| Exploitation of coal resources by mining increases the rate of mineral weathering, particularly pyrite oxidation, which can lead to the contamination of groundwater and surface waters with iron, sulfuric acid, and cations and anions. Treatment of mine drainage requires the oxidation of the Fe(II) and precipitation of the ferric iron in engineered systems. This study was conducted to investigate heterogeneous Fe(II) oxidation (HetOX), at slightly acidic pH and in the presence of high ferric iron oxide (FIO) concentrations, in order to identify ways to improve existing or develop new mine drainage treatment methods. Fe(II) oxidation experiments in this study were conducted in field and laboratory conditions using an existing mine drainage discharge containing elevated levels (>1 mM) of Fe(II). Conditions varied and controlled, included pH from 5.2 to 6.8, FIO concentration from 0 to 2.4 g/L as Fe, temperature from 10 to 22°C, and dissolved oxygen from 0.15 to 0.35 mM. The Fe(II) oxidation results indicated: (1) HetOX was the dominant process for Fe(II) oxidation at the acidic pH and FIO concentrations used in the study, (2) the rate of Fe(II) oxidation was first-order with respect to FIO concentration, (3) the Fe(II) oxidation rate increased with increasing pH, but the rate dependence varied as a function of {lcub}H+{rcub}, and (4) the Fe(II) oxidation rate increased with increasing temperature, and the apparent activation energy of reaction was a function of pH. A deterministic HetOX model was developed that accurately estimated the total HetOX rate (Rtotal) in M s −1 based on two separate oxidation (R1 and R 2) and sorption (S1 and S2) mechanisms. The modeling result was consistent with the presence of strong-Type 1 and weak-Type 2 surface complexation sites for the iron oxides. A new HetOX treatment process was developed, based on recirculation of FIO solids, to maintain high Fe(II) oxidation rates. The process achieved rates that were 1000-fold faster than typically observed in passive treatment systems. HetOX rates were similar to those achieved in conventional chemical treatment of AMD, with improved sludge quality and without the addition of alkaline chemicals. |
