Tuesday 1:40 Danielle M C Huminicki, Virginia Polytecnic Institute and State University
The Effect of Gypsum Coatings on Dissolution Rate of Calcite in AMD solutions
Danielle M C Huminicki
A common passive treatment for acid mine drainage (AMD) is to react it with limestone to neutralize its acidity and increase its net alkalinity. During treatment, secondary mineral coatings form on the limestone (armoring), which decreases its dissolution rate and reduces the effectiveness of this treatment. This research shows that gypsum can form epitaxially on calcite dissolving in sulfuric acid solutions by the coupled reaction CaCO3 (cal) + H2SO4 (aq) + 2H2O = CaSO4•2H2O (gyp) + H2CO3 (aq) An array of batch reactor experiments measured the dissolution rates of Iceland spar calcite (Asp = 0.015m2/g, 40-60 mesh fraction) at room temperature in 0, 0.1, 0.3, or 1.0 M solutions of sodium sulfate with pH values of 1.5, 2.0, 2.5, 3.0 or 3.5. Scanning electron microscopy (SEM) showed that gypsum coatings formed on calcite at low pH where sulfate concentrations were high. Rates of dissolution are reported in terms of moles of H+ consumed. The apparent rate, r', was calculated by numerical differentiation and the rate, r = r'/A (mol/m2sec). The general rate law selected as the best fit to the rate data was r = kt^-0.5. Calcite dissolution rates decline with time as the growing gypsum layer greatly reduces the rate of H+ transport to the calcite surface. This physical behavior can be derived from Fick’s first law of diffusion such that k = 1/2[DCA/f(gyp)porosityV]^0.5 for the rate law stated above.
BiographyDanielle M. C. Huminicki completed a B. Sc. (Hons.) degree in geology at the University of Manitoba in 2000 under the supervision of Dr. Frank Hawthorne on the crystal chemistry of beryllium minerals. Part of this work was published in a chapter in the Reviews in mineralogy and geochemistry series. Following her B.Sc., she continued to work on the crystal chemistry of phosphate minerals, which is also published in a reviews in mineralogy and geochemistry chapter. After receiving her M. Sc. degree in 2002 she began her Ph. D. at Virginia Tech with Dr. J. D. Rimstidt as her advisor. She is working on iron and sulfur mineralogy and geochemistry and the important kinetic processes associated with the treatment and prevention of acid mine drainage.