B31F-01
Perspectives on the Environmental Impact of Nanomaterials: What Do We Know Now, and Where Are We Going?
Nanoscience and nanotechnology are rapidly expanding fields that are driving revolutionary advances in all major science and engineering disciplines. To put this in perspective, in the long term it is projected that the scale of these efforts may one day rival what genomics and proteomics have done in recent decades for the biological, medical, and biotechnology disciplines. Nanoscience is based on the fact that the chemical and physical properties of materials change as a function of their physical dimension, and nanotechnology takes advantage of this by applying selected property modifications of this nature to some beneficial endeavor. At the same time, nanoscience is also relevant to the geosciences, in that naturally occurring nanomaterials are ubiquitous in the atmosphere, hydrosphere, pedosphere, and lithosphere. Not surprisingly, natural nanomaterials are thought to be relevant in a diverse range of Earth systems including the biosphere. Therefore, the amount of research concerning environmental, ecological, and biogeochemical impacts of natural and synthetic nanomaterials is rapidly growing, with not only great scientific, but also large economic and political consequences. Our general knowledge of the behavior of nanomaterials, particularly in an environmental context, will be reviewed, and the possible directions of future research pathways will be discussed.
B31F-02 INVITED
Structural and Chemical Transformations in Nanocrystals
Structural and Chemical transformation of nanocrystals from one state to another can proceed rapidly and with remarkably simplified kinetics compared to bulk solid state transformations. We have investigated the Wurtzite to rock salt pressure induced solid-solid structural transformation in CdSe nanorods of 4 nm diameter as a function of rod length. Rods of length less than 10 nm transform reversibly, while rods greater than 10 nm in length fracture during the structural transformation. Two other recent examples we have investigated are the formation of hollow nanocrystals via addition reactions and cation exchange reactions. Hollow nanocrystals of many transition metal oxides and sulfides have been studied, and from these we are learning how to predict the shell thickness. Cation exchange from CdSe to Ag2Se proceeds completely, but is reversible, and the number of anions per nanocrystal remains invariant. Above a critical size, the nanocrystals retain non-equilibrium shapes over multiple cycles of cation exchange.
B31F-03
The Role of Mobile Surface Ions in Nanomaterial Formation
Biogenic and abiotic nanomaterial formation alters the electrical layout of mineral surfaces. Complex dielectric and surface diffusional regimes that affect water sorption, metal co-precipitation, and possibly cell attachment are created. Polarization force microscopy measurements reveal the importance of mobile, surface associated ions in nanomaterial formation, particularly in subaerial conditions. Mass and charge transport between nanostructures can occur via two-dimensional diffusion of surface ions within mono- to multi-layer water. Further, nanostructure stability depends on epitaxy with the underlying substrate. On calcite under humid air, we observe the formation of a 1 to 1.5 nm thick, hydrated calcium carbonate film. This nanophase, which is typically protein- stabilized as an intermediate within biomineralization schema, is instead stabilized by the calcite 104 surface. Moreover, nanostructure dissolution on rhodochrosite is initiated by the accumulation of hydrated, edge- associated ions, most likely representing partially mobilized, positively-charged film material. We connect these molecular-level observations to large-scale, biogeophysical measurements that hold promise of remotely sensing bacteria in the subsurface (e.g., induced polarization (IP)). Here, mobile surface ion diffusion at the cell- mineral interface and biogenic nanomaterial formation are identified as key contributors to the poorly understood IP signature of bacteria in porous media.
B31F-04
Nanoparticle-host interactions in natural systems
Natural nanoparticles are attracting a great deal of attention due to their unique role as agents of elemental transport and their increased reactivity in geologic systems. Although significant progress has been made in understanding their behavior in the Earth's critical zone (i.e. near-surface environments), there is a severe lack of information on their stability for a wider range of geologically relevant temperatures. Here, we describe the first direct observations of the dynamic behavior of natural nanoparticles at near atomic scale, revealing that their thermal stability is not only dependent on particle size, but also on the surrounding host mineral. Native Au nanoparticles (mean diameter ~4 nm) incorporated in an As-rich pyrite from "invisible" Au ores were observed during in-situ heating up to 650°C. While isolated Au nanoparticles melt, with their melting point being a function of size, we show that when incorporated in a sulfide host, Au nanoparticles react to increased temperature by dissolving into the pyrite matrix and forming larger particles in an Ostwald-type ripening process. The dissolution temperatures are much lower than melting temperatures of isolated nanoparticles and are as well strongly size-dependent. These findings provide new insights into the fate of nanoparticulate Au and other metals during geological processes and throughout their metallurgical recovery from refractory ores. The size distribution of the particles may be an indicator of the geologic history of the ore because, as we show, the mean particle diameter sets un upper limit to the maximum temperature of the host rock. Furthermore, results suggest that nanoparticulate minerals, usually documented in low-temperature (T<100°C) aqueous environments, can also occur and survive at higher temperatures when incorporated into refractory host phases.
