Abstract
Kinetics, equilibrium and thermodynamic studies of clavulanic acid adsorption onto hydrotalcites have been conducted with the aim of selecting the best conditions for biomolecular separation. For this purpose, hydrotalcites of different compositions (containing 30, 63 and 70% MgO in their compositions) were tested, with and without pre-treatment (uncalcined and calcined). A model was obtained to predict the degradation constant. This model may be useful in predicting the extent of clavulanic acid hydrolysis in the adsorption process. Adsorption studies were performed in stirred batch glass reactors. Assays were performed using solutions under different conditions, as well as variable solid/liquid ratios.
It was shown that distilled water and a solid/liquid ratio of 15.0 mg/ℓ generated the most favourable conditions for adsorption. A calcined hydrotalcite containing 70% MgO was selected for further study since it presented the best adsorption performance. The adsorbents were characterized by X-ray diffraction methods, Fourier-transform infrared spectroscopy and scanning electron microscopy. Adsorption equilibrium was evaluated from adsorption isotherms determined at different temperatures. The experimental isotherm data were well fitted by a linear equilibrium model with the corresponding adsorption constants being K = 0.404, 0.602, 0.849 and 1.083 ℓ/g at 16.5, 19.0, 21.5 and 24.0 °C, respectively, showing an increased adsorption capacity at higher temperatures.
Thermodynamic evaluation of the process allowed the Gibbs' free energy (ΔG0), the standard enthalpy change (ΔH0) and the standard entropy change (ΔS0) to be estimated as follows: ΔG0 = 2.100, 1.301, 0.503 and −0.295 kJ/mol at 16.5, 19.0, 21.5 and 24.0 °C, respectively; ΔH0 = 94.602 kJ/mol; and ΔS0 = 0.319 kJ/(mol K), respectively. The positive value of ΔH0 confirmed the endothermic nature of clavulanic acid adsorption onto HT70c hydrotalcites, which was considered to be mass transport reaction-controlled process.