Geochimica n Cosmochimica Acfa Vol. 57, pp. 1409-1417 Copyright0 1993 Pergamon Press Ltd.Printedin U.S.A. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 0016-7037/93,‘$6.00 + .OO Calcite precipitation in seawater using a constant addition technique: A new overall reaction kinetic expression SHAOJUN ZHONG and ALFONSO MUCCI Department of Geological Sciences, McGill University, Montreal, Quebec zyxwvutsrqponmlkjihgfedcbaZYXWVU H3A 2A7, Canada f Received April 8, 1992; accepted in revisedform September 27, 1992) Abstract-A simple “constant addition” system was developed to study calcite precipitation reaction kinetics in seawater under steady state conditions. It can be applied to carbonate-trace element copre- cipitation studies and may also provide an interesting alternative for kinetic studies of calcite dissolution reactions and other mineral-solution interactions. Calcite precipitation in seawater can be represented by a reversible overall reaction: Ca2+ + CO:- ti CaCO 3(S)* The measured precipitation rate, R, is adequately described by a classic kinetic model of the form: R = Rf - Rb = kf(rca[Ca2’])“1(yco,[CO:-])“2 - kb, where Rr, Rb and kf, kb are the forward and backward reaction rates and rate constants for the overall reaction, respectively; [i], yi , and ni are the total concentration, activity coefficient, and reaction rate order, respectively, for each species involved in the reaction. If [Ca”] is held constant throughout the precipitation experiments, the above equation reduces to R = Kf[CO:-]“2 - kb. The equation was used to fit calcite precipitation rate data measured over a wide range of saturation states and extending to near saturation conditions. The least-squares fit to the above expression yields values of Kr = 103.5 ~mol(kg)3/m2h(mmol)3, n2 = 3, and kb = 0.29 rmol/m’h with a correlation coefficient of 0.99 at 25”C, when Pto, = 0.0031 atm. and [Ca2*] w 10.5 mmolfkg SW. The partial reaction order for the carbonate ion confirms that calcite precipitation in seawater proceeds through a complex mechanism, as suggested by previous calcite-seawater interaction studies. The calcite dissolution rate constant derived from this study is significantly lower than values obtained in dilute solutions. This observation is in agreement with results of previous studies which indicate that calcite dissolution is much faster in dilute solutions than in seawater under identical saturation conditions. I~ODU~ION NATURAL FLUIDS ARE NOT always in equilibrium with solid carbonate phases with which they are in temporary or per- manent contact. The deviation from ~uilib~um is usually small and the reaction kinetics under this condition are often very sensitive to environmental factors and solution com- position. Therefore, it is desirable to obtain accurate reaction rate data of individual carbonate minerals in various envi- ronmentally relevant solutions and conditions. Ideally, lab- oratory kinetic experiments should be conducted when the system under study is at steady state so that reactions such as precipitation, di~olution, and trace element coprecipita- tion occur at a constant rate, in an invariant environment, and following the same reactional pathway. Under these conditions, any measurable thermodynamic and kinetic property can be reasonably obtained in the time frame re- quired by the measurements without having to take into con- sideration changes of the reaction parameters with time. Fac- tors which may directly or indirectly influence the reactions can be studied by conducting a set of ex~~ments while vary- ing a particular parameter and keeping others constant. A detailed kinetic description of the reaction mechanism can be derived from a series of inv~tigations of individual pa- rameters. Experimental techniques have always played an important role in the evolution of our understanding of calcite-solution reaction kinetics. Various experimental techniques have been applied to achieve and maintain steady state conditions. Ex- amples of such techniques include ( 1) the “free-drift” method utilizing a single calcite crystal in a large volume of solution (e.g., NANCOLLAS et al., 198 1; BUSENEERG and PLUMMER, 1986); (2 ) the “pH-stat” technique (e.g., MORSE, 1974; INS- KEEP and BLOOM, 1985); (3) the “chemo-stat” system (e.g., Muccr and MORSE, 1983; ZHONG and Mwca, 1989) or “constant com~sition” system (e.g., IZAZMLERCZAK et al., 1982); and more recently, (4) the “fluidized bed” reactor (CHOU et al., 1989). Of these experimental techniques, only the “chemo-stat” or “constant composition” system and the “ihridized bed” reactor provided actual steady state conditions for the calcite-solution reaction. However, to apply the “chemo-stat” system, preliminary knowledge of the rates of the reactions under study is essential for the preparation of “titrant” solutions. It often requires tag-and~~or experi- mentation before a successful run can be conducted. More importantly, a nonsteady state period exists at the beginning 1409