Coupled dissolution/precipitation rates in the system CaCO_3-CdCO_3

摘要

Dissolution rates of calcite, aragonite, and ground bivalve shells, in the presence and absence of aqueous Cd were measured in mixed-flow reactors. Experiments in Cd-free solutions indicate that geometric surface area normalized dissolution rates of (a) abiogenic and biogenic aragonite and (b) abiogenic and biogenic calcite can each be described as a single function of chemical affinity. Experiments in Cd-bearing solutions allowed characterization of (a) Cd inhibition of CaCO_3 dissolution rates, (b) otavite (CdCO_3) precipitation rates as a function of chemical affinity, and (c) the effect of surface precipitates on the dissolution rate. Cd inhibits aragonite dissolution rates by less than 40%, whereas it inhibits calcite dissolution by an order of magnitude or more. Otavite precipitation rates are ～7 x 10~(-11) mol/cm~2/s at far-from-equilibrium conditions decreasing to 0 at equilibrium. Surface precipitates are found not to affect dissolution rates until they cover more than 90% of the original surface.