Final Project Report
This report provides a description of the main accomplishments of the EMSP funded research, including products such as conference presentations and publications (including those still in preparation). The purpose of this study was to better understand the chemical interactions between dissolved aqueous contaminants and carbonate minerals occurring as coatings on mineral grains in the vadose zone beneath the Hanford reserve. This information is important for construction of improved reactive transport models intended to predict the subsurface migration of contaminants. We made improvements to the hydrothermal atomic force microscope (HAFM) design to be used in this project. The original HAFM was built with funding from the U.S. DOE, Office of Basic Energy Sciences. Improvements include operating limits of 70 bars and 170 C, from an original limit of 12 bars and 150 C. This product is patented. We completed a series of studies of magnesite, MgCO3, because this mineral is structurally equivalent to calcite but reacts much more slowly, allowing us to study carbonate reactivity under pH conditions (i.e., low pH) that are much more problematic for studies of calcite but which are nevertheless relevant to in-situ conditions. We found that dissolving magnesite exhibits a dramatic change in step orientation, and therefore etch pit shape, as pH is lowered through 4.2 to 3 and 2. This change in step orientation is NOT accompanied by an increase in step velocity with decreasing pH. We also found that, after growing magnesite on a magnesite substrate, the newly grown magnesite dissolved much more readily than the underlying substrate magnesite, and exhibited far larger etch pit densities. This effect may have been related to the rate of growth or to the presence of an Fe impurity in the growth solutions. We studied the dissolution of magnesite and calcite (104) surfaces under a wider variety of conditions with a new hydrodynamically defined hydro thermal AFM fluid cell, and we have observed the precipitation of a strontium-containing carbonate phase on dissolving calcite. We have applied the advection-diffusion equation coupled to proposed homogeneous and heterogeneous kinetic models to test rate laws for dissolution observed by HAFM. Our main conclusions in the magnesite studies are that step density, rather than step velocity, is a strong function of pH near the surface and that the step orientation is sensitive to pH. In these studies, we definitively demonstrate that diffusive mass transport is only important at very low fluid velocities for magnesite, but that studies of calcite dissolution are generally in the mixed transport-kinetics controlled regime (even at high fluid velocities) where quantitative information can only be obtained by accounting for the transport components. We also have found that alkaline earth carbonate secondary precipitate formation on calcite surfaces significantly alters the net flux o f Ca2+ and may passivate the CaCO3 surface from further reaction. The research has so far resulted in 5 conference presentations and 3 published journal articles, with several manuscripts still in preparation. The project supported graduate student Briana Deeds and postdoctoral researcher Steven R. Higgins.
- Research Organization:
- University of Wyoming (US)
- Sponsoring Organization:
- USDOE Office of Environmental Management (EM) (US)
- DOE Contract Number:
- FG07-99ER15019
- OSTI ID:
- 820094
- Report Number(s):
- DOE/ER/15019final; TRN: US200706%%684
- Country of Publication:
- United States
- Language:
- English
Similar Records
Acidic dissolution of plagioclase: In-situ observations by hydrothermal atomic force microscopy
Forsterite [Mg 2 SiO 4 )] Carbonation in Wet Supercritical CO 2 : An in Situ High-Pressure X-ray Diffraction Study