Kinetic and thermodynamic analysis of thermo-oxidative degradation of seashell powders with different particle size fractions: compensation effect and iso-equilibrium phenomena

Abstract

The issue of accumulated seashell waste has been increasingly addressed in the studies of its applicability as a secondary source of calcium carbonate. In this work, the mechanism of the thermo-oxidative degradation of two types of mollusk shells (Dosinia exoleta-DE and oyster Ostrea edulis-OE) was investigated using simultaneous thermal analysis. Physicochemical characterization of raw and calcined materials was performed by XRD and SEM techniques. Results show that increasing powder particle size and heating rate affect the degradation of investigated shells differently and have a significant contribution to released quantities of CO2. It was proposed that the calcination rate of DE powders is determined by the strain-induced collapse of CO2-deficient calcite at its interface with crystalline CaO rods-like structure, with simultaneous expulsion of CO2. A much higher CO2 concentration was identified during the degradation of OE powders, favoring the carbonation reaction and, consequently, increasing the temperature and activation energy of the investigated reaction. Under elevated CO2 concentration, the CaO aggregation step is enhanced by slower reaction kinetics at lower temperatures since both CO2 desorption and structural transformations are probably hindered. The proposed mechanism leads to a meaningful correlation between thermodynamic quantities (enthalpy-entropy compensation) in the form of an iso-kinetic relationship.