B31F-05 INVITED
Nucleation and Growth of Environmental Nanomaterials at Water-Mineral Interfaces
The nucleation and growth of metal oxide nanoparticles and films on mineral surfaces can markedly affect aqueous metal contaminant transport and other surface-controlled reactions. Recently, grazing incidence small angle x-ray scattering (GISAXS) has been used to analyze the size, shape, and distribution of quantum dots and polymers on substrates. However, no work has been attempted thus far using GISAXS for in situ observations of environmental interfacial processes---such as the nucleation, growth, or aggregation of nanoparticles on mineral surfaces. In this work, we devised the first environmental application of GISAXS in aqueous systems and studied the kinetics of nucleation and growth of iron oxide nanoparticles at water-quartz interfaces, using in situ time- resolved simultaneous SAXS/GISAXS technique. The changes in the sizes and shapes of nuclei and the interspacing between nuclei on quartz surfaces are determined as a function of exposure time, iron concentration, ionic strength, and presence of steps at the mineral surfaces. The iron oxide nuclei started to grow close to steps rather than on terraces (diameter 5.3 ± 0.5 nm, height 1.9 ± 0.2 nm, at 4 min reaction time with [Fe3+] = 10-4 M). At 31 min, the nuclei began to coalesce with each other and form larger surface clusters. We found that the surface steps direct the iron oxide nucleation and affect the kinetics of nucleation and growth of iron oxide nanoparticles at water-quartz interfaces. In addition, we were able to distinguish the quantitative contribution between homogeneous and heterogeneous nucleation under different ionic strength and aqueous iron concentrations. For comparison, we generated simulations of the nanoparticle scattering and conducted measurements with AFM and TEM. GISAXS can provide statistically improved morphological information about the early stages of environmental nanoparticle growth compared with AFM and TEM, and allows real-time geochemical kinetic analysis of nanoparticle growth and reactions.
B31F-06
Nature's Semiconductors: Electronic Structure of Biogenic Manganese Oxides
Many bacteria produce Mn(IV) oxides (MnO2), which are environmental nanoparticles having a layer structure. Biogenic Mn oxide minerals are known to participate importantly in both redox reactions and metal- scavenging in soil and aquatic environments. An important structural characteristic of the oxides is the presence of Mn(IV) cation vacancies whose charge deficit is typically compensated by metal cations or protons. These vacancies have long been identified as strong adsorption sites for heavy metals, but they may also play an important role in redox biogeochemistry, particularly in photo-induced redox reactions occurring in natural waters. Because electronic structure is the key to understanding (photo) redox transformations, we examined the electronic structures of Mn oxides both with and without Mn(IV) vacancies using quantum mechanical density functional theory (DFT) simulations. Our DFT results show that Mn(IV) vacancies compensated with protons effectively narrow the band gap energy of MnO2, facilitating the formation and transfer of photo-induced electrons and holes. This study also suggests that the photoconductivity of natural and synthetic MnO2 nanoparticles can be optimized by control of cation vacancies.
B31F-07
The Effect of Nanoparticle Aggregation Processes on Aggregate Structure and Metal Uptake
Nanoscale oxide and oxyhydroxide minerals are commonly found in the natural environment, and play important roles in adsorbing and sequestering aqueous ions including nutrients such as phosphates and contaminants such as heavy metals. After formation, these materials are typically subjected to natural flocculation events that reduce the nanoparticle surface area that is accessible by aqueous ions. However, no studies have addressed the impact of different aggregation processes on the capacity of the nanoparticles to sorb aqueous metal ions. We synthesized a suspension of ~6 nm iron oxyhydroxide nanoparticles and subjected portions of this suspension to analogues of natural aggregation processes. These included: pH variation around the point of zero surface charge (simulating the neutralization of acid mine drainage); ionic strength elevation (simulating mixing of aquifer and saline water); drying; and freezing. The effect of aggregation on metal ion uptake was then studied by exposing batches of aggregated and control samples to 0.5 mM Cu(NO3)2 at pH 6.0 for 24 hours. In addition, we used in situ small-angle x-ray scattering to quantify and visualize the aggregate morphology. We found that the aggregates produced by the different mechanisms varied considerably in their interior porosity and their ability to sequester aqueous ions. In particular, the results demonstrate the important role of water in preserving hydrated channels among aggregated nanoparticles that are permeable to aqueous metal ions.