Octaphenylcyclotetrasiloxane

Biodegradation in water: screening tests

Link to relevant study record(s)

Reference

Endpoint:

biodegradation in water: ready biodegradability

Type of information:

experimental study

Adequacy of study:

key study

Study period:

2012-04-11 to 2012-05-10

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

according to guideline

Guideline:

OECD Guideline 301 B (Ready Biodegradability: CO2 Evolution Test)

Deviations:

no

Qualifier:

according to guideline

Guideline:

other: SEPA, P.R. China. The Guideline for testing of chemicals No. 301B: CO2 Evolution Test 2004

Deviations:

no

Qualifier:

according to guideline

Guideline:

other: SEPA, P.R. China. The Guidelines for the hazard evaluation of new chemical substances (HJ/T 154-2004)

Deviations:

no

Qualifier:

according to guideline

Guideline:

other: SEPA P.R China. The Guidelines for testing of chemicals (HJ/T 153-2004)

Deviations:

no

GLP compliance:

yes

Oxygen conditions:

aerobic

Inoculum or test system:

activated sludge, domestic (adaptation not specified)

Details on inoculum:

A fresh sample of activated sludge was collected from the aeration of Shenyang North Sewage Treatment Center 5 days before the start of the study.

On the day of sample collection, coarse particles were removed by filtration through a fine sieve and the sample was allowed to settle. The supernatant was discarded and the concentrated sludge in the mineral medium was suspended.

Five replicate of 10 ml sludge was weighed and dried at 105°C for approximately 2±0.5 hours, from this result, the suspended sludge was prepared with minimal medium to yield a concentration of 4.2 g suspended solids (SS)/L and the sludge was kept aerobic at 22±2°C thereafter and 2 L of mineral medium was prepared for inoculum suspension. On the day of test starting, five replicate of 10 ml sludge was weighed and dried at 105°C for approximately 2±0.5 hours. The measured concentration of the suspended sludge was 4552 mg/l, from this result, the added amount of wet sludge was calculated.

Duration of test (contact time):

29 d

Initial conc.:

13.3 mg/L

Based on:

test mat.

Parameter followed for biodegradation estimation:

CO2 evolution

Details on study design:

Equipment and apparatus:
Normal laboratory equipment and especially the following were used:
a) Brown flasks 2.5 L, each fitted with aeration tube reaching nearly to the bottom of the vessel and an outlet
b) DL50 Mettler Toledo Titrator
c) Electronic Balance: Mettler Toledo XS205DU, Mettler Toledo AL204-IC, Sartorium BS 124S
d) Mettler Toledo SG2 pH meter
e) Flow meter
f) Magnetic stirrer

Test conditions:
Concentration of test item: 13.3 mg C/L (n =2)
Concentration of microbial inoculums: approximately 30 mg suspended solids (SS)/L
Test temperature: 22±2°C
Period: 29 days
The test was performed in diffuse light

Test Method:
A) Preparation of mineral medium:
The stock solutions were prepared as the following using analytical grade reagents and distilled water:
i) Dissolve 8.5 g of potassium dihydrogen orthophospahte (KH2PO4), 21.75 g dipotassium hydrogen orthophosphate (K2HPO4), 33.4 g disodium hydrogen orthosphosphate dihydrate (Na2.HPO4.2H2O) and 0.5 g ammonium chloride (NH4Cl) in water and make up to 1 L. The pH of the solution was 7.54.
ii) Dissolve 2.75 g calcium chloride anhydrous (CaCl2) in water and make up to 100 ml
iii) Dissolve 2.25 g magnesium sulfate heptahydrate (MgSO4.7H2O) in water and make up to 100 ml.
iv) Dissolve 0.025 g iron (III) chloride hexahydrate (FeCl3.6H2O) in water abd make up to 100 ml.

For each liter of mineral medium, 10 ml stock solution (i) was added to approximately 800 ml distilled water and the mixture stirred before adding, sequentially, 1 ml each of stock solutions (ii), (iii) and (iv). After mixing, the mixture was made up to 1 L with distilled water. The pH was checked and adjusted to the range of 7.4±0.2 with HCl. 20 L mineral medium was prepared for test mixture preparation.

B) Preparation of CO2 pre-absorption bottles
Five 500 ml scrubbing flasks were were chosen as CO2 pre-absorption bottles and scrubbing bottle, while the 1st, 2nd and 3rd bottles with 350 ml 10 mol/L NaOH, the 4th with 350 ml 0.05 mol/L Ba(OH)2 and the 5th bottle with distilled water.

C) Preparation of test flasks
1800 ml mineral medium was added to each 2.5 L brown flask. 13.185 ml of the prepared activated sludge was added to give a concentration of approximately 30 mg SS/L in the final 2 L of inoculated mixture. These inoculated mixtures were aerated with CO2-free air overnight to purge the system of carbon dioxide.

