Dissolution kinetics of calcium carbonate minerals in H2OCO2 solutions in turbulent flow: The role of the diffusion boundary layer and the slow reaction H2O + CO2 → H+ + HCO3−
Dissolution kinetics of calcium carbonate minerals in H2OCO2 solutions in turbulent flow: The role of the diffusion boundary layer and the slow reaction H2O + CO2 → H+ + HCO3−
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DOI：
10.1016/s0016-7037(97)00143-9
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发表时间：
1997-07
影响因子：
5
通讯作者：
Zaihua Liu;Wolfgang Dreybrod
中科院分区：
文献类型：
--
作者：
Zaihua Liu;Wolfgang Dreybrod
碳酸钙矿物在湍流水溶液中的溶解和沉淀受矿物表面附近的扩散边界层（DBL）控制，通过分子扩散影响传质。采用旋转圆盘法研究了介质阻挡层对碳酸钙溶解速率的影响。该技术允许通过控制CaCO 3的圆形样品的旋转速度来精确调节DBL的厚度。在不同CO2分压（1·10− 3 ~ 1 atm）下，对H2O <$CO2 <$Ca2 +-溶液中的溶解速率进行了测量，结果表明，速率R与旋转频率ω有关，由R <$ωn给出。指数n从低Pco 2时的0.25变化到Pco 2为1大气压时的约0.01。这表明速率不仅受质量传递控制，这需要n = 0.5。实验数据可以用一个理论模型来解释，该模型还考虑了CO2+ H2O → H++ HCO 3 −的缓慢反应和表面的化学反应（Dreybrodt和Buhmann，1991）。根据该模型对实验数据的解释表明，CO2的转化率在控制反应速率中起着重要作用。在高PCO 2和大DBL厚度（λ> 0.001 cm）下，CO2的转化主要发生在DBL中，因此成为速率限制。这通过以下观察得到证实：在添加催化CO2转化的碳酸酐酶后，溶解速率提高了1个数量级。从我们的实验观察，我们得出结论，上述理论模型，使人们能够预测具有令人满意的精度的溶解速率。由于从过饱和溶液的沉淀速率是由相同的机制溶解，我们推断，该模型也是有效的预测沉淀速率。溶解和沉淀的预测速率可以近似为线性速率定律R = α ·（ceq− c），其中ceq是方解石的平衡浓度，a是速率常数，取决于温度、Pco 2、DBL厚度（δ）和在矿物上流动的水层厚度。列出了可用于各种地质相关条件的α值。
Dissolution and precipitation of calcium carbonate minerals in aqueous solutions with turbulent flow are controlled by a diffusion boundary layer (DBL) adjacent to the surface of the mineral, across which mass transfer is effected by molecular diffusion. A rotating disk technique was used to investigate the effect of the DBL on the dissolution rates of CaCO3. This technique allows an exact adjustment of the thickness of the DBL by controlling the rotation speed of a circular sample of CaCO3. Measurements of the dissolution rates in H2OCO2Ca2+-solutions in equilibrium with various partial pressures of CO2from 1·10−3up to 1 atm showed a dependence of the rates R on the rotation frequency ω, given by R ∝ ωn. The exponent n varies from 0.25 at low Pco2to about 0.01 at a Pco2of 1 atm. This reveals that the rates are not controlled by mass transport only, which would require n = 0.5. The experimental data can be explained employing a theoretical model, which also takes into account the slow reaction CO2+ H2O → H++ HCO3−and the chemical reactions at the surface (Dreybrodt and Buhmann, 1991). Interpretation of the experimental data in view of this model reveals that conversion of CO2plays an important role in the control of the rates. At high PCO2and large DBL thickness (ϵ > 0.001 cm), conversion of CO2occurs mainly in the DBL and, therefore, becomes rate limiting. This is corroborated by the observation that upon addition of the enzyme carbonic anhydrase, which catalyzes CO2-conversion, the dissolution rates are enhanced by 1 order of magnitude. From our experimental observations we conclude that the theoretical model above enables one to predict dissolution rates with satisfactory precision. Since the precipitation rates from supersaturated solutions are determined by the same mechanisms as dissolution, we infer that this model is also valid to predict precipitation rates. The predicted rates for both dissolution and precipitation can be approximated by a linear rate law R = α · (ceq− c), where ceqis the equilibrium concentration with respect to calcite and a a rate constant, dependent on temperature, Pco2, DBL thickness (ϵ), and the thickness of the water sheet flowing on the mineral. Values of α are listed that can be used for a variety of geologically relevant conditions.