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Environmental fate & pathways

Hydrolysis

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Endpoint:
hydrolysis
Type of information:
experimental study
Adequacy of study:
other information
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Reason / purpose:
reference to same study
Reason / purpose:
reference to other study
Qualifier:
equivalent or similar to
Guideline:
OECD Guideline 111 (Hydrolysis as a Function of pH)
Deviations:
yes
Remarks:
different test conditions (pH, buffer, temperature range)
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,were investigated.
Experiments carried out with a well-defined amorphous silica (Monospher 250 (Merck Darmstadt) included dependence on pH (pH range 1.1 to 8.9) and NaCl concentration at 40 °C.
GLP compliance:
no
Radiolabelling:
no
Analytical monitoring:
yes
Transformation products:
yes
No.:
#1
Details on hydrolysis and appearance of transformation product(s):
orthosilicic acid and silicates
Remarks on result:
not determinable
Key result
Remarks on result:
not determinable because of methodological limitations
Results with reference substance:
not applicable

The surface of silica may be covered by a partial hydrolysed gel layer when in contact with water (p. 4389 in the publication). 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 (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: Monospher).

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

Validity criteria fulfilled:
not applicable
Conclusions:
The surface of silica may be covered by a partially hydrolysed gel layer when in contact with water. This layer is in equilibrium with the outer aqueous phase and constitutes a diffusion barrier for ions and water. For the dissolution kinetics, the cleavage and formation of Si-oxygen bonds is a rate-limiting step. Proton and hydroxide ion promoted dissolution play a more important role as compared to a simple hydrolysis of siloxane bridges.
Executive summary:

The dissolution of a commercial available silica, Monospher 250, as function of time, pH, and sodium chloride concentrations of 0.1, 0.01, and 0.001 mol/L at 40 °C has been measured to derive a general dissolution model considering all possible rate-limiting steps and including protolysis reactions on the surface and in solution. The measured data confirm that proton and hydroxide ion promoted dissolution plays a more prevailing role in the dissolution process than a simple hydrolysis of siloxane bridges. The degree of hydrolysis that may be involved in the dissolution process could not be quantified.

Endpoint:
hydrolysis
Data waiving:
study technically not feasible
Justification for data waiving:
other:

Description of key information

Hydrolysis, the chemical breakdown due to the reaction with water, releases several silica species from the silica matrix into solution according to the simplified reaction: nSiO2 + 2nH2O <-> nSi(OH)4 and H4SiO4(s) + H2O <-> H3O+(aq) + H3SiO4- (aq). The hydrolysis process is considered a rate-limiting step in the dissolution of silica in water. Current standard analytical techniques are however not suitable to quantitatively determine the extent of hydrolysis and to measure transformation products.

It is noteworthy that the current OECD test guideline is applicable to solutions; it is not known how the results are influenced by the presence of colloidal suspensions which are generated by incomplete dissolution of the test material.

Results of dissolution tests are reported in IUCLID section 4.8. (water solubility).

Key value for chemical safety assessment

Additional information

The surface of silica may be covered by a partially hydrolysed gel layer when in contact with water. This layer is in equilibrium with the outer aqueous phase and constitutes a diffusion barrier for ions and water. For the dissolution kinetics, the cleavage and formation of Si-O bonds is a rate-limiting step. Proton and hydroxide ion promoted dissolution play a more important role as compared to a simple hydrolysis of siloxane bridges. A certain, not yet quantifiable degree of hydrolysis is assumed to be involved in the dissolution process (cf. Löbbus M, Vogelsberger W, Sonnefeld J, Seidel A 1998: Current considerations for the dissolution kinetics of solid oxides with silica. Langmuir 14(16), 4386-4396, American Chem. Society)