Tetraethyl orthosilicate

Toxicity to terrestrial arthropods

Administrative data

Endpoint:

toxicity to terrestrial arthropods: long-term

Data waiving:

study scientifically not necessary / other information available

Justification for data waiving:

other:

Justification for type of information:

Please see the cross-referenced supporting information to justify the waiving of terrestrial toxicity data.
Further discussion on the ecotoxicity of silicic acid producers can be found in the attached report “PFA, 2013x Analogue report Ecotoxicity of (poly)silicic acid producers_20130516”.

Cross-reference

Cross-reference 1

Reason / purpose for cross-reference:

data waiving: supporting information

Reference

Assessed substance:

transformation product

Composition of assessed substance:

Tetraethyl orthosilicate (CAS 78-10-4; EC No. 201-083-8)

PBT status of the assessed substance:

PBT assessment does not apply

Cross-reference 2

Reason / purpose for cross-reference:

data waiving: supporting information

Reference

There are no in vivo data on the toxicokinetics of tetraethyl orthosilicate (CAS 78-10-4, EC No. 201 -083-8).

 

The following summary has therefore been prepared based on validated predictions of the physicochemical properties of the substance itself and its hydrolysis products and using these data in algorithms that are the basis of many computer-based physiologically based pharmacokinetic or toxicokinetic (PBTK) prediction models. Although these algorithms provide a numerical value, for the purposes of this summary, only qualitative statements or predictions are made.

 

The main input variable for the majority of these algorithms is log Kow. So, by using this parameter and, where appropriate, other known or predicted physicochemical properties of tetraethyl orthosilicate or its hydrolysis products, reasonable predictions or statements may be made about their potential absorption, distribution, metabolism and excretion (ADME) properties.

 

In contact with water, tetraethyl orthosilicate hydrolyses very rapidly, with a hydrolysis half-life of 0.11 hr (6.6 min), 4.4 hr and 0.22 hr (13 min) at pH 4, 7 and 9 respectively and at 25°C, to form monosilicic acid. Monosilicic acid exists only in dilute aqueous solutions and readily condenses at concentrations above approximately 100 - 150 mg/L as SiO2 to give a dynamic equilibrium between monomer, oligomers and insoluble polysilicic acid. The timing and rate of condensation is regulated by the concentration, pH, ionic strength, temperature etc. The non-silanol product of hydrolysis is ethanol. The toxicokinetics of ethanol have been extensively studied previously and therefore are not discussed further in this summary.

 

Human exposure to tetraethyl orthosilicate can occur via the inhalation or dermal routes.

Some toxicokinetic work may help identify or predict the predominant chemical entity (monosilicic versus polysilicic acid) present in the small intestine, the primary site of gastrointestinal absorption or in the nasal mucosa, the site of respiratory inflammation in the reliable repeat inhalation studies (Omae et al. 1995, Nakashima et al. 2014).

 

Absorption

 

Oral

Significant oral exposure is not expected for tetraethyl orthosilicate.

When oral exposure takes place, it can be assumed, except for the most extreme of insoluble substances, that uptake through intestinal walls into the blood occurs. Uptake from intestines can be assumed possible for all substances that have appreciable solubility in water or lipid. Other mechanisms by which substances can be absorbed in the gastrointestinal tract include the passage of small water-soluble molecules (molecular weight up to around 200 g/mol) through aqueous pores or carriage of such molecules across membranes with the bulk passage of water (Renwick, 1993).

 

If oral exposure of tetraethyl orthosilicate occurs, its molecular weight (208 g/mol) and water solubility (8600 mg/L) favour absorption following oral (neat or aqueous) ingestion. However, tetraethyl orthosilicate will rapidly hydrolyse (hydrolysis half-life at pH 4 is 6.6 minutes; and approximately 5 seconds at pH 2) to monosilicic acid in the stomach, thus making the absorption of tetraethyl orthosilicate following ingestion unlikely. The hydrolysis product monosilicic acid would be absorbed from the gastrointestinal tract (primarily small intestine); whereas absorption of the condensed insoluble polysilicic acid will be insignificant when compared to the absorption of the soluble species (Carlisle, 1986).

 

In the repeated dose oral study with the tetraethyl orthosilicate condensed hydrolysis product polysilicic acid (equivalent to synthetic amorphous silica [SAS], Kim et al. 2014), there were no significant adverse effects identified (author conclusion of a systemic NOAEL at the highest dose tested). Although systemic (renal) findings were observed in the repeated dose oral study with tetraethyl orthosilicate (CIT Safety & Health Laboratories 2005), this result is considered a consequence of the corn oil dosing (see repeated dose endpoint summary).

 

Dermal

The fat solubility and therefore potential dermal penetration of a substance can be estimated by using the water solubility and log Kow values. Substances with log Kow values between 1 and 4 favour dermal absorption (values between 2 and 3 are optimal) particularly if water solubility is high.

 

With a water solubility of 8600 mg/L and log Kow of 1.4, tetraethyl orthosilicate is in the favourable range for dermal absorption, and therefore could be absorbed prior to hydrolysis in the absence of moisture on the skin. After or during deposition of a liquid on the skin, evaporation of the substance and dermal absorption occur simultaneously such that the vapour pressure of a substance is also relevant. With a vapour pressure of 20 Pa at 20°C, evaporation of tetraethyl orthosilicate is not likely to be a major factor influencing potential dermal absorption.

 

Although the molecular weight of the hydrolysis product monosilicic acid favours absorption across the skin, it is water soluble (approximately 100 - 150 mg/L as ‘SiO2 equivalent’ with condensation to insoluble polysilicic acid occurring at higher concentrations). This solubility suggests that monosilicic acid is too hydrophilic to cross the lipid rich stratum corneum. Thus, absorption of the soluble monosilicic acid and the condensed insoluble polysilicic acid across the skin is unlikely.

