EFFECT OF ALUMINUM OXYHYDROXIDE COATINGS ON THE PERFORMANCE OF ANOXIC LIMESTONE DRAINS

Neutralization by limestone is a commonly used treatment for acid mine drainage. The effectiveness of Anoxic Limestone Drains (ALDs) and Successive Alkalinity-Producing Systems (SAPS) is dramatically reduced by iron and aluminum oxyhydroxide coatings that form on the limestone, which inhibit the transport of hydrogen ions (H⁺) from acid mine drainage. We have designed several laboratory experiments to determine the effect of coatings on the diffusion of (H⁺) to the limestone surface and to quantify how these Al coatings affect the limestone dissolution rate.

Initially, batch experiments were used to evaluate what Al minerals form during the reaction of acidic Al-rich solutions with calcite. Aluminum concentrations ranged from 0.002 M to 0.01 M and the pH ranged from 3.6 to 3.8. Examination of the produced coatings with x-ray diffraction identified poorly crystalline gibbsite as the primary Al coating. Subsequent mixed flow reactor (MFR) experiments are being used to quantify the effect of the Al mineral coatings on the calcite dissolution rate. We are measuring pH to determine the rate of H⁺ consumption during the reaction of the acidic Al sulfate solutions with calcite. Initial MFR results show that the consumption of H⁺ declines after 4 to 6 hours, suggesting that the formation of a coating is limiting the diffusion of H⁺ to the calcite surface. Additional experiments using different Al concentrations are currently being conducted. Results from an initial experiment showed that the diffusion coefficient for H⁺ in the gibbsite layer (7.8x10^-13 m²/sec) is four orders of magnitude smaller than that in water (9.3x10^-9 m²/sec). Results of these experiments will be used to estimate the dissolution of limestone in ALDs under different pH conditions and Al concentrations similar to those found in discharges from pyrite-bearing materials.