Silicon tetrachloride

Toxicity to soil microorganisms

Administrative data

Endpoint:

toxicity to soil microorganisms

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

Adsorption/desorption: Low potential for adsorption

Testing is waived in accordance with Column 2 of REACH Annex IX. The substance has a low potential for adsorption.

The substance hydrolyses very rapidly in contact with water, generating monosilicic acid and hydrochloric 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 amorphous polysilicic acid. These hydrolysis products are inorganic substances which enter natural biogeochemical cycles; adsorption/desorption studies are not relevant. Based on their structure and predicted water solubilities, 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.

Cross-reference 2

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 is expected to hydrolyses very rapidly to form monosilicic acid [Si(OH)₄] and hydrochloric acid.

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.

Silicic 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 3

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:

key study

Study period:

1993-10-11 to 1993-10-15

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: 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:

other: hydrolysis products

Basis for effect:

mobility

Duration:

48 h

Dose descriptor:

EC50

Effect conc.:

> 844 mg/L

Nominal / measured:

meas. (initial)

Conc. based on:

test mat.

Remarks:

measured concentration of stock solution.

Basis for effect:

mobility

Remarks on result:

other: Exposure is to the hydrolysis products

Duration:

48 h

Dose descriptor:

LOEC

Effect conc.:

> 844 mg/L

Nominal / measured:

nominal

Conc. based on:

other: hydrolysis products

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 4

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 behavior 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:

other: hydrolysis products

Basis for effect:

mortality (fish)

Duration:

96 h

Dose descriptor:

LC50

Effect conc.:

> 245 mg/L

Nominal / measured:

meas. (arithm. mean)

Conc. based on:

other: hydrolysis products

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 5

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 analysed by GC/FID for ethyl silicate concentration.  Samples analysed on day 0 were removed from the test solutions in the volumetric flasks prior to filling the individual test flasks.  Samples analysed 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 colorless 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 milliliters 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 dilution water 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

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)	Inhibition of yield (biomass) at end of test (%)	Inhibition of growth rate at end of test (%)
0 (Control)	-	10000	34600	261300	1030100	-	-
2.6	1.8	10000	40000	288300	1236900	-20	-3
6.4	0.92	10000	35000	314200	930000	10	2
16	3.6	10000	45800	365000	1343600	-31	-6
40	8.6	10000	40800	289200	1677800	-63	-11
100	22	10000	48300	287500	1685600	-64	-11

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

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). 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

There are no in vivo or in vitro data on the toxicokinetics of silicon tetrachloride.

The following summary has therefore been prepared based on the physicochemical properties of the substance itself and its hydrolysis products and using this data in algorithms that are the basis of many computer-based physiologically based pharmacokinetic or toxicokinetic (PBTK) prediction models. The main input variable for the majority of these algorithms is log Kow so by using this and, where appropriate, other known or predicted physicochemical properties of silicon tetrachloride or its hydrolysis products, reasonable predictions or statements may be made about their potential absorption, distribution, metabolism and excretion (ADME) properties.

Silicon tetrachloride is a moisture-sensitive, volatile liquid that hydrolyses very rapidly in contact with water (half-life approximately 5 seconds at pH 7), generating HCl and silicic acid; hydrogen gas is a further by-product of the hydrolysis reaction. At concentrations above about 100 -150 mg/l (measured as SiO2 equivalents), condensation products of monosilicic acid can also form. At concentrations >100 -150 mg/l of SiO2, monomeric monosilicic acid condenses into insoluble colloidal particles of polysilicic acid (silica sol) or a highly cross-linked network (silica gel). These forms of polysilicic acid are equivalent to synthetic amorphous silica. Most, if not all, hydrolysis will have occurred before absorption into the body, therefore relevant systemic exposure is limited to the hydrolysis products.

Human exposure can occur via the inhalation or dermal routes. Relevant inhalation exposure would be to the hydrolysis products (hydrolysis would occur rapidly when inhaled, even if a mixture of parent and hydrolysis products were present in air). The substance would also hydrolyse rapidly in contact with moist skin. The resulting HCl hydrolysis product would be severely irritating or corrosive.

Potential systemic exposure to hydrochloric acid is not discussed.

Absorption

Oral

Significant oral exposure is not expected for this corrosive substance.

However, oral exposure to humans via the environment may be relevant for the hydrolysis product, silicic acid and then silica. When oral exposure takes place, it is necessary to assume that except for the most extreme of insoluble substances, that uptake through intestinal walls into the blood takes place. Uptake from intestines can be assumed to be 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) through aqueous pores or carriage of such molecules across membranes with the bulk passage of water (Renwick, 1993).