 

Inhalation

There is a Quantitative Structure-Property Relationship (QSPR) to estimate the blood: air partition coefficient for human subjects as published by Meulenberg and Vijverberg (2000). The resulting algorithm uses the dimensionless Henry coefficient and the octanol: air partition coefficient (Koct: air) as independent variables.

 

Using these values for tetraethyl orthosilicate results in a blood: air partition coefficient of approximately 910: 1 meaning that if respiratory exposure occurred there could be uptake of tetraethyl orthosilicate into the systemic circulation. However, given the rapid hydrolysis of tetraethyl orthosilicate, it is expected that the entity absorbed across the respiratory (nasal) mucosa would be the monosilicic acid hydrolysis product.

 

Although systemic (renal) effects were identified in reliable repeated-dose inhalation toxicity studies with tetraethyl orthosilicate (Omae et al., 1995, Nakashima et al. 1994), these findings are considered consistent with a physical effect of insoluble polysilicic acid formation within the kidneys, rather than tetraethyl orthosilicate toxicity (see repeated dose endpoint summary).

 

Distribution

As discussed above, the predicted absorbed entity is the monosilicic acid hydrolysis product, rather than tetraethyl orthosilicate or polysilicic acid. Monosilicic acid, is a small polar molecule, and therefore has potential to be widely distributed, but its hydrophilic nature will limit its diffusion across membranes (including the blood-brain and blood-testes barriers) and its accumulation in fatty tissues. Human blood contains 1 mg SiO2/L of monosilicic acid (Iler RK, 1979).

 

For blood: tissue partitioning of the parent substance, a QSPR algorithm has been developed by De Jongh et al. (1997) in which the distribution of compounds between blood and human body tissues as a function of water and lipid content of tissues and the n-octanol: water partition coefficient (Kow) is described.

 

Tetraethyl orthosilicate quickly hydrolyses to monosilicic acid thus limiting its systemic absorption and therefore distribution. So, the following predicted distribution of tetraethyl orthosilicate to the fatty tissues is not considered likely.

 

Table 5.1.1 Tissue: blood partition coefficients

	Log Kow	Kow	Liver	Muscle	Fat	Brain	Kidney
Tetraethyl orthosilicate	1.4	25.12	1.3	1.1	18.4	1.3	1.1

 

Metabolism

Besides the aforementioned hydrolysis, there is no information on the potential metabolism of tetraethyl orthosilicate.

 

Silicon is an essential trace element participating in the normal metabolism of higher animals. It is required in bone, cartilage and connective tissue formation as well as participating in other important metabolic processes. The silicon is present almost entirely as free, soluble monosilicic acid (Carlisle 1986).

 

The available and negative genetic toxicity tests in vitro showed no observable differences in effects with and without metabolic activation.

 

Excretion

As discussed in the repeated dose endpoint summary, all of the tetraethyl orthosilicate would be expected to hydrolyse to monosilicic acid in the gastrointestinal tract after oral exposure or in the nasal mucosa following inhalation. Any absorbed soluble monosilicic acid would either condense to polysilicic acid in the kidneys or be excreted via urine as monosilicic acid. Condensed and insoluble polysilicic acid formed in the gastrointestinal or nasal mucosa would not be absorbed systemically, and thus only excreted in faeces.

Cross-reference 3

Reason / purpose for cross-reference:

data waiving: supporting information

Reference

Bioaccumulation: aquatic/sediment: Low potential for bioaccumulation

Testing is waived in accordance with Column 2 of REACH Annex IX. Direct or indirect exposure of aquatic organisms to the registered substance is very limited due to the instability of the substance in water. The substance hydrolyses rapidly in contact with water to form monosilicic acid [CAS 10193-36-9; EC No. 233-477-0, Si(OH)₄] and ethanol (CAS 64-17-5; EC No. 200-578-6). In addition, the substance has a low predicted log K_ow value of <3.

Ethanol is readily biodegradable and has very low log K_ow.

Silicic acid condenses at concentrations above approximately 100-150 mg/L as SiO₂ to give insoluble amorphous polysilicic acid. These hydrolysis products are inorganic substances which enter natural biogeochemical cycles.

Monosilicic acid is the bioavailable form of silica that can be absorbed by certain organisms in the environment. In these organisms, silicic acid, precipitated as insoluble amorphous silica, plays a structural and defensive role. In animals, silica is a trace nutrient.

Cross-reference 4

Reason / purpose for cross-reference:

data waiving: supporting information

Reference

Endpoint:

short-term toxicity to aquatic invertebrates

Type of information:

experimental study

Adequacy of study:

weight of evidence

Study period:

2006-09-06 to 2006-09-08

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

according to guideline

Guideline:

OECD Guideline 202 (Daphnia sp. Acute Immobilisation Test)

GLP compliance:

yes

Analytical monitoring:

yes

Details on sampling:

- Sampling method: Prior to the start of the definitive exposure, one water sample was removed from the high, middle and low treatment levels and the control solution and analyzed for ethyl silicate concentration. Results of these pretest analyses were used to judge whether sufficient quantities of ethyl silicate were being delivered to the test vessels and whether the appropriate test concentrations were being maintained in order to initiate the definitive exposure.

During the in-life phase of the definitive study, one water sample (alternating between replicates A and B) from each treatment level and the control solution was collected, extracted and analyzed for ethyl silicate at 0 hour (test initiation) and 48 hours (test termination). Samples were collected from the approximate midpoint of the test vessel by pipet.

