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Environmental fate & pathways

Biodegradation in water and sediment: simulation tests

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Endpoint:
biodegradation in water and sediment: simulation testing, other
Type of information:
read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
supporting study
Justification for type of information:
Please refer to the justification for grouping of substances provided in IUCLID Section 13.
Reason / purpose for cross-reference:
read-across source
Remarks on result:
other: no degradation observed
Transformation products:
no
Details on results:
For each condition, the highest amount of radioactivity is recovered in the liquid phase, even at the end of the incubation. For the Aerobic Surface Water condition, the radioactivity recovered in the liquid phase does not significantly decrease with time. This can be expected since little microbial activity is assumed to be present in these reactors. For all other conditions, the radioactivity recovered from the liquid phase slowly decreases. Only under the Ferri-Reducing and Methanogenic conditions, the decrease seems to be limited to the first period of 90 days.
At the end of the 13-18 months of incubation, for all conditions, there is a very low but significant cumulated release of 14C marked molecules in the gas samples. The detected non-condensable radioactivity can be attributed to volatile 14C-DMSD that was not retained by the cold trap. Indeed, even at -70 °C, DMSD has a positive vapor pressure (unknown value), and some molecule can escape.
At this stage of the incubation, no clear trend emerges yet from the evolution of radioactivity recovered from the solid phase with time; neither for the total radioactivity nor for the ratio between 14C-DMSD by-products extractable and not extractable by THF.
The HPLC analysis aimed at detecting the presence of different 14C-metabolites of 14C-DMSD. However the results don’t show any evidence for the presence of any metabolite other than 14C-DMSD in the liquid phase, in the THF extract or in the acid extract of the residual solids.
Conclusions:
There is no evidence for any significant degradation or conversion of the source substance Dimethylsilanediol (DMSD) in aquatic or terrestrial conditions representative for a diversity of environmental conditions. As explained in the read across justification provided in section 13, no biodegradation in water or soil is expected for the target substance.
Endpoint:
biodegradation in water: sediment simulation testing
Data waiving:
other justification
Justification for data waiving:
other:
Transformation products:
yes
Remarks:
Degradation is principally expected via abiotic transformation (aqueous). The transformation products expected are those identified in IUCLID section 1.10 "Transformation of the registered substance".
No.:
#1
No.:
#2
Endpoint:
biodegradation in water: simulation testing on ultimate degradation in surface water
Data waiving:
other justification
Justification for data waiving:
other:
Transformation products:
yes
No.:
#1
No.:
#2
Endpoint:
biodegradation in water and sediment: simulation testing, other
Type of information:
experimental study
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Reason / purpose for cross-reference:
reference to same study
Qualifier:
no guideline followed
Principles of method if other than guideline:
The degradation of Radio-labeled 14C-DMSD was tested in closed microcosms mimicking eight different environmental matrices. The matrices were surface water (Aerobic Surface water), Composite soil suspended in water (Aerobic soil slurry), Composite sludge slurry (Aerobic sludge), Composite sludge slurry + calcium nitrate (Anoxic sludge), Composite sludge slurry + goethite (Ferri-reducing sludge), Composite sludge slurry + iron(II)sulfate (Sulfidogenic sludge), Composite sludge slurry (Methanogenic sludge), Bottom 1 cm layer of solid organic sludge, covered by a 5 cm layer of solid composite soil, at field capacity. Bottom 0.5 cm layer saturated with water (Gradient microcosm). The microbial communities were supplied from environments where microorganisms have high chance to be in contact and possibly degrade silanols. The degradation was followed measuring the 14CO2 and 14CH4, in the headspace and the exhaust gas of the microcosms. The reactors were incubated for 13 to 18 months. Sacrifices were performed at 4 times: directly after starting the incubation to characterize the initial situation (d0), after 3 months of incubation (d87- 94), after 6 months of incubation (d180-196) and after a total incubation of 13 months to 18 months (d398-557) depending on the tested condition.
GLP compliance:
no
Remarks:
Study was not conducted for registration purposes. Thus no GLP compliance was necessary.
Oxygen conditions:
aerobic/anaerobic
Inoculum or test system:
other: mixture of surface water, sludge, leachate and soil
Details on inoculum:
- Source of inoculum/activated sludge (e.g. location, sampling depth, contamination history, procedure):
The microorganisms were from different sites known or expected to be contaminated by silanols
Source 1: Anton (Belgium), Landfill, inoculum matrix: Wastewater sludge, Landfill leachates, Surface water
Source 2: Mont-Saint-Guibert (Belgium), Landfill, inoculum matrix: Landfill leachates
Source 3: Hallembaye (Belgium), Landfill, inoculum matrix: Wastewater sludge, Landfill leachates
Source 4:Beaumont (Belgium), Landfill, inoculum matrix: Wastewater sludge, Landfill leachates, Surface water
Source 5: Munich (Germany), Stagnant pond, inoculum matrix: Wastewater sludge
Source 6: Essen (Germany), Stagnant pond, inoculum matrix: Biofilm developed at the surface of wood substrates
Source 7: Namur (Belgium), Mixture of garden soils, inoculum matrix: Composite soil

