Silica dissolution at low pH in the presence and absence of fluoride
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SiO2 is the most abundant oxide in the earth and its properties, behaviors and interactions are of immense scientific and technological importance. Of particular importance are the interactions of silica with aqueous fluids because these fluids are present in nearly every natural setting. The dissolution of silica and glass by HF plays a very important role in technology and is widely used for the etching of silica and silicate glasses in the glass industry, in the flint industry, in surface micromachining, in etching of glass fibers for near-field optical probes, in the creation of frosted surfaces for decorative applications like frosted glass and cosmetic vials.
I performed 57 batch reactor experiments in acidic fluoride solutions to measure the dissolution rate of quartz. Quartz dissolution rate data from other published studies were combined with the rate data from my experiments and these 75 data were analyzed using multiple linear regression to produce an empirical rate law for quartz
rqz = 10-4.53 (e-18932/RT) aHF1.18 aH+-0.39
where -5.13 < aHF < 1.60, -0.28 < pH < 7.18, and 25 < T < 100 Â°C.
Similarly, 97 amorphous silica dissolution rate data from published studies were analyzed using multiple linear regression to develop an empirical rate law for amorphous silica
ras = 100.48 (e-34243/RT) aHF1.50 aH+-0.46
where -5.13 < aHF < 1.60, -0.28 < pH < 7.18 and 25 < T < 70 Â°C.
An examination of the empirical rate laws suggests that the rate-determining step in the reaction mechanism involves a coordinated attack of HF and H+ on the Si-O bond where the H+ ion, acting as a Lewis acid, attacks the bridging O atom, while the F end of a HF molecule, acting as a Lewis base, attacks the Si atom. This allows a redistribution of electrons from the Si-O bond to form a O-H and a Si-FH bond, thus â breakingâ the Si-O bond.
In order to quantify the effect of fluoride on the dissolution of silica, I also performed a series of 81 quartz dissolution and 20 amorphous silica dissolution experiments in batch reactors over a pH range of 0 to 7 to investigate the effect of H+ on silica dissolution rates. Between pH 3.5 and 7 silica dissolution rates are independent of pH, but they increase significantly below pH 3.5, so that the dissolution rate of both quartz and amorphous silica at pH 0 is more than an order magnitude faster than the dissolution rate at pH 3.5. I found that the empirical rate law for the dissolution of the â disturbed surfaceâ of quartz in the pH range of 0 to 3.5 is
rqz,pH = 10-0.23 (e-59392/RT) aH+0.28
where 0 < pH < 3.5 and 25 < T < 55Â°C. The empirical rate law for amorphous silica dissolution in the pH range 0 to 3.5 is
rqz,pH = 100.56 (e-64754/RT) aH+0.40
where 0 < pH < 3.5 and 25 < T < 55Â°C.
Based on the empirical rate laws I suggest that the rate-determining step in the reaction mechanism involves a coordinated attack of H3O+, acting as a Lewis acid reacts, on a bridging O atom and the O end of a H2O, acting as a Lewis base, on the Si atom. This results in a redistribution of electrons from the Si-O bridging bond to form two Si-OH surface species.
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