X-Ray Diffraction (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 (XRD) ADVANTAGES:
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.
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.
XRD results will provide the relative abundances of the crystalline mackinawite, pyrite, various iron oxides, and other minerals.
HOW TO USE X-RAY DIFFRACTION (XRD)
As part of a multiple lines of evidence approach, use X-Ray Diffraction to identify aquifers where it is plausible that chlorinated compounds may be degraded through an abiotic mechanism.
Use XRD 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?
Abiotic degradation of PCE, TCE, cis-DCE, and carbon tetrachloride by magnetite can be an important attenuation mechanism in long, dilute plumes. While no direct chemical test is available, magnetite is the most abundant mineral in natural sediments that exhibits magnetic behavior. Therefore, magnetic susceptibility provides an inexpensive and valuable estimate of the quantity of magnetite in environmental samples.
While less well studied than the other iron-bearing minerals, various phyllosilicate clays have been shown to be capable of degradation of PCE, TCE, cis-DCE, vinyl chloride, and carbon tetrachloride.
14C TCE Rate Constant
Prof. David Freedman from Clemson University has developed an innovative 14C assay to determine TCE degradation rate constants from environmental samples as part of a recent ESTCP project.
QuantArray®Chlor or CENSUS® qPCR
In addition to abiotic degradation, aerobic cometabolism (co-oxidation) of TCE and other chlorinated solvents can be an important component of MNA. QuantArray®Chlor and CENSUS®qPCR are used to quantify functional genes encoding enzymes such as soluble methane monooxygenase that are capable of cometabolism of TCE.
Compound Specific Isotope Analysis (CSIA)
While contaminant and pathway dependent, aerobic cometabolism and abiotic degradation can result in significant isotopic fractionation and CSIA can be a strong supporting line of evidence in evaluating degradation mechanisms that contribute to MNA.