Nanomaterials, Sustainability, and Risk Minimization 

WIESNER et al
Introduction to IWA International Conference on Nano and Microparticles in Water and Wastewater Treatment 22sep03

Zurich, Switzerland
22 - 24 September 2003

Mark R. Wiesner, Hélène Lecoanet, Maria Cortalezzi
Environmental and Energy Systems Institute
Rice University
6100 Main Street, Houston, Texas 77005

Emerging technologies, including nanotechnologies, affect the social, economic, and environmental dimensions of our world, often in ways that are entirely unanticipated. There is considerable effort underway to explore uses of nanomaterials in applications such as membrane separations, catalysis, adsorption, and analysis with the goal of better protecting environmental quality. Along with the growth of a nanochemistry industry there is also the need to consider impacts of nanomaterials on environment and human health. Recent advances in nanomaterial-derived membrane technologies and catalyst supports are presented as examples of how nanomaterials might be used to create new environmental technologies. Preliminary data on nanoparticle transport in porous media are presented in the context of the need for studies on environmental exposure and effects of nanomaterials.

The vision of building objects from the atomic scale up combines many scientific areas of inquiry with wide implications for technology development. The production, use, and disposal of of nanomaterials can be anticipated to engender a wide range of benefits and unintended consequences in social, economic, and environmental terms. It is very likely that applications of nanoscience will lead to new means of reducing the production of wastes, using resources more sparingly, remediating industrial contamination, providing potable water, and improving the efficiency of energy production and use. So-called "bright-side" implications of nanotechnology include a tremendous potential for introducing new tools that will help resolve economic growth and environmental protection. However, "dark-side" issues associated with any emerging technology must also be considered. Specifically, it is essential that developments in nanotechnology be accompanied by thoughtful assessments of the potential impact of an emerging nanochemistry industry on human health and the environment.

An "bright-side" example of the use of nanomaterials to achieve near term benefits to the environment and human health is the application of nanochemistry to membrane science. Membrane technologies are playing an increasingly important role as unit operations for environmental quality control, resource recovery, pollution prevention, energy production, and environmental monitoring. Membranes are also key technologies at the heart of fuel cells and bio-separations. Nanoscale control of membrane architecture may yield membranes of greater selectivity and lower cost.

One strategy for producing nanostructured membranes involves an environmentally-benign alternative to the sol-gel process for ceramic membrane formationⁱ. The aluminum based sol-gels formed during the hydrolysis of aluminum compounds belong to a general class of compounds: alumoxanes. The structure of alumoxanes was proposed to consist of linear or cyclic (II) chains (i.e., analogous to that of poly-siloxanes)' Recent work has shown that they are not chains but three dimensional cage compoundsⁱⁱ. For example, siloxy-alumoxanes, [Al(O)(OH)_x(OSiR₃)1-x]n, consist of an aluminum-oxygen core structure (III) analogous to that found in the mineral boehmite, [Al(O)(OH)]_n, with a siloxide substituted periphery (IV).

Figure 1. Proposed structures for alumoxane nanoparticles.

The sol-gel approach of reacting small inorganic molecules to form oligomeric and polymeric materials has several limitations such as difficulties in controlling the reaction conditions, and the stoichiometries, solubility and processability of the resulting gel. It would thus be desirable to prepare alumoxanes in a one-pot bench-top synthesis from readily available, and commercially viable, starting materials, which would provide control over the products. This has been accomplished using an approach that is a significant departure from the traditional synthetic methodologies.

Based on the fact that the carboxylate anion, [RCO₂]^-, is an isoelectronic and structural analog of the organic periphery found in our siloxy-alumoxanes (IV and V), alumoxanes can be produced based upon the reaction of boehmite, [Al(O)(OH)]_n, with carboxylic acidsⁱⁱⁱ. The physical properties of the alumoxanes are highly dependent on the identity of the alkyl substituents, R, and range from insoluble crystalline powders to powders that readily form solutions or gels in hydrocarbon solvents and/or water. Thus, a high degree of control over the nanoparticle precursors is possible.

The carboxylate-alumoxane materials (and, we speculate by extension, other metal-oxane precursors such as iron) are an attractive class of pre-ceramic materials. They are indefinitely stable both in solution and the solid state, and whereas the choice of solvents in sol-gel synthesis is limited, the solubility of the carboxylate alumoxanes is dependent on the identity of the carboxylic acid residue, which is almost unrestricted. The solubility of the alumoxanes may therefore be readily controlled so as to make them compatible with a co-reactant.

The alumoxanes are adaptable to a wide range of processing techniques. They can be dip-coated, spin coated, and spray-coated onto various substrates. Like traditional sol-gel methodology for ceramic synthesis the use of carboxylate-alumoxanes offers the advantages of atomic scale mixing of metals and processability. Furthermore, the incorporation of metals into the alumoxane core structure allows for the formation of meta-stable phases.

