to create the an/aerobic conditions: Flasks were attached to an oxygen (O2) supply manifold.

5. SAMPLING DETAILS
- Sampling intervals: Plugs and KOH solutions removed every 3 weeks for analysis for total 14C and 14CO2 (scintillation counter).

- Sampling method for soil samples: Duplicate 5g aliquots extracted with CaCl2,HCl and KOH for 14C analysis. Soils were air-dried and combusted for total 14C. Reversed-phase HPLC was performed on selected water and HCL extracts.

- Method of collection of CO2 and volatile organic compounds: Fresh plugs and KOH solution were placed in the flasks after each sampling.
Soil No.:
#1
% Degr.:
ca. 0.36 - ca. 0.42
Parameter:
CO2 evolution
Remarks on result:
other: A soil biodegradation of 0.36 - 0.42% per week was obtained for Pipestone soil
Soil No.:
#2
% Degr.:
ca. 0.42 - ca. 0.5
Parameter:
CO2 evolution
Remarks on result:
other: A soil biodegradation of 0.42 - 0.50% per week was obtained for Londo soil
Soil No.:
#3
% Degr.:
ca. 0.64 - ca. 0.74
Parameter:
CO2 evolution
Remarks on result:
other: A soil biodegradation of 0.64 - 0.74% per week was obtained for Cohoctah soil
Soil No.:
#4
% Degr.:
ca. 1.59 - ca. 1.7
Parameter:
CO2 evolution
Remarks on result:
other: A soil biodegradation of 1.59 - 1.70% per week was obtained for Sloan soil
Transformation products:
no

Rates of 14CO2 production were calculated from the first 12 weeks of data since studies show that microbial degradation rates decrease over time under controlled conditions, meaning that initial rates are most representative of field conditions. Rates varied by roughly a factor of 4 for the different soils. Rates increased with microbial biomass for Pipestone (0.36 to 0.42% per week), Londo (0.42 to 0.50% perweek) and Cohoctah soils (0.64 to 0.74% per week). A higher rate of 14CO2 production was found for the Sloan soil (1.59 to 1.70% per week) however, which has the lowest microbial biomass. This suggests that these organisms may be more active in degrading DMSD than organisms in the other soils.

After 30 weeks the 14C was partitioned among numerous fractions. Acid extractable 14C is interpreted as soil bound silanols; HPLC analysis indicated DMSD:degradate(s). Base extractable 14C may represent 14C which has been sequestered in the soil humus; HPLC analysis indicated DMSD: degredate(s).

Conclusions:
An initial rate (12 weeks) of soil biodegradation of 0.36 - 1.70% per week was determined in a reliable study conducted according to generally accepted scientific principles.

Description of key information

Initial rates of degradation in soil simulation tests ranged from 0.16 to 2.1% per month

Key value for chemical safety assessment

Additional information

No soil simulation test data are available for methylsilanetriyl triacetate. However, available hydrolysis data indicate that this substance hydrolyses very rapidly in contact with water to form methylsilanetriol and acetic acid.

Soil simulation studies, which are considered reliable, are available for dimethylsilanediol and methylsilanetriol (Lehmann et al. (1994), Lehmann et al. (1998), Sabourin et al. (1996a and b)).

These studies were conducted according to generally accepted scientific principles, and are used as weight of evidence that degradation in soil does occur but not to a significant extent. Initial rates of soil degradation from these studies are considered to be the data of most use, and most representative of field conditions, as rates tended to slow over the course of the experiments. Initial rates of degradation ranged from 0.16 to 2.1% per month. The decrease in rates of degradation over the course of the experiments may be due to some form of binding of organosilane compounds to soil, as indicated by HPLC analysis of HCl-extractable fractions from the soil in some of the studies.

It is considered relevant to read-across the soil degradation study for dimethylsilanediol to methylsilanetriol because both silanol hydrolysis products are structural analogues. Dimethylsilanediol contained two hydroxy groups and two methyl (alkyl) groups attached to silicon while methylsilanetriol has three hydroxy groups and one methyl (alkyl) group attached to silicon. Both dimethylsilanetriol and methylsilanetriol contained the same function groups, Si-OH. Both are expected to have similar physicochemical properties such as high water solubility, low log Kow and low vapour pressure.