- Storage conditions: at 4 °C in darkness
- Storage length: up to 3 months
- Preparation of inoculum for exposure: The 4 types of matrices sampled (surface water, sludge, leachate and soil) from each site were mixed together to get 4 groups with an initial microbial diversity as broad as possible.
- Pretreatment:
anoxic: stored in a container closed to limit access to oxygen, but regularly opened in order to bring back the inner pressure to the atmosphere pressure
oxygenated: stored in a container closed to avoid desiccation regularly flushed with fresh air

- Initial cell/biomass concentration: Wet matter content 41 g (171 g in gradient microcosm)
Duration of test (contact time):
>= 180 - <= 540 d
Initial conc.:
110.35 other: ppm DMSD
Based on:
test mat.
Parameter followed for biodegradation estimation:
radiochem. meas.
Details on study design:
TEST CONDITIONS
8 different matrices were tested (please see table 1 in "any other information on materials and methods"):
- Composition of medium:
surface water: 40 g of surface water
Composite soil suspended in water: composite soil (mixture of wastewater sludge, leachates and soil inocula in similar proportions on a DM basis)
Composite sludge slurry: composite sludge (mixture of wastewater sludge and leachates inocula in similar proportions on a DM basis)
Composite sludge slurry + calcium nitrate: composite sludge (mixture of wastewater sludge and leachates inocula in similar proportions on a DM basis + 148 μL of 1.69 M Ca(NO3)2.4H2O)
Composite sludge slurry + goethite: composite sludge (mixture of wastewater sludge and leachates inocula in similar proportions on a DM basis+ ca. 4 g of goethite (FeO(OH)))
Composite sludge slurry + iron(II)sulfate: composite sludge (mixture of wastewater sludge and leachates inocula in similar proportions on a DM basis + 285 μL of 0.72 M FeSO4.7H2O)
Composite sludge slurry: composite sludge (mixture of wastewater sludge and leachates inocula in similar proportions on a DM basis)
Bottom 1 cm layer of solid organic sludge, covered by a 5 cm layer of solid composite soil, at field capacity. Bottom 0.5 cm layer saturated with water
- Test temperature: 18 - 22 °C
- Continuous darkness: yes

TEST SYSTEM
- Culturing apparatus: 250 mL GL-45 glass bottles closed with a stainless steel plate cap pressed on a Viton® o-ring with a PBT pierced GL-45 screw cap. The stainless steel plate was crossed by two ¼” outer diameter stainless steel tubes.
- Number of culture flasks/matrix: 2
- Method used to create aerobic conditions: vented with air
- Method used to create anaerobic conditions: vented with N2
- Measuring equipment: gas samples were analyzed by gas chromatography (CompactGC, Interscience, Belgium) for their content in CO2, CH4, O2, N2 and H2.
- Details of trap for CO2 and volatile organics if used:
Trap for 14CO2: 1 M NaOH
Trap for 14CH4: CH4 and any volatile carbonaceous molecule was injected into an Oxidizer furnace ((Carbolite, MTT 12/38/850/3508P1 Tritium & Carbon-14 Capture Furnace) and converted to CO2 which was then trapped

SAMPLING
- Sampling frequency: directly after starting the incubation to characterize the initial situation (d0), after 3 months of incubation (d87-94), after 6 months of incubation (d180-196) and after a total incubation of 13 months to 18 months (d398-557)
- Sampling method: At sacrifice, each reactor content was separated into its different gas, liquid and solid phases.
- Other:
Liquid phase was analysed by liquid scintillation counting (LSC) and HPLC.
Solid Phase extraction by Tetrahydrofuran (THF) and subsequent analysis by LSC and HPLC.
Residual solid Phase was combusted in the Oxidizer, 14CO2 + 14CO2 was then collected into the two alkaline bubblers traps and analysed by LSC

CONTROL AND BLANK SYSTEM
- Inoculum blank: 2
Remarks on result:
other: no degradation observed
Transformation products:
no
Details on results:
For each condition, the highest amount of radioactivity is recovered in the liquid phase, even at the end of the incubation. For the Aerobic Surface Water condition, the radioactivity recovered in the liquid phase does not significantly decrease with time. This can be expected since little microbial activity is assumed to be present in these reactors. For all other conditions, the radioactivity recovered from the liquid phase slowly decreases. Only under the Ferri-Reducing and Methanogenic conditions, the decrease seems to be limited to the first period of 90 days.
At the end of the 13-18 months of incubation, for all conditions, there is a very low but significant cumulated release of 14C marked molecules in the gas samples. The detected non-condensable radioactivity can be attributed to volatile 14C-DMSD that was not retained by the cold trap. Indeed, even at -70 °C, DMSD has a positive vapor pressure (unknown value), and some molecule can escape.
At this stage of the incubation, no clear trend emerges yet from the evolution of radioactivity recovered from the solid phase with time; neither for the total radioactivity nor for the ratio between 14C-DMSD by-products extractable and not extractable by THF.
The HPLC analysis aimed at detecting the presence of different 14C-metabolites of 14C-DMSD. However the results don’t show any evidence for the presence of any metabolite other than 14C-DMSD in the liquid phase, in the THF extract or in the acid extract of the residual solids.
Validity criteria fulfilled:
not applicable
Conclusions:
There is no evidence for any significant degradation or conversion of DMSD in aquatic or soil conditions representative for a diversity of environmental conditions. Environmental DMSD degradation, if any, should be sought in another environmental compartment.

