The availability of clean drinking water is a critical factor in determining quality of life in human society. 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, but chemical contamination is a 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. Another weakness of these traditional sorbent materials is that metal ion sorption to a ceramic oxide surface is an equilibrium process so 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 important. In addition, as acceptable drinking water contamination limits become more stringent, the need to make analytical methods more sensitive and selective becomes more critical.
New developments in the synthesis of nanostructured materials— particularly in mesoporous ceramic materials—allow materials to be prepared in a variety of morphologies, including lamellar, cubic and hexagonal, 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 also is structurally more robust than is a polymer-based ion exchange resin, so particle attrition is less of an issue. Pacific Northwest National Laboratory has been a leader in developing this understanding, as well as the synthetic tools needed to build these functional nanomaterials.
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 SAMMS (self-assembled monolayers on mesoporous supports). Supercritical carbon dioxide (SCCO2) effectively solvates the siloxane monomers, but it doesn't inhibit the attractive (van der Waal's) forces between the hydrocarbon chains since CO2 is a small linear molecule, allowing self-assembly to proceed smoothly. High pressure accelerates the self-assembly process. 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. There is no 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 high affinity for many forms of mercury, including oxidized and organic, as well as "soft" heavy metals, such as cadmium and gold. The kinetics of mercury sorption by thiol-SAMMS™ are extremely fast, with equilibrium generally achieved in just a few minutes.
In addition, thiol-SAMMS™ is the only known technology that's 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 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).