Characterization of Colloidal Material from Natural and Engineered Waters

Investigating Structure-Function Relationships through Microanalysis

R. David Holbrook1, Scott Wight1, Dale Newbury1

1 Surface and Microanalysis Science Division

National Institute of Standards and Technology, Gaithersburg, MD 20899-8371

The fate, transport and ultimate impact of toxic compounds in aquatic systems is governed by their distribution between the particulate, colloidal and dissolved phases. Colloids, conventionally defined as solid material between 1 nm and 1 µm in size, constitute a relatively small fraction of the total waterborne particle mass but possess large surface areas that facilitate binding of other colloids and dissolved species. Partitioning of hydrophobic organic compounds (HOCs) and inorganic species to colloidal material is influenced by colloidal composition, and variations in composition are concomitant with changes in morphology (i.e., physical structure). Yet despite the importance of understanding the colloidal phase in the fate and transport of toxic compounds, there are few methods presently available that enable morphological and compositional analysis of individual colloids. As a result, the relationship between colloidal nanostructure/composition and physicochemical function (e.g., pollutant transport) is virtually unknown.

The purpose of this research is two-fold. First, the use of the environmental scanning electron microscope (ESEM) for investigating the structure and composition of aquatic colloids is demonstrated. The ESEM allows the samples to remain in a hydrated state during imaging and therefore minimizes artifacts caused by, for example, sample desiccation. ESEM images of colloids derived from different wastewater treatment plants indicate unique morphological properties that may correspond to process configuration and sewage characteristics. The structure of aquatic colloids from natural systems appears to vary both temporally and spatially, indicating that land use and hydrologic conditions are an important factor in colloid morphology.

Secondly, a field study that investigates the variability of colloidal material in a watershed will be presented. The Occoquan Watershed (OW) is part of a drinking water system serving over one million people. The tributary watershed drains nearly 148,000 hectares, and receives flows from both rural and urban nonpoint sources, as well as the discharge from an advanced wastewater reclamation facility (WRF). Intensive studies have been conducted in the Occoquan since 1972, and both the reservoir and watershed are among the best characterized (both hydrologically and chemically) in the nation. Samples will be collected from various points within the Occoquan reservoir and imaged using the ESEM. This data will be helpful in providing a basis for predicting colloidal behavior in complex ecosystems as samples will be obtained from various locations during different hydrologic conditions.

Corresponding Author

R. David Holbrook
Surface and Microanalysis Science Division, Chemical Science and Technology Laboratory
Room A 103, Building 222
Mail Stop 8371
Telephone: 301-975-5202 Fax: 301-417-1321

Category for poster: Chemistry