Description of key information

The simulation test on ultimate degradation in surface water/sediment and identification of degradation products do not need to be conducted as:

- in accordance with Column 2 of REACH Annex IX, the chemical safety assessment according to Annex I indicates that this is not necessary,

and

- in accordance with Section 3 of REACH Annex XI, exposure of the aquatic/sediment compartment is not significant.

Key value for chemical safety assessment

Additional information

The simulation test on ultimate degradation in surface water and sediment as well as identification of degradation products does not need to be conducted as:

-             in accordance with Column 2 of REACH Annex IX, the chemical safety assessment according to Annex I indicates that this is not necessary, and

-             in accordance with Section 3 of REACH Annex XI, exposure of the sediment compartment is not significant. 

 

The available evidence regarding biodegradation and persistence of organosilicon compounds is summarised in an attached document in Section 13 (PFA 2021). Many organosilicon compounds hydrolyse rapidly to a silanol and a by-product such as ethanol, methanol or HCl. For most organosilicon compounds, little or no degradation is observed in ready biodegradation studies once degradation of any readily biodegradable hydrolysis by-product is accounted for. This is supported by a small number of simulation studies that show limited biodegradation. Therefore, most organosilicon compounds either meet the criteria for persistence or produce transformation products that may meet the criteria for persistence based on currently available data. 

 

The registeredsubstance hydrolyses rapidly in the environment (DT50 < 0.6 h at pH 7 and 25°C), forming dimethylsilanediol (CAS 1066-42-8, DMSD) and methanol (CAS 67-56-1). Thus, the environmental fate assessment is based on the hydrolysis products dimethylsilanediol (DMSD) and methanol rather than the parent substance. Methanol is readily biodegradable and therefore negligible for the environmental fate assessment of the registered compound (OECD SIDS, 2004).  The exposure assessment and risk characterisation are carried out for the silanol hydrolysis product of the substance. For the silanol hydrolysis product, DMSD, extensive higher tier biodegradation tests for soil, sediment and water are available, which show no evidence for any significant degradation or conversion of DMSD in aquatic or soil conditions representative for a diversity of environmental conditions. On the basis that the silanol hydrolysis product DMSD is not biodegradable, risk characterisation ratios are below 1.

 

Therefore, further testing for biodegradation would not affect the outcome of the environmental risk characterisation.  Since degradation of the registration substance is expected to be principally via abiotic transformation under the aqueous conditions of a degradation simulation study, the transformation products expected in the environment are those identified in IUCLIDsection 1.10 “Transformation of the registered substance”. The registered substance is considered to be not B based on the physicochemical property, i.e., rapid hydrolysis and predicted log Kow of < 4.5. Therefore, further testing for persistence would not affect the overall outcome of the PBT/vPvB assessment. 

The chemical safety assessment does not indicate any need to investigate further the degradation of the substance, or either of its hydrolysis products, at the present time.

 

Additioinal evidence is available from hier tier simulation studies conducted with the silanol hydrolysis product of the registered substance. In her PhD thesis, M. Fischer-Reinhard (2007) investigated the biodegradation of DMSD by microbial consortia (activated sludge from the wastewater treatment plant of a silicone oil producing plant, and a mix of several activated and digested industrial and municipal wastewater sludges) under aerobic, anoxic, sulfato-reducing and methanogenic conditions. While the experimental set-up and duration was not optimal to detect slow DMSD degradation rates, there was also no evidence for a significant conversion of DMSD to CO2 under these environmentally relevant conditions.

The biodegradation of DMSD (dimethylsilanediol) was further investigated under any of various water environmental matrices in contact with soil or sediments, in the presence of microorganisms that have high chance to degrade DMSD (CES, 2016). Different sites known or expected to be contaminated by silanols have been selected in order to maximize the probability to obtain samples with various organisms previously exposed and potentially adapted to silanols. The four types of matrices sampled (surface water, sludge, leachate and soil) from each site were mixed together to get four groups with an initial microbial diversity as broad as possible. Eight different conditions that cover a range of environmentally relevant redox conditions were tested in duplicate.

The experimental set-up and procedure was designed to ensure to close the 14C balance as much as possible, by minimizing and checking the 14C losses in order to unequivocally interpret the results. The reactors were incubated for more than 1 year and sacrifices were performed four times: directly after starting the incubation to characterize the initial situation (d0), after 3 months of incubation (d87-94), after 6 months of incubation (d180-196) and after a total incubation of 13 months to 18 months (d398-557) depending on the tested condition.

As a result of this extensive study, there was no evidence for any significant degradation or conversion of DMSD in any of the aquatic or soil conditions representative for a diversity of environmental conditions.

In consequence, there is no evidence for significant DMSD biodegradation in the aquatic or sediment compartment.