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Environmental fate & pathways

Hydrolysis

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Endpoint:
hydrolysis
Type of information:
other: evidence from degradation product
Adequacy of study:
weight of evidence
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
data from handbook or collection of data
Qualifier:
no guideline available
Principles of method if other than guideline:
Data from scientific publication. It is assumed that basic scientific principles where followed for data aquisition.
GLP compliance:
not specified
Specific details on test material used for the study:
Sodium hexafluorosilicate is tested in this study as well. Focus lies on silicate anion properties and behaviour.
Transformation products:
yes
No.:
#1
No.:
#2
Details on hydrolysis and appearance of transformation product(s):
In water, the compound readily dissociates to sodium ions and hexafluorosilicate ions and then to hydrogen gas, fluoride ions, and hydrated silica. At the pH of drinking water (6.5-8.5) and at the concentration usually used for fluoridation (1 mg fluoride/L), the degree of hydrolysis is essentially 100%. Fluorosilicic acid is a moderately strong acid that can corrode glass and stoneware. Like its salt, its degree of hydrolysis is essentially 100% in drinking water, and when reacted with steam or water or when heated to decomposition or highly acidified, toxic and corrosive fumes of fluorides (e.g., hydrogen fluoride and silicon tetrafluoride) are released.
Key result
Remarks on result:
other: At the pH of drinking water (6.5-8.5) and at the concentration usually used for fluoridation (1 mg fluoride/L), the degree of hydrolysis is essentially 100%.
Conclusions:
The degree of hydrolysis is essentially 100% at environemental pH values.
Endpoint:
hydrolysis
Type of information:
other: evidence from degradation product
Adequacy of study:
weight of evidence
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
data from handbook or collection of data
Qualifier:
no guideline available
Principles of method if other than guideline:
Data from scientific publication. It is assumed that basic scientific principles where followed for data aquisition.
GLP compliance:
not specified
Specific details on test material used for the study:
Sodium hexafluorosilicate is tested in this study as well. Focus lies on silicate anion properties and behaviour.
Transformation products:
yes
No.:
#1
No.:
#2
Details on hydrolysis and appearance of transformation product(s):
Hexafluorosilicate ion reacts with water to produce fluoride ion and an assortment of silicon oxyanions,7-8 e.g., SiO3 2-–, SiO4 4–, Si(OH)O3 3–. We represent the oxyanions as SiIV(aq) without further speciation at this time.
SiF6 2– + n H2O º SiIV(aq) + 6 HF(aq) (4)
The actual speciation of silicon oxyanions is a function of acidity, i.e., [H+]. Busey et al.9
showed that virtually 100% of the hexafluorosilicate is hydrolyzed to silicon oxyanions at pH 6, even
when there is a free fluoride concentration of 0.01 M. Meanwhile, fluoridated drinking water
contains only -1 mg/L of fluoride, which equates to 5 × 10–5 M. Previous investigations10-11 found
a non-negligible concentration of residual SiF4 when this gas was passed through water. Ciavatta10
et al. investigated fluorosilicate equilibria with 0.3 # [H+] # 3 molal [m, mol F– (kg water)–1] and
ionic strength fixed at 3 M, adjusted with LiClO4. They concluded that the mixed ligand species
SiF(OH)3 and SiF(OH)2(H2O)+ are significant contributors to total silicon(IV) in addition to SiF4,
SiF6 2–, and HSiF6 – under these conditions. Nonetheless, their results showed that fluoro-complexes
comprised less than 5 mol% of the total silicon(IV) in 0.01 m H+ and 10–4 m F–. Korobitsyn et al.11
examined the hydrolysis of sodium hexafluorosilicate in sodium carbonate solution. Their work was
geared towards an industrial process for producing sodium fluoride and is not directly applicable
here.
The use of chemical shift information derived from 19F NMR spectrometry in understanding
the formation of fluoro-ligated species is well-established.12-17 Fluoride ligand exchange occurs
rapidly between HF and SiF6 2– at temperatures above –10 °C,14 and the identification of aqueous
fluorosilicate species and the measurement of the concomitant equilibrium constants has been done
almost entirely by 19F NMR spectroscopy and spectrophotometry.15-17 The Gmelin Handbook of
Inorganic Chemistry tabulates values for the equilibrium constants expression (6) of the hydrolysis
reaction (5) at temperatures from 0 to 60 °C.18
SiF6 2– + 4 H2O º Si(OH)4 + 4 H+ + 6 F–

