『Abstract
The forward dissolution rate of San Carlos forsterite Fo91 was measured at 25℃ in a mixed-flow reactor
as a function of pH (1 to 12), ionic strength (0.001 to 0.1 M),
ΣCO2 (0 to 0.05 M), aqueous magnesium (10-6
to 0.05 M) and silica (10-6 to 0.001 M) concentrations.
In CO2-free solutions, the rates decrease
with increasing pH at 1≦pH≦8 with a slope close to 0.5. At 9≦pH≦12,
the rates continue to decrease but with a smaller slope of 〜0.1.
Addition of silicon to solution at pH above 8.8 leads to reduction
of up to 5 times in the dissolution rate. Magnesium ions have
no effect on forsterite dissolution rate at pH from 3 to 6 and
10-5＜[Mg2+]tot＜0.04
M. Aqueous carbonate ions strongly inhibit dissolution in alkaline
solutions when a CO32-＞10-4
M. In acidic and slightly alkaline solutions, forsterite dissolution
is controlled by the decomposition of a silica-rich/magnesium-deficient
protonated precursor complex. This complex is formed by exchange
of two hydrogen ions for a Mg atom on the forsterite surface followed
by polymerization of partially protonated SiO4
tetrahedra and rate-controlling H+ penetration into
the leached layer and its adsorption on silica dimers. This accounts
for the observed 0.5 order dependence of dissolution rate on H+
activity. In alkaline solutions, dissolution is controlled by
the decomposition of Mg hydrated sites in a Mg-rich layer formed
by silica preferential release. Within this conceptual model,
forsterite forward rate of dissolution can be accurately described
for a wide variety of solution compositions assuming two parallel
reactions occurring at silica-rich and hydrated Mg surface sites:
R + (mol/cm2/s)
＝ 2.38×10-11 {＞Si2O-H+}
＋1.62×10-10 {＞MgOH2+}
where {＞i } stands for surface species concentration (mol/m2).
This equation describes the weak dependence of dissolution rates
on pH in alkaline solutions and the inhibiting effect of carbonate
ions and dissolved silica when the hydration of surface Mg atoms
with formation of ＞MgOH2+ is the
rate-controlling step for dissolution. It follows that the decrease
of forsterite dissolution rate with increasing carbonate concentration
at pH≧9 in natural aqueous systems results in a decrease of atmospheric
CO2 consumption, i.e., unlike for feldspars,
there is a negative feedback between pCO2
and forsterite weathering rate. This should be taken into account
when modeling the effect of mafic mineral weathering on CO2 global balance.』
1. Introduction
2. Materials and methods
3. Results and discussion
3.1. Dissolution rates as a function of solution composition
3.2. Forsterite dissolution mechanism
3.2.1. Acidic solutions
3.2.2. Alkaline solutions
3.3. Comparison with previous results
3.3.1. Acidic solutions
3.3.2. Basic solutions
4. Applications
5. Concluding remarks
Acknowledgments
References