『Abstract
We ran a series of 124 semi-batch reactor experiments to measure
the dissolution rate of forsterite in solutions of nitric and
oxalic acid solutions over a pH range of 0-7 and total oxalate
concentrations between 0 and 0.35 m at 25℃. We found that the
empirical rate law for the dissolution of forsterite in these
solutions is
r = (10-7.03aH+0.46)
+ (10-5.44mOX2-0.40aH+0.47)
where aH+ is the activity
of hydrogen ions, mOX is the concentration
of oxalate in molal, and the rate is given in (mol/m2
s). The first term of the rate law expresses the dissolution rate
for solutions with no oxalate and the second term expresses the
additional rate produced by the presence of oxalate ions.
Our experiments show that oxalate-promoted dissolution rates
depend upon both oxalate concentration and pH. Based on this,
we propose a reaction mechanism in which a hydrogen ion and an
oxalate ion are simultaneously present in the activated complex
for the reaction that releases H4SiO4 into solution. By analogy, we propose that water
acts as a ligand in the absence of oxalate, so the first term
of our rate law has aH2Om (=1)
as a hidden term. Thus, our results suggest that both a proton
and a ligand (either an anion or a water molecule) must be present
in the activated complex for forsterite dissolution, suggesting
that proton-promoted and ligand-promoted dissolution are simply
two aspects of the same process, and that the distinction between
the two is artificial.
The increased dissolution rate of forsterite produced by oxalate
ions at a concentration of 0.001 m, a concentration comparable
to the amount of organic acids in typical soils, translates into
an approximately a 6-fold increase in forsterite dissolution rates
at pH＞4.2. This suggests that organic acids have a measurable,
but not profound, effect on chemical weathering.』
1. Introduction
2. Materials and methods
2.1. Materials
2.2. Experimental methods
2.3. Correction for sample withdrawal
2.4. Data filtering/data selection
2.5. Initial rate method
2.6. Calculation of oxalic acid speciation
3. Results
4. Discussion and conclusions
Acknowledgments
Appendix A
References