EFFECT OF IRON COATINGS ON LIMESTONE AND SURFACE AREA ON DISSOLUTION RATES

Effectiveness of passive limestone treatment systems to neutralize acid mine drainage is dependent on the dissolution rate of the limestone. The limestone dissolution can be retarded by a layer of metal-hydroxides that can coat the limestone surface. The coatings are comprised of predominantly iron hydroxides. Determination of the dissolution of the coated limestone under field conditions is difficult because of access, variability in the flow of water, and the heterogeneity of the mine water chemistry. Laboratory experiments were developed to determine the dissolution rate with respect to surface area of limestone. Both uncoated and iron coated rates were determined using pH-Stat methods. A calibration curve of dissolution rate vs. surface area was produced using uncoated limestone marbles with calculated surface areas. Dissolution rates of limestone chips of unknown surface area were calculated. These same limestone chips were coated with goethite from an Fe⁺² environment. The dissolution rates after coating were obtained. Results showed that the apparent surface area of the coated limestone was smaller than when uncoated.