KINETICS OF NEAR-EQUILIBRIUM CALCITE PRECIPITATION AT 100-DEGREES-C -AN EVALUATION OF ELEMENTARY REACTION-BASED AND AFFINITY-BASED RATE LAWS

Three affinity-based rate models based upon physical growth mechanisms were used to fit surface-controlled precipitation rate data for calci te using a continuously stirred tank reactor in NaOH-CaCl2-CO2-H2O sol utions at 100 degrees C and 100 bars total pressure between pH 6.38 an d 6.98. At higher stirring speeds, when alpha(H2CO3) was smaller than 2.33 x 10(-3), rate showed a parabolic dependence upon exp(Delta/RT) for exp(Delta G/RT) < 1.72. However, the rate increased exponentially for exp(Delta G/RT) > 1.72 and followed a rate law based upon the assu mption that surface nucleation is rate-limiting. When alpha(H2CO3) wa s greater than 5.07 x 10(-3), the rate showed a linear dependence upon exp(Delta G/RT), suggesting growth by a simple surface adsorption mec hanism. The rate of these three mechanisms at 100 degrees C can be exp ressed by the following equations: [GRAPHICS] The mechanistic model of Plummer et al. (1978) given by R(net) = k(1)a(H+) + k(2)a(H2CO3) + k (3)a(H2O) - k(4)a(Ca2+)a(HCO3-) also describes the precipitation rate when growth followed the spiral growth equation. The rate constant for precipitation, k(4), ranges between 7.08 X 10(-4) to 1.01 X 10(-3) mo les cm(-2) s(-1) in the a(H2CO3) range studied. This work shows that precipitation at 100 degrees C in the spiral growth regime is well fit by both the mechanistic model of Plummer et al. (1978), based on mult iple elementary reactions, and by a model derived for growth at screw dislocations. Outside of the regime of spiral growth, however, the mod el of Plummer et al. (1978) fails, suggesting that different elementar y reactions control growth in the adsorption or two-dimensional nuclea tion regimes. However, the model of Plummer et al. (1978), based upon individual elementary reactions, accurately predicts both dissolution and precipitation of calcite under certain conditions; tests of the af finity based models reveal that none of these models accurately predic t dissolution. Therefore, although affinity-based models may yield ins ights concerning the physical mechanism of growth, they may not be as useful in modelling dissolution and growth over the full range of Delt a G.