As silica is water soluble (approximately 100 - 150 mg SiO2/l with condensation occurring at higher concentrations) and has a molecular weight of approximately 60.08 g/mol it meets both of these criteria, so should oral exposure occur it is reasonable to assume systemic exposure will occur also. Gastrointestinal absorption of insoluble silica will be insignificant as compared to the absorption of the soluble species (Carlisle, 1986).

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. Due to the very rapid hydrolysis silicon tetrachloride on contact with skin, systemic exposure via this route is predicted to be minimal. After or during deposition of a liquid on the skin, evaporation of the substance and dermal absorption occur simultaneously so the vapour pressure of a substance is also relevant and because silicon tetrachloride is volatile this would further limit the potential for absorption.

Silicon tetrachloride hydrolyses rapidly on the skin, thus producing silicic acid and HCl. The molecular weights of the hydrolysis products favour absorption across the skin. However, silica is water soluble (approximately 100 - 150 mg SiO2/l with condensation occurring at higher concentrations), which suggests that it is too hydrophilic to cross the lipid rich stratum corneum. Since the other hydrolysis product, HCl is corrosive to the skin, damage to the skin might increase penetration. Absorption of the insoluble condensation products is not expected.

Available dermal studies did not show evidence of systemic availability, as effects (such as those on body weights) are generally thought to be secondary to corrosion of the skin.

Inhalation

Inhalation exposure would be to the hydrolysis products as silicon tetrachloride would hydrolyse rapidly when inhaled, even if a mixture of parent and hydrolysis products were present in air. Once hydrolysis has occurred, significant uptake would be expected into the systemic circulation, as the silicic acid hydrolysis product is highly soluble (approximately 100 - 150 mg SiO2/l with condensation occurring at higher concentrations). Due to the hydrophilic nature of silicic acid, it is likely that some will be retained within the mucous of the lungs and thus absorption will be limited. Condensation to silica might lead to some precipitate being retained in the lining of the respiratory tract, although this cannot be confirmed from results of the only reliable experimental animal study available.

As with dermal exposure, damage to membranes caused by the corrosive nature of the hydrochloric acid hydrolysis product might enhance the uptake. In the available acute inhalation toxicity studies, the only adverse effects appeared to be secondary to corrosive effects of the test substance.

Distribution

All absorbed material is likely to be in the form of the hydrolysis products, HCl and silicic acid, which rapidly precipitates to insoluble silica (SiO2) when the concentration is sufficiently high. Silicic acid is a small 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).  Hydrogen and chloride ions will enter the body’s natural homeostatic processes.

Metabolism

Silicon tetrachloride is rapidly hydrolysed to HCl and silicic acid, which rapidly precipitates to insoluble silica (SiO2) when the concentration is sufficiently high. Most if not all of this will have occurred before absorption into the body. Silicic acid is not metabolised, but forms a precipitate, as previously described. 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). Genetic toxicity tests in vitro showed no observable differences in effects with and without metabolic activation.

Excretion

A determinant of the extent of urinary excretion is the soluble fraction in blood. Given the hydrophilic nature of the hydrolysis product, silicic acid, the soluble fraction of silicic acid in blood is extremely high suggesting it is likely to be effectively eliminated via the kidneys in urine and accumulation is very unlikely.

Following oral ingestion precipitated silica will be eliminated in faeces. The low molecular weight and high water solubility of silicic acid suggest that it is likely to be rapidly eliminated via the kidneys in urine. There is therefore no evidence to suggest that this substance will accumulate in the body.

References

Renwick A. G. (1993) Data-derived safety factors for the evaluation of food additives and environmental contaminants. Fd. Addit. Contam. 10: 275-305.

Iler, Ralph K. (1979) The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties and Biochemistry of Silica, Wiley, p. 13.

Carlisle EM. Silicon as an essential trace element in animal nutrition. Ciba Found Symp. 1986;121:123-39.

Cross-reference 7

Reason / purpose for cross-reference:

data waiving: supporting information

Reference

Endpoint:

activated sludge respiration inhibition testing

Type of information:

experimental study

Adequacy of study:

key study

Study period:

2009-09-04 to 2009-09-04

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Remarks:

The study was conducted according to an appropriate OECD test guideline, and in compliance with GLP, and was considered to be reliability 1 (reliable without restrictions).

Qualifier:

according to guideline

Guideline:

OECD Guideline 209 (Activated Sludge, Respiration Inhibition Test

Deviations:

no

GLP compliance:

yes

Analytical monitoring:

no

Vehicle:

no

Details on test solutions:

PREPARATION AND APPLICATION OF TEST SOLUTION
- Method: 54 µl of the test substance, corresponding to 50 mg, was pipetted below the surface of the
Milli-RO-water and stirred briefly to mix (5 minutes). Subsequently, synthetic sewage feed and sludge were added (final volume 500 ml) and the test was initiated. A loading rate of 100 mg/l was tested in duplicate. Optimal contact between the test substance and test medium was ensured by applying continuous aeration and stirring during the exposure period.

