Date of Graduation

1997

Document Type

Dissertation/Thesis

Abstract

Treatment of acid mine drainage (AMD) through the use of anoxic limestone drains (ALDs) is a technology that has shown mixed success. Precipitation of solid phase materials, primarily Fe{dollar}\\sp{lcub}3+{rcub}{dollar} and Al{dollar}\\sp{lcub}3+{rcub}{dollar} hydroxides, inhibits limestone dissolution by presumably creating a barrier between the limestone surface and the AMD, which impedes neutralization at an optimum rate. A laboratory project was initiated to: monitor geochemical transformations of reactive species ({dollar}\\rm Al\\sp{lcub}3+{rcub},\\ Fe\\sp{lcub}3+{rcub}{dollar}, and SO{dollar}\\sb4\\sp{lcub}2-{rcub}{dollar}), determine solid phases controlling precipitation reactions, and determine a kinetic model for limestone dissolution in ALDs. Laboratory ALDs were constructed and filled with graded, washed limestone. Three sampling ports were installed for extraction of AMD along the flow path. AMD was made by adding the desired concentrations of {dollar}\\rm Fe\\sp{lcub}3+{rcub},\\ Al\\sp{lcub}3+{rcub}{dollar}, and SO{dollar}\\sb4\\sp{lcub}2-{rcub}{dollar} to distilled, deionized water. AMD was pumped upward through the ALD for a 12-hour period, with samples taken at various times. Ferric iron and Al{dollar}\\sp{lcub}3+{rcub}{dollar} experiments were completed as single reactive ions at three concentrations (2.2, 2.7, and 3.2 mM). Experiments containing SO{dollar}\\sb4\\sp{lcub}2-{rcub}{dollar} were completed with Fe{dollar}\\sp{lcub}3+{rcub}{dollar} or Al{dollar}\\sp{lcub}3+{rcub}{dollar} as the acid cation. Ferric iron or Al{dollar}\\sp{lcub}3+{rcub}{dollar} were held constant while SO{dollar}\\sb4\\sp{lcub}2-{rcub}{dollar} was added in stepped molar ratios. Results show that the majority of dissolution and precipitation occurred within the first quarter of the flow path. Lower acidity levels combined with the precipitation barrier inhibited dissolution in the last three quarters of the flow path. The addition of SO{dollar}\\sb4\\sp{lcub}2-{rcub}{dollar} enhanced dissolution. Equilibrium calculations indicate that experiments not containing SO{dollar}\\sb4\\sp{lcub}2-{rcub}{dollar} were controlled by Fe(OH){dollar}\\sb{lcub}\\rm 3(a){rcub}{dollar} and Al(OH){dollar}\\sb{lcub}\\rm 3(a){rcub}{dollar} precipitation reactions. The addition of SO{dollar}\\sb4\\sp{lcub}2-{rcub}{dollar} showed that other minerals were oversaturated at various pH values throughout the experiments. Hydronium jarosite was oversaturated at pH {dollar}{dollar} 4.6. Schwertmannite may have precipitated in the pH interval of 3.1 to 4.6. Jurbanite was oversaturated at pH {dollar}<{dollar} 5.13 in Al{dollar}\\sp{lcub}3+{rcub}{dollar} experiments containing SO{dollar}\\sb4\\sp{lcub}2-{rcub}{dollar}, while Al(OH){dollar}\\sb{lcub}3(a){rcub}{dollar} was oversaturated at pH {dollar}>{dollar} 5.13. Identification of solid phases by x-ray diffraction could not further elucidate the composition of sediments. A concentration dependent kinetic model was unattainable because of the lack of data when the dissolution reaction occurred.

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