ENHANCED ZN REMOVAL FROM AMD BY RAPID OXIDATION AND PRECIPITATION OF IRON OXIDES AT NEAR NEUTRAL PH
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 MnII oxidation or precipitation or sorption of Ni and Co.