AN EXPERIMENTAL-STUDY OF CALCITE AND LIMESTONE DISSOLUTION RATES AS AFUNCTION OF PH FROM -1 TO 3 AND TEMPERATURE FROM 25 TO 80-DEGREES-C

Dissolution rates of single calcite crystals, limestones, and compress ed calcite powders were determined from sample weight loss using free- drift rotating disk techniques. Experiments were performed in aqueous HCl solutions over the bulk solution pH range -1 to 3, and at temperat ures of 25 degrees, 50 degrees, and 80 degrees C. Corresponding rates of the three different sample types are identical within experimental uncertainty. Interpretation of these data using equations reported by Gregory and Riddiford [Gregory, D.P., Riddiford, A.C., 1956. Transport to the surface of a rotating disc. J. Chem. Sec. London 3, 3756-3764] yields apparent rate constants and H+ diffusion coefficients. The log arithms of overall calcite dissolution rates (r) obtained at constant disk rotation speed are inversely proportional to the bulk solution pH , consistent with r = k(2') a(H+,b), where k(2)' stands for an apparen t rate constant and a(H+,b) designates the hydrogen ion activity in th e bulk solution, This variation of dissolution rates with pH is consis tent with corresponding rates reported in the literature and the calci te dissolution mechanism reported by Wollast [Wollast, R., 1990. Rate and mechanism of dissolution of carbonates in the system CaCO3-MgCO3. In: Stumm, W. (Ed.), Aquatic Chemical Kinetics. Wiley, pp. 431-445]. A pparent rate constants for a disk rotation speed of 340 rpm increase f rom 0.07 +/- 0.02 to 0.25 +/- 0.02 mol m(-2) s(-1) in response to incr easing temperature from 25 degrees to 80 degrees C. H+ diffusion coeff icients increase from (2.9 to 9.2) x 10(-9) m(2) s(-1) over this tempe rature range with an apparent activation energy of 19 kJ mol(-1). (C) 1998 Elsevier Science B.V. All rights reserved.