CaCO_3-CO_2-H_2O岩溶系统的平衡化学及其分析

EQUILIBRIUM CHEMISTRY OF THE CaCO_3-CO_2-H_2O SYSTEM AND DISCUSSIONS

在介绍了CaCO3-CO2-H2O岩溶系统平衡化学的原理后,对平衡化学的控制因素,包括温度、CO2分压、体系的开放程度、离子强度效应、同离子效应、酸效应、碱效应、离子对效应进行了分析,并与CaMg(CO3)2-CO2-H2O岩溶系统平衡化学作了对比。结果显示,天然开放的岩溶系统的平衡pH值范围为6.80～8.40,在此pH值范围内,水中的碳组分主要以HCO-3形式存在;与开放系统相比,在其它条件相同情况下,封闭系统的平衡pH值较高,而平衡[Ca2+]和平衡[HCO-3]较低,特别是在低CO2分压时,两者的差异更明显;在封闭系统条件下,两种不同的纯CaCO3-CO2-H2O饱和溶液相混合,将导致溶液对CaCO3重新具有侵蚀性;离子强度效应、酸效应和离子对效应使方解石的溶解度增加,而同离子效应和碱效应使方解石的溶解度降低;与方解石溶解平衡相比,其它条件相同时,白云石溶解平衡pH较高,在温度<70℃时溶解度较大,但在温度>70℃时溶解度较小。

After introducing the principle of the equilibrium chemistry of the CaCO_3-CO_2-H_2O system, the authors discussed the controlling factors of the equilibrium pH, equilibrium [HCO^-_3] and [Ca^(2+)], such as temperature, CO_2 partial pressure, the availability of CO_2 in the system, ionic strength effect, common-ion effect, acid effect, base effect, and ion-pair effect. And then, comparison of the equilibrium chemistry between calcite and dolomite dissolution in CO_2-H_2O solutions was made. It shows that the equilibrium pH value of water in natural open karst system ranges from 6.80~ 8.40 and so HCO^-_3is the major type of carbon compositions in solutions of the system. The equilibrium pH value of water in closed karst system is higher than that in open system under the same other conditions, whereas the equilibrium [Ca^(2+)] and [HCO^-_3] are lower in closed system. Under closed system conditions, mixing of two differently saturated waters leads to the solution in a pure CaCO_3-CO_2-H_2O system renew calcite aggressive ability. Ionic strength effect, acid effect, and ion-pair effect will increase calcite solubility, while common-ion effect and base effect will decrease calcite solubility. The equilibrium pH value of water and the solubility of dolomite in open CaMg(CO_3)_2-CO_2-H_2O system is higher than that in open CaCO_3-CO_2-H_2O system when temperature lower than 70℃ under the same other conditions.