July 2006 Symposium on Nanotechnology and the Environment: Fate and Transport of Nanomaterials: Highlights, Question and Answer Session
July 12, 3:30-4:30 pm
Dr. Gregory V. Lowry, Carnegie Mellon University, Associate Professor, Civil and Environmental Engineering, Pittsburgh Pennsylvania
Highlights The two general types of sources are point sources (manufacturing effluent, landfills) and non-point sources (stormwater runoff, tire wear, wet deposition).
Aggregation: nanoparticles can aggregate in water through Van der Waals interactions, chemical bonding, hydrophobic effects, and magnetic attraction. High diffusion coefficients lead to many collisions, and frequent contact between particles promotes aggregation. Coating nanoparticles decreases aggregation by two mechanisms: charge stabilization, or steric stabilization. Nano-sized iron in aqueous suspension readily aggregates. Particle concentration can affect the stable size of aggregates formed and the speed of aggregation. As concentration increases, aggregation is more rapid and aggregates may become large enough to settle out via gravity.
Attachment of nanoparticles to surfaces limits mobility and bioavailability, and may affect transformation/degradation. Attachment is a function of the particle and its surface properties. Bare nano-sized iron particles stick to sand grains and then begin to stick to each other (they have a higher affinity for each other than for silica).
Some important questions include: How long do nanoparticles last, and what do they become once transformed? What kinds of reactions take place (redox, photolysis, biotransformations)?
Redox transformations change the surface characteristics of nanoparticles. Processes that may alter the surface properties on nanomaterials include oxidation, hydroxylation, and sorption of organic matter. Biotransformations are likely but have not yet been demonstrated. Surface modifications that could affect particle toxicity and/or mobility include surface functionalization (either by redox reactions at the surface, through engineered coatings, or by sorption of dissolved organic materials to nanoparticles), and loss of engineered surface coatings on nanoparticles (coatings could be biodegraded or desorbed depending on their makeup).
Factors that limit nanoparticle mobility in porous media (e.g., aquifer) include aggregation, straining (particles or aggregates exceed pore size), and attachment (particles stick to soil).
Physical and chemical factors that need to be considered when assessing mobility include pH, particle surface chemistry, velocity, grain size, heterogeneity, particle size, particle concentration, and ionic strength.
At low concentrations, bare iron particles have limited mobility; at high concentration, the particles have no mobility. A surface coating increases mobility. Coating materials produce either electrostatic repulsion between particles or steric repulsion. With these coatings, stable suspensions are possible and particles do not attach to aquifer grains.
The relative mobility of particle types can be estimated using a standard formula which includes a sticking parameter (designated as alpha). From alpha, one can estimate the travel length for a specified tolerance under conditions specific to the experiment. A smaller negative alpha value indicates that a chemical sticks more. For particles with surface coatings designed specifically to enhance mobility, transport distances are anticipated to be on the order of meters to 10’s of meters under normal groundwater conditions. These are rough estimates as they are highly specific to the conditions of the laboratory tests and should be used cautiously. Overall, mobilty in porous media is low under typical groundwater conditions. Surface modification may enhance mobility. Mobility of nanomaterials in surface waters is unknown. Dilution in receiving water may limit aggregation or promote disaggregation. Attachment to other suspended solids and/or photolysis from surface waters is also possible.
It appears that nanoparticles can be cycled in organisms. In a study in which single-walled nanotubes were ingested by copepods, nanotube aggregates were detected in the copepods’ digestive tracts and feces. Nano-sized iron has been observed in Medaka fish gills.
Questions regarding the fate and transport of nanomaterials include:
- Will they bioaccumulate or facilitate the bioaccumulation of other contaminants?
- How significant are biotransformations?
- Is photolysis significant?
- What role does heterogeneity play in particle mobility?
- Is incineration effective in destroying nanomaterials?
- What is the fate of surface coatings on nanomaterials?
Questions regarding the potential toxicity of nanoparticles include:
- What are the environmentally relevant concentrations of nanomaterials?
- Despite aggregation is the low population of single particles responsible for toxicity? There are bound to be some single particles present – do these cause the bulk of toxicity?
- Do surface coatings enhance or mitigate the toxicity of the particles?
Question-and-Answer Session When asked about the basis for low mobility of landfill leachate, Dr. Lowry indicated that calcium and magnesium promote aggregation, and landfill leachate contains Ca and Mg. Clay is less porous than sand, and nanomaterials should be less mobile in clay. Therefore, transfer through the clay barrier in landfill leachate containing high concentrations of divalent cations would be expected to be limited. When asked about mobility of coated particles, Dr. Lowry answered that particles must be able to attach to dense non-aqueous phase liquid (DNAPL). Coated particles have been shown to move tens of meters in bench-top experiments. It is an engineering challenge to get nanoparticles to move a certain distance and then stop.
A questioner asked about the difference between aggregation and agglomeration. Dr. Lowry indicated that aggregation implies strong attractive forces and is irreversible; agglomeration is not as strong as aggregation and is more readily reversible, i.e., they are easier to break apart into smaller agglomerates or individual particles. Bare particles aggregate strongly. Surface-coated particles agglomerate eventually, but can be broken up readily.
A commenter noted that development of nanoparticles passes through stages. In stage 1, particles are passive; research then progresses to create dual functional particles, then to interconnected or intelligent systems. We need to consider not only passive nanomaterials but also polyfunctional materials. When asked whether a drug delivery molecule could get delivered to some unintended site in the body, Dr. Lowry answered that this is possible.
When asked whether the ability of agglomerates to penetrate crevasses would depend on the flow of water, Dr. Lowry answered that effectiveness does depend on flow. One hundred meters down is too far to treat effectively. If particles sit at interface, DNAPL can be destroyed as it flows out of crevasses.
When asked whether the model for bioaccumulation of nanomaterials was similar to that of Polychlorinated Biphenyls (PCBs), Dr. Lowry responded that it is tempting to think that traditional models might apply, but there is enough difference between nanoparticles and regular chemicals to suggest that their bioaccumulation might differ.