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Minerals and MNA – Biogeochemical Reduction of Chlorinated Solvents

Reactive iron minerals are Fe(II)-containing solid phases are often present in anoxic subsurface environments and engineered systems designed for groundwater remediation. Reactive minerals are commonly formed biogenically under reducing conditions at petroleum contaminated sites, landfills and wetlands, and at sites where biodegradable carbon was added to support biological reductive dechlorination of CVOCs.  Reactive iron minerals can also be present naturally in the geologic material of anoxic aquifers.  Degradation of compounds such as CVOCs via abiotic reactions in contact with these naturally occurring and biogenically formed minerals is often referred to as in-situ biogeochemical transformation.  These abiotic reactions have recently been recognized as important natural attenuation mechanisms for CVOCs (USEPA, 2009).

Figure 1: TCE degradation rates for various iron minerals indicates that Mackinawite (FeS) is the most reactive followed by and pyrite (FeS2) Green Rust GR(SO4) are superior to Magnetite (Fe3O4) – Data from He et al. 2015

Not all Iron Minerals are Created Equal for MNA

Reactive minerals with demonstrated reactivity towards CVOCs include iron sulfides (e.g., mackinawite – FeS, pyrite -FeS2), magnetite, green rust and biotite (USEPA, 2009). Reaction rates for CVOCs with reactive minerals present in aquifers depend on the type of mineral, the content in the aquifer matrix, degree of crystallinity and specific surface area. Mineral reactivity for degradation of CVOCs vary and has been ranked from most reactive to least reactive phases: disordered FeS > FeS > FeS2 > sorbed Fe2+ > green rust = magnetite > biotite (He et al., 2015). The same authors reported that common minerals such as iron sulfides and magnetite present in CVOC-impacted aquifers at concentrations as low as 0.1% wt and 0.3% wt, respectively, may result in environmentally significant rates of natural attenuation of CVOCs.

Ranked from most reactive to least reactive are disordered FeS > FeS > FeS2 > Sorbed Fe2+ > Green Rust = Magnetite > Biotite

Magnetite an iron mineral which is reactive with CVOCs and can be important in NA, Magnetite can be quantified by magnetic susceptibility 

 

Laboratory Testing for Reactive Minerals

The SiREMNA™ package includes analyses developed to identify and quantify reactive minerals in aquifer solids, including:

  • Bulk chemical analyses for total iron and sulphur
  • Acid volatile sulfur (detects FeS) and chromium reducible sulfur (detects total iron sulfides, FeS and FeS2)
  • Magnetic susceptibility (magnetite and greigite)
  • X-ray diffraction (detects crystalline mineral phases)
  • Scanning electron microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDX) – identifies mineral phases at low detection limits regardless of the degree of crystallinity, detects mineral particle size, morphology and elemental composition

It is crucial that the site samples delivered to the laboratory are representative of the original material present in the environment. The recommended methods for preserving the redox status of freshly collected solid materials are freezing and/or storage in an inert atmosphere such as nitrogen. Once received by SiREM, samples are handled in an anaerobic atmosphere to preserve natural conditions, to the extent practicable, during sample preparation for the requested analyses.

References
He, Y.T., J.T. Wilson, C. Su, and R.T. Wilkin. 2015. Review of abiotic degradation of chlorinated solvents by reactive iron minerals in aquifers. Groundwater Monitoring and Remediation 35, no. 3: 57–75.

USEPA (United States Environmental Protection Agency) 2009 Identification and Characterization Methods for Reactive Minerals Responsible for Natural Attenuation of Chlorinated Organic Compounds in Ground Water EPA 600/R-09/115 www.epa.gov/ada office of Research and Development National Management Research Laboratory, Ada, Oklahoma.

Pam Wood

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