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Hydrolysis

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Endpoint:
hydrolysis
Type of information:
experimental study
Adequacy of study:
key study
Study period:
Jun. 4 - Nov. 18, 2019; experimental phase: Jun. 4 - 27, 2019
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Qualifier:
according to guideline
Guideline:
OECD Guideline 111 (Hydrolysis as a Function of pH)
Version / remarks:
2004
Deviations:
no
Qualifier:
according to guideline
Guideline:
EU Method C.7 (Degradation: Abiotic Degradation: Hydrolysis as a Function of pH)
Version / remarks:
2008
Deviations:
no
GLP compliance:
yes (incl. QA statement)
Specific details on test material used for the study:
Name: Silicic acid, magnesium aluminium sodium salt
Batch no.: 201810LC007
Appearance: white fluffy powder
Composition: Sodium; Magnesium; Aluminium; Silica
Purity: >95%
Homogeneity: homogeneous
Specific surface area: 57.9 m²/g
Radiolabelling:
no
Analytical monitoring:
yes
Details on sampling:
The same sample preparation was performed on day 0 and 5.
Before measurement of the solutions via ICP-OES for Si (blanks and test item solutions) tenfold dilution was performed: 0.5 mL of the solution were added to 4.5 mL water p.a. and measured via ICP-OES.
Buffers:
Buffer-Solution, pH 4
Parameter Amount
CH3COOH, 2 M 80 mL
CH3COONa, 1 M 40 mL
Water p.a. ad. 1000 mL
Three replicates were prepared and combined; pH was measured as 4.02 (batch A20190603).
For batch A20190613 (used in the Tier 1), two replicates were prepared and combined; pH was measured as 4.0.

Buffer-Solution, pH 7
Batch A20190604:
Parameter Amount
KH2PO4 8.7085 g
Water p.a. 500 mL
NaOH, 2 M 14.9 mL
Water p.a. ad. 1000 mL
Three replicates were prepared and combined; pH was adjusted with 2 M NaOH to 7.04.
Batch A20190613 (used in the Tier 1):
Parameter Amount
KH2PO4 8.7089 g / 8.7079 g (for replicate 1 and 2)
Water p.a. 500 mL
NaOH, 2 M 14.9 mL
Water p.a. ad. 1000 mL
Two replicates were prepared and combined; pH was adjusted with 2 M NaOH to 7.00.

Buffer-Solution, pH 9
Batch A20190604:
Parameter Amount
H3BO3 3.093 g
KCl 3.729 g
Water p.a. 500 mL
NaOH, 2 M 10.75 mL
Water p.a. ad. 1000 mL
Three replicates were prepared and combined; pH was adjusted with 2 M NaOH to 9.00.
Batch A20190613 (used in the Tier 1):
Parameter Amount
H3BO3 3.0927 g / 3.0926 g (for replicate 1 and 2)
KCl 3.7298 g / 3.7294 g (for replicate 1 and 2)
Water p.a. 500 mL
NaOH, 2 M 10.75 mL
Water p.a. ad. 1000 mL
Two replicates were prepared and combined; pH was adjusted with 2 M NaOH to 9.03.
Estimation method (if used):
The analytical determination of the test item was performed via measurement of Silicon (Si), Magnesium (Mg) and Aluminium (Al) via ICP-OES.
Duration:
5 d
pH:
4
Temp.:
50 °C
Initial conc. measured:
41.67 mg/L
Remarks:
silicon
Duration:
5 d
pH:
7
Temp.:
50 °C
Initial conc. measured:
46.35 mg/L
Remarks:
silicon
Duration:
5 d
pH:
9
Temp.:
50 °C
Initial conc. measured:
36.6 mg/L
Remarks:
silicon
Duration:
5 d
pH:
4
Temp.:
50 °C
Initial conc. measured:
384.7 mg/L
Remarks:
magnesium
Duration:
5 d
pH:
7
Temp.:
50 °C
Initial conc. measured:
20.21 mg/L
Remarks:
magnesium
Duration:
5 d
pH:
9
Temp.:
50 °C
Initial conc. measured:
3.948 mg/L
Remarks:
magnesium
Number of replicates:
2 (+ blank controls)
Positive controls:
no
Negative controls:
yes
Preliminary study:
The test item was considered to be stable and no further examination was made.

