Properties of iron oxides in streams draining the Loess Uplands of Mississippi

Iron oxide precipitates are abundant in small stream systems of NW Mississippi, USA especially during the wet winter months. The properties of these specific materials are unknown even though they have the potential to influence soil physical properties and adsorb chemical pollutants in sediment environments. Streamwater and associated precipitates were collected from 4 representative streams at Cedar Creek (CC), Leeʹs Creek (LC), Spring Creek (SC), and Toby Creek (TC) during winter flow periods. Precipitate specimens were characterized for mineralogy, color, and solubility in oxalate (o), dithionite (d), and HNO3. Chemical composition of the water was dominated by Ca, Na, Mg, and K, in that order, at an average pH of 7.0. X-ray diffraction (XRD) and differential scanning calorimeter (DSC) data indicated that the precipitates were primarily poorly ordered ferrihydrite (CC, TC) and lepidocrocite (LC, SC). The Feo/Fed ratios were 0.40 (CC), 0.68 (LC), 0.66 (SC), and 0.67 (TC). Organic C contents were 80.6, 38.0, 63.0, and 51.3 g kg−1 for the same samples. Precipitate color was uniform among sites, averaging 6.7 YR 4.8/6.2. After oxalate extraction, redness increased slightly in the CC and SC specimens, and decreased in the others. Extraction with dithionite depleted the red color in all specimens, but had less effect on the CC and SC samples which retained hues at 7.9 and 7.3 YR, respectively. Dithionite extractable P equaled 1.02 (CC), 0.72 (LC), 0.56 (SC), and 0.99 (TC) g kg−1. The results from this study indicated that: (1) the precipitates are either primarily poorly ordered ferrihydrite or lepidocrocite; (2) the solubility of ferrihydrite in both oxalate and dithionite is influenced by C contents; and (3) the redder, ferrihydrite specimens contain the greatest P concentrations.