Formation of ferric oxides from aqueous solutions: A polyhedral approach by X-ray absorption spectroscdpy: I. Hydrolysis and formation of ferric gels

X-ray absorption spectroscopy (XAS) was used to follow the evolution of local structural environments around ferric ions during the formation of ferric hydrous oxide gels from 1 M chloride and 0.1 M nitrate solutions. Fe K-XANES and EXAFS confirm that ferric ions remain 6-fold coordinated during this evolution. EXAFS spectra obtained on partially hydrolyzed ferric chloride solutions indicate the presence of aqua-chloro ferric complexes up to OHFe = 2.2. When OHFe < 1 in the solution, iron mainly occurs in trans-[FeCl2(H2O)4]2+ octahedra with Fe-Cl and Fe-O distances of 2.31 and 2.01 A, respectively. With increasing OH availability in the solution, Cl− anions tend gradually to be exchanged for (O,OH,OH2) ligands. Below OHFe = 1, no structural order is detected beyond the first coordination sphere. Above this ratio, two Fe-Fe distances at 3.05 A and 3.44 A are observed and correspond to the presence of edge- and vertex-sharing Fe-octahedra. XAS results show that ferric gels and highly polymerized aqueous species are short-range ordered. The main contribution to disorder in the gels arises from the small size of coherently scattering domains also responsible for their X-ray ‘amorphous’ character. From the initial to the final stage of hydrolysis, particles possess a nearly spherical shape with a minimum average diameter ranging from 10–30 A for polymers formed from chloride and nitrate solutions. As polymerization proceeds, the local order extends to several tens of angstroms and the particle structures becomes progressively closer to that of akaganeite (β-FeOOH) or goethite (α-FeOOH). This local structure is distinct from that of the lepidocrocite (γ-FeOOH)-like structure of ferric gels precipitated after oxidation of divalent Fe solutions. The growth of the crystalline Fe-oxyhydroxides from gels takes place by the progressive long-range ordering in the ferric polymers without modifying the short-range order around Fe.