Title page for ETD etd-05122008-150550


Type of Document Dissertation
Author Mitra, Arijit
Author's Email Address amitra@vt.edu
URN etd-05122008-150550
Title Silica dissolution at low pH in the presence and absence of fluoride
Degree PhD
Department Geosciences
Advisory Committee
Advisor Name Title
Rimstidt, james Donald Committee Chair
Chermak, John A. Committee Member
Dove, Patricia M. Committee Member
Schreiber, Madeline E. Committee Member
Keywords
  • dissolution kinetics
  • dissolution rate
  • quartz
  • hydrofluoric acid
  • silica
  • low pH
Date of Defense 2008-04-29
Availability unrestricted
Abstract

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 < style='mso-bidi-font-style:normal'>T < 100 °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 < style='mso-bidi-font-style:normal'>T < 70 °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 “breaking”

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 “disturbed

surface” 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 < 55°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 < 55°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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  Chapter_2_Raw_Data.pdf 35.39 Kb 00:00:09 00:00:05 00:00:04 00:00:02 < 00:00:01
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