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Functionalized Nanomaterials

The most important factor in determining quality of life in human society is the availability of pure, clean drinking water. Wars have been fought, and will continue to be fought, over access to and control of clean water. Drinking water has two major classes of contamination, biological contamination and chemical contamination. Bacterial contamination can be dealt with by a number of well-established technologies (e.g., chlorination, ozone, UV), but chemical contamination is a somewhat more challenging target. Organic contaminants, such as pesticides, agricultural chemicals, industrial solvents, and fuels can be removed by treatment with UV/ozone, activated carbon or plasma technologies. Toxic heavy metals like mercury, lead and cadmium can be partially addressed by using traditional sorbent materials like alumina, but these materials bind metal ions non-specifically and can easily be saturated with harmless, ubiquitous species like calcium, magnesium and zinc (which are actually nutrients, and don't need to be removed). Another weakness of these traditional sorbent materials is that metal ion sorption to a ceramic oxide surface is an equilibrium process, meaning they can easily desorb back into the drinking water supply.

A chemically specific sorbent material, capable of permanently sequestering these toxic metal ions from groundwater, is needed. Since we consume vast quantities of water every day, the kinetics of heavy metal sorption need to be fast, allowing for high throughput in the process stream. A high binding capacity for the target heavy metal is clearly of value. In addition, as acceptable drinking water contamination limits get lower and lower, the need to make analytical methods more and more sensitive (and selective) is rapidly becoming of critical importance.

Nanostructured Materials. There has been a great deal of developments in the synthesis of nanostructured materials recently, particularly in the area of surfactant templated synthesis of mesoporous ceramic materials. Synthetic methods have been developed to prepare these materials in a variety of morphologies (lamellar, cubic, hexagonal, etc.) with structural features ranging from about 20Å to as much as 300Å. A huge amount of surface area is condensed into a very small volume in these nanoporous ceramics, making them well suited for catalytic, sorbent and sensing applications. In addition, the rigid ceramic backbone precludes solvent swelling and allows facile diffusion throughout the entire porous matrix. The ceramic backbone is also structurally more robust than is a polymer-based ion exchange resin, so particle attrition is less of an issue. PNNL has been a leader in developing this understanding, as well as the synthetic tools needed to build these functional nanomaterials.

Self-assembled monolayers. The self-assembly of a monolayer onto a surface is the spontaneous aggregation of molecules into an ordered, organized array, one molecule thick. The self-assembly process is driven by the attractive forces between the molecules themselves (e.g., van der Waal's interactions, hydrogen bonding or dipole-dipole interactions), as well as the attractive forces between the molecule and interface (e.g., hydrogen bonding, acid/base interactions, etc.). There must be sufficient water on the interface to hydrolyze the silane. Only through the judicious choice of reaction conditions (i.e., solvent identity, water concentration, water location, and reaction temperature) can self-assembly take place, resulting in a dense, uniform coating of the surface.

Supercritical fluids have been found to be a particularly powerful, and "green", reaction medium in which to make SAMMSTM. Supercritical carbon dioxide (SCCO2) effectively solvates the siloxane monomers, but it also doesn't inhibit the attractive (van der Waals) forces between the hydrocarbon chains since CO2 is a small linear molecule, allowing self-assembly to proceed smoothly. In addition, self-assembly can be accelerated by carrying it out under conditions of high pressure. In addition, the monolayers so formed have a lower defect density as a result of some novel annealing mechanisms that come into play under these conditions. The low viscosity of SCCO2 also facilitates mass transport of silane throughout the nanoporous ceramic matrix. When the monolayer deposition is complete, the pores of the SAMMS are clear and dry, and not filled with residual solvent that must be removed before the materials can be used.

By varying the chemical nature of the monolayer interface, it is possible to tailor the chemical affinity of the SAMMS materials for specific classes of target analytes. For example, thiol terminated SAMMS have been shown to have extremely high affinity for many forms of mercury (oxidized, organic, chelated, colloidal, etc.), as well as other "soft" heavy metals (e.g., Cd, Au, Ag, etc.). The kinetics of mercury sorption by thiol-SAMMS are extremely fast, with equilibrium generally be achieved in just a few minutes. In addition, thiol-SAMMS is the only known technology that is effective for mercury removal from hydrophobic oil phases (such as the contaminated vacuum pump oil at Savannah River). Installation of ligands analogous to the CMPO (octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide) extractants provides the SAMMS materials with excellent selectivity for lanthanides and actinides, even at low pH and high nitrate concentrations. Once again, selectivity is high and kinetics are rapid (minutes). Functionalizing the SAMMS surface with ferrocyanides creates a sorbent material that is highly effective, and selective, for cesium (radiocesium is one of the principal daughter nuclides resulting from actinide decay, and hence a key issue in nuclear waste clean-up).

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