KINETICS OF CALCITE PRECIPITATION FROM SEAWATER - II - THE INFLUENCE OF THE IONIC-STRENGTH

To characterize the influence of ionic strength on the kinetics of cal cite precipitation from seawater solutions we carried out a set of exp eriments at four different ionic strengths (I = 0.10; 0.34; 0.55; 0.93 m) ln NaCl-CaCl2 solutions, at the temperature of 298.15K and a CO2 p artial pressure of 100 Pa. The constant addition technique was used in order to maintain [Ca2+] at congruent to 10.5 mmol/kg, while the [CO3 2-] was varied to isolate its role on the precipitation rate of calcit e. Assuming that the calcite precipitation in this solution is dominat ed by the reaction: Ca2+ + CO32- (kb)double left right arrow(kf) CaCO3 (1) where k(f) and k(b) are, respectively, the forward and reverse re action rate constants, the net precipitation rate, R, can be described at any ionic strength by R = k(f)(aCa(2-))(n1)(a(CO32-))(n2) - k(b) ( 2) or, in its logarithmic form Log (R + k(b)) = Log K-f + n(2) Log [CO 32-] (3) where n(i) are the partial reaction orders with respect to th e participating ions, a and gamma are, respectively, the ion activitie s and activity coefficients and, K-f = k(a(Ca2+))(n1)(gamma(CO32-))n(2 ), a constant at a given ionic strength. Results of this study indicat e that, when the ionic strength is increased from 0.10 to 0.93 m, the partial reaction order with respect to the CO32- concentration increas es from 1 to 3 and the forward reaction rate constant, k(f), increases by several orders of magnitude. This is interpreted as both a change in the calcite precipitation mechanism and a catalysis generated by th e presence of inert electrolytes. Applying our model to the rate measu rements carried out by Zhong and Mucci (1989) in seawater solutions at various salinities, under the compositional condition [Ca2+] much gre ater than [CO32-], we find that the partial reaction order with respec t to the carbonate ion and the forward reaction rate constant increase as a function of the total ionic strength of the seawater solutions. A 50% increase of the total ionic strength of the parent solution resu lts in an increase of the precipitation rate by 2 orders of magnitude. Finally, we propose that ion interactions in solution and a concomita nt change of the precipitation mechanism may contribute to the develop ment of nonequivalent kink sites of different sizes on the surface of the growing crystal but which still satisfy the symmetry. Variations i n the amount of foreign ions adsorbed and incorporated into marine cal cites could, therefore, be determined by the density of these less con strained sites, which, in turn, would be dependent on the total ionic strength of the solution. Copyright (C) 1998 Elsevier Science Ltd.