In the study, the following flasks were used:
Inoculum controls 1 and 2 (identifed as IC-1 and IC-2) - containing mineral medium and inoculums
Test suspension 1 and 2 (identified as TS-1 and TS-2) - containing mineral medium, test item and inocula
Toxicity control (identified as TC) - containing mineral medium, test item, reference item and inocula
Procedure control (identified as PC) -containing mineral medium, reference item and inocula.

The method of test addition was as followed: 0.0420 g and 0.0419 g of weighted test item were added to TS-1 and TS-2 flask directly and brown bottles were used during exposure. The flasks were made up to 2 L with further mineral medium. The volume in flask was 2 L to give a final concentration of 13.3 mg carbon/L.

In PC flask, 0.0398 g of weighed sodium benzoate was added directly. The flask was made up to 2 L with further mineral medium. The volume in flask was 2 L to give a final concentration of 11.6 mg carbon/L.

Both 0.0420 g of weighed test item and 0.0413 g of weighed sodium benzoate were added to TC flask directly, the flask was made up to 2 L with further mineral medium. The volume in flask was 2L to give a final concentration of 25.4 mg carbon/L.

IC-1 and IC-2 flask were made upt o 2 L with further mineral medium.

D) CO2 absorption bottle
Three 250 ml scrubbing flasks containing 100 ml of 0.0125 mol/L Ba(OH)2 solution were connected to absorb CO2 produced from the test suspensions and inoculum controls. The concentration of freshly prepared Ba(OH)2 solution was determined at every absorbed change.

E) Bioassay conditions
Brown flasks were used to avoid direct light. The test suspensions were stirred with magnetic stirrer during the test and bubbled with CO2-free air at a rate of 30-100 ml/min. The room temperature were kept at 22±2°C. Stirring, aeration and temperature were checked on every work day.

F) CO2 determination
During the test period, analyses of CO2 were made at 1, 2, 3, 5, 7, 9, 12, 14, 17, 21, 26 and 29 day.

On the days of CO2 measurement, the barium hydroxide absorber closest to the test vessel was disconnected and 50 ml of the hydroxide solution was pipetted and titrated with 0.05262 M HCl using titrator (the concentration of HCl was determined with titrimetric analysis). The remaining absorbers were moved one place closer to the test flask and a new absorber containing 100 ml fresh 0.0125 mol/L barium hydroxide was placed at the far end of the series.

On day 28th, the pH values were measured in each flask and then 1 ml of concentrated HCl was added. Test flasks were aerated overnight to drive off the carbon dioxide present in the test suspensions. On the 29th day, all CO2 absorption bottle were disconnected and titrated with 0.05262 M HCl for the last analyses of evolved carbon dioxide.

Reference substance:

benzoic acid, sodium salt

Key result

Parameter:

% degradation (CO2 evolution)

Value:

ca. 9.7 - ca. 9.9

Sampling time:

28 d

Details on results:

The accumulated percentage biodegradation in activated sludge after treatment of 28 days were 9.7% and 9.9%. The test item did not exhibit a percentage biodegradation exceeding the pass level of 60%.

In the toxicity control (TC) test mixture containing both test item and reference item, 27.9% degradation occurred within 5 days based on total ThCO2 and the value exceeded 25%, which indicated that the test item can be assumed to be not inhibitory under the conditions of the test.

Results with reference substance:

The percentage degradation of the reference substance calculated from the CO2 evolution was 68.4% at day 5.

Table 1: Volume of HCl titrated by Ba(OH)₂ Absorber during test period

Day	Volume of HCl titrated: ml (concentration of HCl: 0.05262 M)
	Fresh Ba(OH)2 absorber					IC		TC		TC	PC
	1	2	3	4	5	1	2	1	2
0	23.302	23.028	23.138	23.032	23.070	-	-	-	-	-	-
1	22.824	22.849	22.857	22.883	22.888	21.719	21.511	21.840	22.080	17.299	15.371
2	23.266	23.124	23.345	23.194	23.359	21.628	21.484	21.354	21.744	11.958	11.642
3	22.883	22.892	22.829	22.971	-	21.734	21.315	21.424	21.379	16.584	15.907
5	23.112	23.129	23.029	23.084	22.932	19.959	20.386	20.012	20.004	16.577	16.793
7	23.207	23.205	23.087	23.226	22.974	19.925	19.959	19.911	19.927	18.926	18.566
9	23.126	22.979	23.129	22.999	22.975	19.292	19.956	19.943	19.918	20.478	19.258
12	22.836	22.867	22.878	23.096	23.016	19.967	19.966	19.921	20.003	19.170	18.846
14	23.131	23.141	22.965	23.198	23.056	12.618	21.566	19.942	21.487	20.356	20.425
17	22.968	22.981	22.992	22.895	22.901	19.976	20.456	21.198	21.607	19.936	19.182
21	22.835	23.015	22.946	23.000	22.876	20.441	19.940	19.939	19.889	20.702	19.151
26	23.109	23.024	23.063	22.982	22.932	21.146	20.713	20.406	19.472	19.912	19.932
29-1	-	-	-	-	-	21.128	19.948	20.478	19.263	19.887	19.928
29-2	-	-	-	-	-	22.619	22.611	21.879	22.022	22.398	22.905
29-3	-	-	-	-	-	22.754	22.870	22.486	22.720	22.640	21.945

