Oxidation-reduction mechanisms in iron-bearing phyllosilicates

Reviewed in this article are the effects of structural Fe oxidation states on the physicochemical properties of smectite clay minerals. Reducing agents selected were dithionite (S₂O₄²⁻), sulfide (S²⁻), thiosulfate (S₂O₃²⁻), hydrazine (N₂H₄), ascorbic acid (C₆H₈O₆), hydroquinone (C₆H₆O₂), and sodium oxalate (Na₂C₂H₂O₄). Clay samples were prepared as aqueous suspensions of < 2 μm particle-size fractions of Na-saturated, freeze-dried ferruginous smectite. The reductive strength of each reducing agent, as determined by measuring the resultant level of Fe(II) in the clay mineral crystal using either a photo-colorimetric method or Mössbauer spectroscopy, decreased in the order S₂O₄²⁻ > S²⁻ > C₆H₈O₆ > S₂O₃²⁻ > C₆H₆O₂ ≈ C₂H₂O₄. The heat of reaction of three of these reducing agents with the clay was measured, and decreased in the order S₂O₄²⁻ > S₂O₃²⁻ > S²⁻. Compared to the order of reductive strength, the heats of reaction with S₂O₃²⁻ and S²⁻ are reversed, suggesting that entropy changes are greater in the S²⁻ treatment. Electron spin resonance (ESR) revealed that free radicals may be responsible for the greatest levels of reductive potential, which provides an important attribute by which potential reducing agents can be screened. Measurements of rheological properties of oxidized and reduced clay suspensions indicated that structural Fe(II) increases the viscosity of clay suspensions as a result of greater attractive forces between clay particles. The type of bonding between particles has yet to be ascertained. Microbial reduction of the clay produces moderate to high levels of reduction and causes changes in physico-chemical properties similar to chemical reduction of the clay.