ENHANCED ZN REMOVAL FROM AMD BY RAPID OXIDATION AND PRECIPITATION OF IRON OXIDES AT NEAR NEUTRAL PH

Metal concentrations in AMD can be attenuated by using biotic, abiotic, active, and/or passive systems. The optimal remediation strategy depends on water quality and quantity plus site characteristics. For example, aeration of net alkaline AMD, with or without inputs of external energy, can be effective for degassing CO₂, increasing pH, and increasing the rate of abiotic Fe^II oxidation. Hydrous Fe^IIIoxide (HFO) flocs may adsorb dissolved trace metals such as Pb, Cu, Ni, Zn, and Co.

In June and July 2013, a series of batch aeration experiments was conducted to evaluate potential treatment strategies for net alkaline AMD from the Oak Hill Boreholes, near Minersville, PA. Three different aeration rates (Aer1 12.6 ml/s; Aer2 16.8 ml/s; Aer3 25.0 ml/s) were tested by bubbling air through porous stone diffusers immersed in the AMD in 20 L insulated cylindrical containers. For comparison, a control with no mechanical or chemical aeration, and a hydrogen-peroxide (5 ml of 3% H₂O₂) treatment were monitored. The temperature, pH, specific conductance, and dissolved oxygen were recorded at 1 minute intervals. Serial samples collected over 5 hours were filtered on-site (0.45 μm Whatman P/P) for dissolved cation, anion, and alkalinity analysis.

The pH increased from 6.3 to 7.3, 7.8, and 8.2 for Aer1, Aer2, and Aer3, respectively, but was unchanged for the control, and decreased from 6.3 to 6.2 for the H₂O₂ treatment. Fe, Al, and Zn removal increased with increased aeration rates. H₂O₂ addition resulted in almost instantaneous removal of Fe and Al with a ~50% decrease in Zn. Phreeqc modeling indicates sorption to HFO was the largest sink for Zn. The ratio of cumulative Zn:Fe removed was greatest for the aerated tests and decreased with time, suggesting the increased pH during the aeration experiments favored rapid adsorption of Zn during the early stages and continued accumulation of HFO during later stages. No changes were observed in Mn, Ni, or Co, most likely due to the pH was not high enough for Mn^II oxidation or precipitation or sorption of Ni and Co.