47. The Influence of Natural Organic Matter on the Fate and Transport of Metal Sulfide Complexes, Clusters and Nanocolloids in the Aquatic Environment

Improved understanding of the geochemistry, fate and transport of natural (for example, HgS and CuS) and engineered nanoparticles ( for example, TiO₂, CdS and CdSe) is critical for assessing ecological and human health effects associated with these materials (Maynard and others, 2006). The complexity of natural systems and the coupled effects of chemical, geological, and biological processes have hampered the development of predictive geochemical, transport, and transformation models for these materials. However, recent studies concerning the bioreactivity and geochemistry of metals (mercury, in particular) and the proliferation of nanotechnology and nanoparticles in commercial products have resulted in increased interest in improving our understanding of the factors that control the occurrence and ecological effects of these materials in ecosystems For instance, to assess the risks of nanoparticles to humans and the environment, the following research needs were outlined by NSETS (2006) for inclusion in the National Nanotechnology Initiative:

1. improving instrumentation, metrology, and analytical capabilities for nanoparticles,
2. determining the effect of nanoparticles on human health,
3. identifying and controlling the effects of nanoparticles on ecosystems,
4. collecting data on the occurrence of nanoparticles in working and natural environments, and
5. developing procedures and practices to minimize exposure to nanoparticles.

Similar research needs were presented as “grand challenges” for the next fifteen years by Maynard and others (2006).

Critical to improving fate and transport models for colloids and nanoparticles is a better understanding of the influences of dissolved organic matter on the geochemistry of these materials. Organic matter controls metal speciation and surface charge of particles, influences sorption interactions with mineral surfaces and drives important photochemical reactions influencing the behavior of both metals and nanoparticles. We seek a Mendenhall Fellow to expand ongoing studies related to dissolved organic matter (DOM) chemistry and mercury cycling to investigate the effect of interactions of dissolved organic matter on the formation, stability, surface properties and reactivity of nanoparticles in aquatic systems.

Research under this Opportunity will provide information critical for understanding influences of DOM on the transport and fate of both naturally occurring and manmade nanoparticles and should be designed to address a key issue related to nanoscale materials–what are the modes of dispersion and aggregation for nanoparticles in the environment? We are interested in understanding the effect of DOM-nanoparticle interactions on the surface charge and aggregation kinetics of a wide range of metal oxides and a select set of metals, metal sulfides, and metal selenides.

Research may be guided by a number of hypotheses including the following: (1) DOM influences the formation of metal sulfides and selenides by terminating the complexation and condensation reactions by which metal and sulfide ions progress through clusters to amorphous solids to crystalline solids, (2)adsorption of natural organic matter will stabilize metal colloid and nanoparticle suspensions and the extent of adsorption depends on the aromaticity of the natural organic matter, (3) exposure of semiconductor nanoparticles (TiO2, ZnO) to light will reduce the adsorption of natural organic matter, and (4) natural organic matter will influence the attachment of nanoparticles to natural mineral colloids, thereby influencing the “colloid-facilitated transport” of the nanoparticles.

Specifically, we seek a postdoctoral scientist who can: (1) quantify the effect of organic matter nature on the inhibition of metal (Hg, Cu, Fe) sulfide precipitation and stabilization of colloids, with an emphasis on mercuric sulfide, (2) examine the influence of natural organic matter sorption on the stabilization of nanoparticle suspensions, (3) define the role of DOM on interactions between colloids and nanoparticles with mineral surfaces that will control transport in aquatic systems, and (4) study the influences of light exposure on the interactions of DOM and semiconductor nanoparticles (TiO₂, ZnO). Experiments will need to be designed to determine the effects of DOM on nanoparticle surface charge, aggregation kinetics, and nanoparticle interactions with clay minerals (kaolinite and montmorillonite). The influence of these reactions on the transport of nanoparticles through porous media will need to be determined by flow-through column experiments. In addition, the effects of photocatalysis of DOM in the presence of photoreactive nanoparticles (e.g., TiO₂) on DOM-particle interactions will also need to be understood.

References

Maynard, A. D., Aitken, R. J., Butz, T., Colvin, V., Donaldson, K., Oberdörster, G., Philbert, M. A., Ryan, J., Seaton, A., Stone, V., Tinkle, S. S., Tran, L., Walker, N. J., and Warheit, D. B., 2006, Safe handling of nanotechnology: Nature, v. 444, p. 267–269.

NSETS, 2006,  Environmental, health, and safety research needs for engineered nanoscale materials:  Washington, D.C., Committee on Technology, National Science and Technology Council, Nanoscale Science, Engineering, and Technology Subcommittee, 62 p.

Proposed Duty Station: Boulder, CO

Areas of Ph.D.: Hydrology, geochemistry, biogeochemistry, geology, chemistry

Qualifications: Applicants must meet one of the following qualifications: Research Hydrologist, Soil Scientist, Research Chemist, Research Geologist

(This type of research is performed by those who have backgrounds for the occupations stated above. However, other titles may be applicable depending on the applicant's background, education, and research proposal. The final classification of the position will be made by the Human Resources specialist.)

Research Advisor(s): George Aiken, (303) 541-3036, graiken@usgs.gov; Joseph Ryan (University of Colorado), (303) 492-0772, joseph.ryan@colorado.edu

Human Resources Office contact: Vanessa Chambless, (303) 236-9584, vchambless@usgs.gov

Summary of Opportunities