Low-pH Fe(II) Oxidation Using a Bioreactor for the Treatment of Acid Mine Drainage
Open Access
- Author:
- Kaley, Bradley Richard
- Graduate Program:
- Environmental Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- November 07, 2013
- Committee Members:
- William D Burgos, Thesis Advisor/Co-Advisor
- Keywords:
- bioreactor
low-pH
acid mine drainage
mine drainage
Fe(II) oxidation
ferrous iron oxidation
GDM - Abstract:
- This thesis examines the ability of a bioreactor to oxidize ferrous iron (Fe(II)) in acid mine drainage (AMD). An abandoned coal and clay mine near Dean, PA produces AMD at a site known as Brubaker Run, and this site was used in the study. Sediment and AMD were collected from Brubaker Run and used to enrich mixed-culture bioreactors in automated chemostats. The reactors were periodically dosed with ferrous sulfate to increase the Fe(II) concentrations. After the bioreactors were established, a series of flow-through experiments, with the reactors operating as completely-stirred tank reactors (CSTRs), were conducted to determine oxidation and removal rates at various operating conditions. Two initial flow-through experiments were performed to determine an optimal hydraulic residence time. These tests were performed the same way, and from the calculated oxidation efficiencies for each hydraulic residence time tested, 6 h was chosen as the optimal value. Using this hydraulic residence time, experiments were then conducted by varying the reactor pH and influent Fe(II) concentration of the bioreactors. Oxidation rates for these experiments increased with a decreasing reactor pH and with an increasing effluent Fe(II) concentration. Although the influent concentration was varied, the effects of the effluent concentration were examined because this is the concentration that was present in the bioreactor and available to the microorganisms. A saturation-like effect appeared to occur when the influent Fe(II) concentration reached the higher set points (1200 and 2400 mg/L). There was also a possible saturation-like effect from the hydrogen ion concentration when the reactor pH was decreased to 2.6 and 2.3. From the experimental data, a general rate law model can be written for biological Fe(II) oxidation. Once simplified, it is only a function of the hydrogen ion and effluent Fe(II) concentrations. Measured pH and effluent Fe(II) values at the different operating conditions can be input into the simplified equation to predict the oxidation rates that should have been measured. The model’s predicted rates are not very accurate in terms of being similar to the actual measured rates. More experimental data or possibly even additional terms in the rate law equation are needed to produce a more accurate model. The recorded addition of acid and base during the experiments can be used to show whether mainly Fe(II) oxidation or both Fe(II) oxidation and Fe(III) precipitation are occurring in the bioreactor. The data show evidence that at pH < 2.9 there is mainly Fe(II) oxidation occurring, while at pH > 2.9 both Fe(II) oxidation and Fe(III) precipitation are occurring. pH=2.9 needs to be explored more because the data do not show evidence for the occurrence of only oxidation or both oxidation and precipitation. The operating conditions are important because of the resulting reactions that they cause, and these reactions can be critical to the success of a treatment system. The loading rates per area of the reactor (GDM) were found to be in the range of 9-448 g Fe(T)/d/m2 (Fe(T)=total iron), depending on the operating conditions. Although this is a large range, the majority of the GDM values were above 60 g Fe(T)/d/m2, and many were even above 100 g Fe(T)/d/m2. The measured bioreactor GDM values were much larger than those reported in the literature. Thus, bioreactors appear to be a sensible choice for removing total iron from AMD.