Neutralization of sulfuric acid solutions by calcite dissolution and the application to anoxic limestone drain design

Batch reactor (BR) experiments were conducted to measure the effect of hydrodynamics and gypsum coatings on calcite neutralization rates. A factorial array of BR experiments measured the H+ concentration change by calcite dissolution over a pH range of 1.5–3.5 and Na2SO4 concentrations of 0–1 M. The rate of H+ concentration change with time was determined by numerical differentiation of H+ concentration versus time. Regression modeling showed that for uncoated calcite, rates are only significantly affected by pH, r=-10-2.32aH+0.76. Whereas, for calcite coated with gypsum only time had a significant effect on calcite dissolution rates, r = −10−1.96t−0.53.Because transport-limited dissolution rates for uncoated calcite are a function of the pH and Reynolds number, a model was developed to express the effects of these two variables on the rate of H+ consumption for a solution with a Darcy velocity, q, through a porous medium with a particle radius, rp, such that r′=1.08×10-3q0.31rp-0.69mH+0.87. This equation was integrated via a numerical model to simulate the performance of an idealized anoxic limestone drain (ALD). This model predicts the pH and alkalinity change along the length of an ALD. The model shows that the efficiency of an ALD is greater when the Darcy velocity is low and the particle radius is small.In addition, the growth of gypsum coatings causes the rate of H+ neutralization to decline as the square root of time as they form and block the H+ transport to the calcite surface. Supersaturation with respect to gypsum, leading to coating formation, can be avoided by diluting the ALD feed solution or by replacing limestone with dolomite.