Calcite precipitation in seawater using a constant addition technique: A new overall reaction kinetic expression

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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 coprecipitation 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: Ca 2+ + CO 3 - *ag CaCO 3(S) . The measured precipitation rate, R , is adequately described by a classic kinetic model of the form: R = R f - R b = f ( Ca [ Ca 2+ ]) n 1 ( CO 3 [ CO 3 2- n 2 - b , where R f , R b and f , b are the forward and backward reaction rates and rate constants for the overall reaction, respectively; [ i ], i , and n i are the total concentration, activity coefficient and reaction rate order, respectively, for each species involved in the reaction. If [Ca 2+ ] is held constant throughout the precipitation experiments, the above equation reduces to R = K f [ CO 3 2- ] n 2 - b . 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 K f = 10 3.5 mol(kg) 3 /m 2 h(mmol) 3 , n 2 = 3, and b = 0.29 mol/m 2 h with a correlation coefficient of 0.99 at 25°C, when P CO 2 = 0.0031 atm. and [ Ca 2+ ] 10.5 mmol/kg 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.

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