Vehicle:

no

Details on test solutions:

PREPARATION AND APPLICATION OF TEST SOLUTION

- Method: During the definitive study, ethyl silicate (density 0.93 g/mL, 930 mg a.i./mL) was delivered directly into the diluter's mixing chamber via syringe. The resulting solution was observed to be clear and colourless with an oily film of undissolved substance on the solution's surface. A Glenco® 50 -mL gas-tight syringe in conjunction with a Harvard Syringe Pump was calibrated to deliver 0.2118 mL/cycle of the 930 mg a.i./mL ethyl silicate solution into the diluter system's chemical mixing chamber which also received 0.390 L of dilution water per cycle. The mixing chamber was positioned over a water-driven magnetic stirrer and was partially submerged within an ultrasonic water bath which continuously mixed the contents of the mixing chamber. The concentration of ethyl silicate in the solution contained within the mixing chamber was equivalent to that of the highest nominal test concentration (500 mg a.i./L) and was proportionally diluted (50%) to produce the remaining nominal test concentrations (250, 125, 63, 3l and 0 (control) mg a.i./L).

Test organisms (species):

Daphnia magna

Details on test organisms:

TEST ORGANISM

- Source: From laboratory stock cultures

- Age at study initiation (mean and range, SD): <24 hours

- Method of breeding: Juvenile daphnids (< 24 hours old) were obtained from the laboratory cultures by removing all immature daphnids from the culture vessels 24 hours prior to test initiation, thereby isolating all mature gravid adult daphnids in the culture. Daphnids produced by these adult organisms were removed from the cultures and used as test organisms. The adult daphnids used to produce offspring for this test (l) did not contain ephippia, (2) produced offspring prior to being l2 days old, (3) were not used in any portion of a previous test and (4) mortality 48 hours prior to test initiation was < 20%.

CULTURE CONDITIONS

- Culture medium: The culture water was prepared by fortifying well water based on the formula for hard water and filtering it through an Amberlite XAD-7 resin column to remove any potential organic contaminants. This water had a total hardness range and total alkalinity as calcium carbonate (CaCO3) of 160 to 170 mg/L and 110 mg/L, respectively, a pH range o f 7.9 to 8.2, a dissolved oxygen concenration range of 7.5 to 9.7 mg/L, a temperature range of 20 to 22ºC and a specific conductance range of 500 to 600 micromhos per centimetre. All water quality ranges presented here were measured during the two weeks prior to testing.

- Lighting: The daphnid culture area received a regulated photoperiod of 16 hours of light and 8 hours of darkness. Light intensity of 68 to 99 footcandles (730 to 1100 lux) at the surface of the culture solutions was provided by fluorescent bulbs.

- Feeding of stock cultures: Daphnids were fed a unicellular green algae (Ankistrodesmus falcatus, 4 x l0E7 cells/ml) at a rate of 0.5 to 1.5mL per vessel daily depending on the age of the adult organisms in the culture vessel and 0.5 mL of a combination of yeast, cereal leaves and flaked fish food (YCT) daily. Daphnids were not fed during the exposure.

- Feeding during test: None

Test type:

flow-through

Water media type:

freshwater

Limit test:

no

Total exposure duration:

48 h

Hardness:

160-170 mg/L as CaCO3

Test temperature:

21 to 23ºC

pH:

8.0 to 8.1

Dissolved oxygen:

7.2 to 8.8 mg/L

Salinity:

Not applicable

Nominal and measured concentrations:

Nominal concentrations: 0 (control), 31, 63, 125, 250 and 500 mg a.i./L

Mean measured concentrations ranged from 15 to 37% of nominal levels and were defined as < LoQ (Control), 11, 14, 34, 70 and 75 mg a.i./L.

Analytical variability in the two highest test concentrations was likely due to solubility of the test substance under the test conditions maintained.

Details on test conditions:

TEST SYSTEM

- Exposure vessel type (e.g., size, headspace, sealed, aeration, # per treatment):  Each test vessel (1600-mL square glass battery jars) had two 2-cm holes drilled in the sides, 15 cm from the bottom, which were covered with Nitex® 40 mesh screen for drainage.  The total test solution volume was maintained at 1400 mL.  Two replicate test vessels were established for each treatment level and a dilution water control.  No aeration was provided to the test vessels. 

- Dilution water source:  Fortified well water based on the formula for hard water (U.S. EPA, 1975). Dilution water chemistry (hardness, alkalinity, pH, TOC):  The dilution water had a total hardness range and alkalinity as CaCO3 of 160 to 170 mg/L and 110 mg/L, respectively, a pH range of 8.1 to 8.2 and a specific conductivity range of 500 to 600 µmhos/cm.  The TOC concentration of the dilution water was 0.38 mg/L for the month of September 2006.  

- Lighting (quality, intensity, and periodicity):  The test area was illuminated with fluorescent bulbs.  The test area received a regulated photoperiod of 16 hours of light and 8 hours of darkness.  Sudden transitions from light to dark and vice versa were avoided.  Light intensity was measured once during the test.

- Water chemistry in test (D.O., pH), in the control, and at least one concentration where effects were observed:  The dilution water control vessels had a measured DO concentration of 8.8 mg/L at test initiation and test termination.  The pH measured in the dilution water control vessel replicates was 8.1 at test initiation and ranged from 8.0 to 8.1 at test termination.  

- Element (unit) basis (i.e., immobilization):  Immobilization

- Test design (number of replicates, individuals per replicate, concentrations):  Twenty daphnids were impartially selected and distributed to each concentration and the control (two replicates, ten daphnids per replicate vessel).  

- Method of calculating mean measured concentrations (i.e., arithmetic mean, geometric mean, etc.):  Arithmetic mean.  

