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Environmental fate & pathways

Hydrolysis

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Reference
Endpoint:
hydrolysis
Type of information:
other: Well documented publication
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Study based on accepted scientific principles, well documented publication
Reason / purpose:
reference to same study
Reason / purpose:
reference to other study
Qualifier:
equivalent or similar to
Guideline:
other: OECD 105 (water solubility)
Principles of method if other than guideline:
Development of a dissolution model for silicon oxides, taking into account Si-O bond cleavage and formation as well as protolysis of silanol groups. Three different models, pH dependent surface potential, diffuse double layer, and gel layer, are investigated.
Experiments carried out with a well-defined amorphous silica (Monospher 250 (Merck Darmstadt) included dependence on pH and NaCl concentration at 40 °C.
GLP compliance:
no
Analytical monitoring:
yes
Preliminary study:
Not relevant
Transformation products:
yes
No.:
#1
Details on hydrolysis and appearance of transformation product(s):
See Report, Table 1 and Fig. 4 (see below: AD under attached background material)
Key result
Remarks on result:
other: see remarks
Remarks:
The experimental part showed that there was a distinct pH dependence in the rate of dissolution, increasing with the pH increasing. However, the free dissolved SiO2 reached a maximum independent of the pH (Report, Table 1, Fig.. 4). It is concluded that the total amount of silica dissolved is relatively constant in a broad range of pH (1.1 < pH < 8.9) (p. 4393).

The surface of silica may be covered by a partial hydrolysed gel layer when in contact with water (p. 4389). This layer is in equilibrium with the outer aqueous phase and constitutes a diffusion barrier for ions and water.

It is known and generally recognised that proton and hydroxide ion promoted dissolution plays a more prevailing role in the dissolution process than a simple hydrolysis of siloxane bridges (p. 4390).

The experimental part showed that there was a distinct pH dependence in the rate of dissolution, increasing with the pH increasing. However, the free dissolved SiO2 reached a maximum independent of the pH (Report, Table 1, Fig.. 4). It is concluded that the total amount of silica dissolved is relatively constant in a broad range of pH (1.1 < pH < 8.9) (p. 4393).

The level of maximum solubility was about 2.7 mmol SiO2/L (here: Monosphere, see also 4.8 Water solubility).

The degree of hydrolysis that may be involved in the dissolution process could not yet be solved.

Validity criteria fulfilled:
yes
Conclusions:
Based on the results of the study performed with silicon dioxide, it can be stated that although ion exchange processes are possible, depending on the surrounding environment, the degree of hydrolysis involved in the dissolution of silicates in water is very limited and not quantifiable.

Description of key information

It is not possible to quantify the very limited hydrolysis involved in the dissolution of silicates in water.

Key value for chemical safety assessment

Additional information

Based on the results of the study performed with silicon dioxide, it can be stated that although ion exchange processes are possible, depending on the surrounding environment, the degree of hydrolysis involved in the dissolution of silicates in water is very limited and not quantifiable.