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Optical Setup Explores Microbes and Minerals

Photonics Spectra
Apr 2002
Brent D. Johnson

Despite concerted efforts by the Department of Energy to clean up radioactive waste at bioremediation sites in Hanford, Wash., and Oak Ridge, Tenn., progress is going very slowly. Jill Scott, a chemist and lead researcher for the Environmental Systems Research and Analysis program at Idaho National Engineering and Environmental Laboratory (INEEL), said part of the problem is that, when this material seeps into the ground, it interacts with both the geologic matrix and microoganisms, creating changes that are not well-understood, partly because the geology of every DoE site is unique.

A major challenge related to the cleanup of radioactive contamination is understanding the complex geological, geochemical, biological and hydrological processes of the subsurface.

"Even though both the INEEL and Hanford have basalt, the mineralogy of the basalts are not the same. So microbes and contaminants might attach differently at the two sites," she said. Various minerals and organisms interact with the waste, selectively binding certain chemicals and affecting their mobility. "If you try to model these things without accounting for the contaminant and microbe population distributions within a specific site's geology, it doesn't match what you get in the field."

Scott and her team attacked these problems by developing an automated laser scanning system that improves the data overlap for taking multiple samples from the same targeted area during successive passes for depth profiling. By obtaining the mass spectra and coding the location with X-Y coordinates on a map, they can return to the same spot. Thus, if microbes have attached to specific minerals, the instrument can identify those minerals.

Crucial to the device's success is an invention by Scott and Paul Tremblay, an engineer at the Idaho lab, called a virtual source and indexing system. Developed for the Fourier transform mass spectrometer, the scanner consists of a series of lenses and prisms that allow users to optically place the laser beam in any position in the X-Y plane. The instrument is indexed to the focusing lens, which provides a uniform laser beam profile that maintains a Gaussian shape rather than changing from circular to elliptical from the center to the edge of the sample. Scott and Tremblay took advantage of the mass spectrometer's high magnetic field to invoke Lenz's law to stabilize the sample, allowing them to scan small spot sizes repeatedly with a resolution of 0.5 µm without moving the sample.

Tremblay used a Surelite Nd:YAG laser to pump a 500-mJ Jaguar C nanosecond dye laser -- both from Continuum of Santa Clara, Calif. -- at either 355 or 532 nm. This allowed them to change the dye and to use any color excitation required to detect the bacteria.

The goal of the researchers is to discover in situ bioremediation processes, whereby uranium, strontium-90, plutonium or organic compounds are transformed into less harmful compounds. To accomplish this, the geological matrix from the actual site must be explored to determine where the contaminants and microbes are attached on the minerals.

"You have to know that the microorganism that can reduce uranium +6 to less-mobile uranium +4 is actually going to be on the same mineral phase before you can use it for bioremediation. If the microbe does not attach to the same place the contaminant does, then stimulating its growth may do little to nothing for remediation," Scott said.

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