Harvard University School of Engineering and Applied Sciences  Environmental Sciences and
Engineering (ESE) Program
Atmospheric Sciences Seminar
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Current Projects
 
  • Phase Transitions of
       Atmospheric Particles


     - Crystallization of SNAP
        Particles

     - 1×3TDMA
     - Atmospheric Nanoparticles
     - Modeling Aerosol Phase
        Transitions and Radiative
        Effects


  • Dissolution and
       Precipitation of Minerals
       in Aquatic Environments

     
  • Chemical Oxidation
       Reactions and Hydrophobic
       -to-Hydrophilic Aging of OAs


     - Aerodyne AMS analysis
     - CCN properties of OAs

  • Origins of Life:
       Mineral Surface Photo-
        Electrochemistry


  • Harvard Environmental Chamber

    AMAZE-08


    Closed Projects

      - Crystallization of Sulfate and
       Nitrate Coatings on
       Tropospheric Mineral Particles

     
      - Tropospheric Mineral
       Particles as Ice Nuclei

     
      - Building Structures at
       the Nanoscale



     

     

    Dissolution and Precipitation of Mineral in Aquatic Environments

    Mineral dissolution and precipitation, especially of Fe and Mn (hydr)oxides and the carbonate mineral family, partially regulate pH and alkalinity of natural waters, affecting the fate and transport of organic and inorganic contaminants. Moreover, precipitation sequesters trace metal ions, including Co, Zn, Cu, Cd, Ni, Pb, Mn, Mg, and U, by adsorption, coprecipitation, or diffusion into the lattice. Many of the hazardous metals occur in aquifers due to human pollution. With seasonal cycles in pH and redox conditions, the (hydr)oxide and carbonate host lattices dissolve, again releasing the trace metals into the aquifer, threatening ecosystems and human health. A detailed understanding of the processes of precipitation and dissolution is essential for a quantitative predictive capability of contaminant fate and transport. Mineral dissolution and precipitation rates are strongly related to surface chemistry and processes.

    A current focus is the behavior of mobile surface ions during calcite dissolution and growth. These phenomena are probed using a technique called Scanning Polarization Force Microsocopy (SPFM). SPFM employs polarization forces between a modified, electrically-biased AFM tip and the sample surface to record changes in surface potential, dielectric properties, and topography.

    AFM RH setup

      AFM RH setup2
    Setup showing equipment and components added to a commercial AFM system (DI Multimode) in order to conduct Scanning Polarization Force Microscopy (SPFM link 1, link 2). The electronic equipment on the left allows a bias (AC or DC) to be applied to the AFM tip and the tip-sample interaction response to be measured. The tip and sample are enclosed in a home-built humidity chamber pictured at right.   Close-up of the humidity chamber enclosing the AFM piezo with a mineral sample sitting beneath the scan head, tip holder, and cantilever. Note the red glow from the reflection of AFM laser off the top of the cantilever and into a photodiode contained in the scan head. Part of the laser light is also being dispersed by the mineral sample. BNC cables are connected to sensors used to monitor relative humidity (RH) and temperature inside the chamber.

    Two modes are employed: 1) Point measurements of the polarization force time response while applying a low frequency ac bias to the tip, and 2) Raster scan SPFM imaging using a dc bias. Trends in the polarization force time response lead to several conclusions regarding the structure of water films and the density and mobility of carbonate surface ions. At RH = 55% the completion of a monolayer of water, the initiation of 3D film growth, and a step increase in the density and the mobility of surface ions is observed for calcite. Results for rhombohedral carbonates (viz. CaCO3, FeCO3, ZnCO3, MgCO3, and MnCO3) show a long relaxation time of the polarization force is connected with rapid dissolution. This finding is rationalized by long lifetimes in terrace positions and hence greater opportunities for detachment of the ion to aqueous solution (i.e., dissolution). For example, MgCO3 has the fastest mobile ion time response but dissolves the slowest. This observation is rationalized by a difference in the hydration of individual ions and their interaction with steps and kinks on the carbonate surface.

    Dissolution rate

    Correlation of the dissolution rate with the relaxation time for the rhombohedral carbonate mineral family. Relaxation times are recorded at 90% relative humidity. Circumneutral dissolution rates are taken from Duckworth and Martin (2004).

