XRD can provide relative abundances of iron bearing minerals including pyrite (FeS2) and the crystalline form of mackinawite (FeS), the most reactive of the iron bearing minerals. Both mackinawite and pyrite will transform PCE, TCE, and carbon tetrachloride. For anaerobic sites and sites transitioning from enhanced anaerobic bioremediation to monitored natural attenuation (MNA), abiotic degradation by reduced iron sulfides can be an important attenuation mechanism.

X-RAY DIFFRACTION ADVANTAGES:

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IDENTIFICATION

XRD is one of the primary methods to identify unknown crystalline materials including crystalline mackinawite (tetragonal FeS) and pyrite. While not quantitative like the magnetic susceptibility test, XRD can also detect magnetite when present at between 2% and 5% on a weight basis. While special sample care to prevent oxidation would be needed, XRD can be used to detect green rust.

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BROAD APPLICABILITY

Mackinawite is known to transform PCE, TCE, carbon tetrachloride, and 1,1,1-TCA as well as tetrachloroethanes. Pyrite is capable of abiotic degradation of PCE, TCE, cis-DCE, vinyl chloride, and carbon tetrachloride

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RELATIVE ABUNDANCE

XRD results will provide the relative abundances of the crystalline mackinawite, pyrite, various iron oxides, and other minerals

HOW TO USE XRD:

As part of a multiple lines of evidence approach, use XRD to identify aquifers where it is plausible that chlorinated compounds may be degraded through an abiotic mechanism.

Use magnetic susceptibility to help answer…

  • Are mackinawite and pyrite present in the impacted zone?
  • Is abiotic degradation of chlorinated compounds plausible?
  • Is abiotic degradation a reasonable explanation for decreasing contaminant concentrations/mass?

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