s. below on detailed results
Transformation products:
no
% Recovery:
> 100
St. dev.:
0
pH:
4
Temp.:
50 °C
Duration:
5 d
Remarks on result:
other: Si content
% Recovery:
92.2
St. dev.:
1.5
pH:
7
Temp.:
50 °C
Duration:
5 d
Remarks on result:
other: Si content
% Recovery:
93.4
St. dev.:
0.5
pH:
9
Temp.:
50 °C
Duration:
5 d
Remarks on result:
other: Si content
% Recovery:
100
St. dev.:
0.6
pH:
4
Temp.:
50 °C
Duration:
5 d
Remarks on result:
other: Mg content
% Recovery:
98.2
St. dev.:
2.7
pH:
7
Temp.:
50 °C
Duration:
5 d
Remarks on result:
other: Mg content
% Recovery:
96
St. dev.:
0.3
pH:
9
Temp.:
50 °C
Duration:
5 d
Remarks on result:
other: Mg content
Key result
Temp.:
25 °C
DT50:
> 365 d
Type:
(pseudo-)first order (= half-life)
Remarks on result:
hydrolytically stable based on preliminary test

The half saturated solutions with the nominal load 50 g/L in pH 4, 7 and 9 buffer solutions were measured for Si, Al and Mg on day 0 and after the storage of the test solutions at 49.7 – 50.2 °C for 5 days. The concentration of Al in pH 4, 7 and 9 buffer solutions was below the LOQ of the respective method at each sampling point (< 0.05 mg/L in pH 4 and 9 solutions and < 0.02 mg/L in pH 7 solutions). Therefore, the calculation of the stability of the test item in buffer solutions based on Al was not possible. All measurements lay below the LOD of the Tyndall device (< 3 mg/L particle concentration).

Recovery rate based on solid content reformation of inorganic salts (ICP-OES measured Si content): pH = 4: 102.9%; pH = 7: 92.2%, pH = 9: 93.4%. As the amount of Si and Mg after five days was more than 90% of the amount at the start of the test, the test item was considered to be stable (following OECD, t(0.5) at 25 °C is then greater than 1 year), and no further examination was made.

Validity criteria fulfilled:
yes
Conclusions:
The concentration of Al in pH 4, 7 and 9 buffer solutions lay below the LOQ of the respective method at each sampling point (< 0.05 mg/L in pH 4 and 9 solutions and < 0.02 mg/L in pH 7 solutions). Therefore, the calculation of the stability of Al in buffer solutions was not possible.
As the amount of Si and Mg after five days was more than 90 % of the amount at the start of the test, the test item was considered to be stable (following OECD, t(0.5) at 25 °C is then greater than 1 year), and no further examination was made.
Recovery rate based on solid content, reformation of inorganic salts (ICP-OES measured Si content):
pH=4: ca. 100 %; pH=7: 92.2 %; pH=9: 93.4 %
Executive summary:

This study was performed in order to determine the hydrolysis behaviour of Silicic acid, magnesium aluminium sodium salt and stability of dissolved test item in dependence of the pH.

Aqueous buffer solutions of different pH values (pH 4, 7 and 9) were treated with the test item and the filtrated test item solutions were incubated in the dark under controlled laboratory conditions (at constant temperature). After appropriate time intervals, buffer solutions were analysed for Silicon (Si), Magnesium (Mg) and Aluminium (Al) via ICP-OES. The concentration of Sodium (Na) was not determined as Sodium is present in all buffer solutions. Potentially available undissolved particles in the filtrated test item solution were measured using Tyndall device.

Endpoint:
hydrolysis
Type of information:
other: reference link
Adequacy of study:
other information
Reason / purpose for cross-reference:
read-across source

Description of key information

One study was performed in order to determine the hydrolysis behaviour of Silicic acid, magnesium aluminium sodium salt and stability of dissolved test item in dependence of the pH (Affolter, 2019, OECD 111):

The concentration of Al in pH 4, 7 and 9 buffer solutions lay below the LOQ of the respective method at each sampling point (< 0.05 mg/L in pH 4 and 9 solutions and < 0.02 mg/L in pH 7 solutions). Therefore, the calculation of the stability of Al in buffer solutions was not possible.

As the amount of Si and Mg after five days was more than 90 % of the amount at the start of the test, the test item was considered to be stable (following OECD, t(0.5) at 25 °C is then greater than 1 year), and no further examination was made.

Recovery rate based on solid content, reformation of inorganic salts (ICP-OES measured Si content):

pH=4: ca. 100 %; pH=7: 92.2 %; pH=9: 93.4 %

Key value for chemical safety assessment

Additional information

Silicic acid, aluminium magnesium sodium salt is an inorganic solid and moderately soluble in water. Due to its chemical nature and stability, hydrolysis is not expected.

However, ion exchange processes are possible depending on the surrounding environment to transfer the substance in another, as very limited, not quantifiable hydrolysis is involved in the dissolution of silicates in water. It is limited to a gel-layer surrounding the silicate particle in aqueous medium. This layer is in equilibrium with the outer aqueous phase and constitutes a diffusion barrier for ions and water.

Assuming that the dissolution products expressed as Si(OH)4, Mg(OH)2, NaOH and Al(OH)3, which is very reasonable, the total dissolution rate can be calculated to approx. 902 mg/L. (Affolter, 2020, OECD 105)