Reaction Kinetics and Mechanisms of Zeolite Dissolution in Hydrochloric Acid

The kinetics of zeolite dissolution in aqueous hydrochloric acid was investigated over a strongly acidic pH range (for pH <= 1 and [H+] = 0.1-6 M) in a batch reactor. The apparent reaction order with respect to hydrogen ion concentration decreased with increasing pH. Existing models do not predict this trend. A fundamental rate model derived from an adsorption and surface reaction mechanism is consistent with the experimental dissolution rates. The correlated model parameters suggest that the hydrogen ion strongly adsorbs onto zeolite surfaces and that zeolite dissolution follows Langmuir-Hinshelwood kinetics. A comparison of the LangmuirHinshelwood kinetic parameters reveals that the zeolite dissolution rates are dependent on the Si/Al ratio in the zeolite framework. Nonstoichiometric dissolution is observed for analcime and type Y zeolites, indicating a unique mechanism in which aluminum atoms are selectively removed. The ratio of the measured silicon dissolution rate to the stoichiometric dissolution rate ranges from 0 to 1 and increases with decreasing Si/Al ratios in the range of 1-2.6, suggesting that the zeolite dissolution phenomenon is controlled by the framework composition.