The application of XPS to the study of MIC Page: 6 of 15
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by Desulfovibrio sp. in the presence of the Fe, Cr, Ni, Mo ions are presented in
Figure 1. The associated total production of gas and reduction of sulfate are pre-
sented in Table 1. Exposure of Desulfovibrio sp. to solutions containing 0.2 mM
concentrations of Fe, Cr, Ni and Mo metal ions was not found to inhibit microbial
growth, as indicated by add production, sulfate reduction, and total gas production
(Figure 1 and Table 1).
The XPS technique involved the irradiation of the cell-pellet sample by an X-ray
beam which induced the ejection of electrons from the outermost layer of the
sample surface. The kinetic energies of the emitted electrons were analyzed to de-
termine their binding energies in the sample. Peaks in the recorded spectra were as-
sodated with specific elements (e.g., S, Fe, Cr, Ni, Mo) by comparison to standards.
The way in which the atoms were bound on the surface was deduced from the shape
and binding energy position of the peaks.
The chemical state of the metal inoculates and sulfur compounds present in the
biomass was determined, where possible, from the deconvolved XPS spectra.
Figure 3 shows the presence of metal sulfides, sulfite, and elemental sulfur in the
biomass that had been inoculated by both cationic (Fe, Cr, Ni) and anionic (Mo)
metal complexes. The highest rate of sulfate reduction by Desulfovibrio sp. occurred
in the culture inoculated with molybdate. This observation indicates that bacteria
may have a significant effect on the efficacy of molybdate corrosion inhibitors.
Compounds that are iso-structural to sulfate, such as molybdate, are generally
regarded as inhibitory to the growth of Desulfovibrio sp. However, the molybdate
concentration (0.2mM) that was used in this study, may have been too low to inhibit
growth. The molybdate concentration utilized is representative of that which might
be present at the surface of a clean, passive stainless steel (approximately 10*5 A/cm2
current density). Further work will demonstrate the critical concentration at
which molybdate will affect the activity of sulfate reducing bacteria.
The formation of metal sulfides from cations in neutral pH medium was not
surprising but evidence of molybdenum disulfide was. One possible mechanism is
that molybdenum disulfide may form from the reaction with biogenic hydrogen
sulfide, but this requires addic conditions.  In the bulk medium with neutral
pH, the formation and stability of a sulfide would require microbial production of
hydrogen sulfide gas in a region of low pH, which could possibly be created by the
acetic add and propionic adds that were observed to be produced by the bacteria.
The S 2p spectra revealed the presence of sulfide in all cases. The smallest
amount of sulfide observed was found in the case of no inoculate. In all cases, the
S 2p spectra revealed four sulfur spedes; sulfur, sulfate and sulfite and sulfide. The
smallest amount of sulfur spedes observed was found in the case of no inoculate.
The XPS metal spectra appear in Figure 4. UiJike the sulfur spectra, the signal -
to-noise ratios for the metal spectra were too low to permit confident deconvolu-
tion. However, the overall shift of the Mo 3d spectra away from the molybdate
binding energy strongly suggests that molybdate has become reduced in favor of a
molybdenum sulfide compound. Further work will improve the signal-to-noise
ratio so that more specific information can be obtained on peak identification. The
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Kearns, J.R. (Allegheny Ludlum Corp., Brackenridge, PA (United States). Technical Center); Clayton, C.R.; Halada, G.P. (State Univ. of New York, Stony Brook, NY (United States). Dept. of Materials Science); Gillow, J.B. & Francis, A.J. (Brookhaven National Lab., Upton, NY (United States)). The application of XPS to the study of MIC, article, January 1, 1992; Upton, New York. (https://digital.library.unt.edu/ark:/67531/metadc1094209/m1/6/: accessed April 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.