Test organisms (species):

activated sludge of a predominantly domestic sewage

Details on inoculum:

- Source: Activated sludge from municipal sewage treatment plant: ‘Waterschap de Maaskant', 's-Hertogenbosch, The Netherlands, receiving predominantly domestic sewage.

- Preparation of inoculum for exposure: The sludge was coarsely sieved to remove large particles. The sludge was then washed by centrifuging at 1000 g for 10 minutes. The supernatant was decanted and replaced with ISO-medium. The washing step was performed three times. A small amount of the sludge was weighed and dried overnight at ca. 105°C to determine the amount of suspended solids (3.8 g/l of sludge, as used for the test). The pH was 7.8 on the day of testing. The batch of sludge was used within two days after collection; therefore 50 ml of synthetic sewage feed was added per litre of activated sludge at the end of each day. The sludge was kept aerated at test temperature until use.

- Initial biomass concentration: 3.8 g/l of sludge, as used for the test

Test type:

static

Water media type:

freshwater

Limit test:

yes

Total exposure duration:

3 h

Test temperature:

18.5 to 19.0 °C

pH:

8.4

Details on test conditions:

TEST SYSTEM
- Test vessel: All glass, approximately 300 ml oxygen bottles and 1 litre test bottles.

- Aeration: Strong aeration with clean, oil-free air.

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

- No. of vessels per control (replicates): Two controls without test substance were tested in each test series, one at the start and one at the end.

TEST MEDIUM / WATER PARAMETERS
- Source/preparation of dilution water: Tap-water purified by reverse osmosis (Milli-RO) and subsequently passed over activated carbon and ion-exchange cartridges (Milli-Q) (Millipore Corp.)

EFFECT PARAMETERS MEASURED (with observation intervals if applicable) :
After the 3-hour contact time, a well mixed sample of the contents was poured into a 300 ml oxygen bottle, and the flask was sealed with an oxygen electrode connected to a recorder, forcing the air out of the vessel. Oxygen consumption was measured and recorded for approximately 10 minutes. During measurement, the sample was not aerated but continuously stirred on a magnetic stirrer. Oxygen meter: WTW inolab Oxi 730 supplied with a WTW CellOx 325 oxygen electrode, electrolyte type ELY/G.

OTHER TEST CONDITIONS
- Adjustment of pH: The pH of synthetic sewage feed was adjusted with 1 M NaOH (Merck) to 7.4.


TEST CONCENTRATIONS
- Spacing factor for test concentrations: Performance of a limit concentration at 100 mg/l is acceptable, because 100 mg/l is far above the water solubility of Tetraethyl orthosilicate and thus exceeds the expected concentration in a Sewage Treatment Plant.

Reference substance (positive control):

yes

Remarks:

3, 5-dichlorophenol

Key result

Duration:

3 h

Dose descriptor:

EC50

Effect conc.:

> 100 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

inhibition of total respiration

Remarks:

respiration rate

Results with reference substance (positive control):

The 3-hour EC50 of 3,5-dichlorophenol was in the accepted range of 5-30 mg/l

Validity criteria fulfilled:

yes

Conclusions:

An activated sludge respiration inhibition 3 hour EC50 value of >100 mg/L was determined in a reliable study conducted according to an appropriate test protocol, and in compliance with GLP.

Cross-reference 8

Reason / purpose for cross-reference:

data waiving: supporting information

Reference

The substance hydrolyses very rapidly in contact with water, generating monosilicic acid and hydrochloric acid (HCl). 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 the 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 silicon tetrachloride (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.

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

Cross-reference 9

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 silicon tetrachloride (CAS 10026-04-7) is approximately 5 seconds at 25°C and pH 4, 7 and 9 (based on read-across data); the substance will therefore undergo very rapid hydrolysis in contact with water. This half-life relates to the rapid hydrolysis of the Si-Cl bonds to Si-OH, giving monosilicic acid and hydrogen chloride, which rapidly dissolves in water to form hydrochloric acid. 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.

The log K_ow of monosilicic acid (Si(OH)₄) is -4 and the water solubility is approximately 100-150 mg/l (limited by condensation reactions) (see Section 4.8 of IUCLID for further discussion).

The non-silanol hydrolysis product, hydrogen chloride (hydrochloric acid), 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 only. 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.

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.

 

READ-ACROSS JUSTIFICATION

No measured data are available for the registration substance. Data have therefore been read across from relevant substances to assess the toxicity of the silanol hydrolysis product to aquatic organisms.

