Ab initio study of dissolution and precipitation reactions from the edge, kink, and terrace sites of quartz as a function of pH

Abstract

In this study, the fundamental concept of mineral surface roughness and the role it plays on the dissolution and precipitation processes using ab initio density functional theory calculations, have been investigated. To understand the topological contribution to the dissolution and precipitation rates of quartz, a systematic study was designed to determine the reaction mechanisms and barrier heights of the forward reaction of hydrolysis of–Si(OH)₃ group from underlying edge, kink, and terrace sites. The edge and kink sites are topological features on mineral surfaces that are bound to the bulk with fewer Si–O–Si bridge bonds than a terrace site, a factor that can be significant in the dissolution process. In this work, the local arrangement of atoms on the underlying sites is included by systematically increasing the Si–O–Si bridging to represent the edge, kink, and terrace sites under ambient conditions in the neutral, deprotonated, and protonated states. Nine reaction profiles were studied using the M05-2X density functional with 6–31 + G(d,p) basis set. The results show that hydrolysis of the–Si(OH)₃ group and the back reactions are not too sensitive to the underlying edge, kink, and terrace site. The barrier height data also explain the experimentally observed dissolution rate over the entire pH range, especially the tremendously high dissolution rate below pH 2.

Keywords:

• ab initio
• dissolution
• precipitation
• surface roughness
• topography

Acknowledgements

We gratefully acknowledge the useful discussions with J.D. Kubicki on this work. This work has been supported by the National Science Foundation NSF under Grant Number CHE-0535656.