However, unlike sol-gel synthesis the alumoxane route is readily performed on a large scale, and the processability is more readily controlled by the choice of organic substituent. Given the advantages observed for the application of carboxylate alumoxanes, e.g., the low price of boehmite ($ 0.5 kg ^-1) and the availability of an almost infinite range of carboxylic acids, make these species ideal as precursors for ternary and doped aluminum oxides. The alumoxanes can be easily converted to g-Al₂O₃ upon mild thermolysis.

Table 1 shows a comparison of the ceramic and sol-gel methods with that of the carboxylate alumoxanes for the synthesis of alumina and ternary aluminum oxides. The ease of modification of the alumoxanes suggests that a single basic coating system can be modified and optimized for use with a range of substrates.

Table 1. Comparison of the alumoxane and sol-gel synthesis methods. 

			alumoxane 		sol-gel
methodology 		simple 			complex
atomic mixing 		yes 			yes
meta-stable phases 	yes 			yes
stability 		excellent 		poor
solubility 		readily controlled	difficult to control
processability 		good 			good
time 			< 8 h. 			>20 h.
cost 			low 			med. - high

Application of a metal-oxane-based approach to creating ceramic membranes reduces the use of toxic solvents and energy consumption. By-products formed from the combustion of plasticizers and binders will be minimized, and the use of acids eliminated. Moreover, the use of tailored nanoparticles and their deposition on a suitable substrate presents an extremely high degree of control over the nanostructure of the resulting sintered film. The versatility of the process can be used to tightly control pore-size distributions the potential for fabricating new ceramic nano- and ultra-filtration membranes with enhanced specificity.

Concentrated mixtures of the alumoxanes with water form rigid gels. Carboxylate alumoxanes decompose above 180°C to give amorphous alumina. Firing above 900°C results in the formation of a mixed phase g-Al₂O₃ and a-Al₂O₃ as expected from the transformation sequence of alumina, and they are converted to a-alumina above 1200°C. Ceramic films of controlled porosity therefore can be formed by depositing alumoxane particles from an aqueous suspension, followed by drying and firing. The MWCO of the first generation of such membranes made approximately 40,000 Daltons^iv. The permeabilities of asymmetric these membranes varied between 0.11 nm² and 0.75 nm², with and average of 0.32 nm².

More recently, we have created iron analogs to the alumoxane nanoparticles, ferroxanesv^,v,vi. The current ferroxane-derived membranes produced have an average pore size, measured by nitrogen adsorption, of 24 nm and a BET surface area of 75 m²/g. The MWCO of these membranes falls in the range of 130,000 to 180,000 g/mole, corresponding to a molecule of diameter between 14.9 nm and 17.3 nm.

We are pursuing a second strategy for creating nano-structured membranes, that of nanoparticle templating. Porous solids can be created from deposits of particles onto substrates. The voids in the deposits can be filled with a polymeric or inorganic material and upon etching of the particles, a porous material with a three-dimensional structure is formed (Figure 2). Particle size, stability and/or depositional trajectories can be controlled to engineer template morphology and yield membranes with a desired structure^vii. The use of nanoparticles in the templating process allows for a high degree of control over chamber and pore size as well as the ability to produce membranes in the nanofiltration range.

Figure 2. Cross-section of a templated membrane.

Commercial applications of nanomaterials currently or will soon include nano-engineered titania particles for sunscreens and paints, carbon nanotube composites in tires, and silica nanoparticles as solid lubricants, and protein-based nanomaterials in soaps, shampoos, and detergents. Industrial applications currently being marketed include the use of alumina nanoparticles in the manufacture of propellants, pyrotechnics, and ceramics membranes, nanoparticles in semiconductor manufacture, and numerous biomedical applications. If the current trend in commercial ventures continues, we will soon find ourselves with a relatively large nanomateriasls industry. Production of significant quantities of anthropogenically-derived nanomaterials will inevitably result in the introduction of these materials to the atmosphere, hydrosphere, and biosphere.

Research is needed to explore the impacts of nanomaterials and nanomaterial production on the environment and public health. One framework for assessing these impacts is that of comparative risk assessment. Applied to an assessment of the production, use and disposal of nanomaterials, a risk assessment typically considers both the potential for exposure to a given material and (once exposed) potential impacts such as toxicity, or mutagenicity. The need to elucidate both of these components of risk in assessing the consequences of nanomaterials on the environment and public health is essential.

