Analysis of the Activation Energy of Dissolution of the Iron-Containing Zinc Sulfide (Sphalerite)

The activation energy of a reaction is a key rate parameter, and understanding the mechanism of a reaction requires an explanation for the activation energy. Iron substitutes for zinc in the zinc blende structure of sphalerite and provides impurity bands within the fundamental band gap. The activation energy of iron-containing sphalerite, (Zn,Fe)S, is dependent in a nonlinear manner on the amount of this substitutional iron in the crystal structure. In addition, there is a linear increase in the rate of dissolution with the substitutional iron. A mechanism of dissolution is proposed in which the oxidant injects holes into both the iron impurity bands and the valence band. These holes in the iron impurity bands and in the valence band then react at the surface to form dissolution products. It is shown that this model describes the data for the effect of the substitutional iron on both the activation energy and the rate constant. The nonlinear dependence of the activation energy arises from the fluctuating energy level model of the oxidant ions in solution. The energy level of the oxidant needs to fluctuate to both levels of the iron impurity bands and the valence band. The proposed mechanism enables a quantitative realization of the electronic structure of the interface and provides a derivation for the activation energy, which is important not only for this reaction, but also for an understanding of dissolution reactions in general.