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Environmental fate & pathways

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Hydrolysis

Hexafluorosilic acid is manufactured as a concentrated solution that contains a significant amount of hydrofluoric acid (HF) to prevent the dissociation and hydrolysis of hexafluorosilicic acid. In an aqueous environment, hexafluorosilic acid dissociates with the formation of hydrogen fluoride:

 

H2SiF6→ 2HF + SiF4

 

and subsequent hydrolysis of silicon tetrafluoride:

 

SiF4+ H2O → 4HF + Si(OH)4

 

The resulting hydrogen fluoride will dissociate to from hydrogen and fluoride ions:

 

HF → H++ F-

 

The behaviour of soluble silicates is complex. The net reaction given below:

 

H2SiF6+ H2O → 6HF + Si(OH)4

 

may actually cover a number of reactions:

 

H2SiF6+ 6 OH-→6F-+ Si(OH)4+ 2 H2O

H2SiF6+ 8 OH-→6F-+ SiO2(OH)22-+ 4 H2O

H2SiF6+ 8 OH-→6F-+ SiO32- +5 H2O

 

The overall rate and equilibrium constant of the overall reaction will be influenced by pH and concentration, but at typical environmental pH, the dissociation and hydrolysis of hexafluorosilicate to fluoride is essentially 100%.