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GROUNDWATER IMPACTED BY ACID MINE DRAINAGE
McNeil*, M S. AND R T. Wilkin*. GROUNDWATER IMPACTED BY ACID MINE DRAINAGE. Presented at Hardrock Mining 2002 Conference, Westminster, CO, May 07, 2002.
To inform the public.
The generation and release of acidic, metal-rich water from mine wastes continues to be an intractable environmental problem. Although the effects of acid mine drainage (AMD) are most evident in surface waters, there is an obvious need for developing cost-effective approaches for ameloriating groundwater impacted by mine wastes. The use of zero-valent iron in subsurface Permeable Reactive Barriers (PRB) presents a possible passive approach for remediating groundwater contaminated by heavy metals. A series of batch studies were carried out to evaluate the performance of zero-valent iron for removing heavy metals from solutions with initial pH from 2 to 5. Zero-valent iron (Peerless Metals, Inc., Fisher Iron) was exposed to solutions containing concentrations of Fe, Al, As(V), Cd, Cu(II), Hg, Ni, Mn, and Zn. In all experiments, the final pH was greater than 5.5. Batch studies (initial pH 2.3) were effective in removing all metals from solution below nominal analytical detection limits, except for Mn and Fe. Ferrous iron concentrations reached approximately 1100 mg/L when exposed to one gram Peerless iron, 670 ppm with 10 grams Peerless iron, and 360 mg/L with 10 grams Fisher iron. Batch studies at pH 3.5 showed that all metals were reduced to concentrations below 0.08 mg/L after 450 hours. Metal concentrations were reduced in the following order: Al>Hg>As>Ni>Cd>Cu>Zn>Mn> Fe>Fe2+, with Fe2+ remaining at approximately 150 mg/L. Less efficient removal of Zn and Mn was observed with decreasing solid/liquid ratio. Results show that all tested metals are effectively removed from the pH 4.5 solution, a pH perhaps most characteristic of groundwaters impacted by mine wastes. In the pH 4.5 batch series, Al, Hg, and As were reduced to below nominal detection limits, within 24 hours, and Cd, Cu, Mn, and Zn concentrations were reduced to <0.03 mg/L within 450 hours. Initial sulfate concentrations for all batch studies ranged from 9500 to 11000 mg/L and final concentrations ranged from 8900 to 10000 mg/L. The filtrates from each batch study were processed anaerobically and analyzed using X-Ray diffraction. The results indicate that the solid precipitate generated during treatment of the metal-rich waters was sulfate-green rust. The decrease in sulfate concentrations over time for each experiment can be directly correlated with the green rust formation.