Nanomaterials may differ from other particulate materials in both size and surface chemistry. These differences may affect both exposure and impacts. Nanomaterials are all surface: a gram of single-walled carbon nanotubes, for example, has over ten square meters of available surface. Control over the chemistry of this interface is essential in developing nanotechnologies. Rarely will bare inorganic solids be present in the solution phase of nanostructures; rather, nano-particles will likely be coated chemically to give them specific desired properties. As in the case of toxicity, the relevance of our current experience in this domain is unclear. Although there are many nanoscale particles produced naturally, it is unclear how anthropogenically created materials might differ in their interactions in ground- and surface waters. In particular, the impact of nanoparticle chemistry, and interactions with surfaces are of great interest since the phenomena normally associated with surfaces of larger particles take place over length scales that are similar to the size of nanoparticles.

The mobility of nano-engineered particles in aqueous environments is a function of particle transport, transformation, and removal mechanisms. Fluid flow, gravity, and diffusion are the primary mechanisms for transport. Transformation includes particles precipitation, dissolution, bio-uptake, and aggregation. Nanoparticles or objects that include nanoparticles such as nanoparticle aggregates may be removed from flow in a two-step process in which particles are transported to the vicinity of a surface followed by attachment to that surface.. Particle transport may occur by advection, diffusion, and gravity. Attachment is considered to be primarily a function of particle and collector surface chemistries and is expected to vary with particle charge and the presence of adsorbed/associated material. These variables are in themselves likely to depend on the nature of the aquatic environment such as the pH and ionic strength.

The potential for nanoparticle mobility in porous media such as ground waters can be quantified in bench-scale experiments in which suspensions of nanoparticles are introduced to a packed bed column and the influent and effluent concentrations are monitored over time. The mobility of three varieties of engineered nanoparticles have been quantified in this fashion (Figure 3).

Figure 3. Breakthrough curves for nanoparticle transport through a porous medium of silica spheres 9.5 cm in depth, at a Darcy velocity of 0.136 cm/sec, pH 7, ionic strength 10^-2M.

Under conditions of pH and ionic strength similar to many ground waters, significant differences in nanoparticle mobility are observed. In contrast with the view of nanomaterials as a single class of materials that may require monolithic treatment in a a regulatory sense, these data underscore the need to evaluate the potential exposure associated with a nanomaterial on a case-by-case basis. Among the nanomaterials considered in this work, silica nanoparticles appeared to present the greatest potential for transport and potentially exposure. Although silica nanoparticles appear to pass readily through the porous medium, their potential for migration in a homogeneous medium is predicted to be relatively limited. Expressions for particle transport in porous media (REF) may be used to calculate an average attachment efficiency from experimental data. This attachment efficiency can then be used to estimate the potential for transport at larger scales and variable flow conditions. Such calculations, assuming a homogeneous groundwater aquifer with sand having similar properties to those of the silica media used in the laboratory porous medium, suggest that most (99.9%) of the silica nanoparticles would transport no farther than 5 meters. However, these measurements and calculations do not account for important factors such as interactions with naturally occurring materials such as organic macromolecules in ground waters, or the heterogeneity of groundwater aquifers.

As nanochemistry emerges as an important force behind new environmental technologies, we are also presented with responsibility of considering the environmental implications of an emerging technology at its inception. Our ultimate goal must be to ensure that nanotechnologies, and the materials which enable them, evolve as instruments of sustainability rather than as environmental liabilities.

The contributions of Professor Andrew Barron and Dr. Jerome Rose to the work on nanostructured ceramic membranes, as well as the contributions of Dr. Vicki Colvin to the work on templated membranes and those of Jean-Yves Bottero to the work on nanoparticle transport in collaboration with the authors are gratefully acknowledged.

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2. A. W. Apblett, A. C. Warren, and A. R. Barron, Chem. Mater., 1992, 4,167.

3. Landry, C. C.; Pappè, N.; Mason, M. R.; Apblett, A. W.; Tyler, A. N.; MacInnes, A. N.; Barron, A. R., J. Mater. Chem. 1995, 5, 331.

4. Cortalezzi, M.M., J. Rose, A.R. Barron, M.R. Wiesner, " Characteristics of ceramic membranes derived from alumoxane nanoparticles," Journal of Membrane Science 205:33-43, 2002.

5. Cortalezzi, M.M., J. Rose, A.R. Barron, M.R. Wiesner, " Ceramic membranes derived from ferroxane nanoparticles: a new route for the fabrication of iron oxide ultrafiltration membranes," Journal of Membrane Science accepted, 2003.

6. Rose, J., Fidalgo, M.M., Moustier, S., Magnetto, C., Jones, C.R. , Barron, A.R., Wiesner, M.R., and Bottero' J.Y., " Synthesis and Characterization of Carboxylate-FeOOH Particles (Ferroxanes) and Ferroxane-derived Ceramics, " Chemistry of Materials, 14:621-628, 2002.

7. Cortalezzi' M.M., V. Colvin and M.R. Wiesner. "Controlling nanoparticle template morphology: effect of solvent chemistry," Colloid and Interface Science, submitted.