 

Note:

Fresh Ba(OH)₂ absorber prepared at 0 day were initial absorber on the 1^st, 2^nd, 3^rd day of CO₂ analysis

Fresh Ba(OH)₂ absorber prepared at 1 day were the initial absorber on the 5^th day of CO₂ analyses and the following were corresponding in sequence 29-1, 29-2, 29-3 was expressed as the 1^st, 2^nd, 3^rd absorber of every treatment at 29^th day titration

 

Table 2: Amount of CO₂ produced and percentage biodegradation during 28 days test

Day	CO2 produced (mg)						Biodegradation% = 100*(CO2(mg) TS/TC/PC –mean IC)/ThCO2(mg)				Cumulative biodegradation %
	IC-1	IC-2	TS-1	TS-2	TC	PC	TS-1	TS-2	TC	PC	TS-1	TS-2	TC	PC
1	3.23	3.71	2.95	2.39	13.46	17.93	0.0	0.0	5.4	17.0	0.0	0.0	5.4	17.0
2	3.44	3.77	4.08	3.17	25.83	26.56	0.5	0.0	11.9	27.0	0.5	0.0	17.3	43.9
3	3.20	4.17	3.91	4.02	15.12	16.69	0.2	0.3	6.1	15.3	0.7	0.3	23.5	59.2
5	6.72	5.73	6.59	6.61	14.55	14.05	0.4	0.4	4.5	9.2	1.1	0.7	27.9	68.4
7	7.72	7.64	7.75	7.71	10.03	10.86	0.1	0.0	1.3	3.7	1.2	0.8	29.2	72.1
9	6.86	6.80	6.83	6.89	5.59	8.42	0.0	0.1	0.0	1.9	1.2	0.8	29.2	74.0
12	7.16	7.16	7.26	7.07	9.00	9.75	0.1	0.0	1.0	3.0	1.3	0.8	30.2	77.1
14	3.52	3.64	7.41	3.83	6.45	6.29	3.9	0.2	1.5	3.2	5.2	1.1	31.7	80.2
17	7.10	5.99	4.27	3.32	7.19	8.94	0.0	0.0	0.3	2.8	5.2	1.1	32.1	83.0
21	5.78	6.94	6.95	7.06	5.18	8.77	0.6	0.7	0.0	2.8	5.8	1.8	32.1	85.9
26	4.52	5.52	6.23	8.40	7.38	7.33	1.2	3.5	1.3	2.7	7.0	5.3	33.3	88.6
28-1	4.21	6.95	5.72	8.53	7.09	6.99	0.1	3.0	0.8	1.7	7.2	8.3	34.1	90.2
28-2	0.73	0.75	2.44	2.11	1.24	0.07	1.7	1.4	0.3	0.0	8.9	9.7	34.4	90.2
28-3	0.62	0.35	1.24	0.70	0.88	2.49	0.8	0.2	0.2	2.4	9.7	9.9	34.6	92.6

 

Note:

If CO₂ production (mg) (TS/TC/PC-mean IC)<0, it is considered that biodegradation is temporarily prohibited.

CO₂ production is considered to be 0, and values of the cumulative CO₂ production are constant

Cumulative percentages are calculated from unrounded values and may be different to those presented above.

Day 28 data presented were the results of the analysis on Day 29, following acidification of the cultures on Day 28.

Table 3: pH measured at the end of the test

Treatment	pH
IC-1	7.17
IC-2	7.15
TS-1	7.24
TS-2	7.26
TC	7.31
PC	7.31

 

IC = Inoculum control; TS = Test suspension; TC = Toxicity control; PC= Procedural control

 

Validity criteria fulfilled:

yes

Interpretation of results:

not readily biodegradable

Conclusions:

In a ready biodegradation study in accordance with OECD 301B Test Guideline, biodegradation of 9.7-9.9% in 28 days were obtained for the substance. The result is considered to be reliable.

Description of key information

Biodegradation in water: screening tests: 9.7-9.9% in 28 days (CO₂ evolution, OECD 301B), based on read-across from a structurally-related substance.

