Intellectual Merit: Kinetic models of mineral dissolution and growth are hampered by our inability to precisely define the mechanisms of relevant reactions at the molecular scale. The net reaction for many mineral surface processes are known, but the pathway(s) and mechanism(s) through which they occur are not well constrained. Calcite is a particularly important mineral in that it is: a dominant buffer of acidity in soils and groundwater, one of several materials used to understand crystal growth generally, and studied widely to understand biomineralization processes. In the abiological dissolution and growth of calcite, specific, molecular-scale, limiting reaction mechanisms have been proposed, but have not been tested.
Similarly, during biomineralization it is known that certain molecules and proteins such as aspartates can influence growth morphology, but quantitative estimates of their kinetic effects remain elusive. The main goal of this proposal is to test proposed reaction mechanisms for calcite dissolution and growth by directly comparing formation and activation energies for step movement measured experimentally to reactions simulated from first principles. Classical crystal growth theories will form the framework to relate the experimentally measured quantities to the simulated chemical mechanisms. The reactions that will be tested are the mechanisms of kink site formation thought to control the movement of steps at circum-neutral pH, atmospheric pCO2, and saturations near equilibrium as well as growth inhibition by a model compound.
The proposed research will take place over three years and will address three main hypotheses that will form the bulk of a Ph.D. dissertation:
*Hypothesis 1: Step movement on calcite surfaces observed by AFM can be fit by the same model for both growth and dissolution, where kink site formation and propagation are limiting. *Hypothesis 2: Accurate kink site formation and activation energies can be simulated from first principles calculations and simulations. *Hypothesis 3: Detachment of the carboxylic acid functional group of aspartate from calcium ions on the step edge controls its ability to poison growth.
The activation and formation energies for kink sites will be estimated from atomic force microscopy measurements of step velocity as a function of saturation and temperature. These will be compared to direct observations of kink site concentrations in equilibrium solutions using high resolution AFM. Simulations will conducted after ensuring the best fit to existing crystal truncation rod date of water structure at calcite interfaces. The mechanism of the reactions will be then be tested by simulating kink site formation and activation energies using ab initio density functional theory and molecular dynamics umbrella sampling. The possible importance of the dissociation of water in the reaction will be explored by protonating surface carbonate and hydroxylating surface calcium ions. Finally, the same AFM measurements and simulation techniques will be made in the presence of aspartate and the experimentally estimated activation energy for growth poisoning will be compared to the simulated activation energy for detachment of aspartate from a calcium on a step edge. The proposed research will combine experimental and computational techniques in a novel way to explicitly test our understanding of growth and dissolution reactions on calcite. This information could be used subsequently in larger-scale rate measurements where assignation of physical meaning to measured rate constants is ambiguous.
Broader Impacts: This project will form the bulk of a graduate student's Ph.D. dissertation. Additionally, in year two, a K-12 teacher will be awarded a Georgia Intern-Fellowship for Teachers (GIFT) and work in the laboratory of the PI for six weeks. The goal is to provide experience for the teacher so that he or she can better incorporate modern environmental science into their K-12 Earth Science class. The results from this work could result in some long-term benefits to science as well. These include an enhancement of our ability to predict the kinetics of mineral surface reactions in environmental settings, the design of new growth inhibitors to tailor crystallization techniques to industrial applications, creation of nano-devices and improved heterogeneous catalysis techniques. These are areas where the current lack of understanding makes it difficult to predict a priori what the effect added components will have on a system. This in turn makes design of new growth inhibitors or modifiers for a given system or application difficult, and the quantitative prediction of the rates of mineral surface reactions is often beyond reach.
This project is supported jointly with the Ceramics Program in the Division of Materials Research.
