Carbonate-Enhanced Transformation of Ferrihydrite to Hematite

An elevated activity of (bi)carbonate in soils and sediments (pCO₂, ∼2%) above current atmospheric CO₂ (∼0.04%) could influence the iron cycling in mineral-water interfacial chemistry. However, the impact of (bi)carbonate on mineral transformation is unclear. Here, a model short range-ordered iron oxyhydroxide, two-line ferrihydrite, was used to evaluate the impact of (bi)carbonate on mineral transformation at near-neutral pH using experimental geochemistry, X-ray diffraction, X-ray absorption spectroscopy, transmission electron microscopy, and Fourier transform infrared spectroscopy. Results showed that (bi)carbonate promoted the transformation of ferrihydrite to hematite and retarded the goethite formation. As pCO₂ increased from 408 to 20,000 ppmv at 40 °C, the transformation efficiency of ferrihydrite increased from 53 to 95%, and the formation of hematite increased from 13 to 76%. During the formation of hematite, a terminal ligand on a Fe(III)O₆ octahedral monomer such as a hydroxyl or water was displaced to form Fe(III)O₆ octahedral dimers and/or polymers. Because the Fe-O bond of ≡(Fe-O)₂-CO is much weaker than that of ≡Fe-O-H, the -O₂CO group can be more easily replaced by two terminal -OH groups; the dehydration/rearrangement between Fe(III)O₆ octahedral monomers was enhanced under high pCO₂. Results suggest that high carbonate activity is an important geochemical parameter controlling the occurrence of hematite in oxic environments and, in turn, iron cycling in the critical zone.