Reference substance (positive control):

no

Duration:

48 h

Dose descriptor:

NOEC

Effect conc.:

>= 75 mg/L

Nominal / measured:

meas. (arithm. mean)

Conc. based on:

test mat.

Basis for effect:

mobility

Duration:

48 h

Dose descriptor:

EC50

Effect conc.:

> 75 mg/L

Nominal / measured:

meas. (arithm. mean)

Conc. based on:

test mat.

Basis for effect:

mobility

Details on results:

- Immobilisation of control: 0

Reported statistics and error estimates:

There were no significant effects on mobility at any of the test concentrations and therefore statistical analysis of the results was not required.

Table 1. Results of analysis of test media

Nominal concentration (mg/L)	Measured concentration at start of test (mg/L)	Measured concentration at end of test (mg/L)	Mean measured concentration (mg/L)	Mean measured concentration as % of nominal
0 (Control)	<0.44	<0.46	Not applicable	Not applicable
31	12	10	11	37
63	14	13	14	22
125	37	30	34	27
250	79	61	70	28
500	59	91	75	15

Table 2. Test results

Mean measured concentration (mg/L)	Percentage immobilisation after 24 hours	Percentage immobilisation after 48 hours
0 (Control)	0	0
11	0	0
14	0	0
34	0	0
70	0	0
75	0	5

Validity criteria fulfilled:

yes

Conclusions:

A 48-hour EC50 value of >75 mg/L and NOEC of ≥75 mg/L have been determined for the effects of the test substance on mobility of Daphnia magna. Under the flow-through exposure conditions used in the test it is likely that the test organism will have been primarily exposed to the test substance.

Cross-reference 5

Reason / purpose for cross-reference:

data waiving: supporting information

Reference

Endpoint:

short-term toxicity to aquatic invertebrates

Type of information:

experimental study

Adequacy of study:

weight of evidence

Study period:

1993-10-11 to 1993-10-15

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

guideline study with acceptable restrictions

Remarks:

NOEC and EC50 are based on a single measurement of the stock solution before dilution and addition to test vessels.

Qualifier:

according to guideline

Guideline:

EU Method C.2 (Acute Toxicity for Daphnia)

Version / remarks:

Cited as Directive 84/449/EEC, C.2 (Acute toxicity for Daphnia)

GLP compliance:

yes

Analytical monitoring:

yes

Details on sampling:

The measured Dissolved Organic Carbon Concentration was determined in the fresh stock solution that was used to prepare the test media..

Vehicle:

no

Details on test solutions:

PREPARATION AND APPLICATION OF TEST SOLUTION

- Method: The hard-to-dissolve test substance was added to the synthetic fresh water to provide a concentration of 1 g/l, and was stirred for 18 hours.  Then the solution was filtered and the carbon content was determined.  This solution served as the initial solution and contained a DOC value of 433 mg/l, which corresponds to a substance content of 938 mg/l.  Test concentrations selected for the main test were 235, 328, 469, 657, and 844 mg/l. 

Test organisms (species):

Daphnia magna

Details on test organisms:

TEST ORGANISM

- Source: Daphnia magna were produced through internal self-production.  The mother specimens were maintained in M4 medium per Elendt in 1-liter beakers for breeding.  The daily amount of nutrient, consisting of Scenedesmus subspicatus, was so measured that the amount added was totally consumed by the specimens.  At regular intervals of about four weeks, the young were isolated from the tanks.  They were the basis for further breeding.  Juveniles (less than 24 hours old) were isolated and used for testing in synthetic fresh water (294 mg/l CaCl2 x 2 H2O; 123 mg/l MgSO4 x 7 H2O; 63 mg/l NaHCO3; 5.5 mg/l KCl; the sum of the Ca2+ and Mg2+ was 4:1, and the ratio of Na+ to K+ was 10:1).

Test type:

static

Water media type:

freshwater

Limit test:

no

Total exposure duration:

48 h

Hardness:

-

Test temperature:

20+/-1ºC

pH:

7.0-7.1

Dissolved oxygen:

7.8-8.5 at test termination

Salinity:

Not applicable

Nominal and measured concentrations:

Test concentrations selected for the main test were 0(Control), 235, 328, 469, 657, and 844 mg/l based on the measured concentration in the stock solution. 

Details on test conditions:

Static exposure tests were conducted at 20 + 1oC.  The tests were conducted in the dark.  The test vessels were glass cylinders graduated to 10 ml.  Four vessels, each containing five neonates, were used for each test concentration and for the control (total of 20 neonates/test concentration).  The test organisms were not fed during the test and no aeration was applied.  Test organisms were observed for mortality (immobilization) at 24 and 48 hours after test initiation.  Dissolved oxygen and pH were measured at test termination for each test concentration and control. 

Reference substance (positive control):

yes

Remarks:

Potassium dichromate

Duration:

48 h

Dose descriptor:

NOEC

Effect conc.:

>= 844 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

mobility

Duration:

48 h

Dose descriptor:

EC50

Effect conc.:

> 844 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

mobility

Duration:

48 h

Dose descriptor:

LOEC

Effect conc.:

> 844 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

mobility

Details on results:

- Immobilisation of control: 0

Results with reference substance (positive control):

The previous test conducted with the positive control, potassium dichromate, showed 20% immobilization at 24-hours with 0.9 mg/l, and 100% immobilization at 24-hours with 1.9 mg/l.

Reported statistics and error estimates:

There were no significant effects in the test and therefore statistical analysis of the results was not required

There was no immobilisation at any test concentration or in the control group.  

Validity criteria fulfilled:

yes

Conclusions:

A 48-hour EC50 value of >844 mg/L and a NOEC of ≥844 mg/L have been determined for the effects of the test substance on mobility of Daphnia magna. It is likely that the test organisms were exposed to the hydrolysis products of the substance.

