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Dissolution kinetics and mechanisms at dolomite-water interfaces: effects of electrolyte specific ionic strength.

Environmental science & technology (2012-06-12)
Man Xu, Katie Sullivan, Garrett Vanness, Kevin G Knauss, Steven R Higgins
ABSTRACT

Elucidating dissolution kinetics and mechanisms at carbonate mineral-water interfaces is essential to many environmental and geochemical processes, including geologic CO(2) sequestration in deep aquifers. In the present work, effects of background electrolytes on dolomite (CaMg(CO(3))(2)) reactivity were investigated by measuring step dissolution rates using in situ hydrothermal atomic force microscopy (HAFM) at 90 °C. Cleaved surfaces of dolomite were exposed to sodium chloride and tetramethylammonium chloride (TMACl) aqueous solutions with ionic strengths (I) ranging from 0 to 0.77 m at pH 4 and pH 9. HAFM results demonstrated that dolomite step retreat rates increased with increasing solution ionic strength and decreasing pH. Comparison of [481] and [441] steps revealed that the anisotropy of [481] and [441] step speeds became significant as solution ionic strength increased, with NaCl exerting more pronounced effects than TMACl for the same I. To interpret the different trends observed for NaCl and TMACl, a dissolution mechanism involving orientation-dependent ion adsorption and consequent edge free energy changes is proposed.

MATERIALS
Product Number
Brand
Product Description

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Tetramethylammonium hydroxide pentahydrate, ≥97%
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Tetramethylammonium iodide, 99%
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Tetramethylammonium bromide, 98%
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Tetramethylammonium bromide, ACS reagent, ≥98.0%
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Tetramethylammonium nitrate, 95%
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