Studies of kinetic models and adsorption isotherms: application on the interaction of insulin with synthetic hydroxyapatite

The non-stoichiometric, calcium-deficient hydroxyapatite was prepared through a low-temperature from aqueous solutions method and characterized using Physico-chemical methods. The potential of this hydroxyapatite to adsorb and release insulin from aqueous solutions was evaluated under physiological conditions. The effect of contact time and initial concentration were studied in batch experiments. The adsorption rate reached up to 81±5% in the first half-hour of contact, while the release rate of insulin incubation was about 41 ± 5% after 1 hour. The pseudo-first-order, pseudo-second-order, Elovich equation, Weber and Morris intraparticle diffusion model and Bangham’s pore diffusion model were applied to study the kinetics of the adsorption process. The pseudo-second-order kinetic model provided the best correlation R2(0.998) of the used experimental data compared to the other models. The adsorption of insulin onto hydroxyapatite was correlated well R2(0.998) with the Langmuir model as compared to Freundlich, Temkin and Dubinin–Kaganer–Radushkevich (D-K-R) models, with a maximum adsorption capacity of 24.46 mg/g. The isotherms parameters values of ΔG0, b_t and E show that the adsorption process is favorable, spontaneous, exothermic, and controlled by physisorption. The point of zero charge (pHZPC) of hydroxyapatite and the isoelectric point (pI) of insulin indicate that the interaction of insulin molecules with prepared apatite can be well described as an ions exchange reaction

Kinetic, isotherm and thermodynamic studies of the adsorption of phenol and tyrosine onto apatitic tricalcium phosphate

The present study was conducted to evaluate the feasibility of apatitic tricalcium phosphate with a Ca/P ratio of 1.50 for the adsorption of phenol and tyrosine from aqueous solutions. The adsorbent was synthesized at room temperature using an aqueous double decomposition method and characterized through physicochemical methods. Batch adsorption studies were conducted as a function of contact time, initial adsorbate concentration, temperature, and pH. The adsorption kinetics of phenol and tyrosine were well fitted to the pseudo-second-order model. The maximum adsorption capacity was found to be 5.56 mg/g for phenol and 9.65 for tyrosine mg/g at 298 K. The adsorption of phenol and tyrosine was well explained using the Langmuir, Freundlich, Temkin, and Dubinin-Radushkevick models. The Langmuir model is the most suitable, with a maximum monolayer adsorption capacity of 7.32 mg/g for phenol and 11.43 mg/g for tyrosine at 298 K. The thermodynamic parameters indicate that the adsorption process is favorable, spontaneous, exothermic, and controlled by physisorption with electrostatic interactions between compounds containing the phenolic group and apatite. The results of this study have demonstrated the potential utility of apatitic tricalcium phosphate, which could be developed into a viable technology for the adsorption of compounds containing the phenolic group from aqueous solutions