Mechanisms of scale formation and carbon dioxide partial pressure influence. Part II. Application in the study of mineral waters of reference

In the first part, we have designed a new model of evolution for the calco-carbonic system which includes the hydrated forms of CaCO3 : CaCO3 amorphous, CaCO3•6H2O (ikaite) and CaCO3•H2O (monohydrate) (J. Eur. Hydr. 30 (1999) 47). According to this model, it is the precipitation of one or other of these hydrated forms which could be responsible for the breakdown of the metastable state. After this first step, the precipitates evolve to dehydrated solid forms. Through the elaboration of computer programs in which the CaCO30 (aq) ion pair formation was considered, this model was compared to experimental data obtained by the critical pH method applied to synthetic solutions. In the present article, the same method was applied for four French mineral waters, at 25°C under study. Three samples formed a precipitation during the sodium hydroxide addition. For these three cases, this precipitation began for the CaCO3•H2O saturation. The added volume of sodium hydroxide was more than what was required for neutralizing free CO2 initially in solution. These results indicate that during a spontaneous scaling phenomenon, the pH rises at the same time by loss of the initial free CO2 and of the one produced by the hydrogen carbonate ions decomposition. Then we calculated, at various temperatures for the three studied scaling waters: CO2 partial pressures and loss of total carbon corresponding to the solubility products of CaCO3 hydrated forms. The results show that the partial pressure monitoring of the carbon dioxide is important in managing the behavior of scaling waters.