Kinetic inhibition of calcite (1 0 4) dissolution by aqueous manganese(II)

This study presents evidence of calcite (1 0 4) dissolution inhibition by dissolved manganese(II) through detailed observations of crystal surface morphology. Rates of dissolution have been quantified by direct measurement of surface-normal retreat and etch pit growth at far-from-equilibrium conditions using vertical scanning interferometry. The approach provides new insight into the control that dissolved inorganic carbon (DIC) exerts on manganese inhibition kinetics. Our results show that at 2×10−6 molal manganese, the rate of overall calcite dissolution is suppressed to close to zero, but only when sufficient CO32− is available in solution. The near arrest of surface-normal dissolution is brought about by the likely reduction in step retreat, inferred through observed variation of etch pit and surface morphology. The inhibition mechanism likely reflects the strong affinity of Mn2+ for carbonate ion. In solution, this affinity is expressed by formation of a strong complexing ligand (MnCO30). On the (1 0 4) surface, this affinity may govern formation of a similar surface complex that stabilizes reactive sites, thereby inhibiting step movement and defect nucleation. Comparison with results from other metals suggests that inhibition may be understood in the context of the relative energetics of dehydration versus carbonation reaction steps.