LABORATORY AND FIELD-SCALE OBSERVATIONS OF FERROUS IRON OXIDATION AND FERRIC IRON PRECIPITATION AT AN ACID MINE DRAINAGE SITE
Open Access
- Author:
- Brown, Juliana F.
- Graduate Program:
- Environmental Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- None
- Committee Members:
- Bill Burgos, Thesis Advisor/Co-Advisor
William D Burgos, Thesis Advisor/Co-Advisor - Keywords:
- low-pH Fe(II) oxidation
acid mine drainage
schwertmannite - Abstract:
- The first study (Chapter 2) analyzed the aqueous chemistry, mineral precipitates, microbial communities, and Fe(II) oxidation rates at an acid mine drainage (AMD) site in the context of a depositional facies model. Both pool and terrace facies at upstream and downstream locations were studied. Fe(III) precipitates were determined to be schwertmannite with pin-cushion morphology at all locations, regardless of facie. Microbial community composition was studied with 16s rDNA cloning and fluorescence in situ hybridization (FISH) and found to transition from a Betaproteobacteria and Euglena dominated environment at the AMD spring to an Acidithiobacillus dominated environment downstream, as pH decreased. Microbial composition at adjacent pool and terraces was similar; thus, microbial community structure was more strongly a function of pH and other geochemical conditions rather than depositional facie. Intact pieces of terrace and pool sediments from upstream and downstream locations were used in laboratory reactors to measure rates of low-pH Fe(II) oxidation at variable residence times (2-10 h). Rates of Fe(II) oxidation were normalized to mass and surface-area. Mass-normalized oxidation rates ranged from 0.74 to 10.4 x 10-9 mol Fe L-1 s-1 g-1 and rates were faster for pool sediments compared to terrace sediments. Surface-area normalized rates ranged from 0.63 to 1.75 x 10-9 mol Fe L-1 s-1 cm-2 and the faster rates were also associated with pool sediments. Upstream sediments were also more efficient at Fe(II) oxidation than downstream sediments, regardless of facie, suggesting that Fe(II) oxidation rates were also dependent upon biogeochemical conditions, not solely the depositional facies environment. Sediments were irradiated with 60Co and analyzed again to determine abiotic Fe(II) oxidation rates. No change in Fe(II) concentration was observed for sterilized sediments, indicating that all Fe(II) oxidation was a result of biological activity. A depositional facies model explained some differences in Fe(II) oxidation kinetics, but this model could not explain differences in water chemistry, mineral composition, crystal morphology, or microbial community composition. In the second study (Chapter 3), three low-pH coal mine drainage (CMD) sites in central Pennsylvania (Lower Red Eyes, Fridays-2, and Hughes Borehole) were studied to determine similarities in sediment composition, mineralogy, and morphology. Water from one site (Lower Red Eyes) was used in a discontinuous titration/neutralization experiment to produce Fe(III) minerals by abiotic neutralization/precipitation for comparison with the field precipitates that were produced by biological low-pH Fe(II) oxidation. Sediments were characterized using X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS) spectroscopy, and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS). Even though the hydrology and chemistry of the CMD varied considerably between the three field sites, the mineralogy of the three iron mounds was found to be very similar. Schwertmannite was found to be the predominant mineral phase precipitated at low-pH with traces of goethite at some sampling locations. Schwertmannite particles occurred as micron sized spheroids with characteristic “pin-cushion” morphology at all sites. No trace metal incorporation was detected in sediments from the field, and no metals (other than Fe) were removed from the CMD at any of the field sites. Minerals formed by abiotic neutralization/precipitation (pH 5.18 – 8.34) were also found to consist primarily of schwertmannite. In contrast to low-pH precipitation, substantial trace metal removal occurred in the neutralized CMD, the subsequent precipitates were found to contain Al, and schwertmannite morphology changed dramatically. While secondary minerals such as schwertmannite essentially store some of the emergent acidity from CMD sources, this mineral may be of industrial value because, at least when precipitated at low-pH, it does not contain trace metal contaminants.