Molecular-level mechanisms of quartz dissolution under neutral and alkaline conditions in the presence of electrolytes

The mechanisms of quartz dissolution are affected intricately by pH conditions and electrolyte types. Most previous studies have focused on the mechanisms of quartz dissolution under one specific condition (e.g., temperature, pH, saturation or electrolyte type); however, this study investigates molecular-level mechanisms under combinations of electrolyte and pH conditions, which are more complex but closer to the reality. Under neutral and alkaline pH conditions with Ca2+, Mg2+ or Na+ aqua ions in the solution, the dissolution of Q1 (Si) and Q2 (Si) sites on the quartz surface, which represents the predominant part of the quartz dissolution story, is investigated by first-principles quantum chemistry calculation methods. Our results confirm that quartz dissolution can be enhanced significantly by the presence of electrolytes under neutral pH conditions. However, under alkaline pH conditions, the surface complexes of aquo ions are different, depending on where and how those electrolytes bond onto the quartz surfaces. The energy barriers of all possible hydrolysis reaction pathways are calculated carefully. The activation energies for the reaction between the negatively charged quartz surface and H2O have never been reported before. Our results of activation energy are closer to experimental values than previous calculations have been, suggesting that the cluster models and theoretical levels used here are more reasonable. Such information provides a molecular-level understanding of the differences of quartz dissolution rates between pure water and ion-containing solutions.