『Abstract
The dissolution and precipitation rates of rhombohedral carbonate
minerals, such as CaCO3, ZnCO3,
MnCO3, FeCO3, and MgCO3, affect the alkalinity and redox state of natural
waters. In the current study, the rates and mechanisms of carbonate
dissolution are investigated utilizing an atomic force microscope/flow-through
reactor. Based on time series changes of the microtopography,
we classify the studied minerals as type I (Ca, Mn), type II (Fe,
Mg), or type III (Zn) carbonates. Type I carbonates develop shallow
rhombohedral pits, and the step-retreat velocities are pH-dependent
for pH＜4. Type II carbonates form deep rhombohedral pits at circumneutral
to alkaline pH. The rhombohedral shape alters for pH＜4, and the
step retreat velocities are weakly pH-dependent. Type III carbonates
develop shallow distorted puts at circumneutral to alkaline pH
and triangular pits at acidic pH. Groups of point defects vs.
line dislocations apparently give rise to type I vs. type II behavior,
whereas the unique tetrahedral surface coordination of adsorbed
Zn2+ may explain type III behavior. For type I at all
pH values and type II carbonates for pH＜4, geometric models employing
the microscopic step retreat velocities are accurate predictors
of the absolute rates of macroscopic dissolution. Macroscopic
circumneutral dissolution rates, which trend with both water exchange
rates and lattice formation energies, vary from 10-5.5
to 10-9.1 mol/m2・s at circumneutral pH and
25℃.』
Introduction
Experimental methodology
Experimental preparation
Simultaneous AFM/flow-through reactor experiments
Results and discussion
Pit shape and distribution
Type I carbonates
Type II carbonates
Type III carbonates
Pit morphology, defects, and chemical composition
Microscopic dissolution rates
Calculation of microscopic dissolution rates
Calculation of step retreat velocities
Macroscopic dissolution rates
Comparing macroscopic and microscopic rates
Type I carbonates
Type II carbonates
Correlation of dissolution rate with other properties
Concluding remarks
Acknowledgments
References cited