| Abstract
| - Dissolution of carbonate minerals has significant environmental effects. Microorganisms affect carbonatedissolution rates by producing extracellular metabolites,including complex polysaccharides such as alginic acid.Using a combined atomic force microscopy (AFM)/flow-through reactor apparatus, we investigated the effects ofalginic acid on calcite dissolution. Macroscopic dissolutionrates, derived from the aqueous metal ion concentrations,are 10-5.5 mol m-2 s-1 for 5 < pH < 12 in the absenceof alginic acid compared to 10-4.8 mol m-2 s-1 in its presence.The AFM images demonstrate that alginic acid preferentiallyattacks the obtuse steps of dissolution pits on thecalcite surface. In pure water, the obtuse and acutesteps retreat at similar rates, and the pits are nearly isotropicexcept under highly acidic conditions. In alginic acid,the acute step retreat rate is nearly unchanged in comparisonto water, whereas the obtuse step retreat rate increaseswith decreasing pH values. As a result, the pits remainrhombohedral but propagate faster in the obtuse direction.To explain these observations, we propose that alginicacid preferentially forms dissolution active surface complexeswith calcium atoms on the obtuse step, which results inanisotropic ligand-promoted dissolution.
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