Biotic vs. Abiotic Structured Mackinawite

Biotic

  • Resulting from living things.
  • Requires an environment conducive for microbial life.
Example – Sulfate Reducing Microorganisms (SRM) produce sulfide, and when in the presence of iron, may react to form ferrors sulfide (FES).

Abiotic

  • Resulting from physical or chemical factors rather than biological (not derived from living organisms) .

Example – a FeS reagent that is manufactured or formulated to a specification in a chemical reactor and delivered to a remediation site.

Biotic and Abiotic Formation - Similarities

The first iron sulfide formed biotically is a reactive ferrous sulfide (FeS) mineral with a “mackinawite” structure. This is also the main ingredient in FerroBlack®.
  • Both biotic- and abiotic-formed iron sulfides are generally effective in:
    • Promoting reductive de-chlorination.
    • Lowering hexavalent chromium concentrations.
    • Binding with individual metal cations (especially those in the “2+” valence state).
  • Pyrite does not form in well-controlled, anoxic, biotic systems, nor is it present in abiotic suspensions of FerroBlack®

Biotic and Abiotic Formation - Differences

  • Biotic FeS is formed in-situ; abiotic FeS (mackinawite) is synthetically manufactured and delivered to the site.
  • As biotic FeS is formed, any phosphate present (which is a required nutrient for biotic microorganisms) may react with soluble iron and form an insoluble iron-phosphate mineral complex (vivanite) and not be available to form FeS.
  • Biotic respiration produces “extra-cellular materials” which are “sticky” leading to larger aggregated particle-sized mackinawite which may potentially change aquifer flow characteristics.
  • Transformation from mackinawite (FeS) to greigite (Fe3S4) is generally accelerated in biotic systems compared to abiotic FeS.
  • Abiotic FeS (FerroBlack®) is appropriate for multi-metal contamination, however the biotic capabilities for multi-metals treatment are less understood.
  • Abiotic (FerroBlack®) can be manufactured stoichiometrically balanced, with excess Fe, or with excess S.
  • In a biotic system, environmental professionals must create and maintain an environment viable for a living microorganism colonies in a dynamic chemical and physical ecosystem.

Why “roll the dice” by assuming the in-situ conditions will be optimal for biotic FeS Formation?

Simply, Redox Solutions abiotically synthesized mackinawite structured iron sulfide products eliminate the unknown factors of biotic iron sulfide formation.

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