Fe(II) sorption on pyrophyllite: Effect of structural Fe(III) (impurity) in pyrophyllite on nature of layered double hydroxide (LDH) secondary mineral formation

• Fe(II) sorption to natural clays in anoxic systems forms Fe(II)-Al(III)-LDH.
• Clay Fe(III) impurities lead to hybrid Fe(II)-Al(III)/Fe(III)-LDH phase formation.
• Fe(II) oxidation mechanism proposed to be interfacial or surface-sorption-induced electron transfer.
• Environmentally relevant factors impact Fe(II) sorption phase variability.
• Joint spectroscopic analyses are advantageous for Fe speciation in complex systems.

Fe(II)-Al(III)-LDH (layered double hydroxide) phases have been shown to form from reactions of aqueous Fe(II) with Fe-free Al-bearing minerals (phyllosilicate/clays and Al-oxides). To our knowledge, however, the effect of small amounts of structural Fe(III) in natural clays on such reactions were not studied. In this study to understand the role of structural Fe(III) in clay, laboratory batch studies with pyrophyllite (10 g/L), an Al-bearing phyllosilicate, containing small amounts of structural Fe(III) and 0.8 mM and 3 mM Fe(II) (both natural and enriched in 57Fe) were carried out at pH 7.5 under anaerobic conditions (4% H2–96% N2 atmosphere). Samples were taken up to 4 weeks for analysis by Fe-X-ray absorption spectroscopy and 57Fe Mössbauer spectroscopy. In addition to the precipitation of Fe(II)-Al(III)-LDH phases as observed in earlier studies with pure minerals (no Fe(III) impurities in the minerals), the analyses indicated the formation of small amounts of Fe(III) containing solids, most probably a hybrid Fe(II)-Al(III)/Fe(III)-LDH phase. The mechanism of Fe(II) oxidation was not apparent but most likely was due to either interfacial electron transfer from the spiked Fe(II) to the structural Fe(III) and/or surface-sorption-induced electron-transfer from the sorbed Fe(II) to the clay lattice. This research provides evidence for the formation of both Fe(II)-Al(III)-LDH and Fe(II)-Fe(III)/Al(III)-LDH-like phases during reactions of Fe(II) in systems that mimic the natural environments. Better understanding Fe phase formation in complex laboratory studies will improve models of natural redox systems.