Oxidation-reduction Mechanism for Structural Iron in Nontronite

Chemical and physical properties of the clay mineral nontronite were studied as the oxidation state of structural iron was altered. Magnesium exchange capacities (CEC) were determined for different nontronite samples that were unaltered, dithionite-reduced, and reduced-reoxidized, and were compared to predicted values calculated from the molecular formula. Observed CEC increased from 125 meq/100 g as predicted with increasing Fe²⁺ content until the Fe²⁺ content reached about 53 mmoles/100 g then the CEC remained constant at 142 meq/100 g even though the Fe²⁺ content was increased further to 139 mmoles/100 g. A two-step mechanism is proposed that involves first, an initial reduction of Fe³⁺ to a Fe²⁺ content of 53 mmoles/100 g with an accompanying increase in layer charge and no structural changes; and second, further reduction to a Fe²⁺ content of 139 mmoles/100 g during which a constant layer charge is maintained by elimination of structural OH and the coordination number of iron in the octahedral sheet is decreased. The mechanism explains changes in oxidation state and electronic environments monitored by Electron Spectroscopy for Chemical Analysis (ESCA) and Mossbauer spectroscopy. The Fe³⁺(2p₃/₂) electron binding energy for oxidized nontronite was observed at 711.8 eV, the Mossbauer isomer shift (I.S.) at +0.44 mm/sec and quadrupole splitting (Q.S.) was zero. Hydrazine-reduced nontronite showed a binding energy at 711.8 eV for Fe³⁺ and 708.6 eV for Fe²⁺. Dithionite-reduced nontronite shifted the binding energy for Fe³⁺ to 711.0 eV, and Fe²⁺ remained at 708.6 eV. Mossbauer I.S. moved down to +0.10 mm/sec for Fe³⁺ and the Q.S. increased to 1.11 mm/sec, and for Fe²⁺ the I.S. was centered at 1.17 mm/sec with Q.S. near 2.77 mm/sec. The proposed mechanism also accounts for shifts in infrared vibrational energies for the O-H stretching mode, the FeO-H deformation, and the Fe-OH octahedral vibrational modes.