[Si(OH)4 ] [H% ] 4 [F & ] 6[SiF 2&6 ] (6)
The smallest value at ambient temperature reported for K is 10–31.6. Using this value at [H+] = 10–6
M and [F–] = 5 × 10–5 M, the ratio [Si(OH)4]/[SiF62–] = 1.6 × 1018. Note that less than 1% of fluoride
exists as HF at drinking water acid levels (i.e., pH > 5.2) since pKa
HF = 3.17.19 Even if the hydrolysis
constant were off by a factor of 1000, it would not matter. There would still be essentially no
hexafluorosilicate ion. A fractional distribution plot in Gmelin18 shows that other fluorosilicates (i.e.,
SiF4 and SiF5–) also drop off dramatically as free fluoride concentration, and not [F–]T, decreases
towards 10–4 M, even in silica-saturated 4 M perchloric acid. For this solution, total fluoride
concentration is expressible as (7), neglecting any mixed fluorohydroxo-ligated species:
[F–]T = [HF] + [F–] + 4 [SiF4] + 5 [SiF5–] + 6 [SiF62–] (7)
Crosby studied the dissociation of sodium hexafluorosilicate and hexafluorosilicic acid in
deionized water.20 He found that about 99 mol% of the hexafluorosilicate had hydrolyzed when
added to water to produce a 1 mg/L fluoride solution; however, the pH of this solution was 4.20,
considerably below a potable water pH. An important factor must be considered in potable water
fluoridation as Crosby explained:
It should be remembered that the actual ionic population of most public drinking-water supplies is somewhat
different from the experimental conditions used in the present and previous studies. Thus, the pH is normally
adjusted to about 7 to 8, and the presence of additional salts may further influence the equilibrium owing to the
formation of complexes with calcium and other metals.
If the pH of a treated drinking water is too low, it is adjusted to comply with regulations (or
consumer complaints) and minimize corrosion. Crosby’s results were obtained in a water that was
demineralized and completely devoid of buffering agents. Consequently, the dissociation of
hexafluorosilicate was hindered by the drop in pH. Thus, Crosby’s fractional dissociation data
cannot be applied directly to a potable water supply without correcting them for pH. Of course, that
correction is the effect we have computed above, namely, the complete hydrolysis of fluorosilicates.
This is precisely what Crosby was emphasizing. This observation hints at the effect on pH, which
we shall come back to shortly.
Interestingly enough, a number of species actually promote the dissociation of
hexafluorosilicate, including ferric ion.21 While the compound PbSiF6•2H2O can be synthesized, it
decomposes quickly in moist air and slowly when dry.22 Perhaps then lead(II) itself promotes
hexafluorosilicate decomposition, such as through the formation of plumbous fluoride. Because
moist air promotes this compound’s destruction, we can infer that it would not be stable in aqueous
solution at all. There is essentially no hexafluorosilicate remaining in drinking water at equilibrium.
Now we must consider how fast hydrolysis takes place. In the 1970s, Plakhotnik conducted
studies into the effects of lithium and calcium cations on the rate of hexafluorosilicate (and tetrafluoroborate) hydrolysis.23-24
Based on Plakhotnik’s results, we calculated4 that the hydrolysis
would be 99 mol% complete in 12 minutes if carried entirely by the uncatalyzed pathway. That
notwithstanding, natural water supplies do contain calcium and other divalent metals as well as
trivalent metal cations (e.g., Al3+, Fe3+); hence, the actual hydrolysis rate would be even faster so that
equilibrium is reached long before water reaches the consumer’s tap.
Based on the above information on both the thermodynamics of the hydrolysis reaction and
its kinetics, we can safely conclude that there is essentially no (« 1 part per trillion) hexafluorosilicate
remaining in drinking water at equilibrium and that equilibrium is rapidly reached from the
combined uncatalyzed and metal-catalyzed reactions.
Key result
Remarks on result:
other: Hexafluorosilicates are hydrolized virtually to 100% under environmental conditions.
Conclusions:
When H2SiF6 is diluted into a water supply, it undergoes dissociation and hydrolysis. Dissociation products are (hydrogen) fluoride and hydrated silica (Si(OH)4). Hydrogen fluoride will deprotonate under environmental conditions.
H2SiF6(aq) + 4 H2O 6 6 HF(aq) + Si(OH)4(aq)
Hexafluorosilicates are hydrolized virtually to 100% under environmental conditions.
Endpoint:
hydrolysis
Type of information:
other: evidence from degradation product
Adequacy of study:
weight of evidence
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
data from handbook or collection of data
Qualifier:
no guideline available
Principles of method if other than guideline:
Data from scientific publication. It is assumed that basic scientific principles where followed for data aquisition.
GLP compliance:
not specified
Specific details on test material used for the study:
Sodium hexafluorosilicate is tested in this study as well. Focus lies on silicate anion properties and behaviour.
Transformation products:
yes
No.:
#1
No.:
#2
Details on hydrolysis and appearance of transformation product(s):
H2SiF6 is sold as a concentrated solution that contains a significant concentration of HF(aq) to prevent dissociation and hydrolysis of the H2SiF6. When H2SiF6 is diluted into a water supply, it undergoes dissociation and hydrolysis. Dissociation products are (hydrogen) fluoride and hydrated silica (Si(OH)4). Hydrogen fluoride will deprotonate under environmental conditions.
Key result
Remarks on result:
other: When diluted in water hexafluorosilicates undergo dissociation and hydrolysis.
Conclusions:
When diluted in water hexafluorosilicates undergo dissociation and hydrolysis.

Description of key information

The substance zinc hexafluorosilicate is produced solely in solution and is not isolated as a solid over its entire life cycle. The solution is adjusted with a pH of <2 to ensure the stability of the hexafluorosilicate anion which decomposes above pH = 3.2 (refer to water solubility: Gmelin, Syst.Nr.32, Zn). The final degradation products are silicic acid (hydrated silica) Si(OH)4and free fluoride anions, depending on pH levels hydrogen fluoride may form.

 

SiF62-+ 4H2O    Si(OH)4(aq) + 6F-+ 4H++ SiF62-+ 2H2O    SiO2+ 6F-+ 2H+

 

Therefore, the risk assessment and toxicological evaluation of zinc hexafluorosilicate is based on zinc cation and fluoride anion.

Refer to read-across justification for further details.

Key value for chemical safety assessment

Additional information