Cross-reference 6

Reason / purpose for cross-reference:

data waiving: supporting information

Reference

Endpoint:

short-term toxicity to fish

Type of information:

experimental study

Adequacy of study:

key study

Study period:

1993-10-11 to 1993-10-15

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

according to guideline

Guideline:

EU Method C.1 (Acute Toxicity for Fish)

Version / remarks:

Cited as Directive 92/69/EEC, C.1

GLP compliance:

yes

Analytical monitoring:

yes

Details on sampling:

Samples of freshly prepared test media were taken at the start of each successive 24 hour exposure period (0, 24, 48 and 72 hours). Samples of old test medium were taken at the end of the first 24-hour exposure period in order to assess the stability of the exposure concentrations.

Vehicle:

no

Details on test solutions:

A stock solution was prepared by adding the test substance to potable water at the rate of 1 g/L. After mixing the medium was filtered and the aqueous phase used as the test medium. The average measured Dissolved Organic Carbon Concentration determined in the fresh test medium over the course of the test (245 mg/L) was used as the basis for expressing the test results.

Test organisms (species):

Danio rerio (previous name: Brachydanio rerio)

Details on test organisms:

TEST ORGANISM

- Common name: Zebrafish

- Source: West Aquarium, Bad Lauterburg, Germany

- Length at study initiation: 3.0+/-0.5 cm

- Feeding during test: none


ACCLIMATION

- Acclimation period: 14-days

- Acclimation conditions: same as test

- Type and amount of food: Tetramin® at 1% of body weight

- Feeding frequency: Daily

- Health and pre-treatment: The specimens were pre-conditioned (treatment: 3x per week with malachite green) and subjected to a 14-day quarantine.  They were used in testing only after this treatment.  Only specimens displaying normal behaviour at the beginning of the test and free of obvious disease were used in the study.  Fish were not fed during the test.       

Test type:

semi-static

Water media type:

freshwater

Limit test:

yes

Total exposure duration:

96 h

Hardness:

10.7 ºdH

Test temperature:

20+/-1ºC

pH:

7.5-8.4

Dissolved oxygen:

4.3-8.9 mg/L (the low value of 4.3 mg/L did not affect the outcome of the test)

Salinity:

Not applicable

Nominal and measured concentrations:

Nominal loading used to prepare test medium: 1000 mg/L

Mean measured DOC concentration in the test medium: 245 mg/L

The mean measured DOC concentration has been used as the basis for expressing the test results.

Details on test conditions:

TEST SYSTEM

- Test vessel: 20 litre aquarium

- Type (delete if not applicable): open

- Aeration: Yes, continuous

- Renewal rate of test solution: daily

- No. of organisms per vessel: 10

- No. of vessels per concentration (replicates): 1

- No. of vessels per control (replicates): 1


TEST MEDIUM / WATER PARAMETERS

- Source/preparation of dilution water: Potable water from Gelsenwasser AG


OTHER TEST CONDITIONS

- Adjustment of pH: No

- Photoperiod: 16 hours light, 8 hours dark


EFFECT PARAMETERS MEASURED: Daily mortalities


TEST CONCENTRATIONS

- Spacing factor for test concentrations: Limit test

Reference substance (positive control):

no

Duration:

96 h

Dose descriptor:

NOEC

Effect conc.:

>= 245 mg/L

Nominal / measured:

meas. (arithm. mean)

Conc. based on:

test mat.

Basis for effect:

mortality (fish)

Duration:

96 h

Dose descriptor:

LC50

Effect conc.:

> 245 mg/L

Nominal / measured:

meas. (arithm. mean)

Conc. based on:

test mat.

Basis for effect:

mortality (fish)

Details on results:

- Mortality of control: 0

Reported statistics and error estimates:

There were no mortalities observed in the test and therefore statistical analysis of the results was not required.

Sublethal observations / clinical signs:

Table 1. Results of analysis of test media

Nominal loading of test substance (mg/L)	Measured DOC concentration in fresh medium at start of test (mg/L)	Measured DOC concentration in old medium after 24 hours (mg/L)	Measured DOC concentration in fresh medium after 24 hours (mg/L)	Measured DOC concentration in fresh medium after 48 hours (mg/L)	Measured DOC concentration in fresh medium after 72 hours (mg/L)	Mean measured DOC concentration (mg/L)
1000	316	247	227	281	156	245

Table 2. Test results

Mean measured DOC concentration (mg/L)	Percentage mortality after 24 hours	Percentage mortality after 48 hours	Percentage mortality after 72 hours	Percentage mortality after 96 hours
0 (Control)	0	0	0	0
245	0	0	0	0

Validity criteria fulfilled:

yes

Conclusions:

A 96-hour LC50 of >245mg/L and a NOEC of ≥245 mg/L have been determined for the effects of the test substance on mortality of Brachydanio rerio. It is likely that the test organisms were exposed to the hydrolysis products of the substance.

Cross-reference 7

Reason / purpose for cross-reference:

data waiving: supporting information

Reference

Endpoint:

toxicity to aquatic algae and cyanobacteria

Type of information:

experimental study

Adequacy of study:

key study

Study period:

2006-08-15 to 2006-08-18

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

according to guideline

Guideline:

OECD Guideline 201 (Alga, Growth Inhibition Test)

GLP compliance:

yes

Analytical monitoring:

yes

Details on sampling:

Analytical monitoring:  A single sample was removed from each test solution and the control at test initiation and test termination (72 hours) and was extracted and analyzed by GC/FID for ethyl silicate concentration.  Samples analyzed on day 0 were removed from the test solutions in the volumetric flasks prior to filling the individual test flasks.  Samples analyzed at 72 hours of exposure were removed from the composite of replicate vessels for each treatment and the control.  

