In the past, scientists have used theoretical equations or bulk-scale column experiments to determine how to remove contamination. "Neither approach explains what happens at the smallest scale," says Cail. And the results of column experiments, in which water composition is studied before and after it passes through a soil-packed column, can not be extrapolated for different environmental conditions such as pH fluctuations and changes in concentration, she explains.
Cail is using an atomic force microscope (AFM) to measure the attraction between a silica glass collector surface and a 2-micron sphere of carboxylated polystyrene, which has the size, shape, and surface charge similar to bacteria. The micro spheres are attached to a cantilever developed for just such AFM studies. "The AFM measures the forces of attraction and repulsion between the particle and the collector as the surfaces are moved toward each other and then apart.We mathematically determine the energy of interaction, which allows us to quantify how much they want to stick," says Cail.
With this experimental design, various environmental conditions can be simulated and their effect on sticking efficiencies can readily be determined.
The collector material of silica glass is similar to the quartz sand present in many soils. Cail plans to introduce actual bacteria in future experiments, but the focus of her presentation at ACS is the new method of measurement. "Sticking efficiency has not been measured experimentally before using the AFM.
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Contact: Tracy Cail
tcail@vt.edu
540-231-3358
Virginia Tech
23-Mar-2003