Kinetics of calcite precipitation from seawater: I. A classical chemical kinetics description for strong electrolyte solutions

Abstract

To characterize the influence of solution composition and specific dissolved components on the complex mechanism of calcite precipitation in seawater, a kinetic study was carried out in NaClCaCl 2 solutions at a total ionic strength of 0.7 m. Experiments were conducted at 298.15 K and at four different CO 2 partial pressures. A constant addition technique was used to maintain [Ca 2+] at 10.5 mmol/ kg, while [CO 2− 3] was varied to isolate the role of this variable on the precipitation rate of calcite. Like in seawater solutions, calcite precipitation in the NaCl-CaCl 2 solution is dominated by the following reaction: Ca 2+ + CO 2− 3 ⇄ k b1 k f1 CaCO 3(s), Rx 1 where k fl and k bl are, respectively, the forward and backward reaction rate constants. The net precipitation rate, R, can be described at any given P CO 2 by R = k f1 ( a Ca ) na ( a CO 3 ) nb − k b1 or log ( R + k b1 ) = log K f1 + 3 log [ CO 2− 3], where α i and n i are, respectively, the activity and partial reaction order of the species involved in the reaction, K f1 = k 1( a Ca 2+ ) na ( γ CO 2− 3) 3 and γ is the activity coefficient. The partial reaction order with respect to the CO 2− 3 ion concentration is, as in seawater solutions, equal to 3. Unlike seawater, however, the intercept, log K f1 , increases with increasing P CO 2 . This is interpreted as a contribution of the HCO − 3 ion to the precipitation reaction. Its role was evaluated from the difference in the net rate R at every P CO 2 and a fixed [CO 2− 3]. Solution of a system of nonlinear equations allowed us to determine that the partial reaction order with respect to the HCO − 3 ion concentration is nearly equal to 2 and represented by the following reaction: Ca 2+ + 2HCO − 3 ⇄ k b2 k f2 CaCO 3(s) + H 2CO 3. Rx 2 Under our experimental conditions (1.1 < Ω c < 8.3, where Ω c is the saturation state of the solution with respect to calcite), the calcite precipitation rate can be described by the following expression: R = K f1 [ CO 2− 3] 3 — k b1 + K f2 [ HCO − 3] 2 − k b2 ( γ H 2 CO 3 ) αP CO 2 , where K f2 = k 2( a Ca 2+) nd ( δHCO − 3) 2 and α is the solubility of CO 2 in the experimental solution. We propose that, since the partial reaction order with respect to [CO 2− 3] is the same in both seawater and our experimental NaCl-CaCl 2 solutions, the precipitation mechanism of pure calcite in the latter solution may be similar to that of a 8 mol% magnesian calcite from seawater. We suggest that Mg 2+ and SO 2− 4ions, which are both major seawater constituents and calcite growth inhibitors, may not modify the precipitation mechanism, but decrease the rate constants of individual reactions. Finally, as opposed to our observations in the NaCl-CaCl 2 solution, the lack of P CO 2 dependence reported for calcite precipitation rates measured in seawater may be explained by a differential inhibition of the reactions participating in the overall growth process.