Vehicle:

no

Details on test solutions:

PREPARATION AND APPLICATION OF TEST SOLUTION

- Method: A 100 mg a.i./L (mg active ingredient/L) stock solution was prepared by placing 0.2025 g of ethyl silicate (0.2005 g as active ingredient) in a 2000-mL volumetric flask and bringing it to volume with AAP medium.  The resulting stock solution was observed to be clear and colourless with no visible undissolved test substance.  Each test concentration was prepared by adding the appropriate amount of the 100 mg a.i./L stock solution to an intermediate vessel and bringing it to a final volume of 1000 mL with dilution water.

Test organisms (species):

Raphidocelis subcapitata (previous names: Pseudokirchneriella subcapitata, Selenastrum capricornutum)

Details on test organisms:

TEST ORGANISM

- Strain: Pseudokirchneriella subcapitata, formerly Selenastrum capricornutum, strain 1648, Class Chlorophyceae.  

- Source: The alga was obtained from the University of Texas, Austin, Texas, and was maintained in stock culture at Springborn Smithers.  

- Culture conditions: The stock cultures were maintained within the following conditions:  a shaking rate of 100 ± 10 rpm, a temperature of 23 ± 1ºC and continuous illumination at the surface of the medium with an intensity of 7000 to 9100 lux (650 to 800 footcandles).  Lighting was supplied by fluorescent bulbs.  Culture flasks were agitated continuously on an orbital shaker.

Test type:

static

Water media type:

freshwater

Limit test:

no

Total exposure duration:

72 h

Hardness:

Not reported

Test temperature:

23-24ºC

pH:

The pH of the test and control solutions ranged from 6.7 to 6.8 at test initiation.  At 72 hours of exposure, the pH of the test and control solutions ranged from 7.1 to 8.5. 

Dissolved oxygen:

Not reported

Salinity:

Not applicable

Nominal and measured concentrations:

Nominal concentrations: 0(Control), 2.6, 6.4, 16, 40 and 100 mg a.i./L 

Measured (geometric mean) concentrations: 0(Control), 1.8, 0.92, 3.6, 8.6 and 22 mg a.i./L

Details on test conditions:

- Growth/test medium:  The culture medium used was Algal Assay Procedure (AAP) medium prepared with sterile, deionized water.  AAP medium used to prepare the exposure solutions was formulated in the same manner as the culture medium.

- Exposure vessel type: The test was conducted in sterile 250-mL Erlenmeyer flasks containing 100 mL of test solution.  All test vessels were fitted with stainless steel caps which permitted gas exchange.  

- Water chemistry in test:  TOC concentration of the AAP sample collected in August 2006 was 0.51 mg/L. 

- Conductivity of the exposure and control solutions measured at test initiation and termination ranged from 80 to 90 µmhos/cm.  

- Light levels and quality during exposure:  7000 to 9100 lux (650 to 850 footcandles).  The photosynthetically-active radiation (PAR) of the test area measured at test initiation ranged from 90 to 140 µE/m2/s.

- Test Design:  One hundred millilitres of the appropriate exposure solution was placed in each replicate flask.  A 0.23-mL inoculum of Pseudokirchneriella subcapitata cells, at a density of approximately 445 x 10E4 cells/mL, was aseptically introduced into each flask.  This inoculum provided the required initial (0-hour) cell density of approximately 1.0 x 10E4 cells/mL.  Three replicate test vessels were established for the treatment levels and six replicates were established for the algal media control.

- Method of calculating mean measured concentrations (i.e. arithmetic mean, geometric mean, etc.): Geometric mean

Reference substance (positive control):

no

Duration:

72 h

Dose descriptor:

NOEC

Effect conc.:

>= 100 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

other: growth rate and biomass

Duration:

72 h

Dose descriptor:

EC50

Effect conc.:

> 100 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

other: growth rate and biomass

Duration:

72 h

Dose descriptor:

NOEC

Effect conc.:

>= 22 mg/L

Nominal / measured:

meas. (geom. mean)

Conc. based on:

test mat.

Basis for effect:

other: growth rate and biomass

Duration:

72 h

Dose descriptor:

EC50

Effect conc.:

> 22 mg/L

Nominal / measured:

meas. (geom. mean)

Conc. based on:

test mat.

Basis for effect:

other: growth rate and biomass

Duration:

72 h

Dose descriptor:

EC50

Effect conc.:

> 5.8 mg/L

Conc. based on:

test mat.

Remarks:

measured final

Basis for effect:

other: growth rate and biomass

Duration:

72 h

Dose descriptor:

NOEC

Effect conc.:

>= 5.8 mg/L

Conc. based on:

test mat.

Remarks:

measured final

Basis for effect:

other: growth rate and biomass

Duration:

72 h

Dose descriptor:

EC50

Effect conc.:

> 63 mg/L

Nominal / measured:

nominal

Conc. based on:

other: silanol hydrolysis product

Basis for effect:

other: growth rate and biomass

Duration:

72 h

Dose descriptor:

NOEC

Effect conc.:

>= 63 mg/L

Nominal / measured:

nominal

Conc. based on:

other: silanol hydrolysis product

Basis for effect:

other: growth rate and biomass

Details on results:

- Exponential growth in the control (for algal test): yes

Reported statistics and error estimates:

Statistical methods: The data were first checked for normality using Shapiro-Wilks' Test (Weber et al., 1989) and for homogeneity of variance using Bartlett's Test (Horning and Weber, 1985).  If the data sets passed the tests for homogeneity and normality, then Williams' Test (Williams, 1971, 1972) was used to determine the NOEC.  If the data did not pass the tests for homogeneity and normality, then Kruskal-Wallis' Test (Sokal and Rohlf, 1981) was used to determine the NOEC.  All statistical determinations were made at the 95% level of certainty, except in the case of Shapiro-Wilks' and Bartlett's Tests, where the 99% level of certainty was applied.  TOXSTAT® version 3.5  (Gulley et al., 1996) was used to perform these calculations.

