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.