Key value for chemical safety assessment

Biodegradation in water:

under test conditions no biodegradation observed

Additional information

There are no reliable ready biodegradation data available for octaphenylcyclotetrasiloxane (CAS No. 546-56-5), therefore good quality data for the structurally-related substance, 1,1,7,7-tetramethyl-3,3,5,5-tetraphenyl-tetrasiloxane (CAS No. 70940-00-0), have been read across.

Octaphenylcyclotetrasiloxane (CAS No. 546-56-5) and 1,1,7,7-tetramethyl-3,3,5,5-tetraphenyl-tetrasiloxane (CAS No. 70940-00-0) are structurally-similar substances, both are tetrasiloxanes with phenyl functionality. The target substance octaphenylcyclotetrasiloxane (CAS 546-56-5) is a cyclic siloxane containing four Si atoms linked by oxygen. The silicon atoms are each substituted with two phenyl groups. The source substance 1,1,7,7-tetramethyl-3,3,5,5-tetraphenyl-tetrasiloxane (CAS No. 70940-00-0) is a linear structure of four Si atoms, the central silicon atoms are substituted with two phenyl groups and the terminal silicon atoms are each substituted with two methyl and one hydrogen group.

 

A comparison of the key physicochemical properties is presented in the table below. The source substance has negligible biodegradability.

Table: Key physicochemical properties of octaphenylcyclotetrasiloxane and surrogate substance 1,1,7,7-tetramethyl-3,3,5,5-tetraphenyl-tetrasiloxane

Property	Octaphenylcyclotetrasiloxane (CAS No. 546-56-5)	1,1,7,7-Tetramethyl-3,3,5,5-tetraphenyl-tetrasiloxane (70940-00-0)
Molecular weight (g/mol)	793.19	530.92
log Kow	9.0	9.0
log Koc	6.0	6.0
Water solubility (mg/l)	<1.0E-05	<1.0E-05
Vapour pressure at 25°C (Pa)	<1.0E-06	<1.0E-06

The registration and read-across substances are within the Reconsile Siloxane Category of substances within which, in general, there is no evidence of any significant biodegradation. Therefore, in the context of the RAAF, Scenario 6 is considered to apply. The Category hypothesis is that read-across of “not readily biodegradable” between siloxanes is appropriate where data are not available for the substance itself. Additional information is given in a supporting report (PFA, 2017) attached in Section 13.

The table below presents ready biodegradation data for substances within the Siloxane Category.

Table: Ready biodegradation data for substances within the Siloxane Category

CAS	Name	Readily biodegradable? Yes/no	Result: Biodegradation after 28 days (%)	Guideline	Test type	Klimisch code
000107-46-0	Hexamethyldisiloxane	No	2%	OECD 301C	BOD	1
000107-51-7	Octamethyltrisiloxane	No	0%	OECD 310	CO2	1
000540-97-6	Dodecamethylcyclohexasiloxane	No	4.5%	OECD 310	CO2	1
000541-02-6	Decamethylcyclopentasiloxane	No	0.14%	OECD 310	CO2	1
000556-67-2	Octamethylcyclotetrasiloxane	No	3.70%	OECD 310	CO2	1
001873-88-7	1,1,1,3,5,5,5-heptamethyltrisiloxane	No	0%	OECD 310/ ISO Guideline No 14593	CO2	1
002627-95-4	1,1,3,3-tetramethyl-1,3-divinyldisiloxane	No	0%	OECD 301D	BOD	1
017928-28-8	1,1,1,3,5,5,5-heptamethyl- 3-[(trimethylsilyl)oxy] trisiloxane	No	0%	OECD 301F	BOD	1
017955-88-3	1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane	No	10 – 32 % after 25 d	OECD 301B	CO2	1
060111-47-9	3-[(dimethylvinylsilyl)oxy]-1,1,5,5-tetramethyl-3-phenyl-1,5-divinyltrisiloxane	No	2.20%	OECD 301F	BOD	1
070940-00-0	1,1,7,7,-Tetramethyl-3,3,5,5,-Tetraphenyl-Tetrasiloxane	No	9.7 and 9.9%	OECD 301B	CO2	1
180073-46-5	Pentasiloxane, 1,1,9,9-tetramethyl-3,3,5,5,7,7-hexaphenyl-	No	5.2 and 4.4%	OECD 301B	CO2	1

It is therefore considered valid to read-across the results for 1,1,7,7-tetramethyl-3,3,5,5-tetraphenyl-tetrasiloxane (CAS No. 70940-00-0) to fill the data gap for the registered substance. Additional information is given in a supporting report (PFA, 2017at) attached in Section 13.

1,1,7,7-Tetramethyl-3,3,5,5-tetraphenyl-tetrasiloxane attained 9.7 -9.9% biodegradation in 28 days (CO₂ evolution).

Reference:

PFA (2017at). Peter Fisk Associates. Siloxane Category Report for Environmental Endpoints. PFA.404.114.001