Table 1. Test results

Nominal test substance concentration (mg/L)	Geometric mean measured concentration (mg/L)	Mean measured cell concentration at start of test (cells/mL)	Mean measured cell concentration after 24 hours (cells/mL)	Mean measured cell concentration after 48 hours (cells/mL)	Mean measured cell concentration after 72 hours (cells/mL)
0 (Control)	-	10000	34600	261300	1030100
2.6	1.8	10000	40000	288300	1236900
6.4	0.92	10000	35000	314200	930000
16	3.6	10000	45800	365000	1343600
40	8.6	10000	40800	289200	1677800
100	22	10000	48300	287500	1685600

Observations: After 72 hours of exposure, cells exposed to all treatment levels tested and the control were observed to be normal. 

Table 2. Test results

Nominal test substance concentration (mg/L)	Geometric mean measured concentration (mg/L)	Yield (0 -72 hours) x 10000 cells/ml	Growth rate (0 -72 hours)	Inhibition of yield (biomass) at end of test (%)	Inhibition of growth rate at end of test (%)
0 (Control)	-	102	1.52	-	-
2.6	1.8	123	1.59	-20	-5
6.4	0.92	92	1.49	10	2
16	3.6	133	1.61	-31	-6
40	8.6	167	1.68	-63	-11
100	22	168	1.69	-64	-11

 

Table 3. Analytical monitoring

Nominal concentration (mg/L)	Measured concentration 0 hours (mg/L)	Measured concentration 72 hours (mg/L)
Control	<0.11	<0.097
2.6	18	0.18
6.4	5.8	<0.29
40	27	2.8
100	82	5.8

The nominal concentrations of the parent substance can be converted into concentrations of the silanol HP using MW conversions.

Validity criteria fulfilled:

yes

Conclusions:

A 72-hour EC50 value of >22 mg/L and NOEC of ≥22 mg/L have been determined for the effects of the test substance on growth rate and biomass of Pseudokirchneriella subcapitata based on geometric mean measured concentrations (>100 mg/L and ≥100 mg/L respectively based on nominal concentration and >5.8 mg/L and ≥5.8 mg/L respectively based on measured final concentrations). It is likely that the test organisms were exposed to the hydrolysis products of the substance. The results of this test were selected in preference to those of a second test (72-hour EC50 value >1039.3 mg/L for Scenedesmus subspicatus (new name: Desmodesmus subspicatus)) because they were considered to be more reliable. The appropriate interpretation of these data is to use nominal concentrations, because rapid hydrolysis during test media preparation means the test organisms would have been exposed predominantly to hydrolysis products. The low recoveries based on GC analysis are due to removal of test substance by hydrolysis. The nominal results in terms of hydrolysis product are, calculated by MW conversion are EC50 value of >63 mg/L and NOEC of ≥63 mg/L.

Cross-reference 8

Reason / purpose for cross-reference:

data waiving: supporting information

Reference

Hazard assessment conclusion:

no hazard identified

Hazard assessment conclusion:

no hazard identified

Hazard assessment conclusion:

no hazard identified

Hazard assessment conclusion:

no hazard identified

Hazard assessment conclusion:

no hazard identified

Hazard assessment conclusion:

no hazard identified

Hazard assessment conclusion:

no hazard identified

Hazard assessment conclusion:

no potential for bioaccumulation

The hydrolysis half-life of tetraethyl orthosilicate (TEOS, CAS 78-10-4; EC No. 201-083-8) is approximately 4.4 hours at 25°C and pH 7 (based on measured data (OECD TG 111)); the substance will therefore undergo rapid hydrolysis in contact with water to form monosilicic acid (CAS 10193-36-9; EC No. 233-477-0) and ethanol (CAS 64-17-5; EC No. 200-578-6). Monosilicic acid (Si(OH)₄) exists only in dilute aqueous solutions and readily condenses at concentrations above approximately 100-150 mg/L as SiO₂ to give a dynamic equilibrium between monomer, oligomers and insoluble amorphous polysilicic acid.

Log K_ow is not relevant for inorganic compounds such as silicic acid. However, on the basis of structure, monosilicic acid has a high affinity with water and low affinity for lipids and organic carbon. The water solubility of monosilicic acid is approximately 100-150 mg/L (limited by condensation reactions) (see Section 4.8 of the IUCLID dataset for further discussion).

The non-silanol hydrolysis product, ethanol, is discussed below.

REACH guidance (ECHA 2016, R.16) states that “for substances where hydrolytic DT₅₀ is less than 12 hours, environmental effects are likely to be attributed to the hydrolysis product rather than to the parent itself”. ECHA Guidance Chapter R.7b (ECHA 2017) states that where degradation rates fall between >1 hour and <72 hours, testing of parent and/or degradation product(s) should be considered on a case-by-case basis.

The substance will be exposed to the environment through wastewater treatment plant (WWTP) effluent. The minimum residency time in the wastewater treatment plant is approximately 7 hours (although this is a conservative figure and wastewater treatment time may be hours longer) with an average temperature of 15°C (assumed to be at neutral pH). Significant degradation by hydrolysis would be expected before the substance is released to the receiving waters.

Direct releases of the registration substance to air are expected to be low. TEOS will hydrolyse on contact with atmospheric moisture to form monosilicic acid and ethanol,

The environmental hazard assessment, including sediment and soil compartments due to water and moisture being present, is therefore based on the properties of the silanol hydrolysis product, in accordance with REACH guidance.

