What does redox mean
and why does it matter?
When field scientists monitor groundwater contamination,
they need to be able to quickly measure the chemical conditions underground.
One important characteristic is the redox potential of the system –
redox potential measures the availability of electrons for transfer between
molecules. The availability of electrons determines the solubility of
many chemicals and also affects the types of organisms that can live in
the system. For example, some types of bioremediation (cleanup that uses
bacteria to break down contaminants) require specific redox conditions.
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?
Click on highlighted words for pop-up definitions and illustrations.
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
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
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.
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
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.
For More Information
Contact Professor James Ingle at firstname.lastname@example.org
or read more in the following references:
Cantrell, K.M. and Ingle, J.D., Jr., 2003, The SLIM Spectrometer,
Analytical Chemistry, Vol. 75, No. 1, p. 27-35.
Jones, B.D., and Ingle, J.D., Jr., 2001, Evaluation of
immobilized redox indicators as reversible, in situ redox
sensors for determining Fe(III)-reducing conditions in environmental
samples, Talanta, Vol. 55, p.699-714.
Lemmon, T.L., Westall, J.C., and Ingle, J.D. Jr., 1996,
Development of Redox Sensors for Environmental Applications
Based on Immobilized Redox Indicators, Analytical Chemistry,
Vol. 68, No. 6, p.947-953.