Precipitation pathways for ferrihydrite formation in acidic solutions

Iron oxides and oxyhydroxides form via Fe³⁺ hydrolysis and polymerization in many aqueous environments, but the pathway from Fe³⁺ monomers to oligomers and then to solid phase nuclei is unknown. In this work, using combined X-ray, UV-vis, and Mossbauer spectroscopic approaches, we were able to identify and quantify the long-time sought ferric speciation over time during ferric oxyhydroxide formation in partially-neutralized ferric nitrate solutions ([Fe³⁺] = 0.2 M, 1.8 <pH <3). Results demonstrate that Fe exists mainly as Fe(H₂O)₆³⁺, mu-oxo aquo dimers and ferrihydrite, and that with time, the mu-oxo dimer decreases while the other two species increase in their concentrations. No larger Fe oligomers were detected. Given that the structure of the mu-oxo dimer is incompatible with those of all Fe oxides and oxyhydroxides, our results suggest that reconfiguration of the mu-oxo dimer structure occurs prior to further condensation leading up to the nucleation of ferrihydrite. The structural reconfiguration is likely the rate-limiting step involved in the nucleation process.