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

Abstract

Abstraet--Submicron-sized (~3-60 nm) powders of Al-substituted magnetite were synthesized in the laboratory by precipitation methods by mixing appropriate molar volumes FeCI2, FeC13 and A1CI3 solutions and precipitating with 20% NH4OH. Precipitates were dialyzed for 48 hr to remove excess salts and then freeze-dried. The nominal A1 mole fractions [AL = A1/(Al + Fe)] in the initial precipitate ranged from 0.001 to 0.42. Portions of the resulting powders were heated sequentially in air at 400* and 500"C. Powders were examined using X-ray diffraction (XRD), transmission electron microscopy (TEM), and visible and near-IR reflectance spectroscopy. Solubilities were determined in ammonium oxalate (pH = 3) and dithionite-citrate-bicarbonate (DCB) solutions. As determined by XRD, the mineralogy of precipitated powder samples was predominantly magnetite. Powders having Al~ > 0.20 contained minor goethite and a poorly crystalline iron oxide phase (ferrihydrite?), and powders having Al~ > 0.25 also contained gibbsite. The color of the magnetites was black throughout the range of Al-substitution. Powders heated to 400~ were reddish brown; Munsell colors ranged from 5R 2/2 to 10R 3/4 for Al, from 0.1-41.5%, respectively. By XRD, these powders were maghemite, but hematite was also detected by Mrssbauer spectroscopy. XRD and Mrssbauer data indicate powders heated to 500"C are hematite; their Munsell colors are not noticeably different from the corresponding 400"C samples. Mean crystallite dimensions (MCDs) of the magnetite powders increase with the A1 mole fraction from ~ 10 nm for Als = 0.001% to a maximum value of 35 nm for A1, = 0.15 and decrease slightly with further increasing A1 substitution. Heating magnetite powders to 400"C did not change the MCDs significantly. Heating to 500~ resulted in hematites having MCDs larger than those for corresponding precursor magnetites for A1, < 0.10. The opposite is true for hematites derived from magnetites having A1, > 0.10. For hematite powders with AI~ > 0.05, MCD decreased with increasing Al-substitution. Solubilities of powders in oxalate solutions were independent of Al content and decreased in the order unheated samples (mostly magnetite) >400*C-heated samples (maghemite + hematite) >500*C-heated samples (hematite). All powders dissolved completely in DCB. The low crystallinity of the magnetite powders and the presence of ferrous iron are responsible for their relatively high solubility in oxalate solutions.

Occurrence and Constitution of Natural and Synthetic Ferrihydrite, a Widespread Iron Oxyhydroxide

“…Modifications or alternatives to the pyrophosphate and ammonium oxalate treatments have been proposed, , but have not been adopted widely. Recent refinements to the ammonium oxalate treatment have mainly involved the duration of the treatment or X-ray diffraction monitoring to determine coincident dissolution of the Fe oxides. ,, X-ray monitoring is particularly useful for soils that may contain magnetite, which also is soluble in oxalate. , The use of ammonium oxalate and EDTA, and the effects of adsorbed anions such as phosphate, were discussed by Borggaard; , the EDTA reagent, however, requires 3−7 months for extraction. For this reason, and because comparative tests on soils indicate that the amounts of Fe extractable by oxalate and by EDTA are similar (Figure ), EDTA seems not to be used extensively.…”

Selective dissolution of magnetic iron oxides in the acid-ammonium oxalate/ferrous iron extraction method-I. Synthetic samples

“…1982, 1985; Phillips & Lovley 1987; Fine & Singer 1989; Borggaard 1990; Canfield et al . 1992; Golden et al . 1994; Pinheiro‐Dick & Schwertmann 1996; Rozan et al .…”

“…1971; Schwertmann 1973; Walker 1983; Borggaard 1988, 1990; Fine & Singer 1989). Others reported that the method can be applied to dissolve specific minerals, such as magnetite, and that differentiation on the basis of mineralogy would be possible with this method (Chao & Zhou 1983; Golden et al . 1994).…”

Characterisation of Soils by the near Infrared Technique

“…The application of NIR has shown significant developments in geology (Golden et al 1 and Post et al 2 ) and in material science (Beck et al 3 ). The NIR analysis of soils and soil components began in early seventies.…”