『Abstract
Laboratory determined mineral weathering rates need to be normalised
to allow their extrapolation to natural systems. The principle
normalisation terms used in the literature are mass, and geometric-
and BET specific surface area (SSA). The purpose of this study
was to determine how dissolution rates normalised to these terms
vary with grain size. Different size fractions of anorthite and
biotite ranging from 180-150 to 20-10μm were dissolved in pH 3,
HCl at 25℃ in flow through reactors under far from equilibrium
conditions. Steady state dissolution rates after 5376 h (anorthite)
and 4992 h (biotite) were calculated from Si concentrations and
were normalised to initial- and final- mass and geometric-, geometric
edge- (biotite), and BET SSA. For anorthite, rates normalised
to initial- and final-BET SSA ranged from 0.33 to 2.77×10-10
molfeldspar m-2 s-1,
rates normalised to initial- and final-geometric SSA ranged from
5.74 to 8.88×10-10 molfeldspar
m-2 s-1 and rates normalised to initial-
and final-mass ranged from 0.11 to 1.65 molfeldspar
g-1 s-1. For biotite, rates normalised to
initial- and final-BET SSA ranged from 1.02 to 2.03×10-12
molbiotite m-2 s-1,
rates normalised to initial- and final-geometric SSA ranged from
3.26 to 16.21×10-12 molbiotite
m-2 s-1, rates normalised to initial- and
final-geometric edge SSA ranged from 59.46 to 111.32×10-12
molbiotite m-2 s-1
and rates normalised to initial- and final-mass ranged from 0.81
to 6.93×10-12 molbiotite m-2
s-1. For all normalising rates varied significantly
(p≦0.05) with grain size. The normalising terms which gave
least variation in dissolution rate between grain sizes for anorthite
were initial BET SSA and initial- and final-geometric SSA. This
is consistent with: (1) dissolution being dominated by the slower
dissolving but area dominant non-etched surfaces of the grains
and, (2) the walls of etch pits and other dissolution features
being relatively unreactive. These steady state normalised dissolution
rates are likely to be constant with time. Normalisation to final
BET SSA did not give constant ratios across grain size due to
a non-uniform distribution of dissolution features. After dissolution
coarser grains had a greater density of dissolution features with
BET-measurable but unreactive wall surface area than the finer
grains. The normalising term which gave the least variation in
dissolution rates between grain sizes for biotite was initial
BET SSA. Initial- and final-geometric edge SSA and final BET SSA
gave the next least varied rates. The basal surfaces dissolved
sufficiently rapidly to influence bulk dissolution rate and prevent
geometric edge SSA normalised dissolution rates showing the least
variation. Simple modelling indicated that biotite grain edges
dissolved 71-132 times faster than basal surfaces. In this experiment,
initial BET SSA best integrated the different areas and reactivities
of the edge and basal surfaces of biotite. Steady state dissolution
rates are likely to vary with time as dissolution alters the ratio
of edge to basal surface area. Therefore they would be more properly
termed pseudo-steady state rates, only appearing constant because
the time period over which they were measured (1512 h) was less
than the time period over which they would change significantly.』
1. Introduction
2. Materials and methods
3. Results and discussion
3.1. Solution composition
3.2. Changes in specific surface area
3.3. Dissolution rates
3.3.1. Anorthite SSA normalised dissolution rates
3.3.2. Biotite SSA normalised dissolution rates
4. Conclusions
Acknowledgments
Appendix A Anorthite solution composition over the duration
of the experiment (μg L-1)
Appendix B Biotite solution composition over the duration
of the experiment (μg L-1)
References