Mineralogy and trace element association in an acid mine drainage iron oxide precipitate: comparison of selective extractions

McCarty, D.K., Moore, J.N., Marcus, W.A

Applied Geochemistry, Vol.13 Issue 2 pp. 165-176

1998

Mineral and trace element characterization of an Fe-rich precipitate from an acid mine system was accomplished by X-ray diffraction (XRD), differential X-ray diffraction, and ICP chemical analysis. A primary objective was to evaluate the effectiveness of common selective dissolution treatments in determining the association of minerals with potentially toxic trace elements. The precipitate consisted primarily of goethite, a poorly crystalline phase resembling synthetic ferrihydrite, dolomite and gypsum in clay-size fractions. The ammonium oxalate and EDTA treatments, which are thought by some workers to dissolve only poorly crystalline phases, were found to dissolve a significant amount of crystalline goethite. However, the oxalate extraction did dissolve more ferrihydrite than the other treatments tested. A solution of 0.1 M hydroxylamine hydrochloride in 0.1 M HNO3, which is thought by some to dissolve mostly the highly soluble Mn-oxides was found to also dissolve goethite, as did 0.25 M hydroxylamine hydrochloride in 25% (v/v) acetic acid, 0.25 M hydroxylamine hydrochloride in 0.25 M HCI, 0.5 M HCI, and Na-dithionite buffered with Na-citrate and 1.0 M NaHCO3. Both trace and major elements that were extracted with the various treatments were found to vary significantly and non-systematically when compared to the proportion of total extractable Fe. These selective extractions cannot be used to make reliable conclusions about trace metal and mineral associations. Gypsum was identified by XRD in claysize separations from the sample, but this phase was not detected in diffraction scans of the bulk sample. This finding indicates that individual phases can be segregated by particle size.