INVESTIGATION OF HETEROGENEOUS OXIDATION OF MANGANESE(II) AND OXIDE FORMATION ON NATURAL AND SYNTHETIC GOETHITE

Goethite can catalyze the formation of Mn-oxide/oxyhydroxide minerals via heterogeneous oxidation of Mn(II) [1,2]. This was observed on the surface of a natural Pike’s Peak, CO, goethite sample and clay-sized synthetic goethite particles. Mn precipitates on the goethite surfaces were investigated using atomic force microscopy (AFM), transmission electron microscopy (TEM) with selected area electron diffraction (SAED), and scanning transmission electron microscopy with X-ray energy dispersive spectroscopy (STEM/EDS).

Real time AFM data suggests that Mn(II) oxidation occurs rapidly in solutions containing 65 ppm to as low as 0.005 ppm Mn, and nonspecifically, covering the entire (010) goethite surface within 5 minutes of the reaction. This is in contrast to Mn-oxide formation of hematite, where growth occurs preferentially along steps and at a much slower rate [1]. Powder X-ray Diffraction (XRD) could not detect the Mn phase(s) on the goethite surfaces. STEM/EDS analyses show that Mn is strongly associated with the goethite particles. Mn precipitates concentrated on the tips and over entire surfaces of the goethite particles. Textural changes along with platy Mn particles were observed using TEM. SAED was used to identify three MnOOH phases, groutite, feitknechtite, and manganite, which had precipitated on the synthetic goethite particles.

This data confirms that goethite acts as a catalyst for the oxidation of Mn(II), and that there is a diversity of Mn-oxyhydroxide phases precipitating on the surface. The variety of Mn-oxyhydroxides identified on the goethite surface, impacts our understanding of the chemistry of groundwater and soil compositions in near earth surface environments. This improved understanding of Mn-oxyhydroxide precipitation on goethite may provide better predictions of pollutant interactions in soils. It may also serve as a basis for technological development regarding Mn(II) removal from water sources.

[1] Junta J. L. and Hochella M. F. (1994) Manganese (II) oxidation at mineral surfaces: A microscopic study. Geochim. Cosmochim. Acta 58, 4985-4999.

[2] Rakovan, J. and Hochella, M.F. Jr., (1998) Heterogeneous oxidation and precipitation of aqueous Mn(II) at the goethite surface: An SPM study. Microscopy and Microanalysis. 4, supplement 2, 600-601.