PII S0016-7037(98)00257-9 COMMENT Comment on “Precipitation kinetics of calcite in the system CaCO 3 -H 2 O-CO 2 : The conversion to CO 2 by the slow process H   HCO 3  3 CO 2  H 2 O as a rate limiting step” by W. Dreybrodt, L. Eisenlohr, B. Madry, and S. Ringer YUPING ZHANG* and CARLOS A. GRATTONI* Centre for Petroleum Studies, T. H. Huxley School, Imperial College, London SW7 2BP, UK (Received April 21, 1998; accepted August 21, 1998) Dreybrodt et al. (1996, 1997) observed in their CaCO 3 mineral precipitation experiments that the precipitation rate signifi- cantly increases with the ratio of the solution volume to mineral surface area (V/A). They used this result to demonstrate that the chemical reaction H + + HCO 3 - 3 CO 2 + H 2 O (1) is the rate limiting step for CaCO 3 precipitation. However, we believe that the increased CaCO 3 mineral precipitation rates under their experimental conditions may be caused by the formation of nuclei on other active surface sites. Reaction 1 as the rate limiting step of CaCO 3 precipitation can be demon- strated by using the previous experimental results from Zhang and Dawe (1998). Calcite seeds (one of CaCO 3 minerals) are normally used to investigate the kinetics of calcite growth (House, 1989; Nan- collas and Reddy, 1971; Zhong and Mucci, 1989). However, when the supersaturation of a CaCO 3 -saturated solution is above the critical value for homogeneous nucleation, the for- mation of extra nuclei inside the solution can occur. Under this circumstance, the measurements of CaCO 3 growth rate cannot be reliable. Even if the supersaturation is below the critical value, the formation of nuclei on the other surfaces, such as the walls of the reaction vessel and suspended particles within the solution, or the surface nucleation on CaCO 3 seed surface, can provide extra sites for CaCO 3 precipitation (Reedy and Gail- lard, 1981; House and Tutton, 1982). It is obvious that when the total calcite seed surface area is much higher than other avail- able surfaces, the reduction in calcium ion concentration can be considered as calcite growth on the added calcite seed surfaces. However, at low calcite seed concentrations, the other active surfaces can be compatible with the added calcite seed surface, so that the amount of Ca 2+ loss on the other surfaces may be significant and cannot be neglected for the calculation of CaCO 3 precipitation rate. The experimental results from Reddy and Gaillard (1981) demonstrated that the observed calcite growth rate increases when the concentrations of added calcite seeds decrease. When calcite seed concentrations are above a certain level (or V/A below a certain level), calcite precipitation rates do not change with calcite seed concentrations. They suggested that the higher growth rates at low seed concentrations are because of the competition between growth on the calcite seed crystal surface and surface nucleation. Similar results were obtained from the experiments by Dreybrodt et al. (1996, 1997) and Buhmann and Dreybrodt (1985), i.e. high CaCO 3 precipitation rates were observed from solutions with higher values of V/A. However from Dreybrodt et al conclusions a stable CaCO 3 growth rate could be obtained when V/A is higher than a certain level (or seed concentration is below a certain level) because transport will control the CaCO 3 growth rate. We feel that the analysis of this phenomenon from Reddy and Gaillard (1981) is reason- able. In addition, the solution saturation indexes used by Drey- brodt et al. (1997) and Buhmann and Dreybrodt (1985) are relatively high, at which a homogeneous nucleation or/and second nucleation may occur. If the surface area for the calcite growth is assumed the same as the initial calcite seed surface area, the calculated calcite growth rate will be higher than the actual values. Another evidence given by Dreybrodt et al. (1997) to support their conclusion is that the enzyme can enhance CaCO 3 pre- cipitation rate dramatically. In reaction 1 the rate limiting step may be the conversion of dissolved CO 2 into free gas. To form a gas phase inside a water solution, an energy barrier has to be overcome to form gas nuclei. The presence of the enzyme may reduce the energy barrier of forming a gas phase. However the enzyme may also act as a catalyst to reduce the energy barrier of forming nuclei of CaCO 3 minerals. These newly formed nuclei will provide additional sites for CaCO 3 growth and consuming calcium ions from the solution, which gives a higher observed CaCO 3 precipitation rate. According to Drey- brodt et al. (1996, 1997) at small V/A ratios the CaCO 3 mineral precipitation rate is limited by releasing CO 2 , whereas at large V/A the rate is controlled by the surface reaction. If they want to prove this mechanism, relative small V/A ratios (say, d = 15 m) should be chosen for their experiments to add enzyme in order to enhance the rate of releasing CO 2 , instead of the relatively larger V/A values (d = 425 or 605 m) they used. The chemical reaction of forming CaCO 3 precipitation is Ca 2+ + 2HCO 3 - 3 CaCO 3 + CO 2 + H 2 O. (2) One mole of CO 2 will be released by each mole CaCO 3 formed. *Authors to whom correspondence should be addressed (yuping.zhang@read.septnet.co.uk or c.grattoni@ic.ac.uk). Pergamon Geochimica et Cosmochimica Acta, Vol. 62, No. 23/24, pp. 3789 –3790, 1998 Copyright © 1998 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/98 $19.00 + .00 3789