Field Tools for Measuring Redox Potential

Imagine conducting an initial site assessment where groundwater contamination is suspected. What if you could accurately measure redox potential with an inexpensive pocket-sized device that you could quickly hook up to a monitoring well? Or, what if you were tracking the progress of in situ bioremediation and you could continuously monitor redox conditions throughout the cleanup process?

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These are the kind of tools James Ingle and his research team envision. Professor Ingle is an analytical chemist at Oregon State University (OSU) and a researcher for the Western Region Hazardous Substance Research Center (WRHSRC). He is pioneering new technologies for measuring redox conditions in environmental systems. “Our goal is to develop a portable field screening tool that will allow practitioners to determine the redox status of groundwater rapidly and inexpensively.”

Rather than the current practice of using platinum electrodes to measure "redox potential", Dr. Ingle’s approach relies on redox indicators – organic dyes that change color in response to certain redox conditions. The indicators are reduced and oxidized when exposed to certain environmental reductants and oxidants. The color change then, provides information about the oxidation state of certain chemical species and the dominant microbial processes in the sample.

When he started his research in the early 1990's, three obstacles hindered the technology:

• First, the indicator dyes have properties that make them difficult to work with in environmental systems. They are hydrophobic and adhere to solids, such as soil particles. At the same time, the indicators need to remain free of solids for the color change to be accurately measured.
• Second, the redox conditions of the soil or groundwater samples must be measured without exposure to oxygen since that exposure would change the sample’s redox condition. Either the measurements must take place in situ or the sample must be retrieved without exposure to air.
• Third, the color change or adsorption spectra of the indicator is measured with a spectrometer, an expensive laboratory device. Dr. Ingle wanted to develop an inexpensive minispectrometer that could be part of a hand-held, all-in-one field screening tool.

Progress in Small Steps

Step by step, Dr. Ingle and his team worked to overcome each obstacle. During the early 1990’s the team focused on identifying reversible indicator dyes. In a project funded by the first Western Region HSRC, Dr. Ingle teamed up with Dr. John Westall, another OSU chemist, and graduate students Theresa Lemmon and Brian Jones. They tested potential indicator substances and developed a way to immobilize them onto a thin, gel-based membrane that could interact with an environmental sample in a flow cell. Bonding the indicator to the gel-based film was critical – the bond prevents the indicator from attaching to other solids in the sample.

The next barrier was mechanical. How could the team retrieve a sample from underground and expose it to the indicator without exposing it to the atmosphere? Dr. Ingle and his team decided that an on-line flow cell was the answer. The indicator is exposed to the sample in the flow cell and the adsorption spectra are measured by passing light through the cell.

The last barrier to the design was to develop a small, inexpensive spectrometer that could measure the adsorption spectra in the field. Dr. Ingle and graduate student Kevin Cantrell describe the results of this research in the January 2003 issue of Analytical Chemistry. They call their design the SLIM Spectrometer – SLIM stands for Simple, Low-power, Inexpensive, and Micro-controller based. The entire unit is about the size of a bar of soap!

The SLIM uses LEDs as a light source. They require little electrical power and are inexpensive – total cost for the LEDs and circuits are about $25. The system also includes a data logger so that redox conditions can be measured over time and then downloaded into a laptop computer. Dr. Ingle points out that the small size and inexpensive nature of the system mean that the whole unit could be installed underground for long-term, in situ monitoring.

Current Research

With a functioning technology, the next steps for Dr. Ingle and graduate student Peter Ruiz-Haas (Figure 2) are to test the utility of the system in a cleanup setting. Their test environment is an anaerobic system with tricholoroethylene (TCE) and tetrachloroethylene (PCE), two toxic organic compounds. Researchers will then add experimental microbial cultures to the system. The cultures, which are the focus of another WRHSRC research project, convert PCE and TCE to ethylene, a harmless substance.

The project, funded by the Western Region HSRC, has dual goals: first, to test and refine the redox sensor technology for use when monitoring bioremediation and second, to develop better ways to monitor and study the experimental cultures.

Initial tests will take place in microcosm bottles, small vessels where the active bacterial cultures and chemical processes can be studied carefully. Once researchers establish optimal conditions for the cultures, they will use the indicator technology to monitor the activity of the cultures in physical aquifer models (PAMs) and determine if the indicators can be used to predict when bioremediation is occurring. PAMs are table-top, closed systems that contain layers of aquifer sediments (Figure 3).

Development of new cleanup technologies comes through hard work and small steps. Dr. Ingle hopes that his research will one day put practical tools into the hands of practicioners.