18 research outputs found
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The Influence of Calcium Carbonate Grain Coatings on Contaminant Reactivity in Vadose Zone Sediments
The primary objective of this project is to investigate the role of calcium carbonate grain coatings on adsorption and heterogeneous reduction reactions of key chemical and radioactive contaminants in sediments on the Hanford Site. Research will ascertain whether these coatings promote or discourage contaminant reaction with sediment mineral particles, and whether calcium carbonate phases resulting from waste-sediment reaction sequester contaminants through coprecipitation. The research will provide new conceptual models of contaminant reaction/retardation processes in Hanford sediments (for 90Sr2+ and Cr(VI)O4 2- primarily) and improved geochemical models to forecast the future behavior of in-ground contaminants
Adatom Fe(III) on the hematite surface: Observation of a key reactive surface species
The reactivity of a mineral surface is determined by the variety and population of different types of surface sites (e.g., step, kink, adatom, and defect sites). The concept of "adsorbed nutrient" has been built into crystal growth theories, and many other studies of mineral surface reactivity appeal to ill-defined "active sites." Despite their theoretical importance, there has been little direct experimental or analytical investigation of the structure and properties of such species. Here, we use ex-situ and in-situ scanning tunneling microcopy (STM) combined with calculated images based on a resonant tunneling model to show that observed nonperiodic protrusions and depressions on the hematite (001) surface can be explained as Fe in an adsorbed or adatom state occupying sites different from those that result from simple termination of the bulk mineral. The number of such sites varies with sample preparation history, consistent with their removal from the surface in low pH solutions
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''The Influence of Calcium Carbonate Grain Coatings on Contaminant Reactivity in Vadose Zone Sediments''
Our component of this project focuses on the reaction of contaminant-containing fluids with carbonate mineral surfaces in order to better understand the dissolution-growth and related solid-solution processes that ultimately affect contaminant mobility in settings containing carbonates or carbonate grain coatings. Our collaborators (Stanford, PNNL) have focused on other aspects of carbonate and carbonate mineral surfaces as part of the overall project. Because some of the sediments through which contaminants leaking from the Hanford waste have carbonate grain coatings; better understanding the chemistry of carbonate-contaminant interaction constitutes fundamental chemistry needed in order to construct better models of contaminant transport through carbonate-containing sediments
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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
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The Influence of Calcium Carbonate Grain Coatings on Contaminant Reactivity in Vadose Zone Sediments
Calcium carbonate (CaCO3) is widely distributed through the Hanford vadose zone as a minor phase. As a result of current and past geochemical processes, CaCO3 exists as grain coatings, intergrain fill, and distinct caliche layers in select locations. Calcium carbonate may also precipitate when high-level wastes react with naturally Ca- and Mg-saturated Hanford sediments. Calcium carbonate is a very reactive mineral phase. Sorption reactions on its surface may slow the migration of certain contaminants (Co, Sr), but its surface coatings on other mineral phases may diminish contaminant retardation (for example, Cr) by blocking surface reaction sites of the substrate. This project explores the behavior of calcium carbonate grain coatings, including how they form and dissolve, their reactivity toward key Hanford contaminants, their impact (as surface coatings) on the reactivity of other mineral substrates, and on their in-ground composition and minor element enrichment. The importance of CaCO3 as a contaminant sorbent will be defined in all of its different manifestations in Hanford sediments: dispersed minor lithic fragments, pedogenic carbonate coatings on gravel and stringers in silt, and nodules in clay and paleosols. Mass action models will be developed that allow understanding and prediction of the geochemical effects of CaCO3 on contaminant retardation in Hanford sediments