Path from Reaction Control to Equilibrium Constraint for Dissolution Reactions

Although dissolution reactions are widespread and commonplace, our understanding of the factors affecting the rate of dissolution is incomplete and consequently the kinetics of these reactions appear complicated. The focus in this work is on the behavior of the rate as conditions approach equilibrium. The reverse reaction is often treated in terms of chemical affinity, or saturation state. However, the implementation of the chemical affinity model fails, requiring arbitrary empirical adjustments. In this study, a mechanism of dissolution is proposed that describes both the fractional orders of reaction with respect to H+ and OH- and correctly describes the approach to equilibrium. The mechanism is based on the separate removal of anions and cations from the surface, which are coupled to one another through their contribution to and dependence on the potential difference across the interface. Charge on the surface, and hence potential difference across the interface, is caused by an excess of ions of one sign and is maintained at this stationary state by the rate of removal of cations and anions from the surface. The proposed model is tested using data for NaCl (halite), CaCO3 (calcite), ZnS (sphalerite), NaAlSi3O8 (albite), and KAlSi3O8 (K-feldspar). An important feature of the proposed model is the possibility of "partial equilibrium", which explains the difficulties in describing the approach to equilibrium of some minerals. This concept may also explain the difficulties experienced in matching rates of chemical weathering measured in laboratory and field situations.