The Comparison of Two Acid Mine Drainage Sites in Central Pennsylvania: Field Site Characterizations and Batch Reactor Experiments
Open Access
- Author:
- Lucas, Melanie Anne
- Graduate Program:
- Environmental Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- April 11, 2008
- Committee Members:
- William D Burgos, Thesis Advisor/Co-Advisor
- Keywords:
- iron oxidation kinetics
coal mines
acid mine drainage
batch reactors - Abstract:
- Biological oxidation of iron(II) in acid mine drainage (AMD) and the subsequent precipitation of iron(III) occurs unaided at Gum Boot Mine in McKean County, Pennsylvania but it is not observed at a large number of AMD sites across the state. To improve passive AMD treatment methods through a better understanding of biological iron(II) oxidation, the Pennsylvania Department of Environmental Protection (PA DEP) selected this and a similar AMD site (Fridays-2 Mine in Clearfield County, PA) for chemical, microbial, and kinetic characterization of the observed abiotic and biological oxidation processes. Sampling transects were established to identify any spatial trends over the iron mounds at each mine. Water chemistry measurements were conducted on mine drainage waters during several sampling events at each mine site. Solids were collected for subsequent SEM and molecular analyses. Iron sediments collected from the sampling locations of highest and lowest activities of iron(II) oxidation (Gum Boot “fastest”, Fridays-2 “slowest”, and Fridays-2 “fastest”) were incubated in batch reactors under different gas mix compositions of O2, CO2, and N2. The batch reactor results were then used to calculate the abiotic and biological iron(II) oxidation rates of the sediments to determine if field trends were observed in controlled laboratory conditions. These results were also used to examine the effects of pO2 and pCO2 on iron(II) oxidation rates. Lastly, the results of the batch reactor experiments were used to model the iron(II) oxidation kinetics of the sediments with STELLA computer program and the combined model for abiotic and biological iron(II) oxidation proposed by Kirby et al. (1999). Water chemistry results indicate similarities between the emergent AMD at Gum Boot and Fridays-2 Mines, with no observed seasonal trends in iron(II) concentrations, pH, temperature, and DO at either mine site. Iron(II) was quickly removed in the first 10 meters of flow over the iron mound at Gum Boot Mine; however, most iron(II) was still present in solution at Fridays-2 Mine immediately before the AMD mixed with Fridays Run. Given the distance of AMD flow and measured rates of AMD emergence, the residence time over the iron mound at Fridays-2 Mine was much shorter than the residence time at Gum Boot. No nutrient limitations were observed at either Gum Boot or Fridays-2 Mine. Microbial characterization of the mine sites included SEM images of sediments, population counts of iron(II) oxidizing bacteria (IOB), and phylogenetic analyses of IOB communities, in addition to batch reactor experiments with glucose additions to examine the contribution of heterotrophic vs. autotrophic IOB at Gum Boot and Fridays-2 Mines. SEM images and phylogenetic analysis reveal greater morphological and genetic diversity in sediments collected from Gum Boot AMD emergence than from Fridays-2 AMD emergence. Additionally, sediments at each mine site from the sampling locations with the largest numbers of IOB and the largest rates of iron(II) oxidation had the greatest phylum-level similarities. Lastly, though batch reactor experiments with glucose additions did not show any change in observed iron(II) oxidation rates, phylogenetic analyses indicated heterotrophs were the dominant IOB present at the sampling locations with the highest levels of iron(II) oxidation activity. Batch reactor iron(II) oxidation rates were observed to increase with O2 concentrations of 21%. However, IOB numbers increased during each batch reactor experiment, with the largest increases observed at O2 concentrations > 10%. There were no noticeable effects of CO2 concentration on abiotic or biological iron(II) oxidation rates. Fridays-2 reactors generally had higher abiotic rates of iron(II) oxidation and were capable of achieving biological iron(II) oxidation rates that equaled or exceeded the rates observed in the Gum Boot reactors. The Kirby et al. (1999) iron(II) oxidation model was successfully modified in STELLA for application to the batch reactor results of this study. Observed iron(II) oxidation rates were less than the rates predicted by the default STELLA model as well as rates published by other previous studies. Additionally, the pO2 of the gas mix did affect the model performance and fit to the batch reactor data. Lastly, the biological rate constant, kbio, for iron(II) oxidation may be dependent on the bacterial communities composition and certain environmental factors of an AMD site. The results of this study indicate that both Gum Boot and Fridays-2 Mines are equally capable of efficient oxidative precipitation of iron(II) from acid mine drainage. Biological iron(II) oxidation was observed at both mine sites and with both batch reactor sediments. However, the shorter residence time at Fridays-2 Mine did not allow enough time for the hydrolysis and precipitation of iron(III) before the AMD mixed with Fridays Run. In conclusion, future remediation strategies at Fridays-2 Mine should focus on increasing the AMD residence time to encourage optimal conditions for biological iron(II) oxidation promote iron(III) precipitation.