Synthesis and properties of manganese-substituted iron oxides from acid and alkaline solutions

Michael Howard Ebinger, Purdue University

Abstract

Three experiments were conducted to examine the substitution of Mn into iron oxide minerals. The first was done at pH 4 or 6 and with Mn mole fractions (X$\sb{\rm Mn}$) of 0, 0.02, 0.05, 0.1, 0.2, 0.35, 0.5 and 1.0. The pH was not adjusted during the synthesis. Mn-substituted geothite, Fe-substituted groutite, hematite, hausmannite, and manganite were identified after 62 days. The X$\sb{\rm Mn}$ in the products was higher in the pH 6 samples than in the pH 4 samples. Magnetic susceptibility increased as X$\sb{\rm Mn}$ increased and decreased only slightly after oxalate extraction. The second experiment was conducted at pH values of 4, 6, 8, and 10, and X$\sb{\rm Mn}$ of 0, 0.2, 0.4, 0.6, 0.8, and 1.0. The pH was adjusted periodically. Mn-substituted goethite and unsubstituted hematite formed in the pH 4 samples and in the pH 6, 8 and 10 samples with X$\sb{\rm Mn} \leq$ 0.2. Goethite was the only crystalline product in the pH 6 samples with X$\sb{\rm Mn} \geq$ 0.2, and jacobsite and hausmannite formed at pH 8 and 10 at X$\sb{\rm Mn} \geq$ 0.4. Manganite formed at pH 4 and 6 in the pure Mn systems, and hausmannite formed at pH 8 and 10 in the pure Mn systems. The X$\sb{\rm Mn}$ in the products increased as the pH increased. The magnetic susceptibility increased as the Mn-substitution increased, and increased dramatically when ferrimagnetic jacobsite was present. IR spectra of the pH 6 samples indicated that groutite formed at X$\sb{\rm Mn}$ 0.4 to 0.8, and that the goethite structure became more disordered as X$\sb{\rm Mn}$ increased. The third experiment was conducted at pH 4 and 6, X$\sb{\rm Mn}$ of 0.4, and with SO$\sb4\sp{2-}$ added in 0, 1, 10, and 100 ppm concentrations. The pH was adjusted periodically, and the suspensions were sampled weekly. At pH 4, SO$\sb4\sp{2-}$ inhibited the formation of hematite, and goethite nucleation was rapid. At pH 6, no hematite formed in the first 28 days of the synthesis, and goethite formed slowly as more SO$\sb4\sp{2-}$ was added. The samples from both synthesis had smaller crystallites as SO$\sb4\sp{2-}$ increased, and the morphology of the crystallites changed from lath-shaped to more equidimensional as the SO$\sb4\sp{2-}$ increased. Sulfate did not influence the amount of Mn that substituted into the structure or the magnetic susceptibilities of the samples.

Degree

Ph.D.

Advisors

Schulze, Purdue University.

Subject Area

Agronomy

COinS