wAbstract
@In light of recent work on the reactivity of specific sites on
large (hydr)oxo-molecules and the evolution of surface topography
during dissolution, we examined the ability to extract molecular-scale
reaction pathways from macroscopic dissolution and surface charge
measurements of powdered minerals using an approach that involved
regression of multiple datasets and statistical graphical analysis
of model fits. The test case (far-from-equilibrium quartz dissolution
from 25 to 300, pH 1-12, in solutions with [Na+]
0.5
M) avoids the objections to this goal raised in these recent studies.
The strategy was used to assess several mechanistic rate laws,
and was more powerful in distinguishing between models than the
statistical approaches employed previously. The best-fit model
included three mechanisms - two involving hydrolysis of Si centers
by H2O next to neutral (Si-OH0)
and deprotonated (Si-O-) silanol groups, and one involving
hydrolysis of Si centers by OH-. The model rate law
is
@dSi/dt (mol/m2s) = e-8.9}0.8 T e(-67.5}2.7
kJ/mol^RT) (Ζ>SiOH) + e3.6}0.7
T e(-82.8}2.1 kJ/mol^RT) (Ζ>SiO-)
+ e6.7}1.8 T e(-77.5}6.0 kJ/mol^RT) aOH- @@@@@@@@@@@@@@(}0.7 log units),
where Ζ>SiOH andΖ>SiO-
are the fraction of surface silanol groups in the neutral and
deprotonated forms, and aOH- is
the bulk activity of OH-. The fitted ’Hφ
value (67.5 kJ/mol) for the dominant low pH mechanism indicates
that the model lacks a fourth mechanism involving protonation
of bridging oxygens on siloxane (Si-O-Si) groups, which cannot
be included because the acidity of bridging oxygens is unknown.
Further progress on this and other, more complex systems requires
development of more predictive and realistic models of surface
speciation.x
1. Introduction
2. Background
3. Methods
@3.1. Surface speciation
@3.2. Aqueous species and their near-surface activities
@3.3. Fitting and statistical analysis
@3.4. Model evaluation
4. Results
5. Discussion
6. Conclusions
Acknowledgments
Appendix A. Supplementary data
References