    Using dc SPFM imaging contact mode AFM in tandem with we have collected time sequenced micrographs which show that the (104) calcite cleavage surface reconstructs over several hours under two to three monolayers of water. Reconstruction consists of 1 nm high, flat-topped islands growing between expanding surface pits. SPFM images show polarization heights of the islands differ from those of the substrate. Because the polarization heights are a surrogate for the local dielectric constant of the sample ε and arise from a convolution of the mobility and the density of surface ions, the polarization heights imply that εsubstrateisland.

    AFM SPFM

    Contact AFM (left) and SPFM (right) micrographs (5 m × 5 m) of a reconstructed calcite surface after 150 minutes under 80% relative humidity. (see the movie!) Reconstruction is characterized by island growth and pitting. The same island is depicted in each micrograph. The lack of dependency of the polarization images on the sign of the tip bias and the negative polarization heights of the island observed in profile collectively indicate that the island has a lower local dielectric constant than the substrate.

    In agreement with this ordinal ranking, in separate point measurements the surface-averaged maximum polarization force decreases as the surface coverage by the film and the island increases. Changes in topographic and polarization heights at 20% and 50% RH suggest that the structures of the islands are in dynamic equilibrium with the adsorbed water. Our evidence suggests that the islands contain loosely bound water and may therefore be a hydrated calcium carbonate phase stabilized by the calcite surface. As such, this work provides a quantitative connection between mobile surface ion diffusion and surface reconstruction.

    Growth and Dissolution of Iron and Manganese Oxide Films

    The goal of the supported work is to understand the growth and dissolution of Fe and Mn oxide films on mineral surfaces, especially as influenced by the adsorption of other ions. We are also exploring the related effects on surface charge distribution. Over the last year, we have characterized the electrostatic heterogeneities that Mn oxide nanostructures (MnOx) induce on the parent substrate. We grew the nanostructures on rhodochrosite (MnCO3) substrates in the presence of dissolved oxygen at pH 6.3. The nanostructures were identified by their morphology and height using atomic force microscopy (AFM). Their electrostatic properties, including surface potential, ion mobility, and interfacial adhesive forces, were characterized using Kelvin probe force microscopy, scanning polarization force microscopy, and force-volume microscopy, respectively. Results were then compared with the same properties of the parent MnCO3 surface.

    The observations show that MnOx nanostructures induce significant electrostatic heterogeneities on the parent substrate. The MnOx nanostructures of 1.3(± 0.7) nm thick have a surface potential that is 271(± 14) mV higher than that of MnCO3. The nanostructures reduce the lateral movement of surface ions by acting act as transport barriers. The nanostructures also have much stronger interfacial forces (referenced to the AFM tip) than the substrate, indicative of changes in the electrical double layer. These results demonstrate that the nanostructures greatly alter the surface physics and chemistry, even though they contain a nearly negligible amount of mass compared to the parent carbonate surface. The electrostatic heterogeneities arising from these changes should influence the interactions of Mn oxide surface coatings with charged contaminants and microorganisms in the environment. FIND OUT THE NEW MOVIE of series of potential images of nanostructures on rhodochrosite for increasing relative humidity.

    dissol

    Recent Update:

    The observations of interfacial forces in 1 mM NaNO3 solution from pH 5 to 9.7, highlighted in the following figure, show that the oxide nanostructures have different surface charges from the rhodochrosite substrate. This surface-charge heterogeneity is probed by force-volume microscopy using a charged silicon-nitride probe. Unlike the rhodochrosite substrate, which has only a modest interaction with the probe across the entire pH range, the oxide nanostructures exert significant repulsive interfacial forces with respect to the probe. The maximum interfacial repulsion between the nanostructure and the probe occurs when they are 2.4(±1.1) nm apart. The magnitude of the interfacial repulsion increases from pH 5 to 6.5, reaches its minimum, and increases with further increase of pH. This pH dependence can be described by a simple linear model: frepmax(pN)= 23(± 4)[6.8(± 2.1)- pH] (for pH <6.5) and 19(± 2)[pH-6.1(± 1.0)] (for pH≥6.5). The results suggest that the oxide nanostructures have a point of zero charge around pH 6.5. The surface-charge heterogeneity of reacted rhodochrosite is further explained by separate mechanisms that regulate the surface charges of oxide nanostructures compared to the rhodochrosite substrate. Quantifying surface-charge heterogeneity is the first step in accounting for its effects on contaminant immobilization and bacterial attachment on surfaces present in aquifers.

    interfacial repulsion

    We are grateful for support from the Department of Energy, the Petroleum Research Fund, the New York Community Trust Merck Fund (DOE DE-FG02-03ER15384 and ACS PRF 37770-AC5).

     

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