As described below and in Section 4.8 of IUCLID, condensation reactions of the monosilicic acid are possible.

 

Silicic acid is a naturally-occurring substance which is not harmful to aquatic organisms at relevant concentrations. Monosilicic acid is the major bioavailable form of silicon for aquatic organisms and plays an important role in the biogeochemical cycle of silicon (Si). Most living organisms contain at least trace quantities of silicon. For some species Si is an essential element that is actively taken up. For example, diatoms, radiolarians, flagellates, sponges and gastropods all have silicate skeletal structures (OECD SIDS 2004, soluble silicates). Silicic acid has been shown to be beneficial in protection against mildew formation in wheat and to be non-phytotoxic in non-standard studies (Côte-Beaulieu et al. 2009).

Silicic acid is therefore not expected to be harmful to organisms present in the environment. To support this view, the available aquatic toxicity studies with organosilicon substances that hydrolyse to monosilicic acid report no effects at 100 mg/l nominal loading in short-term toxicity studies (PFA 2103x).

The non-Si hydrolysis product, ethanol, does not have the potential to cause harm at high treatment levels and therefore the hazard assessment and Predicted No Effect Concentrations (PNECs) are concluded as ‘no hazard identified’.

 

Considerations on the non-silanol hydrolysis product, ethanol:

Ethanol is well-characterised in the public domain literature and is not hazardous at the concentrations relevant to the studies; the short-term EC₅₀ and LC₅₀ values for this substance are in excess of 1000 mg/L (OECD 2004b).

References

Côté-Beaulieu C, Chain F, Menzies JG, Kinrade SD, Bélanger RR (2009). Absorption of aqueous inorganic and organic silicon compounds by wheat and their effect on growth and powdery mildew control. Environ Exp. Bot 65: 155–161.

ECHA (2016). REACH Guidance on Information Requirements and Chemical Safety Assessment Chapter R16: Environmental Exposure Assessment Version: 3.0. February 2016.

ECHA (2017). European Chemicals Agency. Guidance on Information Requirements and Chemical Safety Assessment, Chapter R.7b: Endpoint specific guidance. Version 4.0 June 2017.

PFA 2013x: Peter Fisk Associates, Analogue grouping report: Ecotoxicity of (poly)silicic acid generating compounds. PFA.300.003.001.

Cross-reference 9

Reason / purpose for cross-reference:

data waiving: supporting information

Reference

Tetraethyl orthosilicate (TEOS, CAS 78-10-4, EC No. 201-083-8) hydrolyses rapidly to form monosilicic acid (CAS 10193-36-9; EC No. 233-477-0) and ethanol (CAS 64-17-5; EC No. 200-578-6). Monosilicic acid exists only in dilute aqueous solutions and readily condenses at concentrations above approximately 100-150 mg/L as SiO₂ to give a dynamic equilibrium between monomer, oligomers and insoluble amorphous polysilicic acid.

Monosilicic acid and its condensation products are inorganic and enter natural biogeochemical cycles. Monosilicic acid and its condensation products are ubiquitous in the environment.

A comparison of the total flux of dissolved silica into rivers can be compared with the input from manufacture and use of tetraethyl orthosilicate (refer to CSR Section 9) and indicates that the input is considered negligible in comparison with the natural flux of silica/silicic acid in the environment. The global natural flux of dissolved silicic acids into rivers has been estimated as 5.6 Tmol Si/y, a fraction (20%) originating from temperate regions. Western Europe represents 6.5% of the temperate regions, the dissolved quantity of monosilicic acid in European rivers is estimated to be 0.073 Tmol Si/y or 4374 Kt SiO₂/y. In the environment, typical concentrations of monosilicic acid are up to 75 mg SiO₂/L in river water and up to 14 mg SiO₂/L in seawater (ECETOC 2006).

Therefore, it is not appropriate to calculate Predicted Environmental Concentrations (PECs) for the hydrolysis product monosilicic acid.

Reference:

ECETOC (2006). Synthetic Amorphous Silica, SAS (CAS 7631-86-9). JACC No. 51. September 2006.

Cross-reference 10

Reason / purpose for cross-reference:

data waiving: supporting information

Reference

Adsorption/desorption: Low potential for adsorption

Testing is waived in accordance with Column 2 of REACH Annex IX. The substance has a predicted log K_ow value of 1.4 and thus has low potential for adsorption.

Tetraethyl orthosilicate (CAS 78-10-4; EC No. 201-083-8) hydrolyses rapidly (t_1/2 = 4.4 h at pH 7 and 25°C) in contact with water to form monosilicic acid (CAS 10193-36-6; EC No. 233-477-0) and ethanol (CAS 64-17-5; EC No. 200-578-6).

Ethanol also has low potential for adsorption due to its low log K_ow.

Monosilicic acid exists only in dilute aqueous solutions and readily condenses at concentrations above approximately 100-150 mg/L as SiO₂ to give a dynamic equilibrium between monomer, oligomers and insoluble amorphous polysilicic acid. These hydrolysis products are inorganic substances which enter natural biogeochemical cycles; adsorption/desorption studies are not relevant. Based on their structure, the hydrolysis products will have a high affinity for water and a low affinity for organic carbon and so a low potential for adsorption to the organic carbon. However, they may interact with the mineral content of soil. Amorphous polysilicic acid is a constituent of most soils.

Materials and methods

Test material

Test material information

Constituent 1

Reference substance name:

Tetraethyl orthosilicate

EC Number:

201-083-8

EC Name:

Tetraethyl orthosilicate

Cas Number:

78-10-4

Molecular formula:

C8H20O4Si

IUPAC Name:

tetraethyl orthosilicate