Topic:
Metadynamics simulation of barium detachment from a monomolecular barite step. Barium atoms are green, sulfate ions ochre (sulfur) and blue (oxygen), oxygens of water red, and hydrogens gray. Colored areas are isosurfaces representing free energies of various states relative to the starting configuration The curved black arrows denote the reactions necessary prior to the breaking of all the bonds between the ion and the mineral surface.
Simulations using metadynamics and other “rare event theories” have revealed the detailed mechanisms of attachment and detachment of aqueous barium ions at the barite (BaSo4) surface in contact with water. The new methodology will improve prediction of rates of mineral reaction rates and enable the design of more efficient growth inhibitors. Although multiple bonds break and form during mineral growth and dissolution, these reactions often are treated as including only a single reversible step that governs the reaction rate. This study showed that multiple intermediate states occur during both processes. Metadynamics was used to explore the reaction mechanism and a sequence of umbrella sampling calculations to obtain the free-energy surface. Then reactive flux was used for stochastic sampling of the number of barrier recrossings from the transition state to calculate the transmission coefficient. The activation energy for the rate-limiting steps was calculated and precisely matched macroscopic experimental estimates for growth and dissolution. The ability to control and predict the growth and dissolution of barite would be beneficial in many cases; barite is a key contributor to scale formation in oil wells and reservoirs in the North Sea, which costs an estimated $1.4 billion per year. This work has significant implications for mineral growth and dissolution generally: the intermediate states could explain anomalous, non-steady-state reaction rates.
For more information, please contact Andrew Stack, stackag@ornl.gov.