Leaching studies of schwertmannite-rich precipitates from the Animas River Headwaters, Colorado and Boulder River Headwaters, Montana

Content Provider	Semantic Scholar
Author	Desborough, George A. Leinz, Reinhard W. Sutley, Stephen J. Briggs, Paul H. Swayze, Gregg Alan Smith, Kathleen Shea Breit, G. N.
Copyright Year	2000
Abstract	X-ray diffraction (XRD) analysis and reflectance spectroscopy show that orange-colored precipitates that have accumulated in pH 3.5 to 4.0 headwaters of the Animas River in Colorado and the Boulder River in Montana may consist chiefly of schwertmannite [Fei 6Oi 6(OH)9 6(SO4)3 2 10H2O]. This mineral is present in streams of coal-mining areas in Ohio but has not been previously reported to occur in the western U.S. These precipitates, which may be accompanied by other solid phases including silicates, goethite, and XRD undetectable amorphous material, were often called "ochreor orange-colored colloids" by other investigators who noted they may contain significant concentrations of Cu, Cd, Pb, Zn, and As. The toxicity of these metals to aquatic organisms was recognized in prior studies as were the potentially harmful effects of colloidal Fe-hydroxides accumulating on the gills of salmonids (trout). Passive serial leaching tests in the laboratory showed that as little as one gram of schwertmannite-rich precipitate generated enough acid to lower the pH of two liters of deionized water from 5 to 3.5 in an initial leach of 24 hours. Acid generation continued for eight successive leaches and was accompanied by the release of metals and sulfate into solutioa Comparable results were obtained when the same amount of precipitate was serially leached with 3and 4-liter quantities of water. The results indicate that 43 to 48 percent of schwertmannite was removed from the precipitates by total water volumes of 18, 27, and 36 liters in the three sets of leaches. Field test along 3.5 km of Cement Creek in the Animas River basin in October of 1998, a time of low flow, indicate the schwertmannite-rich precipitates accumulated at a rate equivalent to 370 kg day"1 of Fe. The results of this study infer that acid formation during subsequent leaching of the accumulated precipitates during spring and summer snow-melt runoff may pose another problem to the aqueous environment INTRODUCTION AND BACKGROUND Prior to this study we found no published reports of schwertmannite in the Rocky Mountain region of the western U.S. However, J. M. Bigham (Pers. Comm., 1999) identified schwertmannite in samples from St. Kevin Gulch, near Leadville, Colorado. Runnells and Rampe (1989), Ranville and others (1989), Kimball and others (1995), and Church and others (1997) described "ochre-colored colloids" in suspension and as "precipitates" coating rocks in both the Arkansas River and Animas River headwaters of Colorado but the phase was not identified. These investigators have referred to the orange precipitates as "hydrous iron-oxide colloids", although some may be schwertmannite because the pH of some of the drainages is favorable for schwertmannite precipitation. High magnification (>50,000 X) transmission electron microscopy is necessary to identify the morphology of schwertmannite crystals (Bigham and others, 1990, 1994). Schwertmannite also has a unique spectral reflectance signature. In the original description of schwertmannite, the empirical formula was given as Fei6Oi 6(OH)9 6(SO4)3.2 .10H2O (Bigham and others, 1994). In later reports, Bigham, Schwertmann, and Pfab (1996a) used Fe8O8(OH)6SO4 and Bigham and others (1996b) used Fe8Os(OH)5 5(SO4)i.25 . The results of this study support a formula with an 8:1.25 molar ratio of Fe:SO4 . A major reason for our interest in schwertmannite-rich material is that metals such as Cd, Cu, Pb, Zn, and As may be sorbed to this phase and may be released during desporption in concentrations that are toxic to aquatic organisms (Lee, 1975; Jenne, 1977; Morel and Gschwend, 1987; Stumm and Morgan, 1996). A second reason for the present report is our recognition that schwertmannite-rich material causes a significant drop in pH by reaction with deionized water because of dissolution of schwertmannite. This acid generation will enhance solubility of toxic metals temporarily sequestered in both the suspended and precipitated Fe-rich solids in the stream bed. Significant acid generation from natural schwertmannite-rich material in a stream bed has not been previously reported However, Bigham and others (1996b) measured significant acid generation in long-term exposure of synthetic schwertmannite to distilled water (initial pH = 3.9) that appeared to equilibrate in about 1,000 days, and traces of goethite were detected as a decomposition product after 65 days. In that experiment, there was a nonstoichiometric release of Fe and SO4, and the molar ratio of H+ :SO4"2 was 2:1. The following was given by Bigham and others (1996b, p. 2114) for the hydrolysis of schwertmannite: Fe8Qj(OH)5.5(SO4), ^ + 2.5 H2O(I) => 8 FeOOH
File Format	PDF HTM / HTML
DOI	10.3133/ofr004
Alternate Webpage(s)	https://pubs.usgs.gov/of/2000/0004/report.pdf
Alternate Webpage(s)	https://doi.org/10.3133/ofr004
Language	English
Access Restriction	Open
Content Type	Text
Resource Type	Article