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Neutralization by limestone is a common treatment for acid mine drainage (AMD). The effectiveness of using limestone to treat AMD can be reduced by aluminum (Al) and iron (Fe) oxyhydroxide coatings that form on the limestone, because the coatings inhibit the transport, and thus neutralization, of hydrogen ions (H+) derived from acid mine drainage. I used mixed flow reactor experiments to investigate the effect of Al coatings on the diffusion of H+ to the surface of limestone and to quantify how those Al coatings affect the limestone dissolution rate. Experiments used acidic Al sulfate solutions with initial Al concentrations ranging from 0.002 M to 0.01 M (32 to 329 ppm) and pH values ranging from 3.7 to 4.2, which are typical of conditions found at AMD sites. Cleaved pieces of Iceland spar calcite were used as a proxy for limestone. The pH was measured in the effluent to determine the rate of H+ consumption. Effluent solutions were analyzed for Al, calcium (Ca) and sulfur (S) using inductively coupled plasma optical emission spectroscopy (ICP OES). Examination of the precipitated coatings using x-ray diffraction indicated that amorphous poorly crystalline gibbsite is the primary Al coating but scanning electron microscope analysis also suggests the possible presence of a poorly crystalline sulfur containing phase, such as hydrobasaluminite. The experimental data were used to calculate the diffusion coefficient of H+ through the Al coatings. The calculated diffusion coefficient for H+, assuming a gibbsite and/or hydrobasaluminite layer, ranged between 10-13 to 10-11 m2/sec, that are significantly lower than in pure water. / Master of Science / Acid mine drainage (AMD) is an acidic discharge characterized by low pH and high concentrations of toxic metals that can have an impact on the aquatic environments. A common treatment method for AMD is the use of limestone drains to neutralize the pH. However, the neutralization capacity of limestone drains can be reduced by coatings of aluminum (Al) that form on the limestone during treatment. I used mixed flow reactor experiments to investigate the effect of Al coatings on the diffusion of H⁺ to the surface of limestone and to quantify how those Al coatings affect the limestone dissolution rate. I measured pH in the effluent to determine the rate of H⁺ consumption during the reaction of the solutions with calcite. I also analyzed effluent solutions for Al, Ca and S concentrations. Examination of the produced coatings with x-ray diffraction suggests amorphous poorly crystalline gibbsite as the primary Al coating but scanning electron microscope analysis also suggests the presence of poorly crystalline hydrobasaluminite, a sulfur-bearing phase. The experimental results were used to model the decline in the limestone neutralization rate as the coatings grow thicker over time under different pH conditions and Al concentrations similar to those found in AMD. Finally, the diffusion coefficient for H⁺ , assuming a gibbsite and/or hydrobasaluminite layer, ranged between 10⁻¹³ to 10⁻¹¹ m²/sec, which is orders of magnitude smaller than the diffusion coefficient in pure water.

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