Detection of Microbial sulfate-reduction associated with buried stainless steel coupons Page: 3 of 12
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4H2+ S042-+ H+-> HS- + 4 H20
Although this is a simple net reaction, reduction of sulfur is a complex process of 6 electron
steps and associated reaction products. The process is very sensitive to system redox
potential and may play an important role in total solution chemistry. The ability to detect very
low levels of sulfate-reducing activity, as demonstrated in this study, has potential applications
for evaluating behavior of buried metals where burial times are intended to be very long.
Storage containers used for burial of nuclear waste are just such materials. Nuclear
industry waste must be isolated to prevent spread of contamination for periods of time that are
much longer than a human lifetime. The first line of defense to prevent spread of radioactivity
is to place waste repositories in remote geologically stable locations that are inaccessible, and
likely to remain so; ideally the location is dry or has very slow groundwater movement. The
second line of defense is to immobilize materials in stable and insoluble forms, and to package
them in strong corrosion resistant containers. Stainless steels that are highly resistant to
corrosion are currently the preferred materials for nuclear waste storage containers.
Although far superior in resistance when compared to other ferrous metals, stainless
steels are relatively new alloys, and there is no observational basis for predicting the long term
integrity of containers made from them. Since its inception the NBS and NIST have acted on a
mandate to provide baseline materials testing including properties of stainless steels. Test
samples buried at sites across the United States in 1970 and 1971 for corrosion study by NBS
have been made available to researchers at the Idaho National Laboratory (INL). Site 'D' from
that NBS project, on a Coast Guard station near Wildwood, N.J., was selected for this study
because of accessibility, and because corrosion processes were expected to be accelerated
there due to the damp saline marine environment.
The imaging process demonstrated in this study was originally developed for mapping
microbial activity on sandstone and shale cores in subsurface microbiology investigations34.
The technique results in a radiographic image of a silver foil that has been incubated in close
contact with a radioactive 35S-sulfate coated surface. Microbial activity on the surface
produces radioactive sulfide that readily reacts to form an insoluble metal sulfide on the foil
surface. Radioactivity from this fixed sulfide produces the radiographic image.
In parallel with imaging, sulfate-reduction activity in soil from within 2 cm of coupons
was quantified using a standard radioactive tracer technique. Trace amounts of radioactive
sulfate were added to soil slurries; sulfide produced from microbial reduction of the labeled
sulfate was quantified by liquid scintillation counting. The strength of this technique relies on
the assumption that the amount of labeled sulfate added to the system is small with respect to
total sulfate present, so the introduction does not stimulate activity5'6.
MATERIALS AND METHODS
Coupons were recovered in May 2004 and shipped to Idaho National Laboratory for
analysis. Collateral analyses from this site and on these coupons are reported elsewhere (1).
(1) M. K. Adler Flitton and T. S. Yoder. Long-term underground corrosion of stainless steels. NACE Corrosion
Expo 2007. Paper # 07595(1)
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Delwiche, Mark E.; Flitton, M. Kay Adler & Olson, Alicia. Detection of Microbial sulfate-reduction associated with buried stainless steel coupons, article, March 1, 2007; [Idaho Falls, Idaho]. (https://digital.library.unt.edu/ark:/67531/metadc888535/m1/3/: accessed April 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.