Dolomite dissolution kinetics at low pH: a channel-flow study

Abstract

The dissolution kinetics of the (100) face of dolomite [CaMg(CO₃)₂] single crystals have been determined at low pH (3–4) in aqueous solution using channel-electrode shielding measurements. An improved channel-flow cell, designed to allow channel depths (and hence transport rates) to be readily varied over a wide range, has permitted the conditions under which mass transfer influences the dissolution process to be determined for the first time. In contrast to calcite (CaCO₃), the dissolution of dolomite displays a half-order, rather than first-order, dependence on the crystal/solution interfacial H⁺ concentration. Complementary microscopy studies of reacted dolomite surfaces indicate that the dissolution process occurs via the formation of etch pits, the geometries of which are very sensitive to the magnitude of local mass transfer, and hence H⁺ concentration at the crystal/solution interface. Under most mass-transfer conditions the dissolution rate, inferred from etch-pit morphologies, is observed to be highly anisotropic at the microscopic level. The general consequences of such effects in macroscopic dissolution studies, which generally assume uniform surface reactivity, are discussed.