Kinetics of Calcite Dissolution in pH-region of Acid Rain.

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The dissolution of limestone collected from Akiyoshidai, Yamaguchi Pref., Japan, In dilute hydrochloric acid solution has been followed by measurement both calcium concentration and pH as a function of time, under various conditions of Initial pH, temperature and stirring rate. The initial dissolution rate increases as the stirring rate increasing. The Arrhenius plot shows that apparent activation energy at 7OOrpm of stirring rate is 15kJ/mol for Initial pH4 and 25kJ/mol for Initial pH5.The viscosity of water depends on temperature and differs about two times within the range from 5ﾟC to 40ﾟC in this experimental conditions, so stirring rate cannot express the real conditions of bulk fluid. In spite of stirring rate, Stirring Reynolds Number Re_s, which is one of non-dimension numbers possible to express fluid condition, defined as pd^2n/mu, where rho is the density of water, d characteristic length of stirring rate, n the revolutions per second, mu the viscosity of water, is proposed.The initial dissolution rate at initial pH4 is almost proportional to Re_s and is independent from temperature. These facts suggest that the dissolution reaction process of calcite is much faster than the mass transport process of species. In the case of Initial pH5, the initial dissolution rate increases as Re_s Increasing and as temperature raising. The apparent activation energy of initial dissolution rate at Re_s = 7X10^4 for initial pH4 is lkj/mol and for initial pH5 is 12kJ/mol. These values are smaller than those obtained from stirring rate, because the previous studies have not considered fluid condition in estimation of the activation energy.The facts mentioned above suggest that dissolution of calcite in weak acid region Is mass transport controlled reaction.