 

In order to reduce testing, read-across is proposed to fulfil up to REACH Annex X requirements for the registration substance from substances that have similar structure and physicochemical properties. Ecotoxicological studies are conducted in aquatic medium or in moist environments; therefore the hydrolysis rate of the substance is particularly important since after hydrolysis occurs the resulting product has different physicochemical properties and structure.

 

In moist medium, silicon tetrachloride hydrolyses very rapidly (half-life approximately 5 seconds at 20-25°C and pH 7), with the final hydrolysis products being monosilicic acid and hydrogen chloride, which rapidly dissolves in water to form hydrochloric acid. The non-silanol hydrolysis product hydrogen chloride (hydrochloric acid) is not expected to contribute to any adverse effects at the relevant dose levels. This is discussed further below.

 

The registration substance and the substance used as surrogate for read-across are part of a class of chlorosilane and alkoxysilane compounds which hydrolyse rapidly or moderately rapidly to produce monosilicic acid (Si(OH)₄) and another non-Si hydrolysis product. Si(OH)₄ has not been isolated and only exists in dilute aqueous solution. It readily and rapidly (within minutes) condenses to give amorphous polysilicic acid. Depending on the pH and concentration, solutions will contain varying proportions of monosilicic acid, cyclic and linear oligomers and polysilicic acid of three-dimensional structure. Further details are given in supporting reports (PFA 2015ao and PFA 2013x) attached in Section 13 of the IUCLID dataset.

 

Analogue approach justification

Silicic acid is a naturally-occurring substance which is not harmful to aquatic organisms at relevant concentrations. Silicic 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, 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, all the available studies with aquatic organisms report no effects at 100 mg/l nominal loading in short-term toxicity studies (see Table 2 in PFA 2013x for key studies). The other non-Si hydrolysis product, HCl 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'.

 

Given that all substances produce silicic acid and no toxicity is observed, it is possible to read-across freely within the analogue group. (Reference PFA 2013x).

 

Please see the attached report in Section 13 for the analogue approach to address ecotoxicity of silicon tetrachloride (CAS 10026-04-7).

 

Considerations on the non-silanol hydrolysis product, hydrochloric acid:

Chloride ions occur naturally (typically at levels 40 – 160 mg/l in environmental fresh waters). Standard test media contain chloride salts at levels equivalent to approximately 20 – 64 mg Cl-/l.

Effects on aquatic organisms arising from exposure to hydrochloric acid are thought to result from a reduction in the pH of the ambient environment (arising from an increase in the H+ concentration) to a level below their tolerable range. Aquatic ecosystems are characterized by their ambient conditions, including the pH, and resident organisms are adapted to these conditions. The pH of aquatic habitats can range from 6 in poorly-buffered ‘soft’ waters to 9 in well-buffered ‘hard’ waters. The tolerance of aquatic ecosystems to natural variations in pH is well understood and has been quantified and reported extensively in ecological publications and handbooks (e. g. OECD SIDS 2002 for CAS No. 7647-01-0, hydrochloric acid). It is not considered appropriate or useful to derive a single aquatic PNEC for hydrochloric acid because any effects will not be a consequence of true chemical toxicity and will be a function of, and dependent on, the buffering capacity of the environment. Physical hazards related to pH effects are considered in the risk management measures (e.g. neutralisation) for effluents/aqueous waste.

 

It is not appropriate for this substance to discuss the combined ecotoxicological potency of the silicon and non-silicon hydrolysis products because:

•            effects arising from exposure to HCl are related to changes in pH and not true chemical toxicity;

•            monosilicic acid has a predicted first dissociation constant around 10 and so does not significantly affect the pH of an aqueous solution;

•            the silicon-containing hydrolysis products are not toxic to aquatic organisms at 100 mg/l in short-term studies.

 

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.

OECD SIDS (2002). SIDS Initial Assessment Report for SIAM 15, Boston, USA, 22-25th October 2002, Hydrochloric acid, CAS 7647-01-0.

OECD SIDS (2004). SIDS Initial Assessment Report for SIAM 18, Paris, France, 20-23 April, 2004, Soluble Silicates, CAS 1344-09-8 Silicic acid, sodium salt; CAS 6834-92-0 Silicic acid (H2SiO3), disodium salt; CAS 10213-79-3 Silicic acid (H2SiO3), disodium salt, pentahydrate; CAS 13517-24-3 Silicic acid (H2SiO3), disodium salt, nonahydrate; CAS 1312-76-1 Silicic acid, potassium salt.

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

PFA, 2015ao, Peter Fisk Associates. The aquatic chemistry of inorganic silicic acid generators, PFA.404.001.001.

Cross-reference 10

Reason / purpose for cross-reference:

data waiving: supporting information

Reference

Assessed substance:

substance itself

Composition of assessed substance:

Silicon tetrachloride

PBT status of the assessed substance:

PBT assessment does not apply