Influence of Fe(II) on the Formation of the Spinel Iron Oxide in Alkaline Medium

Abstract

Fe(II) and Fe(III) in various proportions were coprecipitated by NH3 at pH ≈ 11. The Fe(II)/Fe(III) ratio (x) was varied from 0.10 to 0.50. After stabilization by aging at pH ≃ 8 in anaerobic conditions, hydrous precipitates were characterized by electron microscopy, Mössbauer spectroscopy, and kinetics of dissolution in acidic medium. At any x value, all stable products exhibited the structure of (oxidized) magnetite. For x ≤ 0.30, two distinct species were coexisting: the one (“m”) was made up of ca. 4nm-sized particles with a low Fe(II) content (Fe(II)/Fe(III) ≈ 0.07), and the other (“M”) consisted of particles of larger, more or less distributed sizes, and composition Fe(II)/Fe(III) ≈ 0.33; “M” increased relative amount with increasing x. For x ≥ 0.35, “M” was the only constituent and its Fe(II)/Fe(III) ratio was equal to x. “M” is identified with (nonstoichiometric) magnetite, whereas “m” is likely to be an oxyhydroxide. Mechanisms of formation are discussed, and a phase diagram is proposed which schematizes the evolution of the coprecipitation products with x and with time. Addition of Fe(II) after the precipitation of Fe(III), instead of coprecipitation, yielded very similar results.

Acidified Oxalate and Dithionite Solubility and Color of Synthetic, Partially Oxidized Al-Magnetites and Their Thermal Oxidation Products

“…The sextet #2 (octahedral) for the 0.001 and 0.054 Als samples at 77 K is Fe 3 § as these samples are highly oxidized (note the low value of IS compared with 0.152 and 0.20 A1s), and the 0 mole % At sample has the two overlapping sextets characteristic of maghemite rather than magnetite (consistent with its greater oxidation). Jolivet et al (1992) in preparing magnetite in a similar fashion to what we have done, observed the formation of two types of particles, some smaller (~ 4 nm) and others larger ( > 25 nm). The greater the oxidation of Fe 2 § the more of the smaller variety formed.…”

“…The ratio Feo/Fed = 0 represents a mature soil, and Feo/Fed = 1 represents an immature soil. As shown in this and other studies (Rhoton et al, 1981;Jolivet et al, 1992), magnetites prepared by precipitation from aqueous solutions (MT-RT samples) have small particle sizes and are strongly cation deficient (partially oxidized). Despite relatively low Fe 2 + contents (< 5 wt.…”

“…Because ferrihydrite is still a doublet at 77 K (Murad et al, 1988), this may be evidence for a ferrihydrite-like phase, which would be X-ray amorphous and could be Alsubstituted. Jolivet et al (1992) also found a similar doublet in unsubstituted magnetites prepared in a similar fashion, but only for low Fe 2+ concentration in the solution. The doublet disappears completely at 77 K for samples with Als = 0.001 and 0.054 and shows relaxation broadening for the 0.101 Als.…”

“…Similar effects have been reported previously (Schwertmann and Murad, 1990) and attributed to stabilization of the magnetite structure by A1 substitution. Particle size, Al content, and Fe 2+ content are related in a complex way because, for unsubstituted magnetites, larger particle sizes are associated with higher Fe 2+ contents (Jolivet et al, 1992).…”

Transformation of 2-line ferrihydrite to 6-line ferrihydrite under oxic and anoxic conditions

“…Peaks other than 6-line in the difference pattern are artifacts of the subtraction procedure. played a Mössbauer spectrum similar to that of abiotically synthesized magnetite with Fe 2+ /Fe 3+ = 0.25 (Jolivet et al 1992). The central doublet disappeared upon cooling the bioreduced sample to 77 K (Fig.…”