Tetrahedral Fe³⁺ in Ferrihydrite: ⁵⁷Fe Mössbauer Spectroscopic Evidence

Studies of the Reactivity of the Ferrihydrite Surface by Iron Isotopic Exchange and Mössbauer Spectroscopy

“…Since the IS values in Table 2 fall within both ranges, the spectral data did not provide enough information to determine whether the ~TFe was octahedrally (sixfold) or tetrahedrally (fourfold) coordinated. Cardile (1988) confirmed the difficulty of determining Fe-O coordination by fitting a ferrihydrite M6ssbauer spectrum with model spectra consisting of either all octahedral Fe(III) or a 2: I combination of octahedral:tetrahedral Fe(III) sites. The author found equal success with both fits and concluded that the coordination of~7Fe in ferrihydrite could not be determined conclusively by room temperature M6ssbauer spectra.…”

“…Many researchers (Murad and Schwertmann, 1980;Childs and Johnston, 1980;Johnston and Lewis, 1983;Cardile, 1988;Murad, 1988) have reported similar doublets for both natural and synthetic ferrihydrite samples. No magnetically split hyperfine component lines are observed because of the small particle size of the ferrihydrite and its correspondingly low magnetic ordering temperature, in the range 115-25 K depending on crystallite size (Murad and Johnston, 1987;Murad et aL, 1988).…”

“…Murad and Johnston, 1987). Two-Lorentzian-doublet fits have been found to be adequate in most ferrihydrite studies because the index of fitting error (either x 2 or MISFIT) does not improve significantly with additional Lorentzian doublets (Cardile, 1988); two doublet fits were, therefore, used in this study. The two doublets should not be interpreted to represent two distinct Fe sites in ferrihydrite, although the fits can be used to probe qualitative differences in Fe environments.…”

“…Iron octahedra at the end of chains can have as little as two nearest Fe neighbors and as many as three; Fe octahedra at edge sites typically have 4 Fe (1) 0.54 (2) Values in parentheses indicate error in the last significant figure. 1 MQS = mean quadrupole splitting, the sum of the products of the individual QS values and their respective areas. 2 Six-line ferrihydrite fit with all octahedral Fe 3 § (Cardile, 1988). 3 Six-line ferrihydrite heated for l0 and 30 min.…”

Modified Inorganic Polymer Flocculant-PFSi: Its Preparation, Characterization and Coagulation Behavior

“…Even if it might be possible for iron to form the same Keggin structure, the stronger hydrolysis trend would lead to the lower charge of this polymer causing it to become unstable for further polymerization or aggregation. However, the possibility of tetrahedral Fe has been suspected to be present in ferrihydrites (Cardile, 1989;Combes et al, 1989). The improvement of coagulation with PACl may due to the specific characteristic of Al 13 both on its union structure and chemical behavior.…”